This study is to elucidate the dominant hydrometeorological processes in Northwest India. The hydrometeorological processes involve orographic forcing, land-atmosphere interactions, and especially soil moisture and vegetation controls, underlying the persistence and spatial extent of a hydrometeorological dryline. For this purpose, simulations of monsoon active and break phases for the 2001 monsoon season were conducted using a mesoscale model (e.g. the MM5). During the active phases of the monsoon, southeasterly depressions from the Bay of Bengal propagate over northern India, maintaining sustained convergence of moist available energy east of the Aravalli range, leading to increased rainfall and cloudiness patterns consistent with deep convective activity. Drier air originating from the Arabian Sea in the Western Indian Ocean is constrained to the west. During monsoon break phases, moisture convergence from the Bay of Bengal to the NICZ decreases dramatically, weakening regional circulations east of the Aravalli range. This allows ventilation of the central portion of the Northern India Convergence Zone (NICZ) through penetration of westerly dry air, leading to reduced rainfall, lower soil wetness, decrease of latent heat fluxes, and finally lower CAPE and humidity in the lower troposphere. The simulated results are consistent with analysis of the climatology of cloudiness and convective activity over the Indian subcontinent, which suggests the existence of a persistent discontinuity aligned with the Aravalli range that separates NICZ to the east from the dry climate of the Thar Desert to the west.

Furthermore, the temporal evolution of the location and spatial reach of this discontinuity exhibit a pattern of eastward excursions and retreats consistent with the space-time trajectories of convective systems during the break and active phases of the monsoon, respectively. From a climatological perspective, data analysis of satellite imagery suggests that the Aravalli mountain range functions as a landform pivot pinning the dryline to the landscape. Whereas SST (Sea Surface Temperature) anomalies in the Bay of Bengal trigger the onset (demise) of active and break periods, the sustainability of either regime requires strong feedbacks between humidity and stability in the lower troposphere and the surface energy balance: negative in the case of monsoon breaks, positive in the case of active periods. This study shows that, albeit relatively modest (< 600 m average elevation), the Aravalli range provide sufficient lift (upwind) and descend (downwind) to organize the spatial distribution of updrafts westward (active phase) and eastward (break phase) of the topographic divide in such a way that low level updrafts are nearly suppressed over the Thar Desert. The sensitivity experiments also shows that daytime latent heat fluxes (and evapotranspiration) play an important role in the spatial organization of CAPE and in triggering light rainfall processes in the semi-arid regions of northwest India, whereas heavy rainfall processes to the east are controlled by large-scale monsoon dynamics.