The Thermal Circulation of a Grand Plateau: Sensitivity to the Height, Width, and Shape of the Plateau

Günther Zängl Meteorologisches Institut der Universität München, Munich, Germany

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Silvia Gonzalez Chico Meteorologisches Institut der Universität München, Munich, Germany

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Abstract

Idealized numerical simulations are presented to investigate the sensitivity of the thermal plateau circulation to the plateau height, width, and the presence of a mountain ridge encompassing the plateau. The study concentrates on plateaus surrounded by land with the same surface properties so that the topography is the only source of differential heating. For low plateaus, it is found that the strength of the plateau circulation increases with the plateau height because the excess heating of the plateau increases with its height. The plateau circulation reaches its maximum strength for plateau heights similar to the mixed-layer depth over the surrounding lowlands. For even higher plateaus, the depth of the circulation decreases whereas the peak intensity exhibits little change. The plateau height also controls the temporal evolution of the plateau circulation. For low plateaus, low-level inflow is primarily found during the night because convection cells forming over the lateral slopes prevent an inward mass flux during the day. With increasing plateau height, the phases of the plateau circulation shift toward earlier times. If the plateau height exceeds the mixed-layer depth over the surrounding lowlands, inflow occurs from afternoon until the late evening. After sunset, the inflow layer tends to assume the characteristics of a density current. The convergence of the density currents propagating from each side of the plateau toward the center helps to fill the heat low formed during the day. Moreover gravity waves are triggered that propagate away from the plateau center and terminate the inflow. The propagation speed of the density currents is found to increase with the plateau height, so that their convergence occurs earlier for high plateaus than for low plateaus. For the parameter range considered in this study, the plateau width is found to have only a minor impact on the mass fluxes associated with the plateau circulation. However, the convergence of the density currents over the plateau center occurs later for a wider plateau, implying that the heat low is more persistent. Substantial changes are obtained when the plateau is surrounded by a mountain ridge. In this case, the inflow toward the plateau gets delayed because a slope wind circulation forms over the mountain ridge. Moreover, the mass fluxes toward the plateau are reduced so that the heat low that formed over the plateau during the day is filled only slowly. As a consequence, the inflow lasts throughout the night in the presence of a high mountain ridge surrounding the plateau.

Corresponding author address: Dr. Günther Zängl, Meteorologisches Institut der Universität München, Theresienstraße 37, D-80333 Munich, Germany. Email: guenther@meteo.physik.uni-muenchen.de

Abstract

Idealized numerical simulations are presented to investigate the sensitivity of the thermal plateau circulation to the plateau height, width, and the presence of a mountain ridge encompassing the plateau. The study concentrates on plateaus surrounded by land with the same surface properties so that the topography is the only source of differential heating. For low plateaus, it is found that the strength of the plateau circulation increases with the plateau height because the excess heating of the plateau increases with its height. The plateau circulation reaches its maximum strength for plateau heights similar to the mixed-layer depth over the surrounding lowlands. For even higher plateaus, the depth of the circulation decreases whereas the peak intensity exhibits little change. The plateau height also controls the temporal evolution of the plateau circulation. For low plateaus, low-level inflow is primarily found during the night because convection cells forming over the lateral slopes prevent an inward mass flux during the day. With increasing plateau height, the phases of the plateau circulation shift toward earlier times. If the plateau height exceeds the mixed-layer depth over the surrounding lowlands, inflow occurs from afternoon until the late evening. After sunset, the inflow layer tends to assume the characteristics of a density current. The convergence of the density currents propagating from each side of the plateau toward the center helps to fill the heat low formed during the day. Moreover gravity waves are triggered that propagate away from the plateau center and terminate the inflow. The propagation speed of the density currents is found to increase with the plateau height, so that their convergence occurs earlier for high plateaus than for low plateaus. For the parameter range considered in this study, the plateau width is found to have only a minor impact on the mass fluxes associated with the plateau circulation. However, the convergence of the density currents over the plateau center occurs later for a wider plateau, implying that the heat low is more persistent. Substantial changes are obtained when the plateau is surrounded by a mountain ridge. In this case, the inflow toward the plateau gets delayed because a slope wind circulation forms over the mountain ridge. Moreover, the mass fluxes toward the plateau are reduced so that the heat low that formed over the plateau during the day is filled only slowly. As a consequence, the inflow lasts throughout the night in the presence of a high mountain ridge surrounding the plateau.

Corresponding author address: Dr. Günther Zängl, Meteorologisches Institut der Universität München, Theresienstraße 37, D-80333 Munich, Germany. Email: guenther@meteo.physik.uni-muenchen.de

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