Energetics of Deep Alpine Valleys in Pooling and Draining Configurations

Gabriele Arduini,Chantal Staquet,Charles Chemel

doi:10.1175/jas-d-16-0139.1

Abstract

Abstract The Weather Research and Forecasting Model is used to investigate the nocturnal atmospheric boundary layer in a valley that opens either on a wider valley (draining configuration) or on a narrower valley (pooling configuration). One draining case and three weak to strong pooling cases are considered. Results show that the structure of the nocturnal boundary layer is substantially different for the draining and pooling configurations. Greater pooling corresponds with a deeper and colder boundary layer. Down-valley winds are weaker for pooling and draining configurations than in an equivalent valley opening directly on a plain. For the strong pooling case, an up-valley flow develops from the narrower to the wider valley during the evening transition, affecting the mass budget of the wider valley during that period. Considering the heat budget of the valley system, the contribution of the diabatic processes, when appropriately weighted, hardly varies along the valley axis. Conversely, the contribution of advection varies along the valley axis: it decreases for a pooling configuration and increases for a draining configuration. Consequently, for a pooling configuration, the heat transfer between the valley and the plain is reduced, thereby increasing the temperature difference between them. For the strong pooling case, this temperature difference can be explained by the valley-volume effect once the down-valley flow has developed. This occurs in a valley when the “extra” heat loss within the valley due to the surface sensible heat flux balances the heat input due to advection.

Highlights

The characteristics of the boundary layer in complex terrain are often tightly coupled with the variations of the underlying orography
In valleys characterized by a decreasing topographic amplification factor (TAF) in the down-valley direction, the along-valley variation of the valley geometry induces a horizontal temperature gradient that promotes the development of a down-valley flow
The purpose of this work was to quantify the impact of along-valley variations of the valley width on the nocturnal boundary layer structure of deep valleys, considering systems of two valleys: one upstream, one downstream, opening on a plain

Summary

Introduction

The characteristics of the boundary layer in complex terrain are often tightly coupled with the variations of the underlying orography. McKee and O’Neal (1989) considered the intravalley variations of the TAF, thereby characterizing temperature gradients along the valley axis. In valleys characterized by a decreasing TAF (i.e., the valley widens) in the down-valley direction, the along-valley variation of the valley geometry induces a horizontal temperature gradient that promotes the development of a down-valley flow. Such valleys are defined as ‘‘draining’’ valleys. When the TAF increases (i.e., the valley narrows) in the down-valley direction, the horizontal temperature gradient changes sign, hindering the development of a down-valley flow

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