Thermodynamics of extreme precipitation related to Atmospheric Rivers in two major Himalayan basins

Abstract

In the extra-tropics, moisture is efficiently transported polewards in the lower troposphere through narrow bands of moisture transport, known as Atmospheric Rivers (ARs). This study identifies ARs that penetrate the southern Himalaya using 6-hourly Integrated Vapor Transport (IVT), and investigates the moisture dynamics, thermodynamics, and moisture sources of the top 8 AR events with substantial precipitation impacts in the Indus Basin (IB) and Ganga Basin (GB), to understand the mechanisms driving extreme AR-related precipitation. Most of these ARs produce over 150 mm/day precipitation (exceeding the 99th percentile of non-zero daily-precipitation) along their central axis and mountain landfall locations, and up to 50 mm/day in the plains. Even the glacierized regions that feed the main basins’ streams receive over 65 mm/day. High-intensity precipitation typically found near southern Himalaya, strongly corresponds to positive moisture convergence and positive advection, with minimal changes in atmospheric moisture content indicating effective translation of AR-moisture to precipitation. Different regions within the ARs experience distinct precipitation mechanisms, with mountains/foothills dominated by orographic uplift even under weak convective conditions, while areas near/on AR-axis are influenced by convective or frontal lifting. The presence of high moisture (IVT), saturated layers, and favourable thermodynamics highlights the ARs' capacity to generate intense precipitation when vertical lift is present. Excess moisture within ARs primarily originates from the Arabian Sea (north, east, and west), followed by Bay of Bengal (north and east), land-based evaporation, and smaller contribution from the Middle Eastern Seas. These case-specific findings demonstrate that ARs are key moisture transport mechanisms to mainland India and the Himalaya, delivering extreme localized precipitation strongly influenced by topography and position of ARs axis. This study offers the first comprehensive understanding of the dynamic and thermodynamic sources of AR moisture, associated precipitation, and the role of the Himalaya in extracting large amounts of moisture from ARs. These findings have important implications for understanding ARs contribution to extreme precipitation in IB and GB, driving mechanisms behind AR-precipitation, and the sources of high moisture content during ARs. These results can help explore potential risks of extreme precipitation, improve AR modeling, and forecasting ARs over these regions.