Washoe Zephyr—A daytime downslope wind in the lee of Sierra Nevada

A daytime downslope flow, known as the Washoe Zephyr, that frequently occurs on the lee slopes of the Sierra Nevada, is investigated climatologically and numerically. The climatological analysis used long-term data from several surface stations and Rawinsonde soundings from Reno, NV and the numerical simulations used the RAMS model. Typically, under undisturbed conditions diurnal winds over mountain slopes blow up slope during the day and down slope at night as a result of differential heating and cooling of the slope and the air adjacent to the slope. The Washoe Zephyr, however, blows downslope during daytime against the local pressure gradient force, and it is not entirely clear what causes this unusual behavior. Two hypotheses have been proposed. The first is that the flow is a result of the pressure difference between the thermal low developed in the interior of Nevada and the high pressure over the coast of California which draws the air from west of Sierra crest down to the eastern slope. The second hypothesis is that the surface downslope wind may be a result of downward transport of momentum from the westerly winds aloft as the convective boundary layer over the eastern slope of Sierra grows high in the afternoon into generally westerly flow layer aloft. An examination of the relationship between the Washoe Zephyr occurrences and the geostrophic wind speed and direction above the Sierra Crest rejects the downward momentum transport of westerly winds aloft as a viable mechanism. This is because on significantly large number of days winds above Sierra Crest are from either north or south when strong afternoon westerly downslope winds occurred in the Great Basin in the lee of Sierra. Even when both surface and geostrophic winds are westerly, the surface wind speeds are oftentimes stronger than winds aloft. Numerical model simulations using RAMS and a particle dispersion model show particles being carried from the Central Valley up the western slope of the Sierra by morning upslope flow and going down the eastern slope in the afternoon as the air being drawn over the slope by regional pressure gradient force. This further confirms the conclusion based on the observations.