Physical model of nocturnal drainage flow in complex terrain

The velocity and temperature fields of the nocturnal drainage flow for the complex terrain in the vicinity of Nogales (Arizona, United States, and Sonora, Mexico, border) have been modeled by a series of laboratory experiments. The experiments employed a square tank, in which the floor is a model of the terrain near Nogales and whose temperature can be controlled to simulate the variation of the surface temperature at night time in the atmosphere. Water was used to simulate the atmosphere. Similarity arguments show that, while a large number of dimensionless parameters appear in the governing equations and boundary conditions governing the behavior of the drainage flow, most are relatively unimportant with the primary dynamic parameter being G ν = H b /H ν , where H b is the characteristic height of the terrain and H ν is the characteristic depth of the drainage flow in the valley near Nogales; the model also requires that the topographic contour function η and the temporal variation of the dimensionless surface cooling function Φ d also be matched. The modeling focuses on the nature of the drainage flow in the valley in the immediate vicinity of the cities of Nogales. The model demonstrates some of the principal features of the drainage circulation system, including the surface streamline pattern, the surface velocities and the vertical profiles of the drainage velocity and temperature near the center of Nogales, Sonora. The streamline pattern changes very little for the range of external conditions considered. The largest drainage flow speed is expected in the valley near the center of Nogales, Sonora, due to the convergence of the drainage flow from the mountainous regions to the Santa Cruz River valley. Quantitative experimental observations were made to determine the dependence of various observables on G v ; these included normalized values of the speed of the drainage flow, the time at which the maximum speed occurs and the depth of the drainage flow in the valley near Nogales. These results are applied to the atmosphere, following similarity relations between the physical model and the atmosphere. The physical model results are in qualitative agreement with limited field observations in the Nogales region.