Exploitation of satellite data in the surface analysis

This paper presents the ECMWF surface analysis system and its recent developments. A short description of the ECMWF land surface model HTESSEL (Hydrology Tiled ECMWF Scheme for Surface Exchanges over Land) is provided. In contrast to the previous version (TESSEL), HTESSEL accounts for geographically varying soil texture and variable infiltration capacity. This improves the simulated soil moisture range and soil moisture dynamics. Model validation is illustrated by using field experiments data set in two contrasted areas. Soil moisture is a key component of the surface-atmosphere water and energy fluxes representation. Its parameterisation in the land surface model influences the surface analysis and controls the strength of the feedbacks between the surface analysis and the forecast model. The ECMWF surface analysis system is presented. Some deficiencies with the current Optimal Interpolation (OI) soil-moisture scheme are demonstrated, which have motivated the development of a new surface analysis scheme. The new scheme is a point-wise Extended-Kalman Filter (EKF) for the global land surface, built on the concepts developed in the European Land Data Assimilation System (ELDAS) project. The EKF surface analysis will facilitate combined use of satellite, in situ and proxy observations. Initial results using the same proxy observations as in the OI (2m air temperature and relative humidity) are shown. Satellite data from active or passive microwave sensors are highly suitable for soil moisture remote sensing. The EKF surface analysis opens the possibility to use these different actual and future data types for the soil moisture analysis. To this end appropriate observation operators have been developed and evaluated. They are presented in this paper. The soil moisture information in the C-band Advanced SCATerometer (ASCAT) and in European Remote Sensing (ERS) scatterometer data (active microwave) has been investigated. ASCAT product is shown to contain valuable information about soil moisture specifically in tropical, dry and temperate climates. An observation operator that matches the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) scatterometer soil-moisture product with the model’s soil moisture has been developed. A demonstration experiment shows the value of this approach and first results of ASCAT soil moisture assimilation using the EKF surface analysis are presented. The future Soil Moisture and Ocean Salinity (SMOS, to be launch in spring 2009) L-band radiances (passive microwave) are to be assimilated directly in the EKF surface analysis system, without first being converted to soil moisture. This approach relies on the availability of an accurate forward model (observation operator) capable of simulating the observations from the quantities represented by the forecast model, i.e. a microwave surface emissivity model. The Community Microwave Emission Model (CMEM) model has been developed at ECMWF for this purpose and it is presented in this paper. It incorporates a large number of parameterisation options. One option reproduces exactly the results of the forward model adopted by ESA’s development team for the SMOS retrieval algorithm. Calibration and sensitivity studies have been performed at different spatial scales with Skylab data, AMSR-E (Advanced Microwave Scanning Radiometer on Earth Observing System) data and SMOSREX (Soil Monitoring Of the Soil Reservoir EXperiment) field data for different microwave emission parameterisations considered in CMEM. In particular a combined inter-comparison between Land Surface Models (LSMs) and surface emissivity models has been conducted and validated against AMSR-E C-band measurements. These studies allow quantitative assessment of the sensitivity of the simulated surface emission to the coupled LSM-microwave emission model components. They point out the optimal microwave modelling configuration that best reproduces the top of atmosphere brightness temperature at different frequencies and at different spatial scales. The new EKF surface analysis system opens a range of further development possibilities, exploiting new satellite surface data and products for the assimilation of soil moisture. It is implemented in the research branch of the ECMWF Integrated Forecast System (IFS) and it is currently under evaluation and validation. Short term objectives for 2009 focus on the operational implementation of the EKF surface analysis. This requires solving the computing time cost issue by revising the structure of the surface analysis within the ECMWF forecast system. Long term objectives encompass an extention of the EKF surface analysis to account for additional variables such as snow mass and vegetation parameters. This work relies on parallel developments in the land surface model concerning the vegetation dynamics, snow and ice. Technical Memorandum No. 576 1 Exploitation of satellite data in the surface analysis