Two-and Three-Dimensional Modelling Studies of the Big Thompson Storm

Abstract

The Big Thompson storm occurred on 31 July?1 August 1976 over Big Thompson Canyon, Colorado, when a secondary cold frontal surge was accelerated and reached the foothills of the Front Range. Two- and three-dimensional moist compressible cloud models developed by Ogura and Yoshizaki are applied to this storm event. Adopting highly simplified terrain shapes, this study addresses two aspects of the storm. One is the distinct characteristics of the storm structure, as schematically depicted by Carasena et al.; the other is that heavy precipitation occurred in the basin area rather than over the mountain peak area. When the model was initialized in such a way that moisture-rich, low-level, strong easterlies impinged upon the orography, the model predicted the development of a storm that not only caused heavy precipitation at the right location relative to the mountain peak, but also reproduced the observed storm in many aspects, both in two- and three-dimensional (2D and 3D) simulations. The major qualitative differences between 2D and 3D simulators is that the model storm in two dimensions is highly transient and exhibits the distinct multicellular structure, whereas the model storm in three dimensions tends to be quasi-stationary. This difference was attributed to the weakness of the induced low pressure inside the storm in three dimensions. Qualitatively, the precipitation accumulation in three dimensions is found to be substantially larger than the 2D counterpart In three dimensions, the low-level easterlies ahead of the mountainous area are deflected as they approach the valley to flow nearly parallel to the elevation contours in each side of the valley and these two airflows eventually converge along the valley to produce heavy rainfall. An interesting finding in the model is the creation of a cold air pool beneath the storm in the situation where the cloud base height is lower than the maximum terrain. The model storm slants severely downstream (particularly in 2D simulations) and precipitating particles fall through the cloud layer, thusthus enhancing evaporating cooling. When the initial distribution of moisture is assumed to be uniform horizontally in the model, the first deep convection (and consequently heavy preciptation) occurs only at or near the mountain peak, in disagreement with the observations.