Wind Flow over a Mountain Ridge in the Central Himalayas during Strong and Low Wind Conditions: Implications on Surface Layer Characteristics

Interaction of the mountain generated winds system with the regional wind flow complicates the understanding of atmospheric boundary layer dynamics over mountainous ridge, and is also of fundamental importance in the discernment of spatio-temporal variability of aerosol and trace gas measurements made over mountain-top observatory. This study presents two-level (12 m and 27 m) fast-response (25 Hz) sonic anemometer measurements made at ARIES, Nainital (29.40 N, 79.50 E, altitude 1926 m above the mean sea level) in the central Himalayas, during two distinct meteorological conditions of spring (March to May 2013) and winter season (November 2013 to January 2014). Since the influence of mountainous topography on atmosphere is more pronounced on cloud-free days and our major focus is to understand surface layer characteristics in fair weather condition, we have considered a total of 46 (59) clear days for the spring (winter) season. In the spring season site is located on the northern fringe of strong north-westerly wind regime (6 to 10 m s-1), whereas during winter regional circulation of winds is much weaker (below 3 m s-1). Due to the variations in mountain circulation during the course of the day, wind direction undergoes a small change in the afternoon in the spring season, whereas a complete reversal of wind direction (from easterly to westerly flow) is observed during the winter season. Tilt corrections using the planar fit method (sector-wise planar fit method) have been applied to convert the measurements to streamline-following coordinate system for the spring season (winter season) before estimating turbulence parameters. The seasonal and diurnal variability in measured fluxes of momentum and heat, along with turbulent kinetic energy (e) has been analyzed. Sensible heat flux (H) exhibits prominent diurnal variations attaining peak values (276 ± 108 W m-2 for the spring and 116 ± 52 W m-2 in winter season) from 1200 to 1400 IST; the seasonal and diurnal mean H (from surface to atmosphere) decreases from spring (50 W m-2) to winter (17 W m-2). Although the seasonal mean value of e is reduced by half, from spring to winter but the momentum flux (τ) remains unchanged. The stability parameter (z/L) analysis displays a sharp contrast in the percentage occurrence of stable and unstable conditions in day and nighttime respectively during spring season, with unstable (85 %) / stable (90 %) conditions in daytime / nighttime respectively for the spring and whereas in winter season the ratio is 75 % / 70 %. Variations of the standard deviations of vertical wind normalized with friction velocity (σw/u*) as a function of stability parameter (z/L) indicate that they follow a form of power law variation during the unstable conditions, with an index of 1/3 for the spring season, whereas for the winter season this index is 1/5. The coefficients defining the above variations are found to be in agreement with those derived over flat as well as complex terrain for spring season. The value of σw/u* is found to be ~1.2 (0.2) during spring (winter) season for neutral stability conditions. A prominent diurnal variation is observed in the vertical wind component as well, depicting almost equal amplitude of variation in both the season, with positive values after 0900 IST onwards attaining peak values of 0.2 to 0.3 m s-1 at 1300 IST and reversal to negative values from 1800 IST (1600 IST) in the spring (winter) season. The disparities in surface layer characteristics further emphasizes the strong seasonality of aerosol vertical distribution influenced by local boundary layer variations over the site.