Satellite Water Vapor
Atmospheric water vapor affects a variety of processes. Water
vapor is like fuel for thunderstorms and is often the main difference
between a cloud which will not grow and one that develops into a
thunderstorm. Water vapor is also a strong green house gas which
absorbs outgoing solar radiation. In order to measure surface
temperatures a satellite channel 12 microns is often used as
there is an atmosphere window in this region. Unfortunately this window
is not perfect and some amount of water vapor absorption still occurs
in this region. In order to correct for this problem, a second channel
is added near the edge of the window where slightly more water vapor
absorption exists. By using the ratio between these two infra-red
wavelengths it is possible to attempt to correct for the effects of
water vapor in deriving surface temperatures. For the same reasons it
is also possible to derive integrated water vapor from these two
channels (Motell et al., 1999). The figures below give several examples
of integrated water vapor observed from satellite using this
approach.
This figure shows trails larger integrated water vapor occuring
behind each Hawaiian island on a typical trade wind day (values ranging
from 18-28 mm of integrated water vapor). This is likely due to the
trade wind inversion forming a lid and forcing the trade winds to flow
around the islands. This results in low level convergence on the lee
side of the island with rising motion in this region. Unique ocean
surface roughness routinely occurs on the lee side of the Oahu (at
the location of the water vapor plume seen in this image) confirming
the the convergence is occuring at the lowest levels. T
This figure shows a trail of larger integrated water vapor amounts
(orange values) being brought up from the south and feeding into the
deep thunderstorms occuring over Kauai (just north of the bow echo).
Above deep clouds the amount of water vapor is small and therefore
lower integrated water vapor values (blue) are seen over deep clouds.
On this day severe flooding occurred over Kauai.
This figure shows the integrated water vapor near Hawaii on the
day of the Helios crash (Porter et al., 2007). What is interesting
about this image is the east-west gradient on integrated water vapor.
In fact very large values are occuring around the deeper cloud SW of
the islands. What could be causing this gradient on water vapor. The
figures below show back trajectories ending in the regions of
large integrated water vapor (SW of islands) and regions of small
integrated water vapor (east of Hawaii). It appears that the region
with larger integrated water vapor spent more time near the surface
while the air from the dryer region spent very little time near the
surface. These changes in integrated water vapor are important for
convection along dry lines (over the US mid-west) but they are poorly
studied over the ocean where one might be surprised to find these vapor
gradients.