A Climatological Study of the Nocturnal Boundary Layer over a Complex-Terrain Station

Abstract

wo years of Doppler sodar measurements are used to study the time?height structure of the nocturnal boundary layer (NBL), its seasonal variation, and the characteristics of different types of NBL. A total of 220 clear-sky nights during which the inversion layer is clearly visible on a sodar echogram are examined. The NBL depth estimated with sodar data using a wind maxima criterion matches reasonably well with radiosonde-based NBL depth estimates. The NBL exhibits clear seasonal variation with greater depths during the monsoon season. Shallow NBLs are generally observed in winter. The evolution of NBL height shows two distinctly different patterns (called type 1 and type 2), particularly in the second half of the night. Type 1 NBL depth is nearly constant and the wind speed in this type is generally weak and steady throughout the night, while type 2 is characterized by moderate to strong winds with considerable variations in NBL height. The local circulation generated by the complex topography is clearly seen in type 1 throughout the night, whereas it is seen only in the first half of the night in type 2. Type 1 NBLs seem to be more prevalent over Gadanki, India, with nearly 61% of total nights showing type 1 characteristics. Furthermore, type 1 NBL shows large seasonal variability with the majority of type 1 cases in winter. The type 2 cases are mostly observed in monsoon (~60%) followed by summer (39%). The surface meteorological parameters during type 1 and type 2 cases are examined. Differences between type 1 and type 2 NBL patterns are discussed in relation to the surface forcing.