Latent Heat Flux Measurements over Complex Terrain by Airborne Water Vapour and Wind Lidars

Abstract

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 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 - 3 km and reveal that the dominant scales of turbulent transport are larger than 700 m in space. Consequently the airborne lidars’ horizontal and vertical resolution of ~200 m is sufficient to seize 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 - 500 W/m² in the CBL and have small vertical divergences. Vertical velocity spectra in the mid-CBL enable 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.