A flood of the Century...See here

Kerala Floods and Heavy Rains August 2018....A Meteorological Explanation

Every year, the south-west monsoon gives heavy to very heavy rainfall over different parts of India spanning over the months from June-September. It has been observed that there is a significant increase in the frequency of very heavy rainfall events over the Indian landmass in the recent decades. In this year, the southwest monsoon seasonal rainfall in spite of being -7.7% below normal till 20 August, there were few episodes of very heavy rainfall in different parts of the country. This caused floods leading to huge loss of livelihood and property. The recent case was of “Kerala floods” which was triggered by very heavy rainfall during the period 14-16^th August 2018. From 0300 UTC of 14^th August to 0300 UTC of 15^th August, the average rainfall recorded in the state of Kerala as a whole was 130 mm. Another 138 mm was recorded in the next 24 hours. This was about 10 times higher than the normal.

It has been seen that anomalous strong westerlies were prevailing at 850 hPa between 5°N - 14°N from 14^th August onwards (Fig. 1a-1c). This strong westerly surge was mainly the response of the depression that had formed over the North Bay of Bengal on 14^th August. Also, there was anomalous moisture transport from the Arabian Sea towards Kerala during this period (Fig. 1d-1f). This anomalous moisture was seen converging over the region which led to a rise in the specific humidity. This acted as a fuel for anomalous high convection. Further, it has been observed that from 14^th August onwards strong divergence at upper levels started building up to the south-west of Kerala. This triggered the low level convergence near Kerala with values reaching up to 20 s^-1. This area of strong divergence at upper levels persisted on 15^th August and moved close to Kerala, leading to sustained convergence at lower levels over the region (Fig. 2a-2f). From Fig. 2g-2i it is evident that the relative vorticity also increased significantly from 14^th August onwards near the Kerala coast with values reaching up to 40*10^-6 s^-1 on 15^th August. This large positive relative vorticity along with strong convergence persisted near the coast of Kerala till the 16^th of August providing favorable conditions for enhanced convection thereby leading to anomalous increase in rainfall.

During this period of heavy rain, Madden Julian Oscillation (MJO) was in phase 6. This was not conducive for enhanced convection over the North Indian Ocean. This shows that MJO doesn’t seem to have played a role in this episode of heavy rainfall over Kerala.  Thus, initial analysis suggests that this heavy rainfall maybe a response of local thermodynamical conditions along with strong westerly surge that transported large amount of moisture leading to persistent widespread heavy rainfall for 2-3 days.

Fig. 1: (a-c) Wind anomalies from 14-16^th August. Color denote the magnitude of the wind anomalies. Vectors denote the direction. (d-f) Moisture flux transport (vectors) and moisture flux convergence/divergence (shading). Negative values denote convergence and positive values denote divergence.

Fig. 2: (a-c) Upper level divergence (s^-1). Thick contour denotes divergence, dashed contour denotes convergence. (d-f) Low level convergence (s^-1), thick contour denotes convergence, dashed contour denotes divergence. (g-i) Relative vorticity (850hPa) s^-1. Color denotes the magnitude of relative vorticity with blue shades as negative relative vorticity and green - red shades denote positive relative vorticity.

 References:

1.      Cooperative Institute for Meteorological Satellite Studies (http://tropic.ssec.wisc.edu/#)

2.      Earth System Research Laboratory Physical Science Division (https://www.esrl.noaa.gov/psd/cgi-bin/data/histdata/fnl.pl?date1=20180816&date2=20180816&days=1&var=Vector+Winds&level=850&type=1&proj=Custom&custproj=Cylindrical+Equidistant&xlat1=0&xlat2=30&xlon1=40&xlon2=100&labelc=Color&labels=Shaded&cint=&lowr=&highr=&scale=&Submit=Create+Plot)

3.      India Meteorological Department, Thiruvananthapuram (https://www.imdtvm.gov.in/)

4.      Tokyo Climate Center WMO Regional Climate Center in RA II (Asia) (https://ds.data.jma.go.jp/tcc/tcc/products/clisys/anim/anim_asia.html)