Convectively-induced secondary circulations and wind-driven heat fluxes in the surface energy balance over land
Increasing horizontal resolution has enabled weather models to partially resolve organized circulation structures within the atmospheric boundary layer, including horizontal convective rolls, cells, and gust fronts. These circulations are common in observations and high-resolution simulations, but their effects on weather prediction are poorly understood. We simulated their impacts using the DOE Energy Exascale Earth System Model (E3SM), showing that horizontal rolls enhance surface winds and increase surface heat fluxes (by 50 W m⁻²) within convergent branches of the circulations. Larger wind-driven flux responses (up to 150 W m⁻²) occur along gust fronts. Doppler lidar and surface turbulence measurements confirmed the presence of roll-like structures associated with surface wind gusts. These results have broad implications for modeling convective boundary layers: widely used subgrid wind-gust parameterizations are not physically consistent with resolved boundary layer circulations and may worsen weather prediction biases at kilometer scales.
Colston, S., & Williams, I. N. (2024). Convectively induced secondary circulations and wind-driven heat fluxes in the surface energy balance over land. Geophysical Research Letters, 51, e2024GL110565. https://doi.org/10.1029/2024GL110565
Long‐Term Observations of Turbulence Vertical Velocity Spectra in a Convective Mixed Layer: Dependence on Land‐Surface Forcing
The formation of cumulus clouds is linked to secondary circulations in the atmospheric boundary layer. We investigated how the land surface influences the types and scales of secondary circulations. Circulation scale is important because wider boundary layer updrafts are more likely to initiate cumulus clouds. In Williams and Qiu (2022), we used ARM Doppler lidar and surface measurements to identify predictors of the length scales of coherent updrafts associated with roll and cellular structures. Past attempts using only a stability parameter have shown mixed success. We found that circulation length scales depend on multiple surface flux variables, helping to explain why it has been difficult to distinguish the onset of rolls from cells using a stability parameter alone. We further used radar clear-air returns to relate the lidar-derived length scales to horizontal rolls in the cloud-topped convective boundary layer, providing additional observational evidence linking boundary layer secondary circulations to cumulus clouds.
Williams, I. N., & Qiu, S. (2022). Long-term observations of turbulence vertical velocity spectra in a convective mixed layer: Dependence on land-surface forcing in the U.S. Southern Great Plains. Journal of Geophysical Research: Atmospheres, 127, e2022JD037137. https://doi.org/10.1029/2022JD037137
Land surface effects on shear balance of squall lines
Existing theory for long-lived squall line thunderstorms (known as RKW theory) does not consider land surface effects. By incorporating the land surface, we have shown that the principle of RKW theory (involving a balance of horizontal vorticity or wind shear between the cold-pool and storm environment) is still useful but needs to be modified to account for effects of surface friction on wind shear within the cold pool. More broadly, the effects of surface friction in our experiments suggest that land cover change can contribute to changes in the intensity, structure, and propagation characteristics of mesoscale convective systems.
Dai, Y., & Williams, I. N. (2022). Land surface effects on shear balance of squall lines. Journal of Geophysical Research: Atmospheres, 127, e2021JD035436.
Detecting land surface feedback on deep convection in cloud-permitting model experiments
We are using numerical model experiments at cloud scales 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.
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. Model simulations 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 at cloud scales.
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
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
Evaluating Soil Moisture Feedback on Convective Triggering: Roles of Convective and Land-Model Parameterizations
We performed model experiments using the Community Earth System Model, to explore land surface influences on the initiation of daytime deep convection. Systematic responses of convective triggering to soil moisture emerged only after changing the convective 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