Effects of all-sky assimilation of GCOM-W1/AMSR2 radiances in the ECMWF System

TitleEffects of all-sky assimilation of GCOM-W1/AMSR2 radiances in the ECMWF System
Publication TypeMiscellaneous
Year of Publication2014
AuthorsKazumori, M, Geer, A, English, SJ
Secondary TitleTechnical Memorandum
Number732
Abstract

This paper assesses the impact of Advanced Microwave Scanning Radiometer 2 (AMSR2) radiances in the all-sky assimilation of the European Centre for Medium-Range Weather Forecasts (ECMWF). Individual impacts of three microwave imagers including AMSR2 were examined by using a base- line experiment that had no microwave imager data. The three microwave imagers brought similar improvements in humidity, temperature, and wind first guess (FG) fields in the troposphere. Im- provements were found both in fits to analyses and to other observations. Moreover, significant improvements of wind and geopotential height fields in the troposphere were found in day 3 to day 6 forecasts. AMSR2 made larger improvements than other microwave imagers to geopotential height forecasts in the northern hemisphere. The addition of AMSR2 radiance data on top of the full ECMWF operational dataset gave mixed results. Consistent improvements in the FG fit to humidity observations were confirmed. However, the FG fit for several channels of the microwave temper- ature sounding instruments was degraded. Mean FG departure of AMSR2 shows biases in certain local time and meteorological conditions. The causes of the biases were identified as insufficient representation of cloud liquid water path (LWP) in the forecast model under unstable conditions and insufficient amplitude of LWP diurnal variation in stratocumulus areas in the tropics. The assim- ilation of too much biased data might start to bring negative effects for the analysis and forecasts perhaps outweighing the improvements in the assimilation. However, similar to what was found in the no-imager baseline experiments, AMSR2 brought significant improvements of the geopotential height field in the southern hemisphere lower troposphere for day 5 to day 7 forecasts.