Regional moisture budget and land-atmosphere coupling over the U.S. Southern Great Plains
Zhang, Yunyan — Lawrence Livermore National Laboratory
Area of research:
Using 10-year warm-season observations at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site, U.S. Department of Energy scientists and collaborators investigated the large-scale atmospheric moisture budget for remotely and locally forced convective events. For each locally generated convection regime, they also quantified the strength of land-atmosphere (L-A) coupling over various land surface conditions.
This study advances our understanding on how the land surface controls the evolution of the planetary boundary layer (PBL), the onset of clouds, and the triggering and development of afternoon precipitation. This study may draw attention from both atmospheric and land communities that it is necessary to examine the L-A coupling in different convection regimes on the related processes in a statistical manner using observational data.
Consistent with previous studies, large-scale moisture flux convergence dominates the SGP regional moisture budget on the daily time scale. However, the strength of L-A coupling is amplified in the locally generated convection regimes. Without distinguishing convection triggering mechanisms, that is, remotely versus locally forced, the derived overall L-A coupling strength might be underestimated.
The study shows that the impact of land surface on the evolution of the PBL depends on the vegetation leaf area index. With similar soil moisture conditions, the cloud fraction over forest is much higher than that over grassland on fair-weather shallow cumulus days. The rain tends to start earlier with larger evaporative fraction on late-afternoon deep convection days; however, a significant statistical relationship between afternoon rain and surface turbulent flux only exists in the northeast part of the ARM SGP site, which is mainly covered by grassland/pasture.