Processes associated with extremely heavy precipitation in the Meghalaya Plateau region: A case modelling study

Abstract

© 2022 Royal Meteorological Society.The Meghalaya Plateau (MP) located in northeast India is one of the rainiest regions in the world. On 18–19 August 2015, Mawsynram on the southern slope of MP received 745 mm of precipitation in 24 hr. This study investigates the dynamical, thermodynamical and cloud microphysical processes associated with this event through numerical simulations with fine horizontal resolutions (1 and 1/3 km). The control (CNTL) simulation with 1 km grid spacing successfully reproduces the observed spatial pattern of accumulated precipitation. A simulation without MP (noMP) is carried out to examine the role of MP in this precipitation. From 1500 LST 18 to 0000 LST 19 (P1) when the low-level jet carrying warm and moist air towards MP is relatively weak, the upslope region receives a moderate amount of precipitation which is initiated over this region due to the orographic lifting, while almost no precipitation is received there in the noMP simulation. Warm microphysical processes play dominant roles in the precipitation in P1. From 0000 to 0900 LST 19 (P2) when the low-level jet is enhanced, the CNTL simulation shows very heavy precipitation in the upslope region, much heavier than that in the noMP simulation. Deep convective systems developed upwind of MP move towards MP. These convective systems merge together and strengthen over the upslope region. The accretion process is substantially enhanced by the vigorous updraughts at low levels over the steep slope of MP, resulting in heavy precipitation. The 1/3 km resolution simulation shows much heavier precipitation in the upslope region than the CNTL simulation. The increased horizontal resolution makes the slopes steeper, resulting in further intensification of the updraughts over this region. This increase in simulated precipitation reduces the deviation from the rain-gauge observation, implying the importance of very high horizontal resolutions in simulating extremely heavy precipitation in MP.