Assimilation of water vapor airborne lidar observations: Impact study on the COPS precipitation forecasts

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 vapor field thanks, in particular, to the deployment of two airborne lidar systems. These lidar observations were assimilated using the 3D VAR assimilation system of the numerical weather prediction mesoscale model AROME. The assimilation was carried out over the period of 4 July -3 August. The impact of the lidar observations was first 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 moist bias in the first 500 m above ground level and by diminishing the root mean square error by roughly 15 percent in the first km. Then, the impact of the lidar observations was assessed by comparing the precipitation forecasts with the gridded precipitation observations provided by the Vienna Enhanced Resolution Analysis. 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.