Airborne Lidar Measurements of Water Vapour and Humidity Fluxes during the Int. H2O Project 2002

During the International H2O Project (IHOP 2002) DLR’s 100 Hz airborne differential absorption lidar (DIAL) enabled two-dimensional scans of small-scale humidity structures in the boundary layer with unprecedented spatial resolution. At an aircraft speed of 140 m/s the spatial (horizontal and vertical) resolution in the humidity field for typical daytime measurements is 200 m, accepting an average random error of 10 %. For the first time the DLR DIAL was flown together with NOAA’s high resolution Doppler lidar (HRDL), both systems pointing vertically downwards. The dominant turbulent structures of the boundary layer are well sampled by both lidars. Applying the eddy covariance technique to both data sets, the upward humidity flux within a cloud-free convective boundary layer over south-western Kansas was estimated to 0.2+-0.04 g/kg m/s, which corresponds to a latent heat flux of 500+-100 W/m2. The flux is found to be nearly constant with height, i.e. the flux divergence is zero, the mean boundary layer humidity is constant over time and the surface evaporation flux equals the entrainment flux at the boundary layer top, representing the boundary layer growth.