Height-resolved variability of midlatitude tropospheric water vapor measured by an airborne lidar

Abstract

Free tropospheric water vapor variability, measured by airborne lidar over Europe during summertime, is analyzed at altitudes from 2 km to 10 km. Horizontal structure functions of specific humidity were computed and show powerlaw scaling between about 10 km to 100 km in range. The second-order structure function shows scaling exponents equivalent to spectral slopes that vary from around 5/3 in the lower troposphere to 2 at upper levels. More specifically humidity smoothness typically increases with height, while intermittency decreases. A classification of the data according to whether the series occurred above or below the level of nearby convective cloud tops gives a separation of the scaling exponents in the two air masses. The results are consistent with a water vapor distribution determined at upper levels by a downscale cascade of variance by advective mixing, but increasingly influenced at lower levels by local injection of humidity by moist convection.