Fig. 1 shows the geography of the Darwin area. In the late pre-monsoon period there is a diurnal cycle of intense convection over the Tiwi Islands (Keenan et al., 1990; Keenan and Carbone, 1992; Carbone et al, 1999; Wilson et al., 1999). This phenomenon is driven by the sea breeze circulation and is so regular as to have earned the local nickname of "Hector". Hector is generally initiated by interaction of convectively-driven cold pools and see breeze fronts and builds into a bundle of intense thunderstorms reaching the tropopause in mid-afternoon, then finally moving off-shore to the west of Bathurst Is. where it dissipates by mid-evening. Updrafts as strong as 40 m/s and cloud tops to 20 km (more typically 17-18 km) have been observed during Hector. The top panel of Fig. 2 displays afternoon radar and surface meteorological observations on a particular day in late November 1995, and shows the organized nature of the convection and surface flow.

The interesting character of the convection in the area led to Darwin being chosen as the site for the MCTEX (Maritime Continent Thunderstorm Experiment) campaign conducted in November and December 1995. This was a joint project of the Australian Bureau of Meteorology Research Centre (BMRC) and researchers at NCAR, NASA Goddard Space Flight Center, Colorado State University, and Monash University, and included contributions from groups at several other institutions in the USA, Australia and Japan. The MCTEX experience provides a very valuable base for understanding the tropospheric convection in the region. MCTEX is described in more detail in Appendix A. The proposed 2002 date for ETCE will allow the 1995 MCTEX results to be thoroughly analyzed and then used in the detailed planning of the new field campaign.

Another key advantage of the proposed venue is that Darwin is a TRMM (Tropical Rainfall Measurement Mission) validation site and, as a consequence, has been extensively instrumented for meteorological monitoring of convection (Keenan and Manton, 1996). Fig. 1 shows the instruments that were available during the 1995 MCTEX campaign, including a dual polarized Doppler radar (C-POL), 50 MHz and 920 MHz profilers, rainguage networks, disdrometers, automatic surface weather stations, radiation instruments and lightning detectors. In addition, regular operational rawindsonde observations are taken near Darwin. Much of this instrumentation remains deployed for the TRMM monitoring. The C-POL radar is not currently near Darwin but BMRC will deploy it in the Darwin area for ETCE.

Darwin has an excellent civilian infrastructure, including weather forecast facilities and access to real time satellite data. There has been considerable local experience in mounting a large field campaign developed during MCTEX. Darwin was also a base for the tropical campaign of the Stratosphere-Troposphere Exchange Project (STEP), during which the NASA ER-2 flew out of Darwin Airport. There is a military airbase at Tindal, 275 km south of Darwin, that can also be used if necessary.

The key aspects of the Hector phenomenon that make it uniquely suitable as a subject for the ETCE experiment are its:

(i) Regularity and predictability. The fact that virtually each day during the late transition season a strong Hector occurs over the Tiwi Islands obviously aids in the formulation of an efficient observational strategy. Even an experiment lasting 5-6 weeks may allow only ~15 flights of a high-altitude aircraft. It is obviously very valuable to know in advance that a strong convective event is likely to develop. It may also make it much more palatable for pilots who are expected to fly near the convection if they can be reassured that the rapidly deepening turrets will be fairly localized. Experience from MCTEX suggests that even the day-to-day differences in the development and propagation of Hector are fairly predictable in real time, given observations in the morning of the ambient temperature and wind soundings. In particular the MCTEX observations showed that the diurnal evolution of the convection generally takes one of two paths. In about 20% of the cases (Type A) the convection is largely initiated by the collision or merging of inward-penetrating sea breeze fronts. This leads to development of somewhat weaker convection (or convection peaking later in the day) than in the more common (Type B) cases. In the Type B development (about 80% of cases) the organized convection is initiated before the actual merging of sea breeze fronts by interaction between cold pools (from early less-organized convection) and the sea breeze fronts. This Type B evolution generally results in the strongest fully-developed Hectors.

(ii) Strong intensity. It is likely that the effects of convection in stratosphere-troposphere exchange and in generating gravity waves depend strongly on the intensity of the convection, notably the degree of stratospheric penetration by the turrets. One obvious example is possible injection of tropospheric air into the stratosphere which presumably is much more effective for more deeply penetrative clouds. Similarly deep clouds are more likely to force strong gravity waves with easily observed momentum fluxes. Given that ETCE will be one of the first detailed studies of these various effects, it is reasonable to focus on a system where the expected convective influences should be strong. Also note that one of the key issues to be studied in the chemistry component of ETCE is the effect of lightning on NOx production, and for this Hector is very suitable as it is extremely active electrically,

(iii) Locally-forced nature. Hector is largely forced by locally-driven sea-breezes and so the convection is somewhat isolated from other deep cloud systems, at least on many days. This is a crucial simplification for many of the measurements that are planned for ETCE. In particular, the chemical measurements will attempt to sample air that has detrained from near the top of the convective core. Similarly the in situ observations of possible tropospheric injections into the stratosphere will require a target region where there is a possibility of finding the signal of irreversibly mixed tropospheric air associated with the convection. Planning a sampling strategy for these aspects will be much simpler if one has to deal only with a well-defined isolated convective system. The isolated nature of the system makes it easier to study the vertical transport of chemical constituents. Also, because the convection is located away from major sources of pollution, it should be possible to isolate NOx production due to lightning. Also very significant is the fact that the locally-forced nature of Hector makes the job of detailed numerical simulation much more feasible than for other more random, synoptically-forced convection. Indeed there has already been some considerable success in simulating the development of Hector as observed on some individual days during MCTEX (see Section 8, below).

The experiment is envisaged as a major international project with contributions from many countries, but principally from the US, and Australia. It is expected that the various international participants will work cooperatively and creatively to design the optimal observing strategies. The support and participation of the Australian Bureau of Meteorology Research Centre (BMRC) is crucial for ETCE, of course. BMRC scientists are principally interested in studying the dynamics of the convection itself (goal 1 of Table 1). In addition, there is strong interest from other groups in Australia in studies of the gravity wave generation and propagation (goal 2), and in cirrus dynamics and microphysics (goal 4). Further details of ETCE-related proposals being prepared in Australia and elsewhere are given in Section 9.

3. Investigation of the Tropospheric Convection During ETCE

a. Proposed Measurements

Detailed monitoring of the time-dependent, three-dimensional structure of Hector is crucial for the goals of ETCE. This will be accomplished primarily through a combination of airborne and ground-based Doppler radar observations. One key resource that will be deployed is the NCAR Electra aircraft equipped with the ELDORA radar. In one mode of operation the Electra would fly some 15-20 km away from the storm near 4-5 km height in straight line legs or curved patterns for airborne multiview-Doppler ELDORA measurements. It will be able to monitor the complete storm life cycle (~8 hours). It will provide unprecedented complete dual-Doppler synthesis of the initial growth, merger, and convective scale processes such as penetrative convection involved in the excitation of gravity waves and possible convective injection of water into the stratosphere. Kinematic details on explosive cell growth and perturbation of the tropopause should be observable. In another mode of operation the Electra would fly close to the base of the storm and use upward-looking ELDORA to obtain rather direct Doppler determinations of vertical velocity in the storm.

While the airborne ELDORA system is extremely powerful, it does have limitations related to the geometry of viewing deep storms at high resolution. For the very complete observation of Hector envisaged for ETCE, the ELDORA radar must be supplemented with ground-based Doppler radars. The BMRC/NCAR C-Band polarimetric/Doppler (C-POL) radar which has been deployed for long periods near Darwin (including during MCTEX) will be available for ETCE. In addition it is proposed to deploy the NCAR S-Band polarmetric/Doppler (S-POL) radar to the Darwin area for ETCE. The exact locations of the radars have not been determined, but one possibility would be to have the C-POL located at Gunn Pt. on the mainland and the S-POL radar placed at Niugiu on Bathurst Is.

In addition to the polarimetric radars, there will be other ground-based instruments available for remote sensing of the troposphere during ETCE. There is another BMRC Doppler radar located 30 km south of Gunn Pt. at Berrimah and this should provide additional storm surveillance information. In addition there are microwave wind profilers now deployed, and an Australian Research Council proposal has been submitted by researchers at the University of Adelaide for deployment of another tropospheric wind profiler during ETCE. Finally, investigators from the University of Massachusetts (who participated in MCTEX) are interested in deploying their upward-looking cloud radar as part of ETCE. Their system operates simultaneously at 33 GHz and 95 GHz through a single antenna aperture and is fully polarimetric. Multi-wavelength analysis of the Massachusetts MCTEX data has permitted estimation of particle size distributions in ice clouds and vertical wind estimation in precipitation. Recent work with full Doppler spectra FFT measurements of precipitation has shown that vertical air motions below the melting layer can be resolved with high precision and high time resolution (less than two seconds).

b. Scientific Issues to be Addressed

The 1995 MCTEX experiment made very impressive strides in the detailed understanding of the initiation and development of the Hector convection. However, even after the extensive analysis of the MCTEX results (see Carbone et al., 1999; Wilson et al., 1999) there remain a number of important questions concerning the dynamics of the convection. The proposed observations for ETCE will allow the MCTEX results to be extended in important ways (in particular ETCE should supplement the C-POL radar data available during MCTEX with measurements from both the S-POL radar and the airborne ELDORA radar). Colleagues involved in MCTEX (both on the US and Australian side) are very excited by the prospect of conducting another extensive study of the transition season over Melville and Bathurst Islands as part of ETCE. The key scientific aims for this component of ETCE are briefly summarized below.

(i) Establish more firmly a dynamically-based mesoscale climatology for the island convection. The MCTEX timeseries, while a major step forward, is too short to establish this with any certainty. The major findings from MCTEX would be the testable hypotheses (e.g. Type A frequency vs Type B frequency) in ETCE. In the MCTEX observations the degree of early convection appeared to regulate whether a Type A or Type B Hector initiation would occur. Spatial measurements of water vapor during MCTEX were insufficient to study the effect of stability on the suppression of early convection. Thus such measurements would be a goal during ETCE. Also longer periods of observations are needed to clarify the relation of the Hector convection to continental influences and the overall large-scale environment. The combined MCTEX/ETCE timeseries should put such findings and others on statistically significant ground (with a total of ~60 diurnal cycles).

(ii) Quantify the land-atmosphere latent/sensible heat fluxes, island boundary layer entrainment from the free troposphere, and storm-scale water budgets, for the purpose of determining which factors influence most the amplitude of organized convection.

(iii) Produce detailed data sets that can be used to evaluate high-resolution models of explicitly-resolved convection.

4. Gravity Wave Excitation

a. Background

There is now widespread appreciation that the distribution of important trace constituents, including ozone, in the atmosphere is very significantly affected by dynamical transport and that any complete model of the chemistry of the stratosphere and of the upper troposphere-lower stratosphere (UTLS) region needs to have a credible treatment of the dynamics. The global-scale mean meridional circulation in the middle atmosphere is driven by a competition between the in situ radiative heating which tries to create a state with very cold winter polar temperatures, and eddy processes that drive the temperatures away from this state (e.g., Fels, 1987). Haynes et al., (1991) showed that the general circulation of the atmosphere was governed by a "downward control" principle, i.e that the mean meridional circulation at any location is largely controlled by the eddy momentum flux divergences at higher levels in the atmosphere.

A realistic simulation of the stratospheric transport in a numerical model thus requires an accurate treatment of the radiative transfer and an adequate representation of the eddies throughout the depth of the atmosphere. It is believed that the radiative transfer codes used in comprehensive general circulation models (GCMs) are now sufficiently accurate, but there are still major deficiencies in the simulated stratospheric transport in current GCMs (see Hamilton, 1996, for a review). In particular GCMs typically produce a simulation in the middle atmosphere characterized by an unrealistically weak meridional circulation and a temperature structure that is too close to radiative equilibrium (i.e. colder than observed in the winter polar stratosphere and mesosphere, and warmer than observed in the summer polar mesosphere). It has become increasingly clear that these discrepancies result in large measure from the omission of the effects of eddies with scales too small to be resolved in current GCMs (e.g., Garcia and Boville, 1994). In particular, gravity waves with relatively small horizontal wavelengths (say from ~10 km to ~several hundred km) play a crucial role in driving the meridional circulation in the stratosphere. Lindzen (1981) pointed out that the qualitative effects of gravity waves on the mesospheric circulation should be to act as a drag on the zonal jets in the extratropical mesosphere and to increase the strength of the mean meridional circulation. The effects of the wave drag are significantly non-local, and so gravity waves which break in the mesosphere affect the stratospheric circulation. Recently evidence has also been presented that gravity wave drag may be acting directly in the extratropical stratosphere to a significant degree (Rosenlof, 1996; Alexander and Rosenlof, 1996). Some of this drag could result from topographically-generated gravity waves (particularly in the Northern Hemisphere extratropics in winter), but nonstationary gravity waves excited by mechanisms such as convection or jet stream excitation must also be very important (and likely dominant in the tropics, in the SH extratropics and in the summer season in the NH extratropics.

Convectively-generated gravity waves probably have the greatest direct impact in the tropics, due to the strong and widespread moist convection there. There is now renewed interest in the notion that relatively small scale gravity waves may be important in forcing the quasi-biennial oscillation (QBO) of the tropical stratosphere (e.g., Sato and Dunkerton, 1997; Alexander and Holton, 1997). The tropical QBO itself is now understood to be an important component of interannual variability in middle atmospheric climate, with effects felt even in the extratropics and polar regions. In fact, all major issues in stratospheric dynamics and transport are now believed to involve the interaction of relatively small-scale gravity waves with the larger-scale circulation.

Current attempts to incorporate the effects of unresolved gravity waves into global models (e.g. Lindzen, 1981; Fritts and Lu, 1993; Hines, 1997) suffer from an almost complete lack of knowledge of the actual spectrum of waves in the stratosphere, its variability and its dependence on tropospheric conditions. Typical of approaches adopted in some current climate models is an assumption that the input spectrum is spatially isotropic and geographically uniform over the entire globe (e.g., McFarlane et al., 1997). Slightly more ambitious is the approach of Manzini et al. (1997) who assume that the wave flux has a weak temporal and geographical dependence on the precipitation at each location. At the other extreme are models which assume that the flux of waves upward into the stratosphere at each individual gridpoint and timestep depends entirely on the instantaneous convective stability of the tropospheric layers below (Rind et al., 1988a,b; Kershaw, 1995). At present there is no way of knowing how reasonable any of these approaches is. The enormous uncertainties involved in formulating subgrid-scale gravity wave drag parameterizations must be reduced if the field of global dynamical/chemical modelling of the stratosphere and UTLS region is to progress on a sound basis.

Recently there has been also been considerable interest in the possibility that the gravity waves in the UTLS region could directly impact ozone chemistry. In particular the very temperature-sensitive heterogeneous chemistry on aerosols could be affected by the temperature fluctuations associated with gravity waves (e.g., Murphy and Gary, 1995; Tabazadeh, 1996; Carslaw et al., 1998). Detailed chemical models (whether employing self-consistently determined temperatures or "observed" temperatures taken from global meteorological datasets) may need parameterizations of the temperature fluctuations associated with gravity waves. For this, however, one needs a detailed knowledge of the spatial and temporal temperature spectra, including geographical variation and temporal intermittency. While the focus of current interest is in the high latitude winter lower stratosphere (where heterogeneous ozone loss can be significantly enhanced by small-scale fluctuations in temperature), the results of ETCE and associated limited-domain modelling studies could provide useful input into the study of this issue. Information on the gravity wave spectrum near the tropopause may also be needed for a complete understanding of the cross-tropopause water vapor transport. In the tropical regions, where subvisible cirrus clouds near the tropopause are believed to play an important role in the dehydration of air entering the stratosphere (Jensen et al., 1996), rapid cooling rates due to gravity wave induced temperature fluctuations may limit such dehydration, since the size of ice particles formed (and thus the fallout rate) is inversely related to the cooling rate.

b. Goals

The overall goal of the gravity wave component of ETCE is to characterize the gravity wave field generated by intense convection in sufficient detail to aid in the formulation of parameterizations of the associated gravity wave effects in global atmospheric models. The campaign would focus on understanding and modeling the gravity wave source spectrum including vertical eddy momentum flux, wave field anisotropies, and the temporal and spatial intermittency. Secondary goals include gaining a better understanding of wave propagation in the middle atmosphere (including the relationship to the background wind and temperature structure) and improved insights regarding the interaction of gravity waves with the mean flow in the stratosphere and mesosphere.

Some specific issues and questions that can be addressed by the intense field program proposed here include:

- How does the wave momentum flux entering the stratosphere scale with the intensity of convection? The answer to this is crucial for determining how much temporal intermittency and geographical variability should be assumed for the source spectra used in gravity wave drag parameterizations. Some recent modelling work (Alexander and Holton, 1997) has suggested that a very limited number of intense African-wave thunderstorms could account for a very significant part of the mean flow acceleration in the QBO. More generally, how does the time and space spectrum of waves entering the lower stratosphere depend on the intensity of convection?

- Still more generally, one could ask how the properties of the storms (e.g., peak intensity, suddenness of onset, duration, aspect ratio, degree of mesoscale organization) as well as the background wind in the UTLS region affect the spectrum of waves entering the lower stratosphere?

While efforts should be made to sample a wide variety of conditions, any field experiment will provide only a small sample of seasonal and geographical variability in the real atmosphere. Thus it is critically important that the results be related to ongoing monitoring and numerical modelling efforts. In particular the following issues should be addressed:

- There are now limited-area numerical models which explicitly simulate tropospheric moist convection and stratospheric gravity wave generation. An important goal of a field experiment would be to provide detailed results for testing/validation of these models. The current state and immediate prospects for limited-area modelling and the role of the proposed field experiment in model testing are described in more detail in Section 8 below.

- How well can one relate detailed observations of the wave flux in the lower stratosphere to satellite observations of cloud properties? This important question can really only be answered with a field program involving detailed in situ measurements above the cloud tops.

- A great deal of effort recently has been aimed at deriving properties of dominant lower stratospheric gravity waves from routinely-available single station radiosonde observations. Impressive results have been obtained (e.g., Allen and Vincent, 1995; Vincent et al., 1997; Guest et al., 1997; Whiteway and Duck, 1997), although there is some degree of controversy concerning interpretation of single station data (e.g., Eckermann and Hocking, 1989). Particularly interesting has been recent work using single station radiosonde data to make inferences concerning the role of gravity waves in driving the tropical QBO (Sato and Dunkerton, 1997; Sato, 1997). The proposed experiment will provide a possibility of comparing a more detailed picture of the wave field (obtained from aircraft, multiple balloons, ground-based profilers etc.) with the inferences from single balloon data. This is especially important as efforts (supported by WCRP/SPARC) are now underway to gather as much radiosonde data from as many locations as possible, in order to characterize the seasonal and worldwide geographical variability of the gravity wave field in the lower stratosphere (Hamilton and Vincent, 1995).

There are also upper stratospheric and mesospheric issues that can be addressed by the proposed experiment:

- Observations with rockets, lidars and airglow imagers often show the presence of quasi-monochromatic waves in the middle atmosphere (e.g., Swenson et al., 1995). A key question still outstanding is whether such features can be related in some fairly simple way to intermittent sources. There have been speculations along these lines for at least two decades (e.g., Clark and Morone, 1981; Taylor and Hapgood, 1988), but clear evidence is lacking. By including upper stratospheric and mesospheric measurements (lidars, imagers, radars) in conjunction with the proposed tropospheric and lower stratospheric measurements, it may be possible to trace the effects of a fairly isolated gravity wave source on the wave field throughout the middle atmosphere. This task will be complicated by the restriction of some observational techniques to nighttime and the large horizontal spread anticipated for the gravity waves by the time they reach the mesosphere, but a systematic effort of this kind should be undertaken and has the possibility of yielding a major advance in middle atmospheric meteorology.

The proposed experiment will provide an excellent opportunity to address these questions. In addition to the proposed ground-based and aircraft-based observations of the storm environment and storm-generated waves, observations of winds in the mesosphere and lower thermosphere from the TIMED (Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics) satellite, due to be launched in early 2000, should be available. Combining these data sources will provide an unprecedented opportunity to place the mesosphere airglow observations in the detailed context needed for their interpretation, and also allow testing the middle atmosphere wave propagation models that now form the basis for gravity wave parameterizations.

c. Proposed Measurements and Resources Required

The key lower stratospheric measurements will be made from the WB-57 in repeated overflights of the convection in its developing, mature and decaying stages. Both in situ observations of winds and temperature and remote-sensing microwave observations of temperature above and below the plane will be taken. Such observations over tropical convection have been made from the NASA ER-2 on occasion. Fig. 3 shows an example from an overflight of the ITCZ during the recent NASA Stratospheric Tracers of Atmospheric Transport (STRAT) campaign. Shown are in situ vertical wind measurements from the NASA Meteorological Measurement System (MMS), temperature profiles determined from the NASA Microwave Temperature Profiler (MTP) measurements and cloud brightness temperatures measured by a downward-looking radiometer. The results show a clear enhancement of the vertical wind variance over the region of low brightness temperatures, which is the locus of a mature convective system. The data also reveal the richness of the horizontal wavenumber spectrum, with vertical wind variance peaking in the 5-10 km wavelength range, and temperature variance more apparent at wavelengths longer than about 50 km. Dominant vertical wavelengths for the latter were estimated at about 5.5 km. Fig. 3 and other previous work (Pfister et al., 1993) from the Stratosphere-Troposphere Exchange Project (STEP) have demonstrated the ability of aircraft-based observations to capture particular mesoscale gravity wave phenomena. However, the results illustrated in Fig. 3 also reveal the limitation of currently available data sets. First, since the focus of the STRAT program was not on gravity waves, there are only two (fortuitous) overflights, 3 hours apart, of a convective system with a characteristic variation time scale (as seen by half-hourly satellite photos) of perhaps 1-2 hours. One feature of the observations was that there was no evidence of the large vertical wavelength (~10 km) gravity waves generally seen in limited-area models of explicitly-resolved convection. However, this may be entirely due to the inability to observe the system throughout its entire evolution, especially since these large vertical wavelength waves propagate upward quite rapidly. Second, there is no auxiliary data to obtain information about the interior evolution of the convective system. ETCE will allow a much more systematic investigation of the gravity wave field in the lower stratosphere and its relation to tropospheric conditions. Ozone measurements near the tropopause, planned for the chemistry component of ETCE, provide a useful tracer in the lower stratosphere for measuring gravity vertical parcel displacements along the flight path. From these data, the characteristic scales of the gravity waves triggered by the convection can be inferred. The planned radiosonde observations will be important for characterizing the longer horizontal wavelength and low-frequency gravity waves associated with the storm.

Observations of mesospheric/lower thermosphere variability during ETCE will be made from a number of ground-based instruments. An Australian Research Council proposal from the University of Adelaide for construction of a Medium Frequency radar near Darwin for deployment during ETCE has been submitted. The CNES group in France plans to deploy a Rayleigh lidar during ETCE. Researchers at the University of Illinois would deploy both lidars and airglow imagers. It is likely that other research groups throughout the world would be interested in contributing optical instruments. This concentration of mesospheric/lower thermospheric instruments near Darwin during ETCE will make possible some very detailed comparisons with results from the TIMED satellite.

5. Upper Tropospheric Ozone Chemistry

a. Background

Photochemical oxidation is central to the control of the chemical composition of the atmosphere. Thus oxidation plays an important role in determining the atmosphere's radiative properties, thereby exerting a controlling influence over atmospheric dynamics and climate. Most trace gases emitted into the atmosphere, whether biogenic or anthropogenic, are transformed via oxidation into other chemical species that are more readily removed from the atmosphere. In so doing, oxidation processes generally determine the lifetimes, and hence the atmospheric abundances, of most trace species emitted into the atmosphere. Furthermore, oxidation is also potentially subject to anthropogenic influences, so not only is it important in its own right, but it is likely to play a significant role in global change as well.

Photochemical oxidation is important all over the globe, but the tropical troposphere is an especially critical region in determining the global oxidizing capacity of the atmosphere. Insolation is high and overhead ozone is relatively low, so the actinic flux is high, and oxidation proceeds at high rates. Indeed, the tropospheric removal of many trace species (by O3-derived OH) occurs predominantly in the tropics. Moreover recent studies show that the upper troposphere is more photochemically active than previously thought, due to the generation of free radicals derived from constituents convectively transported from the lower to the upper troposphere. Relatively short-lived species that are abundant in the boundary layer, such as peroxides and aldehydes, when transported rapidly upward in convection, are able to have a significant impact on the upper troposphere (Singh et al., 1995; McKeen et al., 1997; Jaeglé et al., 1997, 1998; Prather and Jacob, 1997; Wennberg et al., 1998; Brasseur et al., 1998; Hauglustaine et al., 1998; Müller and Brasseur, 1999). Also, tropical lightning is an important source of NOx, and so plays a major role in the budget of ozone (e.g., Emmons et al., 1998). The tropics are also of interest from the perspective of global change, because the global oxidizing capacity is very sensitive to NOx emissions from tropical continents (Jacob et al., 1996), and as the population in this region grows dramatically in the next few decades, the increase in NOx emissions may lead to significant change.

Not only is convection a major source of chemically active species to the upper tropical troposphere, but also it has recently been pointed out that tropical convection can replace the air in the upper tropical troposphere in a time of about ten days (Prather and Jacob, 1997). This time is not long relative to the time scales of important photochemical processes. As a result, the upper troposphere is in a state of perpetual chemical imbalance, because the rapidity of convection replaces the air faster than it can achieve chemical equilibrium. In this manner convective processes play a dominant role in shaping the photochemical environment in the upper troposphere. Thus it is impossible to understand and predict future anthropogenic changes in oxidation in this important region of the globe without also having an understanding of tropical convection and its impact on oxidation processes.

b. Goals

The production and loss rates of O3 in the upper troposphere are functions primarily of the concentrations of members of the NOx and HOx families.

The upper tropospheric abundances of species within both families are significantly influenced, or even dominated, by convection, HOx primarily via transport of source species from below, and NOx via the lightning source and, when there is a polluted boundary layer, transport of boundary layer NOx. In order to assess the impact of convection on NOx and HOx, and hence on O3, we propose to measure these species and their precursors (for HOx) from a high altitude aircraft in air masses with and without recent convective impact. We also propose to make measurements from a low altitude aircraft, and on the ground, in order to know the species abundances in air entering the storm from below. The measurements at high altitude will suffice for the determination of the instantaneous in situ production and loss rates for O3, and, if instrumentation is available, there will also be high-altitude measurements of HOx source species, so that a longer term assessment of O3 chemistry is possible. The measurements at lower altitudes and on the ground yield the HOx sources and the NOx entering the cloud from below.

In the case of NOx, lightning is expected to be the dominant source for the upper troposphere in the region of the experiment. Convection of boundary layer NOx to the upper troposphere is another potential source, but the lack of strong anthropogenic sources on the Tiwi Islands perhaps makes this an unlikely source for the region of the planned experiment, but it is still possible that polluted air may enter from below after being transported from more distant sources, so low-altitude measurements are needed. A study of the NOx budget will require measurements of NOx in the thunderstorm outflow in the upper troposphere from the WB-57. These measurements may occur in the anvil attached to active convection, if this is logistically possible, or they may occur in the anvil remaining after convection has occurred. Ideally the WB-57 would fly in the anvil attached to the convection while, or not long after, the lightning is active. These chemical measurements would be related to lightning measurements from a time-of-arrival Lightning Mapping System which will provide information on discharge locations, timing, and whether discharges are intracloud or cloud-to-ground. The lightning discharge measurements will be interpreted within the context of the flow field provided by Doppler radar measurements, so that the high-altitude NOx measurements can be tied to the lightning measurements.

It is recognized that there may be operational constraints limiting the ability of the WB-57 to fly in the attached anvil, especially very close to the convection. However, given the large size of the anvil, the predictability of Hector, and the degree of real-time radar coverage, it is hoped that an operational scenario could be developed that would allow penetration of the anvil at a safe distant from the convection. Given the reproducibility of Hector, it may be possible to work toward shortening this distance, while maintaining safety, as the project progresses. Indeed the predictability of Hector will make flight planning easier and permit the use of several flights to study different aspects (or the same aspect repeatedly) of effectively the same convection on different days.

In the case of HOx, convection is expected to transport source species from the lower to upper troposphere, species such as H2O (in conjunction with O3), CH3C(O)CH3, peroxides, and aldehydes. Fig. 4 shows global model results for the relative contributions (zonal means in percent) of these different source species for the production of HOx (Müller and Brasseur, 1999). These results demonstrate the significance, from a global perspective, of the acetone, peroxide, and aldehyde sources, relative to O3 photolysis, for the upper troposphere. For the case of the upper troposphere over the United States, Fig. 5 (Jaeglé et al. 1998) shows the estimated impact of some of these HOx sources species on the O3 budget. In this study HOx concentrations were up to a factor of 3 higher that could be calculated from the oxidation of water vapor and the photolysis of acetone, suggesting the significance of other sources such as peroxides and formaldehyde.

For ETCE some of these source species will derive from biogenic emissions from the Tiwi Islands, while some may derive from oceanic emissions transported over the islands. The Tiwi Islands are known to be a strong source of isoprene [A. Guenther, private communication], making this a good region for pursuing such a study of HOx source species. We propose to measure at least some of these species in the inflow region and on the ground, and in the thunderstorm outflow as well. The Electra will be used for the low-level measurements, and this is consistent with its use in the Doppler radar measurements, as the Electra will, at times, be positioned under Hector for the purpose of obtaining Doppler measurements of updraft velocities. The Electra measurements will be supplemented with measurements from an Australian King Air and by measurements on the ground. This will enable an assessment of whether, or to what extent, the hydrocarbon-rich air in the lower boundary layer actually makes it into the updraft. Unfortunately there are currently no WB-57 measurement capabilities for the HOx source species, but it may well be possible to measure some of them by the time of ETCE. For example, the application of the CIMS (Chemical Ionization Mass Spectrometer) technique to the measurement of peroxides may be possible. The development of such an instrument would be of great value for ETCE, as it would allow a more direct determination of HOx source strengths in the upper troposphere than would be possible by relying on less direct inferences from the Electra measurements in the inflow region. If measurements in both regions (by the WB-57 and the Electra) are made, it allows for an assessment of the removal of different species during convection. For example, this would test assumptions common to modeling studies about the preferential removal of H2O2 in comparison to CH3OOH.

Summary of questions:

1. What are the upper tropospheric production and loss rates for O3 in air with and without recent convective impact? This requires measurements of NO, NO2, O3, OH, HO2, H2O, J values, pressure and temperature to evaluate the instantaneous in situ production and loss rates, plus hydrocarbon measurements to allow an estimate of RO2. To assess the production and loss rates on longer time scales, as the air ages over days to weeks, it is necessary to have measurements of HOx source species (this leads to question 3 below).

2. What is the strength of the lightning NOx source for these convective outflows? Relate this to lightning discharge rates and locations with respect to the flow field in the convective system. This requires measurements of NOx from the WB-57, lightning discharge rates and locations from the lightning mapping system, and the flow field from the multiple Doppler radars. Also, the flight patterns of the WB-57, in the outflow, and the Electra, in the inflow, must encompass adequate areal coverages of the two region to enable a calculation of the fluxes of species into and out of Hector.

3. What are the strengths of the various HOx sources for these convective outflows? This requires measurements of HOx from the WB-57, HOx source species at low altitude and at the ground, and, most importantly, from the B57, as is possible, given the available instrumentation. As for the NOx question, fluxes of species into and out of Hector are required.

c. Proposed Measurements and Resources Required

Four sets of measurements are crucial to achieving the goals related to the chemistry of ozone in the upper troposphere: (a) in situ measurements of species from the WB-57 flying in the convective outflow at 17-20 km, (b) measurements of the flow field (as required by all the other components of the ETCE project as well), (c) measurements, from the ground, of lightning flash rates and locations, and (d) measurements of HOx source species from a low-altitude aircraft flying in the updraft region below cloud base, and on the ground as well. The platform for inflow measurements will be the Electra when it flies in this region for the purpose of making direct measurements of the vertical Doppler velocities in the updraft. It is critical that there be sufficient overlap of instruments between the WB-57 (see Table 2) and the Electra (see Table 3) to reliably infer whether or not the air sampled at high altitudes is related to that sampled in the inflow region and to make inferences about sources and sinks for that air, from the time of sampling in the inflow region to the time of sampling in the outflow.

Also, it is critical that there be adequate areal coverage of both the inflow region by the Electra and the outflow region by the WB-57 to allow a reliable measurements of the fluxes of the various species into and out of Hector. For the WB-57, this means that the anvil needs to be sampled no further from the updraft core than approximately two core diameters. It needs to be ascertained whether this can be safely and routinely accomplished with the WB-57. For the Electra, too, there may be some concerns about encountering hail below cloud base. These are operational issues that obviously require discussion with the flight crews and those overseeing operations to determine the likelihood of being able to accomplish the measurement of fluxes. Also, this may lead to requirements for real-time communication between flight crews and those with access to real-time radar data.

From the WB-57, it will be necessary to measure those species that control the rate-limiting steps for the production and loss of O3, plus the actinic flux. These species include NO, NO2, OH, HO2, H2O, and O3. In addition, it will be useful to have WB-57 measurements of tracers that will serve to distinguish between stratospheric and tropospheric air, tracers such as CO and N2O, as well as tracers of boundary layer air, such as methyl halides and hydrocarbons. To the extent possible (that is, depending on the availability of instrumentation), HOx source species will be measured from the WB-57 as well. Doppler radar measurements from ELDORA on the Electra and from radars on the ground will provide the flow field, which will be essential to tie together the high-altitude in situ measurements with low-altitude in situ measurements of species and with measurements of lightning. A ground-based lightning mapping system based on the time of arrival of RF radiation at multiple stations will provide locations of NOx-producing discharges with respect to the measured flow field. Long-path measurements of NO2 from the ground or an aircraft would also be useful for providing an integrated measurement of the NOx produced,

especially in the updraft core. From the Electra flying in the inflow region, CH3C(O)CH3, peroxides (H2O2, CH3OOH), and aldehydes (CH2O, CH3C(O)H), J-values, NO, NO2, and O3 will be measured. Some of the HOx source species (acetone, peroxides, aldehydes) might also be measured from the ground.

Depending on the practicalities of (a) instrument development schedules and (b) the operational challenge of flying into anvils attached to active convection, it may prove useful to conduct a "pre-experiment" with the WB-57 prior to the full-blown ETCE deployment to Darwin. Some of the proposed instruments have been flown on other aircraft, but not yet on

the WB-57. Other, hoped-for, instruments have yet to fly at all. It may be too risky to fly these instruments for the first time in ETCE at Darwin. If this is the case, then it would make sense to conduct flights of a more local nature, say, out of the WB-57 home base at Ellington Field. Also, this would enable the acquisition of some operational experience with flying into anvils attached to active convection. Both of these factors would increase confidence in the likelihood for the success of the ETCE deployment to Darwin. This would also lead to substantial scientific payoff as well. It would be interesting to study the effects of storms other than just Hector. For example, if the flights occurred adjacent to deep convection over the central United States, abundant HOx sources may be expected (Brune et al., 1998), and such mid-latitude storms and their assorted impacts could be contrasted with Hector.

6. Tropical Cirrus: Dynamical, Microphysical and Radiative Interactions

Tropical cirrus are widely recognized to have an important influence on tropospheric-stratospheric exchange (Danielson 1982; Doherty et al. 1984), on the atmospheric heating rate (Ackerman et al. 1988; Webster and Stephens 1980), on the water budget (Chahine 1992) and on climate in general. However, the processes which control their generation, evolution and lifetimes have been little studied. ETCE has two general objectives related to tropical cirrus. First, improving understanding of the convective generation of tropical cirrus, which involves the mesoscale dynamics of convective systems and, second, the evolution of the tropical cirrus per se which involves the interaction among dynamics, microphysics and radiation. These objectives will be realized through the collaborative efforts of the US, European and Australian contingents.

Some of the outflow from anvils is maintained near the tropopause in the form of small particles that have a very low fall-speed. Such layers can remain for many hours after the primary convection (Jensen et al., 1996). Lidar and infrared radiometric (LIRAD) observations on tropical cirrus during MCTEX did indeed observe and track such cirrus layers for many hours. The clouds had a laminar appearance without any indication of the fall-streaks typical in warmer cirrus. On the other hand, the clouds were no means 'sub-visual'. They assumed depths of up to 4 km, and had visible optical depths that approached 1 or 2 at times. The LIRAD - determined effective radii were in the region of 10 microns, in agreement with Jensen et al. (1996) and implying a high particle concentration. Such clouds are obviously important to the radiation balance within the atmosphere and at the surface.

Widespread cirrus not associated directly with anvil clouds was also evident on some days, and appeared to impede the development of morning convection, thus being an important part of the daily cycle of convection. Such cirrus was lower than the thin tropopause cirrus, and was also observed over Darwin in 1981 (Platt et al., 1984). It is important to understand the origin and incidence of this non-anvil cirrus.

The investigation of cirrus generation will involve the late-stage life-cycle of Hector and will be collaborative with the study of the convection initiation and early-stage life cycle. Extensive tropical anvils are a product of organized convection in shear, and so particular attention will be paid to the upper-tropospheric dynamics of the attendant convective systems, how they interact with the troposphere and lower stratosphere, the dynamics of anvils and their horizontal spread rate; this will also involve idealized theoretical studies.

Critical to the understanding of anvil cloud and other cirrus cloud evolution and radiative properties are measurements of the ice particle size distribution and shapes, preferably over long horizontal transects at various height levels, and at different stages of the anvil/cirrus evolution. In the case of long-lasting upper tropospheric cirrus, measurements will be required also during convectionally quiescent periods when ground-based lidar observes the layers. The aircraft observations should be complemented by ground-based lidar and millimeter radar, from which size distributions can be retrieved (Sekelsky et al., 1999) and validated from the aircraft observations. When the cloud particles are too small to be sensed by mm radar, the LIRAD method will be used.

Some important measurements require an aircraft platform. The cirrus cloud layers must be characterized by measurements of particle size distribution and ice crystal habit together with observations of downward and upward longwave and shortwave radiative fluxes and radiances. The observations need to be made at straight and level flights at various altitudes through the cloud depth. Also desirable would be a measurement of the asymmetry parameter using a Gerber probe. These observations could be made from the WB-57 or the Australian Grob Egrett. Simultaneous thermodynamic observations and observations of vertical velocities would also be of great benefit.

A central observational focus will be to characterize the ice crystal sedimentation velocities, as they directly affect the buildup or decay of cloud ice mass and cloud lifetime, and the anvil radiative properties. A sensitivity study by Christian Jakob at ECMWF has recently demonstrated that the tropical net radiative flux divergence is extremely sensitive to the ice crystal sedimentation rate, with relatively small changes in the fallspeed, from 20 to 70 cm-s-1, leading to changes in the net flux divergence of nearly 30 W-m-2. Petch et al. (1997) found that large differences in ice water path cloud base heights, and cloud top and base heating rates resulted in tropical anvils by varying the ice crystal sedimentation rate. Wu et al. (1999) also found that changing the fall velocities had a major impact not only on the outgoing longwave radiation budgets but also on the atmospheric energy balance.

Observationally, information is needed from Doppler radar on air velocity, and mean and standard deviation of the particle velocity as a function of distance downwind of their convective origin. The NOAA/ETL dual wavelength profilers, and the NCAR S-POL radar are ideally suited to collect these data. In situ particle size spectra measurements from the WB-57 aircraft can provide information to "calibrate'" the radar results, allowing an investigation of the water budget. Local measurements of the solar and infrared radiative fluxes from aircraft can help in assessing the influence of radiative forcing on cloud dynamics.

Ground-based observations should be made as in MCTEX, including lidar, IR radiometry, microwave radiometry (to measure water vapor path) and two-frequency millimeter radar. There is also interest in utilizing a Doppler lidar, possibly at 2 microns or 10 microns wavelength to measure vertical velocities (of aerosol or cirrus) below, and in, the upper tropospheric cirrus layers. These would help to validate modeling studies of these layers (Jensen et al., 1996).

This aspect of ETCE will also have a strong computational component, because the interaction among microphysics, dynamics and radiation is highly nonlinear and difficult to quantify by other means. Of particular interest are the small-scale dynamical circulations associated with gravity waves within the anvils, which will affect cirrus cloud extent and lifecycle. Also of interest is the evaluation of what little theory now exists on anvil dynamics (e.g., Lilly 1988), and the development of new ideas based on nonlinear gravity wave theory.

Cloud-resolving models will be used to simulate the evolution of anvil particles and dynamics. Particle information from the observations will be incorporated into the model, and the modeling results and observations will be compared. Critical factors which determine anvil lifetime will be extracted from these comparisons. The model will then be applied to less vigorous anvil-producing situations in the tropics. Factors which control anvil lifetime, and the rate of sedimentation of anvil crystals, will be evaluated over a broad range of environmental conditions.

7. Other Scientific Issues

a. Aerosol Chemistry and Microphysics

Aerosols play a key role in atmospheric processes. They act as cloud condensation nuclei for cloud formation. Aerosols can scatter (or absorb) atmospheric radiation, affecting climate and photolysis rates of many chemical species. Aerosols can also be sites for chemical reactivity. Because of their importance in the atmosphere, aerosol properties have been measured in many field campaigns. One aspect of aerosol properties that remains an important question is where are new particles formed. In general, sulfuric acid will condense onto pre-existing particles before producing new particles, thus locations of new particle formation must be where pre-existing particle concentrations are very low. Hegg et al. (1990) and Clarke et al. (1998) have shown that the outflow region of the cloud is a location of new particle formation. Most earlier studies have been performed on field campaigns with complicated cloud fields. The aircraft platforms involved in ETCE offer the potential for examining the issue of production of new particles by performing a sulfate aerosol budget around a fairly isolated storm.

There are several measurements needed to perform this budget analysis. First the inflow region must be characterized. Measurements in the inflow region should include DMS, SO2, MSA, H2O2, O3, and aerosol composition and size. The outflow region should also be characterized with measurements of DMS, SO2, MSA, OH, and aerosol composition and size. To complete the budget, the precipitation should be analyzed for pH and ion composition. With these measurements, an understanding of how much sulfur is transported from the inflow region of the storm to the outflow can be estimated. Investigations of aerosol nucleation can be done in the outflow region. The budget of sulfur species that are processed by a thunderstorm can also be determined.

Modeling of the sulfur chemistry in and around the thunderstorm can also be done with a cloud resolving model. Evaluation of the model can be accomplished by comparing model results to both measurements in the outflow region and measurements in the precipitation. The model can then be used to understand the processes that affected the concentrations of the various species measured.

To measure the constituents in the inflow region, a low-flying aircraft with instruments that can measure DMS, SO2, MSA, H2O2, O3 and aerosol composition and size is needed. Similarly, a high-flying aircraft with instruments that can measure DMS, SO2, MSA, OH, and aerosol composition and size is needed. Lastly, precipitation needs to be collected and analyzed with pH indicators and ion chromatography.

b. Stratosphere-Troposphere Exchange

One of the most crucial unresolved issues in stratospheric chemistry is how tropospheric air gets into the tropical stratosphere (e.g., Holton et al., 1995). This is a particularly important question for understanding (and ultimately modelling and predicting) seasonal and interannual variations (including trends) in water vapor input into the stratosphere. There are a number of plausible paths for tropospheric air to enter the stratosphere, but there is a severe lack of empirical constraints on how much air actually passes through each pathway. One possibility is that significant amounts of air (and perhaps even cloud particles) could be irreversibly mixed into the stratosphere at the top of deep tropical convective turrets. The availability of stratospheric aircraft during ETCE makes it possible to perform at least a preliminary evaluation of the importance of this pathway in the vicinity of very deep convection. In particular, stratospheric flights of the WB-57 during the development stage of Hector aimed primarily at studying gravity wave generation could be extended to the decaying phase during which the composition of the air in the lower stratosphere could be observed (particularly concentrations of ozone and cloud aerosols). This is another aspect of the experiment in which observations would be analyzed in close conjunction with detailed model simulations.

8. The Role of Limited-Area Modelling

a. General Considerations

Even the ambitious field program proposed here cannot hope to observe all of the details of the three-dimensional, time-dependent development of a convective system and its effects on the surrounding atmosphere. Also, of course, any field campaign will provide direct information only on the phenomena observed in the limited area and time period of the experiment. High-resolution mesoscale simulation models will be needed to place the various observations in context, thereby greatly aiding in their interpretation. In addition detailed models that have been validated against the ETCE observations will be used in efforts to generalize the lessons learned from ETCE. Numerical modelling will be a crucial component of ETCE.

High-resolution numerical models have been applied to simulation of extratropical mesoscale convective systems with some degree of success since the 1970's (e.g., Klemp and Wilhelmson, 1978). Simulations of deep tropical convection soon followed (e.g., Soong and Tao, 1984; Redelsberger and Lafore, 1988; Lafore and Moncreiff, 1989; Tao and Simpson, 1989). These models typically employ grid spacing of the order of one km in the horizontal and 300 m in the vertical, and explicitly include cloud water, rain water and ice phase precipitation as variables. The cloud physics in such models is handled by bulk parameterizations. At present there are at least ten groups worldwide who are engaged in detailed numerical modelling of atmospheric moist convection. Some specific studies related to modelling the Hector convection over the Tiwi Islands will be briefly reviewed in Section 8d below.

One important general consideration is that the data collected in the ambitious field campaign envisaged for ETCE will likely be used as the basis for developments in limited area modelling for many years. Again the analogy with the GATE experiment noted in Section 1 is relevant: 25 years after the field program, GATE data are in some ways still the "gold standard" for comparisons with model studies of tropical oceanic convection (and for formulation of sub-grid scale convective parameterizations for global models). Mesoscale modelling capability (including chemical modelling) will certainly improve over the next several years, and the need for the extremely detailed observations proposed here for ETCE will also become ever more pressing.

Just as the observations provide invaluable constraints for the models, the models in turn provide assistance in the interpretation of the observations. The proposed mesoscale modeling efforts provide the means to assess the relationships between the storm dynamics and thermodynamics, chemical transport, cloud particle formation and ice evolution, aerosol sources and transport, gravity wave generation and propagation, and stratosphere-troposphere exchange.

b. Gravity Wave Generation

The investigators who have conducted numerical experiments with cloud-resolving models have generally not studied the upward flux of gravity waves produced by the convection. In some cases the models use an approximate "anelastic" form of the governing equations that limits the validity of the solutions to some relatively shallow depth. And, of course, modellers have been reluctant to devote a significant fraction of their model domain to the region above the tropospheric convection that they aimed to simulate. The first papers to examine gravity waves in cloud model simulations appear to be those of Clark et al. (1986) and Hauf and Clark (1989) who studied gravity waves near the tropopause resulting from fairly shallow convection as simulated by a 2D model and a 3D model, respectively. Fovell et al. (1992), Holton and Durran (1993), Alexander et al. (1995), and Alexander and Holton (1997) have performed more extensive investigations of the stratospheric gravity wave field forced by deep convection in high-resolution 2D models. Fig. 6 shows instantaneous results from two of the Alexander and Holton (1997) simulations of intense tropical squall lines. In each panel the heavy solid curve outlines the simulated cloud, the thin lines show isentropic surfaces and the background shading represents the vertical velocity field. The two cases shown are for nearly identical tropospheric squall lines, but with different stratospheric wind profiles: one appropriate for the extreme west phase of the QBO, and one for the east phase. In each case the convection has disturbed the material surfaces in the lower stratosphere and produced a complicated pattern of upward-propagating gravity waves. As Alexander and Holton (1997) note, the differing patterns of gravity waves in west phase and east phase QBO winds are associated with a strong QBO modulation of the eddy momentum transports associated with the waves. This, in turn, provides a mechanism by which the convectively-excited gravity waves can contribute to the dynamical forcing of the QBO.

This modelling research at U. Washington has recently been extended to high-resolution, three-dimensional simulations of deep tropical convection with initial conditions like those occurring during the Hector season. These are still somewhat idealized simulations in terms of the topography and surface fluxes imposed, but they include a deep stratospheric layer above the convection in order to resolve and study the vertically-propagating gravity waves generated. Preliminary results suggest that the gravity wave flux from a relatively small number of intense isolated "Hector-like" mesoscale convective events could have significant effects on the global-scale momentum budget of the tropical stratosphere, and in particular could explain a significant portion of the mean flow accelerations observed in the QBO. However, these simulation efforts suffer from a severe lack of constraining data on the relationship between the storm strength and the gravity wave activity above. The observations obtained during ETCE would be the first to provide the needed constraints on both the storm dynamics and the gravity waves generated.

The kinds of numerical experiments of interest to the gravity wave component of ETCE fall into two broad categories. One would be somewhat idealized experiments designed to examine the gravity wave spectrum above convection in a variety of conditions (mean winds, initial mean sounding, initial perturbations). Kershaw (1995) has attempted something along these lines for rather shallow extratropical convection, and has even used the results as a basis for a simple gravity wave parameterization for GCMs. This kind of work should be extended to a range of tropical convection situations, and the results should be examined in light of the observations that ultimately would be available from the ETCE field campaign. The second category of experiments would be those aimed at gaining the ability to simulate a particular observed convective event as closely as possible. Initial results for the Hector convection are discussed in Section 8d.

c. Mesoscale Transport and Chemical Simulations

Chemical species transport along with convective dynamics have been examined in other cases such as in STERAO (Skamarock et al, 1999), CLEOPATRA (Hauf et al, 1995), and in numerous other programs. For ETCE, the modeling approaches for tropospheric chemistry studies can proceed as in past studies; candidate models appear suitable. With respect to some ETCE goals, the modelling approaches and requirements may require significant model testing and development. For example, the cloud-resolving mesoscale models are typically run with relatively coarse vertical resolution in the upper troposphere and stratosphere. Studies of mixing across the tropopause will likely require much higher vertical (and horizontal) resolution to resolve the mixing processes.

There is strong interest from NCAR/ACD in the chemical modelling issue. Some of the chemistry simulations for ETCE would likely be carried out with a model now being developed that will couple the WRF (Weather Research and Forecast) meteorological model and the NCAR/ACD regional chemistry model. The WRF itself is now being developed collaboratively among NCAR/MMM, NOAA/NCEP, NOAA/FSL, and a number of university scientists, as a next-generation mesoscale forecast model and assimilation system. This model will be used to improve the simulation of significant weather features across scales ranging from cloud to synoptic, with priority emphasis on horizontal grid spacings of ~1-10 km. The model will incorporate advanced numerics and data assimilation techniques, multiple relocatable nesting capability, and improved physics, particularly for treatment of convection and mesoscale precipitation. It is intended to be appropriate for a range of applications, from idealized research to operational forecasting, and have flexibility to accommodate future enhancements. As soon as the WRF Model becomes functional, it will be maintained and supported by NCAR as a community model and freely distributed.

The WRF model is currently being developed to accommodate the transport of chemical species, and the coupled WRF/chemistry model will incorporate the required sub-grid physical parameterizations including convective and PBL transport schemes, microphysical parameterizations that include the transfer of chemical species from one type of hydrometeor to another, and detailed radiation treatments suitable for photochemistry.

d. Current Progress in Simulating the Hector Convection

Efforts are currently underway at NCAR/MMM to perform detailed simulation of the development of convection over the Tiwi Islands during specific days during the 1995 MCTEX period (Crook, 1997; see also http://www.rap.ucar.edu/staff/crook/trop97.html for more details). Thus far this work has used a version of the Clark (1977) model with realistic topography and high spatial resolution (1 km in the horizontal, and 50 m in the lowest part of the atmosphere). Integrations are initialized with the early morning balloon-sounding data and are forced with specified surface heat and moisture fluxes. Fig. 2 shows a result from one such simulation, in this case an attempt to reproduce the observed behavior on 27 November 1995. As noted earlier, the top panel shows low-level radar reflectivity and observed surface meteorological fields in the early afternoon. The bottom panel shows the result of one simulation for the surface winds and the near-surface rainwater field. The general agreement between model and observations is clear. Studies are continuing to examine the sensitivity of the solution to details of the initial conditions and the specified surface fluxes. While there are still some deficiencies in the current simulations, it is reasonable to aim for simulations (with surface fluxes, initial and boundary conditions that are tuned post facto) that agree in detail with the observed three-dimensional evolution of the meteorological fields in the troposphere. Such meteorological simulations can then be used as the basis for simulations of aspects of the ozone chemistry, of the stratospheric gravity wave field, and of stratosphere-troposphere exchange.

9. Summary of Resources Needed

To meet the ambitious goals of ETCE it is proposed that US agencies fund the following components of the experiment:

a. Major Equipment Deployments

The chemical and gravity wave measurements for the US component of ETCE require an aircraft that can fly near the tropopause and in the lower stratosphere as a platform for both in situ and remote sensing observations. The NASA ER-2 or the WB-57 would be appropriate platforms for these measurements. The present White Paper has been written with the assumption that the WB-57 would actually be deployed. From a purely scientific and operational point of view there are particular merits in each of the planes, and the choice of the WB-57 for the present proposal was made on the assumption that competing demands for the ER-2 are likely to be a more significant problem than competition for the WB-57. A tentative list of proposed instruments for the WB-57 is given in Table 2. The aircraft would be deployed in Darwin for the roughly six-week intensive observing period (IOP) of ETCE (late October-early December 2002) and about 100 flight hours would take place during that time (i.e. 100 hours exclusive of the ferry flights from the US).

In addition, the present plans call for the deployment of the NCAR Electra with ELDORA radar and other instrumentation for the IOP. The ELDORA data would be used in the detailed characterization of the development of the precipitation. In addition the Electra would be outfitted with instruments for whole air sampling as well as in situ observations of various trace constituents. A proposed list of instruments for the Electra is given in Table 3.

For a more complete characterization of the development of the convection, it is proposed to also deploy the NCAR S-POL radar to the Darwin area for the IOP.

b. Funding for Aircraft Instrument Development/Deployment

Most of the instrumentation proposed for the WB-57 and Electra already exists. In some cases funds will be needed to support the instrument integration and deployment on the aircraft. One major issue is the provision of high-quality in situ wind observational capability on the WB-57 which is important for the aims of ETCE. What would be needed is a capability comparable to that now available in the NASA Meteorological Measurement System (MMS) on the ER-2. There is interest among the ER-2 MMS engineering team at NASA Ames in working on developing this capability for the WB-57, but funding will have to be found for this. This is a major project in that it would involve several calibration flights of the aircraft. It also would represent one of the earliest major expenditures as the experiment plans proceed. Of course, the development of such capability for the WB-57 would be useful for a large array of possible applications, and if the WB-57 is to have a long-term future as a high-altitude research platform, then the MMS capability will have to be developed in any case.

c. Funding for Deployment of Other US Ground-Based Instrumentation

There is interest from some US university groups in participating in the experiment using ground-based instrumentation (or instrumentation that could fly on one of the Australian aircraft or possibly Australian vessels). Examples include the U. Massachusetts group with cloud radars, the New Mexico Institue of Mining and Technology group with their lightning mapping system, and researchers at U. Illinois with airglow imagers and sodium lidars for middle atmospheric observations. In some cases these instruments would be deployed just for the IOP, in others (particularly the middle atmospheric instruments) there may be interest in having longer deployments.

d. Funding for Modelling Studies and Mission Planning

As noted in the discussion of scientific goals, detailed numerical modelling is a very important component of ETCE. Funds would be required to support some modelling efforts prior to the actual observing period to help refine the operational plans for the experiment.