How Well Does an Earth System Model Represent the Occlusion of Extratropical Cyclones?

Extratropical cyclones are the main providers of midlatitude precipitation, but how they will change in a warming climate is unclear. The latest NASA Goddard Institute for Space Studies (GISS) Earth system models (ESMs) accurately simulate the location and structure of cyclones, though deficiencies in the depiction of cloud and precipitation are found. To provide a new process-level context for the evaluation of simulated cloud and precipitation in the midlatitudes, occluded cyclones are examined. Such cyclones are characterized by the formation of a thermal ridge, maintained via latent heat release in the wider three-dimensional trough of warm air aloft (TROWAL) in the occluded sector. Using a novel method for the objective identification of occluded cyclones, the simulation of occlusions in the latest GISS-E3 model is examined. The model produces occluded cyclones, adequately depicting the thermal and kinematic structures of the thermal ridge, with realistic depth and poleward tilt. Nevertheless, E3 occlusions are less frequent than observed and systematically shifted poleward and toward the exit region of the climatological storm tracks. Compared to CloudSat–CALIPSO cloud retrievals across the thermal ridge, the dependence of cloud properties on thermal ridge strength is well represented, though at the expense of producing low ice mass clouds too often at high altitudes (i.e., “too many, too tenuous”). Overall, E3 produces significantly more precipitation in occluded versus nonoccluded cyclones, demonstrating the importance of accurately representing occlusions and associated hydrological processes in ESMs.