Simulating the Effects of Surface Energy Partitioning on Convective Organization: Case Study and Observations in the US Southern Great Plains. Issue 2 (21st January 2021)

• Record Type: Journal Article
• Authors: Dai, Yi; Williams, Ian N.; Qiu, Shaoyue
• Is Part Of: Journal of geophysical research. Volume 126:Issue 2(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-01-21
• Subjects: Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2020JD033821
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Realistic cloud‐resolving simulations were performed to study the effects of surface energy partitioning (surface sensible and latent heat fluxes) on the organization of isolated convection into larger mesoscale convective systems (MCSs) near the US Southern Great Plains. The role of cold pools in mediating surface‐convection interactions was explored. Better organized MCSs tended to occur in the experiments with perturbed wetter soil (and more active vegetation), regardless of the effects of soil moisture on the diurnal timing of convective triggering. Wetter soil led to shallower boundary layers and more convective available potential energy than drier soil. The roles of cold pools on convection are lifecycle‐stage dependent: A dry surface allows more numerous colliding cold pools, thereby aiding in convective triggering by reducing entrainment in the early stages, and providing gust‐front uplift in later stages. However, horizontal propagation of the cold‐pool density current can outrun the convective system, creating a slantwise updraft and thus weakening the gust front uplift in later stages. This effect calls into question previous cold pool parameterizations, in which the gust front uplift is mainly proportional to the negative buoyancy of cold air. Lastly, both the model and observation show an enhancement of surface latent heat flux during the passage of a gust front at night, suggesting that a positive feedback between the surface and convection helps MCSs to persist into the nighttime. Plain Language Summary: Numerical simulations explicitly resolving thunderstorms were used to study the effects of the land surface (soil moisture/vegetation) on rainfall in the US South Great Plains. Better convective organization (larger, stronger thunderstorm clusters with heavier rainfall) occurred more often in perturbed wetter soil/vegetation experiments than in dry experiments, indicating a positive feedback of soil moisture on rainfall. A case study was conducted to explore the mechanism of this feedback. Although dry soil is initially more favorable for triggering thunderstorms, wetter soil can modify environmental conditions (convective available potential energy) to become more favorable for further development of isolated thunderstorms into larger organized clusters. Current climate models do not fully capture the feedback mechanisms indicated here. The results provide insight into how climate models can be improved to more realistically represent processes connecting rainfall to future land‐surface change. Key Points: Wet soil can lead to better organized convection than dry, constituting a positive rainfall feedback, despite earlier triggering over dry Cold pools diminish or contribute to feedback; their effects vary with convective lifecycle stage and are not captured in parameterizations Enhanced surface latent heat flux along gust fronts provides additional positive feedback that may help convection persist after sunset