Assessment of the forecast impact of suface-sensitive microwave radiances over land and sea-ice

The present forecast impact of surface-sensitive microwave sounder radiances over land and sea-ice has been assessed in the ECMWF system, using observing system experiments as well as adjointbased diagnostics. The assimilation relies on surface emissivities retrieved from window channel observations. Short-comings of the current use of the data in specific regions are also highlighted.

Surface-sensitive microwave radiances over land and sea-ice have a significant positive forecast impact in the ECMWF system (2-3 % reduction in forecast error for the 500 hPa geopotential over the extra-tropics). When added incrementally to an otherwise full observing system, observations over sea-ice, humidity-sounding radiances over land, and temperature-sounding radiances over land all contribute significantly to this positive forecast impact. The impact shows some seasonal dependence, and the Northern Hemisphere impact over land is smaller during winter, most likely related to a more restricted and less optimal use of the observations over snow.

Short-comings are nevertheless apparent in specific areas. Desert regions show diurnal biases, most likely due to biases in the temperature used to specify surface radiation, likely arising from a combination of penetration effects and diurnalmodel biases. Snow-covered regions show biases that appear consistent with assuming specular reflection when diffuse reflection is prevalent. The quality control currently applied is mostly successful in protecting the analysis from the deficiencies in these areas. Neglected cloud signals can have a significant effect on the retrieved emissivities and the subsequent quality control. Potential avenues to improve the identified short-comings are outlined.