NGM Performance during Cold-Air Outbreaks and Periods of Return Flow over the Gulf of Mexico with Emphasis on Moisture-Field Evolution

The National Meteorological Center's Nested Grid Model (NGM) analyses and 24-48-h forecasts of cold-air outbreaks and their associated return flows are examined from January through March 1988 coincident with the Gulf of Mexico Experiment (GUFMEX). Seven episodes of moderate-to-strong cold-air outbreaks and associated return flows are isolated. A good index of these cycles is the 950-mb meridional wind component. Composites and individual cases of the horizontal, vertical, and temporal structure of wind, temperature, and moisture are diagnosed for NGM analyses and 24-48-h forecasts of these events. Primary focus is on air and moisture modification over the Gulf during the return-flow cycle. Comparisons among observed inversion layers capping the low-level moisture and those present in NGM analyses and forecasts are examined. These differences highlight model strengths and weaknesses. With regard to humidity-field evolution during the return-flow cycle, NGM forecasts are consistently too dry over the Gulf during both offshore and onshore flow phases. Isentropic trajectory computations suggest that advective processes clearly dominate the moisture modification processes in the NGM forecasts. Comparisons indicate that low-level moisture errors in the model forecasts result more from shortcomings in model physics than from inadequate or poor model initialization. Recently, changes to the NGM input analyses and physics package have been implemented. While the affects of these changes have yet to be determined, this study illustrates strengths and weaknesses of the NGM during the 1988 cool season and emphasizes the necessity for accurate simulation of boundary-layer processes during return-flow events and times of airmass modification.