Separation of Upslope Flow over a Plateau

Roni Hilel Goldshmid,Christopher Hocut,Stef Bardoel,Harindra Fernando,Qiang Zhong,Dan Liberzon

doi:10.3390/atmos9050165

Abstract

A laboratory study was conducted in order to gain an understanding of thermal convection in a complex terrain that is characterized by a plateaued mountain. In particular, the separation of upslope (anabatic) flow over a two-dimensional uniform smooth slope, topped by a plateau, was considered. The working fluid was homogeneous water (neutral stratification). The topographic model was immersed in a large water tank with no mean flow. The entire topographic model was uniformly heated, and the width of the plateau, the slope angle, and the heating rate were varied. The upslope velocity field was measured by the Particle Tracking Velocimetry, aided by Feature Tracking Visualizations in order to detect the flow separation location. An analysis of the resulting flow showed a quantitative similarity to separating the upslope flow over steeper slopes, in the absence of a plateau when an effective angle that incorporates the normalized plateau width, the slope length, and the geometric slope angle, was used. Predictions for the dependence of the separation location and velocity on the geometry and heat flux were presented and compared with the existing data.

Highlights

Complex terrain, which includes slopes, valleys, canyons, escarpments, gullies, and buttes covers about 70% of Earth’s land surface [1]
The transition period was followed by a quasi-steady period of 80–130 s, depending on the set buoyancy flux and slope angle values, during which the upslope flow and the rising plume experienced minimal end wall influence, as the intrusion was propagated
The typical quasi-steady period lasted for about five minutes, with small variations based on the prescribed buoyancy flux and plateau width feeding the plume after the detachment of the upslope flow

Summary

Introduction

Complex terrain, which includes slopes, valleys, canyons, escarpments, gullies, and buttes covers about 70% of Earth’s land surface [1]. Slope and valley flows are important characteristics of complex terrain in the absence of synoptic effects, with downslope (katabatic) and down valley flows occurring at night, and upslope (anabatic) and up valley flows occurring during the day [2,3]. The downslope winds blow through the gaps and canyons [4], which separate out from the slope as intrusions [5], and collide with each other. During the morning transition, when the nocturnal stable boundary layer breaks down and the daytime convective boundary layer is developed, the downslope flow reverses to form an upslope flow, as a result of the heating of the Atmosphere 2018, 9, 165; doi:10.3390/atmos9050165 www.mdpi.com/journal/atmosphere. In the case of flow separation, a thermal plume is formed, and deep convective clouds can often be observed directly over the mountain peak [7,8]

Methods

Results

Conclusion