Recent Modifications of the Emanuel Convective Scheme in the Navy Operational Global Atmospheric Prediction System

摘要

The convective parameterization of Emanuel has been employed in the forecast model of the Navy Operational Global Atmospheric Prediction System (NOGAPS) since 2000, when it replaced a version of the relaxed Ar-akawa―Schubert scheme. Although in long-period data assimilation forecast tests the Emanuel scheme has been found to perform quite well in NOGAPS, particularly for tropical cyclones, some weaknesses have also become apparent. These weaknesses include underprediction of heavy-precipitation events, too much light precipitation, and unrealistic heating at upper levels. Recent research efforts have resulted in modifications of the scheme that are designed to reduce such problems. One change described here involves the partitioning of the cloud-base mass flux into mixing cloud mass flux at individual levels. The new treatment significantly reduces a heating anomaly near the tropopause that is associated with a large amount of mixing cloud mass flux ascribed to that region in the original Emanuel scheme. In another modification, the selection of the updraft source level is changed in a manner that takes into consideration the assumed connection between updraft mass flux and parcel buoyancy at cloud-base level in the Emanuel scheme. Test results suggest that the modified scheme may in some cases better represent precipitation during the middle and latter stages of convective events. The scheme has also been modified to eliminate cloud-top overshooting. The parameterization changes are supported in part by diagnostic tests, including semiprognostic model tests using observed data and single-column model tests using cloud-resolving-scale simulation data. The modifications showed significant positive impacts in forecast experiments over the original designs and have been implemented into the operational NOGAPS.