MOISTURE PATTERNS IN DEEPENING MARITIME EXTRATROPICAL CYCLONES - PART-I - CORRELATION BETWEEN PRECIPITATION AND INTENSIFICATION

Coincident satellite passive microwave (SSM/I) observations and 48-h n umerical simulations of 23 intensifying extratropical cyclones located over the North Atlantic or North Pacific Oceans during a single cold season are examined in order to identify systematic differences in the moist processes of storms exhibiting rapid and ordinary intensificati on rates. Analysis of the observations and simulations focus on the 24 -h period of most rapid intensification for each case as determined fr om European Centre for Medium-Range Weather Forecasts 12-h mean sea le vel pressure analyses. SSM/I observations of area-averaged precipitati on and an index that responds to cold-cloud (convective) precipitation to the northeast of surface cyclone centers were previously shown to correlate well (similar to 0.80) with the latitude-normalized deepenin g rate (NDR) of the study sample. This large correlation is replicated by the numerical model, although the area-averaged precipitation regi on yielding the maximum correlation coefficient differs significantly from that determined using microwave imagery. A similar correlation em erges between model-derived area- and vertically averaged vertical mot ion fields and NDR. The similarity of these correlations for nearly co incident averaging regions relative to the storm center implicates unr ealistic rainfall patterns as the reason for the failure of the model to accurately capture the observed optimal area-averaging region. This region is located near the storm triple point and occluded (bent-back ) front, both potentially strongly convective environments.