Identification and Climatology of Alpine Pumping from a Regional Climate Simulation

Abstract

The thermally driven circulation between the European Alps and the Alpine foreland-named Alpine pumping-occurs regularly under clear and calm weather conditions. While previous studies focused on the impact of Alpine pumping on moist convection and transport of air pollutants, this study was motivated by its ventilation effect for Munich, located about 50 km north of the Alps in undulating and only slightly inclined terrain, where local thermal circulations are weak. Hourly data from a reanalysis driven regional climate simulation with COSMO-CLM model for the period 1989-2008 were analyzed to identify days with Alpine pumping and to determine the mean diurnal characteristics of this regional thermal circulation. Four literature derived combinations of meteorological criteria were tested to identify days favorable for Alpine pumping from COSMO-CLM results. The first criterion selects days with a daily sum of solar radiation ≥20 MJ/m 2 and has been used in an earlier observational study. On average 60 day/year are fulfilling the criterion in the model simulation, which compares well to the 67 day/year determined from observations. The other three criteria combinations consider a maximum wind velocity at 850 hPa, a maximum daily precipitation sum, and/or a maximum mean cloud cover. The mean annual number of selected days is lower for these criteria combinations and ranges between 20 and 52. Diurnal wind reversals occur on 77-81% of the selected days, depending on the criteria combination. The daily solar radiation sum of 20 MJ/m 2 is only exceeded during April to September, while days satisfying the criteria combinations without the radiation threshold occur all year round. In agreement with observations, the simulated regional thermally driven wind field extends up to ∼100 km north of the Alps with average near-surface wind speeds of 0.5-1.5 m/s in the Munich area. With increasing distance from the Alps, the diurnal cycle of Alpine pumping is delayed by up to 3 h. The simulated mean depth of the daytime inflow layer ranges between 500 and 1500 m, whereas the depth of the nocturnal outflow layer typically reaches up to a few hundred meters.

The Impact of Three-Dimensional Effects on the Simulation of Turbulence Kinetic Energy in a Major Alpine Valley

“…Observations and regional climate simulations suggest that this phenomenon occurs on average around 60 days per year (Graf et al. 2016 ).…”

Current Challenges in Understanding and Predicting Transport and Exchange in the Atmosphere over Mountainous Terrain

“…For example, idealized daytime simulations by Schmidli [69] showed local cooling in the slope-wind layer due to advection together with heating from turbulent exchange, which in combination with turbulent heating in the valley ML and advective heating due to subsidence above the ML, led to relatively homogeneous heating throughout the whole valley and a net heat export out of the valley. Going beyond the scale of individual valleys to the scale of entire mountain ranges, the mountain-plain circulation (or Alpine pumping, [72]) also contributes to a heat transport [73].…”

Atmospheric patterns associated with sub-daily rainfall extremes in western Europe

“…Interestingly, for Switzerland and Austria the top 5 WPs account for only 56.9% and 48.2% of 3hr events over 40 mm respectively, whereas for Germany, Belgium and the Netherlands it is near to or over 60% (60.9%, 69% and 70% respectively -Table 1). This suggests that 3hr extreme rainfalls occur under a wider range of large-scale atmospheric conditions in the Alps than in the Low Countries, possibly due to the potential for convection initiation through the 'Alpine pumping' mechanism (Graf et al, 2016;Kahraman et al, 2022) . This mechanism aids in the triggering of thunderstorms over mountainous regions where local winds converge (due to mountain-valley circulations), leading to low-level rising motions and enhanced probability of convection-driven rainfall, especially during periods where large-scale circulation is weak and moisture is already present (Giorgi et al, 2016 2022).…”