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Research highlights

Detecting land surface feedback on deep convection in cloud-permitting model experiments

We are using numerical model experiments at 'cloud-permitting' spatial resolution to identify land surface influences on the initiation of deep convection (thunderstorms). The results demonstrate cases in which a perturbed wetter land surface can shut down convective initiation, by trapping unstable air in a shallower boundary layer beneath the level of free convection. The thermal stratification of the lower troposphere emerges as a key factor in identifying cases where convection is sensitive to the state of the land surface; this stratification determines the depth to which air may be lifted in the boundary layer given high surface sensible heating.

Animation:  Perturbed land surface experiments showing successful convective initiation over (left) a dry surface and (right) failed convective initiation over a wet surface.

 

Modeling Spatial Heterogeneity in Surface Turbulent Heat Flux

We used observations to improve vegetation input datasets and inform parameters in the Community Land Model (CLM). The new model configuration predicts spatial patterns of surface evapotranspiration that more closely resemble the distribution of crops along the 'winter wheat belt' across central Oklahoma and Kansas. Predictions show that spatial patterns of surface water and energy exchanges are determined not just by those of past rainfall and soil moisture, but by the distribution of crops and other vegetation. This new dataset gives a more accurate and detailed depiction of vegetation properties and surface fluxes that can be used in cloud-resolving and LES models.

Williams, I. N., Lee, J. M., Tadić, J., Zhang, Y., & Chu, H. (2020). Modeling spatial heterogeneity in surface turbulent heat flux in the US Southern Great Plains. Journal of Geophysical Research: Atmospheres, 125,e2019JD032255. https://doi.org/10.1029/2019JD032255

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Figure: Surface observations from the DOE Atmospheric Radiation Measurement (ARM) Climate Research Facility were used to improve vegetation input datasets for numerical models.

 

Observational Evidence of State‐Dependent Positive and Negative Land Surface Feedback on Deep Convection

The transition from shallow to deep (precipitating) cumulus is influenced by surface water and energy fluxes, and by the state of the overlying atmosphere. Using satellite observations of clouds, we found that the shallow-to-deep cumulus transition is more frequent over a dry surface (a negative feedback) in a weakly stratified atmosphere, where condensation can be reached by lifting and cooling air to saturation. In contrast, shallow-to-deep transition and precipitation events are more likely over a wetter surface (a positive feedback) in a more stratified and humid atmosphere. The strong negative feedback of a dry land surface on the transition from shallow to deep convection can play a role in ending droughts, while the positive feedback of a wet surface on precipitation occurrence may prolong periods of heavy rainfall and flooding.

Citation: Qiu, S., & Williams, I. N. (2020). Observational evidence of state‐dependent positive and negative land surface feedback on afternoon deep convection over the Southern Great Plains. Geophysical Research Letters, 47,e2019GL086622. https://doi.org/10.1029/2019GL086622

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Figure: Observed diurnal frequency of cumulus event days by cloud top height. The transition from shallow to deep cumulus is more likely over a dry surface in a 'negative feedback regime' defined by tropospheric state.

 

Evaluating Soil Moisture Feedback on Convective Triggering: Roles of Convective and Land-Model Parameterizations

We performed model experiments using the Community Earth System Model in 'single column' mode, to explore land surface influences on the initiation of daytime deep convection. Systematic responses of convective triggering to soil moisture emerged only after switching the convective parameterization from a 'CAPE-based' parameterization to one that accounts for turbulence kinetic energy and convective inhibition energy. This suggests that the choice of convective mass-flux closure in climate and Earth system models determines the response of modeled clouds to drought.

Williams, I.N. (2019), Evaluating soil moisture feedback on convective triggering: Roles of convective and land-model parameterizations, J. Geophys. Res. Atmos., https://doi.org/10.1029/2018JD029326

 

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Figure:  Models that suppress boundary layer responses to variations in surface evaporative fraction (EF) are not able to represent the mechanisms of negative feedback between soil moisture and rainfall.