Structure of the Cold Front Observed in SESAME-AVE III and its Comparison with the Hoskins-Bretherton Frontogenesis Model

Abstract

A cold front which passed through the dense network of the SESAME-AVE (Severe Environmental Storms and Mesoscale Experiment?Atmospheric Variability Experiment) on 25?26 April 1979 was investigated. Rawinsonde data collected from 23 special stations and 19 National Weather Service stations at three-hour intervals for a 24-hour period were used along with hourly surface data, radar summary charts and GOES-East satellite images. Severe storms formed along the surface front during this period. The analysis focused on the vertical circulation across the frontal surface at low levels. The major features of the cold frontal system that emerged from an analysis of this unique data set include a familiar direct vertical circulation, with moist warm air ascending just above the surface front. However, the upgliding motion was intercepted by a secondary circulation at middle levels. The analysis result was compared with model predictions of Hoskins and Bretherton (1972) as calculated by Blumen (1980). Several features of the observed front were found to agree qualitatively well with the model prediction. These include: a) Both the horizontal temperature gradient and the vertical component of vorticity have their maxima near the ground surface; b) The horizontal gradient of potential temperature is smaller in the warm air region than in the cold air region; c) The temperature inversion layer representing the frontal surface is located behind and below the axis of the maximum cyclonic relative vorticity. However, the model is found to be less successful in predicting the low-level convergence field; the observed surface convergence and cyclonic vorticity are of the same order of magnitude and concentrated in zones of approximately the same width of 300 km. The observed maximum ascending motion is located at low levels, rather than in middle levels as predicted. The subsidence in the cold air region is also much stronger than the model prediction.