Afternoon convection favored over drier or wetter land surface depending on atmospheric state
Williams, Ian N. — Lawrence Berkeley National Laboratory
Area of research:
General Circulation and Single Column Models/Parameterizations
A strong negative feedback of dry soil on convective initiation was found in atmospheric states supporting deep boundary layers. Convection and precipitation show positive relationships to surface evapotranspiration only in more humid, thermally stratified atmospheres.
The negative feedback of a dry surface on shallow to deep cumulus transition (and rainfall) may play a key role in ending droughts. Dependence of feedback sign on tropospheric state and vegetation explains why weak or no relationship was found between soil moisture and cumulus convection in prior studies, in the US Southern Great Plains.
Connections between cumulus clouds and the land surface are critical in determining the impacts of climate and land-surface change on drought and extreme precipitation events. In particular, the transition from shallow (non-precipitating) to deep (precipitating) cumulus clouds depends on the state of the land surface and that of the overlying atmosphere. In this study, we applied a new approach that combines model simulation with satellite cloud and ARM surface observations to reveal the feedback mechanisms that were missed in earlier studies. After distinguishing the atmospheric states using model experiments, we found that the transition is more frequent over a dry surface (a negative feedback) in a dry but less stratified atmosphere, where clouds can be formed by lifting and cooling air to saturation. In contrast, there are more shallow-to-deep transition and precipitation events over a wetter surface (a positive feedback) in a more stratified and humid atmosphere. The strong negative feedback of a dry surface on the transition from shallow to deep convection has not been reported previously for this region, and may play a key role in ending droughts, while the positive feedback of a wet surface on precipitation occurrence may prolong periods of heavy rainfall and flooding.