Estimation of Water Vapor and Clouds Using Microwave Sensors

Abstract

Both water vapor and clouds play important roles in the energy and water cycle, and their accurate observation is required. Several techniques to measure water vapor distributions and cloud properties exist with each of them having their specific limitations. Measurements in the infrared and visible spectral range are strongly influenced by clouds and cannot be used to observe water vapor under cloudy conditions. Information about cloud properties is limited to the cloud top and base depending on the use of a space‐ or ground‐based system. Microwave techniques can provide information about water vapor and liquid water also under cloudy conditions, but satellite observations are limited to retrievals above ocean surfaces. Therefore, the combination of different measurements needs to be further pursued in order to gain insight into the three‐dimensional distribution of water vapor and cloud properties as well as their temporal variability. Recently, information about the cloud vertical structure including the occurrence of multiple cloud layers and cloud overlap has become available at a few ground‐reference sites through sensor synergy including active and passive instruments. In the near future, the first global measurements of this kind will be carried out from satellites.

Retrieval of Cloud Liquid Water Using Microwave Signals from LEO Satellites: A Feasibility Study through Simulations

“…Understanding the role of clouds is of great importance in many areas such as numerical weather prediction and climate studies, as clouds are not only vital in the hydrological cycle, but also crucial for the Earth's radiation budget. Various instruments have been developed and deployed, such as microwave radiometers, cloud radars and lidars, to measure cloud liquid water content (LWC) on a fine scale [1]. However, due to the high cost and scarce distribution of such instruments, the real-time global seamless high-resolution observation of clouds is still a challenge.…”