Sensitivity of L-band NWP forward modelling to soil roughness

This article investigates the sensitivity of the European Centre for Medium-Range Weather Forecasts (ECMWF) simulated L-band brightness temperatures (TB) in response to different soil roughness parameterizations. To this end, the ECMWF operational conditions during the year 2004 have been used to force the Hydrology Tiled ECMWF Scheme for Surface Exchanges over Land (HTESSEL) coupled to the Community Microwave Emission Model (CMEM). The coupled HTESSEL-CMEM system is then run at five different incident angles (20 degrees, 30 degrees, 40 degrees, 50 degrees and 60 degrees) and for five soil roughness parameterizations available in CMEM. The performance of the simulated TB is analysed at ground point scale over the Surface Monitoring Of Soil Reservoir Experiment (SMOSREX) site in south-west of France. For this particular data set, both ground-based vertical profile of soil moisture and L-band radiometric observations are available for evaluation of the ECMWF forecast over the nearest grid box. In particular, the results show that the simple Choudhury parameterization best fits the observations for both horizontal (H-pol) and vertical (V-pol) polarization and for most of the incidence angles tested. The best forward modelling configuration is at 50 degrees for the V-pol, with coefficient of determination between modelled and observed TB of 82.9% and root mean squared error of 7.9 K. The sensitivity of the L-band TB errors to the empirical soil roughness parameter is also investigated. Strong sensitivity to this parameter is shown, mainly at H-pol for the least rough surfaces. The investigation carried out in this article gives an insight into the soil roughness model to be used in the operational configuration of the CMEM L-band forward operator, for future assimilation of the Soil Moisture and Ocean Salinity (SMOS) satellite data of the European Space Agency.