Impacts of terrain on convective surface layer turbulence over central Himalaya based on Monin–Obukhov similarity theory

Abstract

In the context of non-uniform Monin–Obukhov similarity scaling of unstable surface layer turbulence over complex terrain, this study attempts to identify the range of slope angles (as a proxy for terrain complexity) over which convective turbulence behaviour is similar to that over flat terrain. Attempts are also made to optimize constants of numerical relationships between integral turbulence parameters (ϕu,v,w) and the stability parameter (z/L) for observations having a significantly strong signature of terrain undulation. High frequency wind observations during the Indian winter season obtained from two field stations (on-slope and ridge-top) in the central Himalaya near Kosi-Katarmal, Uttarakhand, India, are analysed. Convective day-time period observations from rain-free days at these two sites are used with measures of terrain heterogeneity, represented by the vertical wind-derived slope angle (θw), vertical wind-derived slope angle normalized with respect to maximum θw (θwˆ), and the terrain-derived slope angle (θt). When the relationship of wind-derived slope angle and terrain derived slope angle was assessed, we found that wind-derived slope angle(θw) sufficiently represents the terrain undulation with a correlation coefficient of 0.86 at a p-value < 0.001. Optimization was undertaken of the numerical relationships between the integral turbulence parameters (ϕu,v,w) and z/L for observations having a significantly strong signature of terrain undulation characterized by |θˆw|> 0.2. Results indicated that for selected runs with increasing terrain complexity, c1 decreases and d1 increases in ϕu,v,w=c1(1−d1zL)1/3 having ranges of c1 and d1 between 0.85–2.39, and 0.98–13.85, respectively.