Water vapor flux measurements from ground-based vertically pointed water vapor differential absorption and Doppler lidars

For the first, time, two lidar systems were used to measure the vertical wa ter vapor flux in a convective boundary layer by means of eddy correlation. This;was achieved by combining a water vapor differential absorption lidar and a; heterodyne wind lidar ina ground-based experiment. The results prove that the combined lidar system can determine vertical flu x profiles with a height resolution of approximately 100 m. Vertical averag ing oi er ii greater height interval reduces the error sufficiently that th e changes in Aux occurring throughout the day as a result of solar heating can;be resolved. Horizontal and, fdr the first time, vertical integral scal es were calculated from the lidar signals. The error analysis based on thes e results indicates that instrumental white noise and sampling error are th e main sources of the statistical error in the flux measurement. Since the lidars measure simultaneously at many levels throughout the boundary layer; these errors can be reduced by vertical averaging to less than 50% for a;4 0 min time series,depending on how much vertical resolution is required in the flux profile. The combined lidar system was used to measure the height-resolved water vap or flux associated with boundary layer circulations induced by active fair- weather cumulus clouds. A cloud-modulated flux of up to 300 W m(-2) was obs erved in the upper third of the boundary layer The measurement also showed the breakdown of that flux during the transition from active to passive cum ulus clouds.