Separating Local and Non-Local Impacts on Convection

Summary

The representation of convective clouds (i.e., clouds formed from rising air motions) is a key uncertainty in climate models due to the small scales of convective elements relative to model grid size and the complex interactions between large-scale circulation and local surface conditions on time scales of less than a day. To properly understand and simulate these complex interactions in numerical models, relationships between the various scales from the local land surface to the large-scale background state of the atmosphere must be consistently quantified. In particular, to improve weather and climate models of convection, scientists need to understand under what conditions convective clouds are triggered by local changes in heat and moisture and when they are more influenced by the larger-scale atmospheric conditions.

Researchers previously introduced the idea of the Heated Condensation Framework (HCF) as a tool for addressing the issue of separating local from non-local impacts on convection. In a recent study, scientists used data from the Department of Energy’s Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site to illustrate the full suite of HCF variables and to demonstrate their capability for 1) quantifying the necessary moisture and heat inputs to trigger convection; 2) identifying the transition height separating two boundary layer regimes; 3) identifying which convective events were triggered locally; and 4) identifying sources of model bias in the convective state. The newly developed relationships provide a comprehensive way of assessing the atmosphere’s convective state, and, in particular, isolate the influence of the large-scale background state on convective initiation. Because the approach only requires atmospheric profiles of temperature and humidity to produce the entire suite of variables, models can be compared directly against observations enabling targeted model development. The capabilities presented here enable better process understanding of how the land surface may influence convective initiation, which can help improve future weather and climate models.

References

Tawfik, A. B., P. A. Dirmeyer, and J. A. Santanello Jr. 2015. “The Heated Condensation Framework. Part I: Description and Southern Great Plains Case Study,” Journal of Hydrometeorology, DOI:10.1175/JHM-D-14-0117.1.