Characteristics of Historical Precipitation in High Mountain Asia Based on a 15-Year High Resolution Dynamical Downscaling

Abstract

The mountains of High Mountain Asia serve as an important source of water for roughly one billion people living downstream. This research uses 15 years of dynamically downscaled precipitation produced by the Weather Research and Forecasting (WRF) model to delineate contrasts in precipitation characteristics and events between regions dominated by the Indian Summer Monsoon (ISM) versus westerly disturbances during the cool season (December to March). Cluster analysis reveals a more complex spatial pattern than indicated by some previous studies and illustrates the increasing importance of westerly disturbances at higher elevations. Although prior research suggests that a small number of westerly disturbances dominate precipitation in the western Himalaya and Karakoram, the WRF-downscaled precipitation is less dominated by infrequent large events. Integrated vapor transport (IVT) and precipitation are tightly coupled in both regions during the cool season, with precipitation maximizing for IVT from the south-southwest over the Karakoram and southeast-southwest over the western Himalaya. During the ISM, Karakoram precipitation is not strongly related to IVT direction, whereas over the western Himalaya, primary and secondary precipitation maxima occur for flow from the west-southwest and northwest, respectively. These differences in the drivers and timing of precipitation have implications for hydrology, glacier mass balance, snow accumulation, and their sensitivity to climate variability and change.

Grand Challenges of Hydrologic Modeling for Food-Energy-Water Nexus Security in High Mountain Asia

“…Similar to Mei et al (2020) and Xue et al (2021), we showed significant improvement in precipitation and temperature accuracy as model resolution increased. In addition, we used the 4 km WRF domain from Dars et al (2021) to assess the characteristics of historical HMA precipitation (Riley et al, 2021). Importantly, our results show differences in amount, frequency, and seasonality of precipitation between valleys and the surrounding higher elevations.…”

“…For example, weather stations in Nepal show mean annual precipitation of around 1,400 mm for elevations below 1 km, increasing to above 2,000 mm at an elevation of 1.6-2.4 km and decreasing to under 1,000 mm by 4 km (Krakauer et al, 2013). The north side of the Himalayas is in a rain shadow and has annual precipitation of only around 50-200 mm (International Centre for Integrated Mountain Development, 2005;Riley et al, 2021). Winter storms that propagate from the Atlantic-Mediterranean sector are a significant portion of annual precipitation in the western HMA, and are critical contributors to snowpack at high elevations further east (Lang and Barros, 2004;Riley et al, 2021).…”

“…The north side of the Himalayas is in a rain shadow and has annual precipitation of only around 50-200 mm (International Centre for Integrated Mountain Development, 2005;Riley et al, 2021). Winter storms that propagate from the Atlantic-Mediterranean sector are a significant portion of annual precipitation in the western HMA, and are critical contributors to snowpack at high elevations further east (Lang and Barros, 2004;Riley et al, 2021).…”

“…The models are initialized with the data describing the initial atmospheric state and then driven by chosen boundary conditions. As described in the precipitation section above, we generated a variety of WRF simulations for HMA that include SWE estimates (Sarangi et al, 2020;Riley et al, 2021). Land surface models (LSM's) are often coupled with these atmospheric models to provide surface heat and moisture fluxes at the ground.…”

Indus River Basin Glacier Melt at the Subbasin Scale

“…The model described in the subsequent subsection requires the following inputs from HAR: incoming radiative fluxes at the surface, 2 m air temperature, total precipitation, surface pressure, 2 m relative humidity, emissivity, and 10 m windspeed. We imposed a precipitation cutoff for drizzle at 0.25 mm per day, consistent with instrument limits on measurement (following, e.g., Riley et al, 2021). The model is driven by meteorological input from HAR and additional variables derived from them that are necessary to calculate the turbulent and outgoing longwave fluxes (e.g., surface saturation vapor pressure).…”