The humidity structure of the convective boundary layer - six weeks measurements with a ground-based Differential Absorption Lidar (DIAL)

The earth's surface is the source of water vapour in the climate system. Water vapour is transported upwards by turbulent and convective eddies, but the strength of the temperature inversion on top of the atmospheric boundary layer (ABL) controlls the transport into the "free atmosphere". Water vapour can in the first approximation be treated as a passive scalar, and thus reflects the temperature- driven mixing processes. Entrainment at the top of ABL in most cases is a sink for the humidity budget of the ABL - contrary to the temperature budget. These processes result in a different humidity than temperature stratification.

During three measuring campaigns in the frame of the project EVA-GRIPS (EVAporation at GRId/Pixel Scale) at an agricultural site in eastern Germany measurements of the absolute humidity have been performed with two Differential Absorption Lidar Systems (DIAL). Time-height sections from 6 to 18 UT and up to 3000 m asl of 25 days under different synoptic situations illustrate the variability of the the humidity ABL appearance. Plots of the backscattered signal - namely, the vertical gradient of the range-corrected backscatter - give some more information on motions in the atmosphere. Radiosonde and surface data complete the data set. Part of the observations fell into an untypically dry period.

- In most cases the evolving ABL is clearly present in the morning, but synoptic- scale or mesoscale features often disturb the 'stationary' daytime ABL.

- Low-level cloud bases are lifted with the growing ABL.

- The variability of the top of ABL due to single convective structures may be as large as 300 m with a time scale of several minutes, depending on the mean wind speed.

- Radiosondes launched at 04:45 UT, 10:45 Ut and 16:45 UT therefore do not always give the exact ABL height.

- Nearly in all cases the high humidity values in the growing ABL decreases before the top height is reached: the supply of water by evapotranspiration is not sufficient for mixing up to the top of ABL. This effect is clearest in situations when the well-mixed residual layer is high and the ABL grows 'explosively' fast.

- Entrainment of dry air from above the ABL results in a humidity gradient in the upper BL.

- Low-level convergence - e.g. during a frontal passage - leads to upwinds which break through the ABL inversion and transport humidity into higher levels.

- During the growing of ABL the layered features in the stably stratified lower kilometers subside.

The features of the humidity ABL may differ strongly from those of the temperature ABL, in particular over land surfaces with low soil moisture. Some observed features - like the last-named - need further investigation in order to explain and assess the participating processes.