Vertical profiles of water vapour fluxes in the convective boundary layer measured by ground-based Differential Absorption Lidar and Heterodyne Doppler Lidar

The transport of water vapour from the earth's surface into the free atmosphere is an important part in the water and energy cycle. Turbulent and convective fluxes of water vapour within the atmospheric boundary layer are still hard to measure. Ground-based remote sensing systems are promising instruments to continuously record turbulent fluxes up to the top of the boundary layer.

In the frame of the project EVA-GRIPS (EVAporation at GRId/Pixel Scale) the field experiment LITFASS-2003 took place in May/June 2003 in an agricultural area southeast of Berlin. Among a variety of near-surface, soil and atmospheric measurements, ground-based lidar systems were operated under various synoptic conditions, actually two Differential Absorption Lidar (DIAL) systems measuring the absolute humidity and one Heterodyne Doppler lidar system measuring the vertical wind. The time resolution is 10 s, the height resolution approximately 90 m. The joint measurements cover 14 days from early morning to late afternoon and the region from 400 m above ground to the top of the boundary layer. Turbulent fluxes are calculated by the eddy-correlation method with averaging times of 1 to 3 h, according to the the mean flow conditions.

Vertical profiles of water vapour fluxes show different structures depending on whether there is entrainment of dry air from above or not. In all situations a decrease of the water vapour flux from the surface to the middle of the boundary layer is observed. On days with dry air lying above the humid boundary layer, entrainment leads to an increase of the water vapour flux in the upper part of the boundary layer. This layer can comprise up to 50% of the boundary layer and entrainment fluxes can exceed the surface fluxes. On days with humid air lying over the boundary layer the entrainment flux is nearly zero and does not contribute to the humidity concentration within the boundary layer.

The combination of ground-based lidar systems to measure turbulent fluxes approved to be a suitable tools for the investigation of transport processes within the boundary layer. Both instruments have the capability to measure continuously.