Assimilation of Water Vapour Airborne Lidar Observations: Impact Study on the COPS Precipitation Forecasts

Abstract

The Convective and Orographically-driven Precipitation Study carried out in summer 2007 over northeastern France and southwestern Germany provided a fairly comprehensive description of the low-troposphere water vapour field thanks, in particular, to the deployment of two airborne differential absorption lidar systems. These lidar observations were assimilated using the 3D VAR assimilation system of the Application of Research to Operations at MEsoscale (AROME) numerical weather prediction mesoscale model. The assimilation was carried out for the period of 4 July-3 August by running a 3-hour forward intermittent assimilation cycle. First, the impact of the lidar observations was assessed by comparing the analyses with a set of more than 200 independent soundings. The lidar observations were found to have a positive impact on the analyses by reducing the dry bias in the first 500 m above ground level and by diminishing the root mean square error. In general, the impact was found to be positive but not significant for the 24h precipitation and positive and significant for the 6h precipitation with an improvement lasting up to 24h. Some selected case studies show that the improvement was obtained through a better depiction of the convection initiation or through a more accurate positioning of the precipitation systems.