Moisture Patterns in Deepening Maritime Extratropical Cyclones. Part I: Correlation between Precipitation and Intensification

Abstract

Coincident satellite passive microwave (SSM/I) observations and 48-h numerical 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 intensification rates. Analysis of the observations and simulations focus on the 24-h period of most rapid intensification for each case as determined from European Centre for Medium-Range Weather Forecasts 12-h mean sea level pressure analyses. SSM/I observations of area-averaged precipitation and an index that responds to cold-cloud (convective) precipitation to the northeast of surface cyclone centers were previously shown to correlate well (?0.80) with the latitude-normalized deepening rate (NDR) of the study sample. This large correlation is replicated by the numerical model, although the area-averaged precipitation region yielding the maximum correlation coefficient differs significantly from that determined using microwave imagery. A similar correlation emerges between model-derived area- and vertically averaged vertical motion fields and NDR. The similarity of these correlations for nearly coincident averaging regions relative to the storm center implicates unrealistic 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.