Water Vapor Flux Measurements from Ground-Based Vertically Pointed Water Vapor Differential Absorption and Doppler Lidars

Abstract

For the first time, two lidar systems were used to measure the vertical water 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 in a ground-based experiment. The results prove that the combined lidar system can determine vertical flux profiles with a height resolution of approximately 100 m. Vertical averaging over a greater height interval reduces the error sufficiently that the changes in flux occurring throughout the day as a result of solar heating can be resolved. Horizontal and, for the first time, vertical integral scales were calculated from the lidar signals. The error analysis based on these results indicates that instrumental white noise and sampling error are the 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 40-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 vapor 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 observed in the upper third of the boundary layer. The measurement also showed the breakdown of that flux during the transition from active to passive cumulus clouds.