Abstract

An interesting feature of the flow pattern over India during the premonsoon months - a frontal surface separating a shallow, warm moist layer from a deeper, hot, very dry layer - is studied in some detail. In particular, its role in the development of severe cumulonimbus over north-east India is investigated. Routine observations are insufficient to resolve many of the features of this 'dry-line' and a simple dynamical model is constructed which suggests an interface in the form of a step with a height (Z_i*) dependent upon the pressure gradient in the moist air (A = 1/ρ ∂p/∂x), surface drag coefficient (C_D) and Coriolis parameter (f), given by the expression Z_i* = 4/15 A C_D/f². An extension of the model to include a frictional stress variable with height suggests a circulation within the shallow moist layer which, together with solar heating, would be likely to lead to cumulus and cumulonimbus development near the edge of the moist air; this is observed on satellite photographs.