Role of Eastern Ghats Orography and Cold Pool in an Extreme Rainfall Event over Chennai on 1 December 2015

Abstract

Chennai and its surrounding region received extreme rainfall on 1 December 2015. A rain gauge in the city recorded 494 mm of rainfall within a span of 24 h—at least a 100-yr event. The convective system was stationary over the coast during the event. This study analyzes how the Eastern Ghats orography and moist processes localized the rainfall. ERA-Interim data show a low-level easterly jet (LLEJ) over the adjacent ocean and a barrier jet over the coast during the event. A control simulation with the nonhydrostatic Weather Research and Forecasting (WRF) Model shows that the Eastern Ghats obstructed the precipitation-driven cold pool from moving downstream, resulting in the cold pool piling up and remaining stationary in the upwind direction. The cold pool became weak over the ocean. It stratified the subcloud layer and decelerated the flow ahead of the orography; hence, the flow entered a blocked regime. Maximum deceleration of the winds and uplifting happened at the edge of the cold pool over the coast. Therefore, a stationary convective system and maximum rainfall occurred at the coast. As a result of orographic blocking, propagation of a low pressure system (LPS) was obstructed. Because of the topographic β effect, the LPS subsequently traveled a southward path. In a sensitivity experiment without the orography, the cold pool was swept downstream by the winds; clouds moved inland. In the second experiment with no evaporative cooling of rain, the cold pool did not form; flow, as well as clouds, moved over the orography.

Impact of Cartosat-1 orography on weather prediction in a high-resolution NCMRWF unified model

“…Figure 4(d-f) shows the temporal evolution of the cyclonic vortex and a gradual intensification of the cyclonic system can be clearly seen in figure 4(d-f), which has a maximum at 9 UTC. The formation of a low-pressure system during the heavy rainfall event is in synergy with the results of the study performed by Phadtare (2018). The difference in the Cartosat-run and the SRTM-run shown in figure 4(g-i) clearly shows the weakening of the cyclonic vorticity from 10 UTC onwards over the northeast part of the Chennai domain as the precipitation over these regions intensify.…”

“…This is in agreement with the increase in mountain heights seen in Cartosat DEM over the northern and western sides of the Chennai domain (see figure 6e). Earlier studies have shown that an enhancement in rainfall can occur over hundreds of kilometres away from the mountains when orographically blocked flow occurs (Houze 2012;Phadtare 2018). The SRTM-run shows a shift in moderate to heavy rainfall patterns over the ocean, which is very much improved in the Cartosat-run.…”

“…Studies performed by Chen et al (2013) over the northeastern Taiwan have also shown the importance of mountains in the enhancement of rainfall over the coastal regions. A detailed study performed by Phadtare (2018) highlights the role of the Eastern Ghats in the enhancement of extreme rainfall events over Chennai city. Hence, it is not possible to fully attribute the observed organisation and the extreme rainfall intensities over Chennai city to the intensity of the prevailing synoptic systems only (Chakraborty 2016;Van Oldenborgh et al 2016;Krishnamurti et al 2017;Phadtare 2018), as it is not sufficient to fully explain the observed heavy rainfall intensity over Chennai.…”

“…A detailed study performed by Phadtare (2018) highlights the role of the Eastern Ghats in the enhancement of extreme rainfall events over Chennai city. Hence, it is not possible to fully attribute the observed organisation and the extreme rainfall intensities over Chennai city to the intensity of the prevailing synoptic systems only (Chakraborty 2016;Van Oldenborgh et al 2016;Krishnamurti et al 2017;Phadtare 2018), as it is not sufficient to fully explain the observed heavy rainfall intensity over Chennai. The present study investigates the effects of improvements in the orography in an operational high-resolution regional weather forecasting model, which can substantially improve the skill of the numerical weather prediction (NWP) models.…”

Large‐Scale Extreme Rainfall‐Producing Synoptic Systems of the Indian Summer Monsoon

“…The southwestern sector of LPSs is known to receive heavy rainfall, often exceeding 300 mm day −1 (Godbole, 1977; Krishnamurthy & Ajayamohan, 2010). In the past two decades, India has experienced many devastating floods triggered by extreme rainfall, namely, the Kerala flood of August 2018 (Baisya & Pattnaik, 2019), Chennai flood of December 2015 (Chakraborty, 2016; Phadtare, 2018), Uttarakhand flood of June 2013 (Houze Jr et al, 2017), Leh flood of June 2010 (Rasmussen & Houze Jr, 2012), and Mumbai flood of July 2005 (Kumar et al, 2008). Though each individual event had unique features, the common thread in all events was the presence of an LPS.…”