Latent heat flux measurements over complex terrain by airborne water vapour and wind lidars



Vertical profiles of the latent heat flux in a convective boundary layer (CBL) are obtained for the first time over complex terrain with airborne water vapour differential absorption lidar and Doppler wind lidar. During the Convective and Orographically-induced Precipitation Study (COPS) over the Black Forest mountains in south-western Germany both lidars were installed nadir-viewing onboard the Falcon research aircraft of the Deutsches Zentrum für Luft- und Raumfahrt (DLR). On 30 July 2007, additional in situ measurements by the Karlsruhe Institute of Technology (KIT) were performed with a Dornier-128 aircraft that flew below the Falcon. This unique instrument configuration allows us to validate the lidar-derived fluxes and to assess lidar-specific issues such as instrument noise and data gaps that impinge on the results. The cospectra of in situ humidity and vertical velocity peak at wavelengths between 1 and 3 km and reveal that the dominant scales of turbulent transport are larger than 700 m in dimension. Consequently the airborne lidars' horizontal and vertical resolution of ∼ 200 m is sufficient to capture most of the flux. The lidar and in situ fluxes of five collocated 45 km flight legs agree within ±20%; the average difference over the total distance of 225 km is 3%. A flux comparison with ground-based water vapour Raman and wind lidars shows agreement within the instruments' accuracies under low-wind conditions. All latent heat fluxes vary between 100 and 500 W/m2 in the CBL and have small vertical divergences. Vertical velocity spectra in the mid-CBL enable us to estimate the dissipation rate of turbulent kinetic energy that amounts to 5 × 10−4 m2 s−3 in the Rhine Valley and 10−3 m2 s−3 over the Black Forest mountains. This new airborne lidar instrumentation proves to be a valuable tool for the study of CBL processes and variability, particularly over complex terrain. Copyright © 2011 Royal Meteorological Society