Towards assimilating surface sensitive microwave channels over land

Towards assimilating surface sensitive microwave channels over land
Date Published
EUMETSAT/ECMWF Fellowship Programme Research Report
Document Number

Microwave channels with a low sensitivity to the surface are assimilated over ocean and land. However, challenges in the description of the land properties (e.g. surface emissivity) mean that surface sensitive microwave channels have, until now, only been assimilated over ocean. In this study we investigate the possibility of assimilating surface sensitive microwave channels over land using the all-sky approach. An emissivity retrieval has typically been used to provide the surface emissivity when assimilating the less surface sensitive microwave frequencies. It is difficult to apply this to the surface sensitive microwave channels in our system, as the bias correction over ocean is affected by assimilation over land, and this interaction results in a moistening of the surface humidity over tropical ocean. We propose to use a spectrally adjusted emissivity retrieval: an emissivity retrieval at 19 v GHz which is adjusted using the dependency of emissivity with frequency obtained from an atlas. In our setup we assimilate additionally 89/92 GHz and 150/166 GHz from GMI and SSMISF17, and 183 GHz channels from GMI over land. The results are quite neutral with small positive improvements in the observational fits to AMSU-A and surface AMVs. However, there remains a need to improve the bias predictors when assimilating microwave radiances over land, to resolve issues over arid land surfaces and to allow the assimilation of 89/92 GHz channels from MWRI and AMSR2.

DOI 10.21957/gdwqzfns