Utilization of the AMSU high frequency measurements for improved coastal rain retrievals

Abstract

This study describes a new rain estimation technique using Advanced Microwave Sounding Unit (AMSU) high frequency measurements and applicable over coastal regions. The existing operational AMSU rain retrieval algorithm utilizes the microwave window frequency measurements at 23, 31, 89 and 150 GHz. These measurements have limited applicability over coast due to ocean surface emissivity effects. This situation often leads to missing rain extent and thus rate retrievals over coastal regions. The new technique utilizes the higher window (89 and 150 GHz), opaque oxygen (53.6 GHz) and water vapor absorption (183 ± 1, ±3 and ±7 GHz) channels that are less impacted by surface emissivity variations. Rain extent is determined using a number of scattering measures computed from linear combinations of the brightness temperature at the selected high frequency channels. Next, the ice water path parameter is estimated from a quadratic relationship with the brightness temperature at the 183 ± 7 GHz water vapor absorption channel and the cosine of the local zenith angle. Rain rate is computed from ice water path. Evaluation and monitoring of the new technique integrated with the existing algorithm has indicated that the upgraded algorithm is robust and superior to the existing one over coast.