The dry‐line of Northern India and its role in cumulonimbus convection

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 /f 2 . 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.