Measurements and Modeling of the Effects of Ambient Meteorology on Nocturnal Drainage Flows

Abstract

An experimental and modeling investigation of nocturnal drainage flows within the Mesa Creek valley in western Colorado revealed their wind and temperature characteristics and the effects of the ambient meteorology on their development. The valley, located about 30 km east of Grand Junction, is situated on the north slopes of the Grand Mesa. It is surrounded by ridges on three sides with low terrain toward the north. The terrain at the higher elevations is characterized by steep slopes that become shallower at the lower elevations. A network of seven meteorological towers and a monostatic solar collected data within the study area from December 1988 through November 1989. Analysis of the experimental data indicated that shallow drainage flows generated over the many individual slopes at the higher elevations converge at the lower elevations to form deeper flows that join with those generated within adjacent drainage areas. The characteristics of the flows generally deviated from those displayed by idealized slope flows due to both internal circulations within the valley and external influences. During the summer, the depths of the flows were typically a few tens of meters along the upper slopes and about 100 m over the upper part of the lower slopes while during the winter, the depths decreased to about 10 and 60 m, respectively. Their frequency of occurrence was highest during the summer or fall, about 50%, when the synoptic-scale influences were minimal. The flows along the upper slopes were particularly susceptible to influences by the ambient meteorology due to minimal terrain shielding. When the larger-scale ambient flows over the Grand Mesa were greater than about 5 m s?1, the surface cooling along the slopes was unable to develop and maintain the surface temperature inversion needed to generate strong drainage flows. The radiative cooling rates of the sloped surfaces, as characterized by net radiation measurements, were correlated with the downslope wind speeds observed along the upper slopes. Thus, a decrease in the observed net radiation level will produce a corresponding decrease in the downslope wind speed. Since temporal changes in net radiation levels are primarily governed by variations in atmospheric moisture, the effect of increased atmospheric moisture is to retard the development of the drainage flows. In order to place the observations in proper perspective, it was necessary to employ numerical models that account for the physical processes governing the dynamics of the flows. The general features of the wind and temperature characteristics of the valley circulations and the influence of strong ambient winds and atmospheric moisture on the drainage flows over the upper slopes could be accounted for by numerical modeling techniques based on solving the equations of momentum, continuity, and energy coupled with a surface energy budget and a radiation module.