An Explanation for Intense Frontal Updrafts and Narrow Cold-Frontal Rainbands

Abstract

Measurements with Doppler radar, and instrumented aircraft and towers, have revealed that surface cold fronts often have cross-frontal circulations organized on a scale of a kilometer or less. These circulations include intense updrafts (1 to 20 m s?1) that result in a narrow band of heavy rainfall. We used a nonhydrostatic model to investigate the mechanism for these updrafts and to isolate those characteristics of the prefrontal environment that result in intense updrafts and narrow bands of heavy rainfall. Our simulations were initialized with a cold reservoir in a manner analogous to that used to produce a gravity current. The similarity between the observations and our simulated frontal flows supports the hypothesis that the flow at the leading edge of surface cold fronts can sometimes be represented by gravity-current dynamics. We also found that the differences between frontal circulations and classical dry gravity currents can be explained by the effects of precipitation and vertical shear. In our simulations, the intense updrafts at the leading edge of the cold air mass were associated with a strong upward-directed pressure force and were not associated with significant parcel buoyancy. The conditions for intense updrafts and heavy rainfall in our simulations were 1) strong deep cold pools, 2) a prefrontal environment that contains deep layers of air that are nearly saturated with a lapse rate that is nearly neutral to moist ascent, and 3) intense low-level vertical shear in the cross-frontal direction of the horizontal wind. These conditions are typical of maritime surface cold fronts that often have strong updrafts and bands of heavy rainfall. In our simulations, the vertical shear in the cross-frontal direction exerted a strong influence on the strength and character of the frontal updraft. For a given magnitude and depth of the cold air an optimal vertical shear existed where the updraft tended vertical, intense, and result in a narrow band of heavy precipitation. With decreasing vertical shear, the updraft tended to weaken, tilt back over the cold air, and result in a broad band of lighter prcipitation. An unsteady system resulted at shears higher than optimal. The dependence of the updraft character on vertical shear is similar to that predicted by recent theoretical work on squall lines.