Towards unified land data assimilation at ECMWF: Soil and snow temperature analysis in the SEKF

Weather centres use a variety of data assimilation schemes to analyze different land variables in their operational forecast systems. Current activities at the European Centre for Medium-Range Weather Forecasts (ECMWF) are working towards a unified and more consistent land data assimilation system to provide more accurate initial conditions for the atmospheric forecasts. The first step is to replace the current 1D optimal interpolation (1D-OI) used for first-layer soil and snow temperature analyses, and integrate multi-layer soil and first-layer snow temperature into the ensemble-based simplified extended Kalman filter (SEKF) currently used for multi-layer soil moisture. This work focuses on the technical developments and the evaluation of the atmospheric forecast skill of a series of numerical weather prediction experiments to compare different SEKF configurations with the former 1D-OI over a three-month summer and winter period. Using the SEKF leads to seasonally varying significant improvements in the 2-m temperature forecast in the verification against own analyses and to slightly improved results in the validation using independent synoptic observations. This work lays the foundation for integrating additional land variables into the SEKF and investigating stronger land-atmosphere coupling. This article presents the first step towards a unified land data assimilation system at ECMWF. This work is on abolishing the 1D optimal interpolation used in operations to analyse first-layer soil and snow temperature and including first-layer snow and multi-layer soil temperature in the simplified extended Kalman filter currently used for multi-layer soil moisture. The focus is on the technical developments and the scientific experimentation to evaluate the impact on atmospheric forecast skill. image