Nocturnal low-level jet and 'atmospheric streams' over the rain shadow region of Indian Western Ghats

Abstract

Spatial and temporal characteristics of a nocturnal low-level jet (LLJ) on the east side of the Western Ghat mountain range over India's west coast and processes leading to the formation of the jet are discussed. The boundary-layer jet has a regional scale extent, as revealed by high-resolution Advanced Research Weather Research and Forecasting (ARW) model simulations, and contributes to the formation of 'atmospheric streams' of water vapor over the selected land regions. Simulations indicate that the formation of LLJ is mainly attributed to the baroclinicity of the valley atmosphere due to the gently rolling terrain, which is assisted by the persistence of an unstable residual layer above the developing stable boundary layer in the valley and cooling over the slopes. Prior to the formation of LLJ, the boundary layer is dominated by deep roll circulations. The LLJ followed a gust front zone associated with a mountain wave. The low-level flow below the jet is decoupled from the upper-level flow as a result of strong vorticity below the jet and suppression of turbulence at the jet core. A conceptual model for the boundary layer interactions, dynamics of the mountain wave, LLJ, etc. are proposed for Western Ghat region.