Flow structure and turbulence characteristics of the daytime atmosphere in a steep and narrow Alpine valley

Abstract

AbstractAircraft measurements, radio soundings and sonic data—obtained during the MAP‐Riviera field campaign in autumn 1999 in southern Switzerland—are used to investigate the flow structure, temperature profiles and turbulence characteristics of the atmosphere in a steep and narrow Alpine valley under convective conditions. On all predominantly sunny days of the intensive observation periods, a pronounced valley‐wind system develops. In the southern half of the valley, the daily up‐valley winds have a jet‐like structure and are shifted towards the eastern slope. These up‐valley winds advect potentially colder air, a process which appears to be balanced by vertical warm air advection from above. The profiles of potential temperature show that, with the onset of up‐valley winds, the mixed layer consistently stops growing or—on days with very strong up‐valley winds—even stabilizes almost throughout the entire valley atmosphere. This is probably due to a pronounced secondary circulation in the southern part of the valley, which induces advection of warm air from above. The secondary circulation appears to be a consequence of sharp curvature in the along‐valley topography.Turbulence variables are calculated from flight legs in the along‐valley direction. Turbulent kinetic energy (TKE) scales surprisingly well (i) if a TKE criterion (TKE >0.5 m2s−2) is employed as a definition of the boundary layer height and (ii) if the ‘surface fluxes’—which exhibit a substantial spatial variability—from the slope sites are used rather than those from directly beneath the profile considered. Significant site‐to‐site differences in incoming solar radiation seem to be the reason for this characteristic behaviour. Profiles of momentum flux—scaled with a surface friction velocity—reveal more scatter than the TKE profiles, but still show a consistent behaviour. A surprisingly strong shear in the cross‐valley direction can be observed and is probably a result of the secondary circulation. Copyright © 2004 Royal Meteorological Society