Three-Dimensional Airflow and Precipitation Structure in a Nondeepening Cyclone

Abstract

During the period 1200 UTC 3 March to 1200 UTC 5 March 1985 a cyclone located over eastern Colorado tracked northeastward across the Great Plains to southern Ontario. Although the low-pressure system decayed considerably during the latter 24 h of the period, notable amounts of precipitation fell, particularly over the New England states. The time of detailed study encompasses 1200 UTC 4 March to 1200 UTC 5 March 1985. Inspection of the observed 6-h precipitation totals during this period indicate three distinct maxima. One is to the north and northwest of the low center, another to the cast of the low center, and a third along the surface cold front extending southwestward from the low center. In order to aid in the explanation of these observed patterns, isentropic trajectories were utilized to illustrate the three-dimensional airflow around the nondeepening cyclone. Low-level easterly flow was found to the north of the cyclone center. Meanwhile, a flow of warm moist air being transported northward from the Gulf of Mexico between 700 and 500 mb bifurcated east of the cyclone center. A branch of this moisture curved cyclonically and flowed to the north and west of the low center while the other branch curved anticyclonically and flowed to the low of the low center. These two streams of warm, moist air were forced to ascend over the layer of cold air in the low-level easterly flow, causing clouds and precipitation to result. These processes help explain the cloud patterns and precipitation structure associated with the non-deepening cyclone. The Sutcliffe-Petterssen development equation is utilized to calculate the surface voricity tendency field. From the calculations, it is seen that warm advection in the lower troposphere rather than midtropospheric vorticity advection is responsible for the persistence of the precipitation over the New England states.