Extreme Precipitation Observations During the 2019 MISO-BOB Field Campaign

The 2019 MISO-BOB field campaign, conducted from 27 May to 31 July in the Bay of Bengal (BOB), focused on investigating tropical waves and intraseasonal oscillations (ISO). Two ocean cruises took place in June and July, complemented by ground-based observations in tropical islands - Sri Lanka, Maldives, and Seychelles. On 18 July, an extreme rainfall event occurred in Sri Lanka, with over 200 mm of precipitation recorded in the mountainous Nuwara Eliya district (\(\sim\)1900 m above mean sea level), while coastal stations experienced minimal rainfall. Concurrently, gale-force westerly winds exceeding 60 km/hr were observed in Colombo, a low-elevation coastal city in the Western Province. This event was associated with a Convectively Coupled Kelvin Wave (CCKW) embedded within an eastward-moving ISO envelope over the Indian Ocean. The sharp contrast in rainfall between mountains and coast highlights the influence of elevated terrain on precipitation distribution. The role of orographic uplifting was studied using field data from sonic anemometers, radiosondes, Doppler Lidar, and rain gauges; the satellite-based rainfall estimates did not capture the intense orographic rainfall. During the event, the Froude number of the onshore flow was greater than 1 suggesting an unblocked flow over the coast transporting moisture toward the mountains; the level of free convection (LFC) was below the mountain peak. Thus, mechanical uplifting of conditionally unstable jet resulted in localized flooding over the mountain slopes. This case study supports the earlier idealized numerical experiments involving moist flow over orography and provides guidelines for forecasting extreme rainfall over the mountains of a tropical island.