Land surface variations influence shallow cloud populations

Science

Shallow clouds occur over many areas of the world and are an important component of Earth’s atmospheric radiation budget. Models, called Large Eddy Simulations (LES), that can represent turbulent motions near the surface as well as the size of small clouds have been used extensively to better understand processes that control the life cycle of shallow convective clouds. However, because LES models can take days to weeks to run on a supercomputer, these models usually simulate clouds over relatively small areas (less than 50 km wide) and use simplified assumptions of the environmental meteorological conditions so that the predicted clouds are nearly uniformly distributed. Researchers at the U.S. Department of Energy used a more realistic modeling approach to represent observed complex cloud distributions over Oklahoma on a day during the U.S. Department of Energy’s recent Atmospheric Radiation Measurement (ARM) Holistic Interactions of Shallow Clouds, Aerosols, and Land Ecosystems (HI-SCALE) atmospheric sampling campaign. Researchers compared the model predictions with satellite images and collected extensive measurements near the ARM Southern Great Plains (SGP) site.

Impact

The high-resolution model experiments show that the convective cloud populations observed on one day of ARM’s HI-SCALE field campaign were influenced by two factors: (1) in the morning, the distribution of drier and wetter soil moisture drove the initial formation of shallow convection, and (2) the land–atmosphere coupling associated with these soil moisture gradients enabled some shallow clouds to grow deeper, precipitating convection during the early afternoon and producing cold pools that further disrupted the spatial distribution and evolution of clouds. The strength of this land–atmosphere–cloud coupling is expected to vary depending on the ambient wind speed. This suggests that weather and climate model predictions can be improved by including the effects of cold pools and small-scale variations in surface properties, such as land use and soil moisture.

Summary

The SGP is one of several land–atmosphere “hotspots” around the world that influence the life cycle of shallow convection. The measurements collected at the ARM SGP site make this region unique in terms of providing information needed to better understand how coupling of the land–surface interface influences the development of cloud populations. By integrating measurements from this site, special aircraft measurements collected during HI-SCALE, and satellite images with higher-resolution modeling, researchers were able to deduce two superimposed processes that contribute to the organization of convective clouds:

• The first factor is soil moisture variability and, although to a lesser degree, cooler lake temperatures, which control cumulus formation early in the day. Regions with drier soil moisture had warmer boundary layers and higher boundary-layer depths had more vigorous shallow cumulus formation earlier in the day than regions with wetter soil moisture. The boundaries of drier and wetter soil regions favor the transition of shallow cumulus to deeper, precipitating convection.
• The second factor is cold pools that form during the early afternoon. There are many precipitating convective cells that last only a few minutes and only perturb the local cloud fields. Fewer, longer-lasting, and stronger convective cells produce cold pools that are more persistent and expand over large areas that are tens of kilometers wide. The cold pools suppress turbulence and cloud formation so that clear skies are present over relatively large areas. The cloud distributions forced by soil moisture variations are thus perturbed by cold pools that expand and overlap each other.

These processes are likely responsible for the complex observed population of convective clouds. In addition, a cloud-tracking algorithm that can quantify the lifetime and size of the clouds in the model showed that more complex soil moisture distributions led to larger clouds with longer lifetimes.