The Causes of Foehn Warming in the Lee of Mountains
AbstractThe foehn effect is well known as the warming, drying, and cloud clearance experienced on the lee side of mountain ranges during “flow over” conditions. Foehn flows were first described more than a century ago when two mechanisms for this warming effect were postulated: an isentropic drawdown mechanism, where potentially warmer air from aloft is brought down adiabatically, and a latent heating and precipitation mechanism, where air cools less on ascent—owing to condensation and latent heat release—than on its dry descent on the lee side. Here, for the first time, the direct quantitative contribution of these and other foehn warming mechanisms is shown. The results suggest a new paradigm is required after it is demonstrated that a third mechanism, mechanical mixing of the foehn flow by turbulence, is significant. In fact, depending on the flow dynamics, any of the three warming mechanisms can dominate. A novel Lagrangian heat budget model, back trajectories, high-resolution numerical model output, ...