Process-based evaluation of occluded extratropical cyclones in climate models using CloudSat-CALIPSO observations

Abstract:

The process of occlusion in extratropical cyclones is largely driven by latent heat release, and therefore, the degree to which a model accurately simulates this cyclone lifecycle stage provides information on how well it represents moist processes. In turn, it provides process-based constraints for model evaluation. Using CloudSat-CALIPSO observations of cloud profiles and surface precipitation, it has been possible to obtain a detailed picture of the three-dimensional structure of extratropical cyclones and their characteristics during occlusion.

We will present a brief introduction to what we have learned about cloud properties across cold and warm fronts, their sensitivity to the cyclone specific environment, and how this information has been used to evaluate climate models. Preliminary results on cyclone occlusions, derived using reanalysis data and CloudSat-CALIPSO observations, will then be shown.

We will describe the technique developed to identify and characterize the occlusion phase of an extratropical cyclone using the MERRA-2 reanalysis. A similar technique is applied to the most recent version of the GISS general circulation model (GCM) to be submitted to CMIP6 (ModelE3). We will discuss a series of tests that are designed to determine whether: 1) the GCM can simulate occlusions; 2) occlusions occur as frequently in the GCM as in the real world; and 3) GCM simulated occlusion characteristics are similar to observations (e.g., time of onset, duration). Finally, we will present a preliminary examination of occluded-sector cloud distributions in observations and in the GISS GCM.