Persistent signatures of meteorological parameters and variation along topographic setup of NWH region

The present study examines the meteorological variables over the North West Himalayan (NWH) region, with special emphasis on rainfall and planetary boundary layer height (PBLH), along with their elevation dependence. Regional reanalysis data IMDAA for the monsoon season (JJAS) from 1989 to 2020 have been used for the study. Long term seasonal climatology unravels the persistent signatures of the meteorological variables and suggests low values of PBLH (<400 m), temperature (20 °C > T > 10 °C) and wind (<2 m/s) as well as high values of relative humidity (RH > 80 %) over the mountain base regions of NWH which happens to be the locations of peak precipitation (>0.7 mm/h). Diurnal variations in rainfall and PBLH show an inverse pattern across most regions. The minimum PBLH is generally followed by peak rainfall after a lag of approximately 0-3 h. A change of rainfall pattern from a bimodal structure to a unimodal structure is observed, corresponding to the change in topography from a two-step structure to a single step structure from the south-east to north-west direction of NWH. The observed peaks of rainfall are located over the valley regions where wind speed and PBLH are low. High relative humidity along with the low wind speed and PBLH over the valley regions cause heavy downpour due to the influence of cross-valley circulations. The current study forms the basis for improving our present understanding of mountainous circulation and the pattern of meteorological variables over a topographically complex region of NWH. This study provides valuable insights into the influence of PBL physics on rainfall peak formation and may contribute to the refinement of PBL parameterization schemes.