Time Variability of Surface-Layer Characteristics over a Mountain Ridge in the Central Himalayas During the Spring Season
Abstract
We present the diurnal variations of surface-layer characteristics during spring (March-May 2013) observed near a mountain ridge at Nainital (29.4°N, 79.5°E, 1926 m above mean sea level), a hill station located in the southern part of the central Himalayas. During spring, this region generally witnesses fair-weather conditions and significant solar heating of the surface, providing favourable conditions for the systematic diurnal evolution of the atmospheric boundary layer. We mainly utilize the three-dimensional wind components and virtual temperature observed with sonic anemometers (sampling at 25 Hz) mounted at 12- and 27-m heights on a meteorological tower. Tilt corrections using the planar-fit method have been applied to convert the measurements to streamline-following coordinate system before estimating turbulence parameters. The airflow at this ridge site is quite different from slope flows. Notwithstanding the prevalence of strong large-scale north-westerly winds, the diurnal variation of the mountain circulation is clearly discernible with the increase of wind speed and a small but distinct change in wind direction during the afternoon period. Such an effect further modulates the surface-layer water vapour content, which increases during the daytime and results in the development of boundary-layer clouds in the evening. The sensible heat flux ( H) shows peak values around noon, with its magnitude increasing from March (222± 46 W m^{-2}) to May (353± 147 W m^{-2}). The diurnal variation of turbulent kinetic energy ( e) is insignificant during March while its mean value is enhanced by 30-50 % of the post-midnight value during the afternoon (1400-1600 IST), delayed by {≈ }2 h compared to the peak in H. This difference between the phase variations of incoming shortwave flux, H and e primarily arise due to the competing effects of turbulent eddies produced by thermals and wind shear, the latter increase significantly with time until nighttime during April-May. Variations of the standard deviations of vertical wind normalized with friction velocity (σ _w/u_{*}) and temperature normalized with scaling temperature (σ _{θ }/T_{*}) as functions of stability parameter ( z / L) indicate that they follow a power-law variation during unstable conditions, with an index of 1/3 for the former and -1/3 for the latter. The coefficients defining the above variations are found in agreement with those derived over flat as well as complex terrain.
- Publication:
-
Boundary-Layer Meteorology
- Pub Date:
- March 2016
- DOI:
- 10.1007/s10546-015-0098-5
- Bibcode:
- 2016BoLMe.158..453S
- Keywords:
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- Atmospheric surface layer;
- Micrometeorology;
- Momentum flux;
- Mountains;
- Sensible heat flux;
- Turbulence;
- Wind shear