Experimental study of katabatic jets over steep slopes: buoyancy effect and turbulence properties

Katabatic winds are gravity flows that develop over sloping terrain due to radiative cooling at the surface. They have been extensively studied, but experimental works have generally been performed over gentle slopes. Some recent papers (eg. [3]) focused on the combined effect of surface angle and buoyancy on turbulence over steep slopes. In such configurations, the vertical component of the turbulent sensible heat flux may differ a lot from the slope-normal component, suggesting that buoyancy may act on turbulent quantities in an unusual way when katabatic jets develop over steep slopes. Such behavior seems to affect stability parameters used in Monin-Obukhov similarity theory applied in most meteorological models.

We study the buoyancy production term in the continuity of the work from [3], drawing on temperature and wind speed measurements acquired during 10 nights in November 2012 [1]. In situ measurements were performed under stable anticyclonic conditions, over an alpine slope of around 21° (French Alps) on a 4 level mast up to 6.5m, at a frequency sampling of 10 to 20Hz.

We conclude that turbulent kinetic energy and turbulent momentum flux are damped below the maximum wind speed height as expected from stably stratified atmospheric boundary layer. Conversely, turbulent kinetic energy can be locally reinforced by buoyancy in the external part of the katabatic jet, which confirms the results from [3]. Buoyancy may also produce turbulent momentum flux around the maximum wind speed due to the asymmetry of the jet. Results compare well with recent numerical modeling of a katabatic jet along a curved alpine slope under similar meteorological conditions [2].

Another field experiment took place during 16 nights in February 2019, over a snow-covered slope of around 34° in a similar location. The 11 wind speed levels and 17 temperature levels up to 12m, associated with a change of the surface level due to packing and melting of the snow, widen the range of analysis of the vertical profile. These data are associated with meteorological measurements and with a tethered balloon up to 50-100m above the ground surface.

Wind velocity measurements with a multi-hole pressure probe (cobra type) close to the ground provided more information than the previous dataset at a high frequency sampling of 1250 Hz. We show that the classical turbulent boundary layer wind speed profile applies well to the inner-layer region of katabatic jets, in spite of the presence of a maximum on the vertical streamwise velocity profiles. We find no significative changes caused by buoyancy on this profile. Roughness effect due to the snow on the surface will be discussed as well.

[1] Blein (2016), PhD

[2] Brun et al. (2017), JAS

[3] Oldroyd et al. (2016), BLM