Retrievals of liquid water path during light rain from ARM microwave radiometer

Description

Liquid water path is one of the geophysical quantities retrieved by the ARM microwave radiometers. However retrievals during rainy conditions are currently discarded because of the deposition of rain on the radome and because the radiative transfer code used in the retrievals assume a non-scattering atmosphere. This assumption does not hold when the drop size is large. The new 3-channel microwave radiometers have a powerful blower that keeps the rain contamination low during very light rain. It may be possible therefore to retrieve liquid water path during light rain events. Simulations were conducted using a new scattering forward radiative transfer code the Passive and Active Microwave TRAnsfer operator (PAMTRA) developed by the university of Cologne. The purpose of the simulations is to study the sensitivity of microwave brightness temperatures to the parameters of the drop size distribution and ultimately to understand what is needed to develop operational liquid water path retrievals during rain. Results show that microwave observations during rain are sensitive to the vertical distribution of the liquid water, to the rain rate, and to the parameters of the drop size distribution, and can not distinguish between rainwater path and cloud liquid water path. Therefore it may be possible to retrieve liquid water path from microwave radiometers during rain provided that additional information is available on the drop size distribution parameters. A roadmap for the development of the retrievals is presented.