The three-dimensional structure of precipitating shallow cumuli. Part two: The effect of precipitation on water vapor distribution

Abstract

Water vapor is radiatively important and abundant in the atmosphere, with great temporal and spatial variability. Spatial and temporal variations of water vapor during precipitation in warm convective clouds from a large eddy simulation are reported here. Results show that precipitation can enhance the variability of water vapor mixing ratio horizontally. The standard deviation of column-integrated water vapor changes from 0.33kgm−2 (when there is no precipitation) to 0.64kgm−2 (when the precipitation is very strong), suggesting that water vapor field becomes less homogeneous horizontally as precipitation develops. The layer near the surface has the largest variability of water mixing ratio compared to the higher layers. Meanwhile, precipitating cumulus clouds tends to smooth the water vapor field vertically. The horizontal cross sections indicate that water vapor field has cellular structure near the surface as precipitation develops. Water vapor mixing ratio is lower in the center of the convective cell than that at the edges. This particular structure is due to the precipitation-dynamics feedback: the regions with precipitation have downdrafts, which transport dry air from the upper layer into the lower layer. The flow diverges near the surface, and collides with the outflow from the adjacent precipitating cloud. Therefore, water vapor mixing ratio near the surface is lower in the precipitating regions and higher in the surrounding regions.