High altitude precipitation dynamics using long-term observations in a Himalayan catchment

Abstract. In the Himalayas, the interaction between climate, orography and land cover results in a highly heterogeneous precipitation pattern. Mountain precipitation, being the main driver of the water cycle, ultimately provides water for millions of people downstream. Our current understanding of precipitation patterns within the Himalaya region is mainly based on past modeling efforts and data ensemble products. Yet due to computational limitations these products either cover only a limited time period or are unable to capture fine-scale precipitation patterns due to their resolution. Furthermore, the lack of long-term meteorological observations due to the remoteness and inaccessibility of the terrain results in poor validation of these products, especially above 3000 m a.s.l. We address this knowledge gap by synthesizing thirteen years (2012–2025) of meteorological field data that spans an altitude range between 1406 and 5350 m a.s.l. in the monsoon-dominated Langtang Valley, Nepal. We show that: (i) the diurnal precipitation cycle is bimodal during the monsoon, with an afternoon and a nocturnal peak, but unimodal during the pre-monsoon; (ii) the relative amplitude of the afternoon (nocturnal) peak increases (decreases) linearly with elevation, such that the monsoon precipitation–elevation gradient reverses diurnally from positive in the afternoon to negative at night; and (iii) the nocturnal precipitation, which dominates the monsoon totals, is highly sensitive to valley topography: under the near-saturated conditions prevailing at night, condensation rates scale directly with terrain-forced ascent, concentrating the nocturnal maximum at the steep, confined valley entrance. This elevation gradient of the diurnal cycle offers a simple validation tool for testing whether models capture the topography–circulation interaction, and the observational record provides a unique reference dataset for studying valley-scale precipitation dynamics in this region.