Reversed flow in the south‐Alpine Toce Valley during MAP‐IOP 8: Further analysis of latent cooling effects

Abstract

AbstractNumerical simulations are presented to examine the physical mechanisms leading to downvalley flow in the south‐Alpine Toce Valley during MAP IOP 8 (20–21 October 1999), continuing a recent study by Asencio and Stein. The downvalley flow occurred during a phase of moderate precipitation, opposing the southerly (upslope) flow at higher levels that generated the precipitation. Based on sensitivity experiments with suppressed latent cooling by melting and evaporation, Asencio and Stein concluded that cooling by melting was important for maintaining a northerly low‐level flow in the exit region of the Toce Valley and the adjacent parts of the Po Valley. Here, the investigation is extended to the Toce Valley itself, using a finer model resolution that allows the valley topography to be properly resolved. The effects of cooling by melting and evaporation are tested separately. In addition, the evaporation of precipitating hydrometeors and cloud water is considered separately. The tests indicate that the effects of cooling by melting and evaporation of precipitating hydrometeors are quite small. A markedly larger impact is found for the evaporation of cloud water, which is advected into the valley area by the subsiding motion compensating for the low‐level downvalley flow. This appears to be related to the fact that cloud water is assumed to evaporate instantaneously in a subsaturated atmosphere via saturation adjustment, whereas larger hydrometeors have a fairly small evaporation rate in a nearly saturated atmosphere. Nevertheless, the sensitivity of the simulated flow field to completely suppressing latent cooling is not as large as found in the preceding study. Though latent cooling substantially intensifies the downvalley flow and prevents temporary changes to upvalley flow, it does not appear to be essential for the development of downvalley flow. An additional driving mechanism based on the evolution of the mesoscale pressure field is proposed. Copyright © 2007 Royal Meteorological Society