The researchers focused on two cases that could hardly be more different in character. The first, designated Case-A, occurred on 17 and 18 August 2019 and was a short-duration, high-intensity event, unleashing torrential rain within a compressed window of time. The second, Case-B, unfolded over three days from 8 to 10 July 2023 and was a multi-day rainfall episode that kept the region saturated and under siege far longer. Both events produced destructive flooding and landslides across the northwestern Himalaya, and both were documented in damage memoranda issued by the Government of Himachal Pradesh. But as the new analysis shows, the atmospheric pathways that delivered the rain in each case diverged in fundamental ways, and those differences carry important lessons for prediction.

To compare the two events rigorously, the team drew on an extensive suite of observational and reanalysis datasets. Rainfall estimates were validated against the gridded precipitation product of the India Meteorological Department, the authoritative reference for Indian rainfall, and cross-checked with satellite-based products including NASA’s GPM-IMERG and the MSWEP multi-source merged precipitation dataset. The three-dimensional structure of the atmosphere was reconstructed using ERA5 reanalysis from the Copernicus Climate Data Store, which assimilates observations worldwide into a physically consistent picture of winds, temperature, and humidity at every level. Topography came from the ETOPO global relief model, and additional satellite observations were accessed through the MOSDAC archive of the Indian Space Research Organisation. The researchers also consulted a modern catalogue of monsoon low-pressure systems derived from ERA5 and a Western Disturbances Atlas, allowing them to place each event precisely within the taxonomy of South Asian synoptic weather systems.

The synoptic analysis revealed a striking common thread: both events were influenced by the southward intrusion of an upper-tropospheric trough, a elongated region of low pressure at high altitudes embedded within the subtropical westerly jet stream. This jet, which circles the globe at midlatitudes, normally remains well north of the Himalaya in summer, but when its embedded troughs dig southward they can interact with the tropical monsoon circulation below. Such monsoon-midlatitude interactions have long been suspected of amplifying extreme rainfall over the Himalayan region, and this study confirms that the upper-tropospheric trough played a role in both disasters. The crucial insight, however, is that the presence of this shared ingredient was not enough to make the two events behave alike. What separated them was the behavior of the monsoon low-pressure systems, the sprawling rain-bearing vortices that form over the warm seas and land surfaces of the Indian subcontinent.

In Case-A, the August 2019 event, the interaction between a monsoon low-pressure system and the upper-tropospheric trough produced what the authors describe as moderate upper-to-lower tropospheric coupling. This vertical linkage between the top and bottom of the atmosphere set off a characteristic chain of dynamical responses: enhanced divergence in the upper troposphere acted like an exhaust system, pumping air away at high altitudes and thereby encouraging convergence of moisture-laden air near the surface. The result was deep convective heating, as towering thunderstorms released latent heat that further reinforced the upward motion. This configuration produced a short-lived but ferociously intense rainfall event, the kind of concentrated deluge that can trigger sudden flash floods in Himalayan valleys with little warning. The rapid onset and brief duration of Case-A exemplify the forecasting challenge posed by such events, where the window for effective warnings is measured in hours rather than days.

Case-B, the July 2023 event, followed an entirely different script. Instead of a compact low-pressure system interacting with a transient trough, this event featured an elongated Tibetan anticyclone aloft and a slow-moving upper-tropospheric trough that lingered over the region. The interaction of this persistent upper-level feature with the monsoon trough, the elongated zone of low pressure that stretches across northern India during the peak monsoon, maintained a continuous pipeline of moisture drawn from both the Arabian Sea and the Bay of Bengal. Rather than a brief convective explosion, the atmosphere settled into a sustained regime of moisture transport and precipitation that persisted for roughly three days. The slow movement of the trough meant that the large-scale forcing did not vacate the region, allowing successive pulses of rain to accumulate on already saturated terrain, a scenario that is particularly dangerous for landslide generation in the Himalaya.

To move beyond a purely descriptive comparison, the researchers conducted a process-wise analysis, examining the dynamical and thermodynamic budgets that govern vertical motion and rainfall production in each case. This approach traces how divergence and vorticity at upper levels, moisture convergence at lower levels, and the vertical distribution of heating combine to determine whether an event will be brief and explosive or prolonged and accumulative. The analysis provides insight into the dynamical pathways by which monsoon-midlatitude interactions influence rainfall extremes in this complex mountainous region, where the terrain itself reshapes the flow, forcing moist air up windward slopes and enhancing precipitation through orographic lift. Understanding these pathways is essential because numerical weather prediction models often struggle over the Himalaya, where coarse resolution and complex topography can misrepresent both the moisture fluxes and the convective processes at work.

The broader significance of the study lies in its demonstration that the fate of a Himalayan heavy rainfall event hinges on two key factors: the degree of upper-to-lower tropospheric coupling and the synoptic persistence of monsoon low-pressure systems. When a monsoon low couples effectively with an upper-tropospheric trough, the vertical circulation can intensify rapidly, favoring short-duration, high-intensity rainfall. When instead a slow-moving upper trough and an elongated anticyclone maintain a quasi-stationary moisture conveyor, the outcome is a multi-day deluge. Distinguishing between these two regimes in real time could help forecasters anticipate not just how much rain will fall, but over what timescale, information that directly determines whether the primary threat will be flash flooding or widespread landsliding. The findings also connect to a growing body of literature on interactions between tropical depressions and western disturbances, the eastward-moving midlatitude storms that dominate winter precipitation over the western Himalaya, suggesting a continuum of monsoon-midlatitude interactions that operate across seasons.

For a region where heavy rainfall events have significant socio-economic impacts, the implications extend well beyond atmospheric science. The 2019 and 2023 monsoon seasons both left trails of destroyed roads, bridges, and homes across Himachal Pradesh, and the state government’s damage memoranda record the cumulative toll of cloudbursts, floods, and landslides in those years. As climate change alters the behavior of the South Asian monsoon and potentially the frequency of monsoon-midlatitude interactions, process-based studies of this kind provide the mechanistic foundation on which better early warning systems, land-use planning, and infrastructure design can be built. By showing that two superficially similar disasters arise from contrasting dynamical and thermodynamic pathways, the work of Sharma and Pattnaik underscores a deceptively simple message: in the Himalaya, predicting the rain requires understanding the entire vertical architecture of the atmosphere, from the moisture-laden winds at the surface to the jet-stream undulations ten kilometers above.

Subject of Research: Dynamical and thermodynamic mechanisms of heavy rainfall events over the northwestern Himalaya of India

Article Title: Contrasting heavy rainfall events and associated mechanisms over the north western Himalayas of India

Article References: Sharma, V., & Pattnaik, S. (2026). Contrasting heavy rainfall events and associated mechanisms over the north western Himalayas of India. Natural Hazards, 122(21), Article 663. https://doi.org/10.1007/s11069-026-08446-6

Image Credits: AI Generated

DOI: 10.1007/s11069-026-08446-6

Keywords: heavy rainfall, northwestern Himalaya, Indian summer monsoon, low-pressure systems, upper-tropospheric trough, flash floods, landslides, monsoon trough, ERA5 reanalysis, monsoon-midlatitude interaction, Himachal Pradesh, forecasting