Measurements of Daytime and Upper Tropospheric Water Vapor Profiles by Raman Lidar

One of the most important atmospheric constituents needed for climate and meteorological studies is water vapor. It plays an important role in driving atmospheric circulations through latent heat release and in determining the earth’s radiation budget, both through its radiative effects (water vapor is the major greenhouse gas) and through cloud formation. The vertical distribution of water vapor is particularly important because in addition to determining convective stability, radiative effects are also strongly altitude dependent. In fact, several one-dimensional radiative convective models1 have shown that although upper tropospheric (8-12 km) water vapor concentrations are 2-3 orders of magnitude less than those near the surface, upper tropospheric water vapor exerts an important influence on climate. What these models show is that for a given absolute increase in water vapor in the upper troposphere, the response or change in surface temperature is extremely disproportionate to the amount of water vapor. At present, considerable controversy exists over the nature of the vertical redistribution of water vapor in a changing climate, and particularly the distribution of water vapor in the upper troposphere. Understanding upper tropospheric moistening processes such as deep convection are therefore of prime importance in addressing the water vapor feedback question. Accurate measurements of the vertical and temporal variations of water vapor are essential for understanding atmospheric processes and hence model refinement.