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

Abstract

Numerical 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.

Impact of melting snow on the valley flow field and precipitation phase transition

“…In particular, they observed that the top of the radar bright band correlated with the shear level where the flow direction changed. On the other hand, Zängl (2007) conducted numerical simulations of the same event and concluded that the melting process only has a small contribution to the change of the wind flow in the valley.…”

“…It was hypothesized that the cooling from the melting snow was associated with the change in direction of the valley flow field. The characterization of the rain-snow boundary in mountainous terrain has been addressed in several studies including Medina et al (2005), Minder et al (2011), Zängl (2007, and Minder and Kingsmill (2013). In particular, Minder et al (2011) performed numerical simulations to study the mesoscale features of the rain-snow boundary along mountainside.…”

Latent cooling of atmosphere as an indicator of lowered snow line: Case study from Planica and Vrata valleys

“…In a study of latent cooling effect in the south Alpine Toce valley Zängl (2007) argues that cooling by evaporating cloud water also influences temperature, especially if valley atmosphere continues to be subsaturated due to down valley wind. If snow melts already above the valley, the contribution of evaporating cloud water to cooling can be of major importance.…”

Role of sublimation and riming on the precipitation distribution in the Kananaskis Valley, Alberta, Canada

“…This was observed in the Toce river valley in the Italian Alps during the Mesoscale Alpine Program (MAP, Steiner et al, 2003) and during the 2010 Vancouver Olympics in the Whistler area (Thériault et al, 2012. However, Zängl (2007) using numerical simulations, suggested that the cooling by melting of snow was of less importance in creating the down-valley flow for the same event because of the impact of cooling associated with sublimation.…”