PNNL

The Science

Clouds need two things to form—upward winds (buoyancy) and water vapor. These can come from the surface, but variable land conditions, such as different vegetation and soil wetness, make predicting cloud formation, including the location and extent of cloud clusters challenging. Researchers used a high-resolution model and in situ soil observations to explore cloud cluster formation. Cloud clusters spread randomly over areas 2 to 4 km wide when there are no surface variations. Observed dry/wet soil variations create hotspots of strong circulations that are 3 to 9 km wide. For the day under investigation, cloud clusters over such “hot spots” can reach 9 km wide in the morning and grow to 20 km or larger in the afternoon.

The Impact

Knowing the size of a target process is critical to designing observational networks and numerical models. But the sizes of the processes involved in land-atmosphere coupling have not been quantified due to limits in observations and computer model resolution. The findings of this study will enable researchers to design a network with sufficient resolution to observe the turbulent air motions that link the soil to the clouds and to guide theoretical development toward faithfully representing that coupling in climate models. These in turn will lead to more reliable future projections of climate, especially about the near-surface temperature and precipitation over land.

Summary

Water stored in sub-surface soil affects clouds through uprising turbulent air motions. Previous studies showed that these land-atmosphere couplings depend on different surface conditions. However, researchers do not fully understand how variations of surface features relate to the size, or scale, of turbulent winds and cloud clusters. Previous work only used idealized numerical experiments to answer such questions. High-resolution model simulations targeted at a case during the HI-SCALE field campaign at the Atmospheric Radiation Measurement Southern Great Plains site provide more real-world relevant answers. Spectral analysis revealed that the scales of surface variations and atmospheric responses do not necessary overlap. Water, urban, and vegetation surfaces exhibit a wide range of scales from 1 to > 30 km but tend to energize turbulent winds in a limited size range (3—6 km). Soil moisture variations at scales larger than 20 km also influence circulation at a similar scale (3—9 km). A critical difference is that soil moisture variations substantially strengthen winds over drier areas and boundaries between different surface types, which become “hotspots” of land-atmosphere interactions. These hotspots produce larger cloud clusters that can generate twice as much precipitation compared to the smaller cloud clusters that form over uniformly wet soils.  

PNNL Contact

Jerome Fast, Pacific Northwest National Laboratory, Jerome.Fast@pnnl.gov

Funding

The Department of Energy (DOE) Office of Science, Biological and Environmental Research supported this research as part of the Atmospheric System Research program. Observations collected during HI-SCALE were supported by the Atmospheric Radiation Measurement User Facility and the Environmental Molecular Sciences Laboratory, both DOE Office of Science user facilities sponsored by the Office of Biological and Environmental Research.