Importance of Water Vapor Measurements

Water vapor is a critical variable for climate studies. The interception by water vapor by a broad band of infrared (IR) radiation from the surface and its subsequent emission at lower temperatures greatly influences the radiative energy balance of the planet. Indeed water vapor is the principal contributor to the greenhouse effect and plays a key role in our understanding of the Earth’s climate and its sensitivity to increasing levels of carbon dioxide. Observational studies have illustrated the importance of water vapor feedback in regulating the greenhouse effect of the atmosphere. More recently, the importance of upper tropospheric water vapor in determining the amount of clear sky IR radiation emitted to space. It has been speculated that the enhanced convection expected from a global warming will lead to increased subsidence and a drying of the upper troposphere and questions whether existing general circulation (GCMs) correctly simulate the vertical transport of water implied by this feedback. Evaluating the reliability of GCMs and investigating the existence of such feedbacks obviously requires accurate observations of upper tropospheric water vapor. Accurate simulation of upper tropospheric water vapor is also necessary if GCMs are to be able to predict the occurrence and distribution of cirrus clouds which play an important role in regulating the IR radiation emitted to space.

Furthermore, defining low-level moisture variability on small (20 - 200 km) scales is beneficial to several disciplines. Detailed water vapor observations are essential to the improved analysis and prediction of convective storms. Thunderstorms have been observed to develop in regions of strong and rapidly evolving moisture gradients. It has been determined that 37% of the variance in mixing ratio was at scales less than 200 km, and 20% at less than 100 km, for non-stormy environments. However, in stormy environments, it has been determined that 68% and 32% of the variance occurs at scales less than 200 and 100 km, respectively. Vertically integrated moisture is also critical in the objective prediction of clouds and precipitation. Furthermore, high resolution moisture data are also needed in hydrometeorology, astronomical observations, and studies of atmospheric radiation and electromagnetic wave propagation. Finally, water vapor plays a key role in the Earth's hydrologic cycle. Therefore, a better understanding of its role will require long-term observations of both small and large scale water vapor features, a major goal of the National Aeronautics and Space and Administration's (NASA's) Mission to Planet Earth (MTPE) program. Unfortunately, water vapor is poorly measured on a global scale, particularly in the upper troposphere where radiosonde measurements are unreliable. Therefore, water vapor measured remotely from satellites should provide a valuable alternative to the conventional radiosonde network. Remote sensing from satellites has the potential to fill this data void. Satellites and aircraft, in particular, provide excellent temporal and spatial resolution.

Last updated on: November 2, 1999