Observations of Thermally Developed Wind Systems in Mountainous Terrain

Abstract

Slope and valley wind systems are local thermally driven circulations that form frequently in complex terrain areas. Recent research has focused on the temperature structure along the slope and valley axes that leads to the wind systems. Two new tools being used in these analyses include the topographic amplification factor, which quantifies the role of the topography in producing bulk temperature gradients along a valley’s axis, and atmospheric heat budgets, which identify key physical processes leading to changes in temperature structure. Both tools are in an early stage of development, are being applied primarily to steady-state nighttime periods, and are leading to new concepts and understanding.

Recent climatological evidence in Austria’s Inn Valley and in several Colorado valleys supports the concept that valley winds are driven by horizontal pressure gradients that are built up hydrostatically by the changing temperature structure along a valley’s length. Topographic amplification factors appear to be useful in assessing the strength of valley wind systems. Several components of valley atmospheric heat budgets have proven difficult to measure, and large imbalances are being experienced. Several recent experiments, in a range of climatological regimes, suggest that measured nighttime surface sensible heat fluxes are too small to result in balances. This may be caused by measurement errors or by nonrepresentative measurements. The advective and radiative flux divergence components are also uncertain.

A simple conceptual model of diurnal wind and temperature structure evolution in deep valleys is presented. During the morning transition period, upslope flows form over heated valley sidewalls and compensatory subsidence over the valley center produces warming that eventually reverses the down-valley winds. The key role of vertical motions in transferring energy through the valley atmosphere during the morning transition period has been demonstrated by field and modeling studies.

The evening transition period has received little observational attention, and the key physical processes are not yet well known. Investigation of slope wind systems has focused mostly on the nighttime flows. Flows on the sides of isolated mountains are reasonably well understood when external flows are weak, but slope flows on valley sidewalls are complicated by the continued evolution of temperature structure within the valley and the strong influence of the overlying along-valley flows.

Recent experiments have shown that thermally driven flows within the topography may be influenced in subtle ways by the overlying circulations. This influence is nearly always present to some extent, but has not yet been systematically investigated. Recent research on strong winds that issue from a valley’s exit at night and on tributary flows is briefly summarized, and some comments are made on Maloja winds and antiwind systems. The chapter ends with a summary of topics needing further research.