Impact of Mountainous Topography on Surface-Layer Parameters During Weak Mean-Flow Conditions

We investigate surface-layer characteristics over a mountainous ridge in the Central Himalayas, utilizing tower-based fast-response micrometeorological observations (at 12 and 27 m above ground level) for the winter months November 2013–January 2014. During this period, the site generally experienced clear skies and weak synoptic flow (wind speed < 2 m s−1), favouring a strong diurnal evolution of the atmospheric boundary layer. The observations show a regular change in wind direction from north-easterly during the night-time to westerly during the daytime throughout the season, indicating the systematic development of a mountain circulation due to changes in heating of the mountain slopes as the day advances. Considering the variations in wind direction and topography of the site, the tilt corrections are implemented sector-wise by segregating the data into three sectors, thus estimating three sets of coefficients for the tilt correction. Observations during fair-weather conditions (59 days only) are analyzed with the sensible heat flux (H) showing large diurnal variations, which are in-phase with the mean vertical velocity. The afternoon peak value of H is found to be ≈ 116 ± 80 W m−2. In contrast, diurnal variations of momentum flux and turbulent kinetic energy are less prominent with rather weak maxima occurring between 0900 and 1300 IST, the period when wind direction changes over the ridge. Variations of the dimensionless standard deviations of the vertical velocity component and temperature are found to scale with the stability parameter z/L under convective conditions, while taking into account the effect of self-correlation. The constancy of fluxes with height, slope-flow buoyancy and stress divergence are also analyzed to provide a rigorous evaluation of Monin–Obukhov similarity theory based on two-level turbulence measurements.