Characterization of Surface Heterogeneity‐Induced Convection Using Cluster Analysis. Issue 20 (12th October 2020) - Chen, Jingyi
Abstract: Land‐atmosphere interactions and boundary layer processes often control the formation of shallow clouds and subsequently deep convective precipitation over the U.S. Southern Great Plains. In this study, we examine the impacts of large‐scale advection on the cloud populations and land‐atmospheric coupling observed during the Holistic Interactions of Shallow Clouds, Aerosols, and Land‐Ecosystems field campaign in 2016. We performed two large eddy simulations using the Weather Research and Forecasting Model for a day in which the transition from clear‐sky to shallow precipitating clouds and isolated deep convection was observed. The control simulation reproduced the overall distribution of cloud populations by using realistic soil conditions with an interactive land model. In the sensitivity simulation where large‐scale advection is removed, a strong relationship between the land and boundary layer is found. To study the timing, location, and intensity of convective initiation and the relationship of clouds with land surface properties, a cluster analysis of equivalent potential temperature is performed for the simulation without large‐scale advection. That analysis shows that convective clouds first form over regions with higher surface sensible heat flux. Precipitation from those convective clouds likely triggers new updrafts nearby about 2 hr later through the lifting associated with cold pools. The cluster analysis also shows that in addition to the spatial pattern of soil moisture, land use and soil texture in western Oklahoma also influence the location of convective initiation. Plain Language Summary: Accurately predicting the timing, location, and intensities of convection over midlatitude land is extremely important. Yet, complete answers to the related questions such as how the land properties impact the atmosphere and cloud properties remain lacking. Models that resolve boundary layer turbulence and clouds have been used extensively to understand processes controlling land‐atmosphere interactions, but computational expense limits the employment of realistic land properties. In this study, we employ a more realistic modeling approach to study abovementioned question. First, we examined the relative role of large‐scale advection and the local effects of land properties in convection and cloud formation. A comparison of two sensitivity studies shows that large‐scale advection is vital in the characterization of land‐atmosphere interactions not only in terms of the magnitude but also the types of relationships. Then, we analyzed the simulation with only local effects by cluster analysis. We found that convection initiates over the regions with high surface sensible heat flux. Precipitation from the earliest convection likely triggers new convection around about 2 hr later by forming a cold surface area due to evaporating cooling. Moreover, soil moisture, land use type, and soil texture play roles in determining the surface heat fluxes. Key Points: Large‐scale advection is vital in characterizing land‐atmosphere interactions not only in magnitude but also types of relationships Convection initiates over regions with high sensible heat flux; precipitation likely triggers new updrafts around through cold pools Soil moisture, land use type, and soil texture play roles in the timing, location, and type of convective clouds