Revisiting mechanisms and regimes of soil moisture influence on convective initiation
Authors
Ian N. Williams — Lawrence Berkeley National Laboratory
Sebastien Christophe Biraud — Lawrence Berkeley National Laboratory
Margaret S. Torn — Lawrence Berkeley National Laboratory
Category
Boundary layer structure, including land-atmosphere interactions and turbulence
Description
Coupling and feedbacks between clouds, precipitation, and the land-surface can affect the hydrologic cycle. The goal of this project is to improve prediction of the hydrologic cycle by advancing understanding of the role of the land-surface in cloud and precipitation processes, using experiments and observations at multiple scales. Here we present paired single-column model experiments (daily hindcasts in summer) having perturbed soil moisture, including interactions between turbulence, clouds, and radiation (in the NCAR Community Atmosphere Model, CAM5), given large-scale forcing prescribed from observations collected at the DOE Atmospheric Radiation Measurement (ARM) site in the U.S. Southern Great Plains. Our goal is to identify observational targets for model evaluation, and select archetypal cases for convection-permitting simulations that are planned to elucidate mechanisms and systematic behaviors of soil moisture-precipitation interactions.
The results support the existence of so-called wet- and dry- soil moisture advantages for daytime deep convection initiation, but with different mechanisms than indicated previously. High evaporative fractions (EFs) led to systematically lower convective inhibition (CIN) on wet-advantage days (as expected), but did not significantly reduce the boundary layer saturation deficit. Moreover, high-EFs increased surface net radiation by 50-60 W m-2 due to less infrared radiation loss from a wetter (cooler) surface, indicating an energetic mechanism for the wet-advantage regime. On dry-advantage days, low-EFs promoted convective initiation by increasing turbulent kinetic energy (TKE) and lowering the ratio of CIN to TKE. Our analyses suggest that the formulation of cumulus mass flux closure largely determines the sign of the simulated precipitation response to soil moisture perturbations. Overall, the results indicate that convective initiation is sensitive to surface energy partitioning, even in the presence of realistic large-scale forcing and interactive radiation.
