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.