Observational Evidence of State-Dependent Positive and Negative Land Surface Feedback on Afternoon Deep Convection Over the Southern Great Plains

The detection of soil moisture-precipitation feedback is complicated by the dependence of feedback sign on atmospheric state and cloud types. We evaluated responses of convective clouds to changes in surface evaporative fraction (EF) from satellite and ground-based measurements over the Southern Great Plains in summer months. Strong responses to EF were inferred only after accounting for tropospheric states supporting systematically positive or negative responses. State dependence (inferred from controlled model experiments) explains the weak response found in previous studies. The probability of transitions from shallow to deep cumulus, and subsequent precipitation onset, increases by 58% over drier surfaces in weakly stratified environments having deeper boundary layers. Boundary layer dynamics play a more important role than low-level relative humidity in these environments. Wetter surfaces yield increased deep convection in moist and thermally stratified environments. The distinct pathways of convective initiation over dry and wet land surfaces can play important roles in drought onset and termination. Plain Language Summary 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 differentiated between atmospheric states that allow either positive or negative responses of the shallow-to-deep transition to increasing evaporation (soil moisture). After distinguishing the atmospheric states, 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 act to prolong periods of heavy rainfall and flooding.