Contrasting Features of Monsoon Precipitation Around the Meghalaya Plateau Under Westerly and Easterly Regimes

Abstract

Precipitation features around the Meghalaya Plateau, northeast India, during summer are investigated using a 17 year (1998–2014) high‐spatial‐resolution Tropical Rainfall Measuring Mission precipitation radar data set. Precipitation around the plateau fell into two distinct regimes based on the low‐level wind direction that fluctuates on intraseasonal time scales over Bangladesh, windward of the plateau: a westerly regime (WR) and an easterly regime (ER). Under the WR, strong low‐level onshore southwesterlies across Bangladesh encounter the plateau, and localized strong low‐level southerlies running parallel to the Arakan Mountains (i.e., the barrier jet) also blow toward the plateau, concentrating convective unstable air onto its southern slopes. The low‐level wind fields and large‐scale upper level divergent fields promote frequent and intense orographic rainfall along the southern slopes due to forced uplift, generating high precipitation. In contrast, under the ER, strong southeasterlies that blow along the Gangetic Plain without encountering the plateau and subsidence inhibit upward motion around the plateau, resulting in low precipitation. Diurnal variations in precipitation significantly affect the daily precipitation around the plateau under both regimes. High rainfall frequency persists over the southern slopes between 2100 and 1200 LT of the next day under the WR, whereas modest rainfall frequency occurs between 0000 and 0600 LT under the ER, with a daytime minimum and nocturnal maximum in both regimes. The atmospheric boundary layer processes over Bangladesh regulate the wind speed and vertical structure of the low‐level wind toward the plateau, with deceleration during daytime and acceleration at night (i.e., nocturnal jet) that result in the nocturnal rainfall maximum.

On the extreme rainfall events during the southwest monsoon season in northeast regions of the Indian subcontinent

“…Fujinami et al . () also reported intensification of rainfall activity due to the westerly regimes of low level wind over the Meghalaya plateau in the northeast regions of India. Low level wind features before and after the extreme events are almost alike and the winds are more prominent during the extreme events over the northeast region.…”

“…In a recent study, Fujinami et al . () found a reversal of the meridional pressure gradient in the lower troposphere around the Gangetic Plain facilitated heavy rainfall during the westerly regime and suppressed rainfall during the easterly regime. Analysis of the vertical velocity also explains the same developments (Figure , right panels).…”

“…This anomaly of the SLP before and after the events (5 days before and 5 days after) is not seen in the figure and therefore it can be concluded that the sudden F I G U R E 5 Spatial structure of composite rainfall during the extreme rainfall activity in the region 88 E-93 E, 23 N-27.5 N. The evolution of rainfall from 4 days before the event and the dissipation of the rainfall activity until 4 days after the event development of this pressure anomaly gradient favours the strong northerly wind at lower levels from the Bay of Bengal with ample moisture towards Tibet and this will interact with the topography of the Himalayas and generate short lived torrential showers over the region. In a recent study, Fujinami et al (2017) found a reversal of the meridional pressure gradient in the lower troposphere around the Gangetic Plain facilitated heavy rainfall during the westerly regime and suppressed rainfall during the easterly regime. Analysis of the vertical velocity also explains the same developments (Figure 7, right panels).…”

Mesoscale precipitation systems and their role in the rapid development of a monsoon depression over the Bay of Bengal

“…In fact, weak subsidence appears in the mid‐ to upper‐level troposphere in the CNTL run (figure not shown). Synoptic‐scale weak subsidence was observed in monsoon depressions to the north–northeast of the low/monsoon depression (Fujinami et al ., ). Weak subsidence might result in the dry air (i.e.…”

“…Figure 1 shows the monsoon depression at 0000 UTC 17 August 2016 when it reached minimum central pressure over the head of the BoB during its life cycle ( Figure 2). Deep convection (<200 K) that reaches above 15 km develops south of its centre, especially in the southwest, while convective activity is suppressed from the north to east of the centre, showing a synoptic-scale cloud distribution that is typical of monsoon depressions (Godbole, 1977;Sikka, 1977;Stano et al, 2002;Hunt et al, 2016b;Fujinami et al, 2017). Active convection extends from the southwestern part of the monsoon depression to the western Tibetan Plateau across the western Himalayas, corresponding to the convergence zone between the prevailing monsoon westerlies and the easterlies in the northern part of the depression in the lower troposphere.…”