Wednesday, July 29, 2020
Meteorological Explanation of July Weather Systems Absence in Bay this year...( for vagaries By Jivan Prakash ji Ex Scientist IITM)
Some facts regarding July 2020 rainfall
(1) Low Pressure Systems (LPS) over India.
July 2020 is typical month in the history that not a single Low Pressure System (LPS) formed.
Study by Mooley (1990) showed that all India rainfall(air) and over central India (CI) are correlated with number of LPS. The correlation coeff. (cc) increases with number of LPS days (.6 for the period 1888-1983) and further increases to 0.7 for total westward longitudinal displacement.
LPS days means the number of days a Low Pressure System is present in any part of India.
The lowest number of LPS days 3 occurred in the July 1903 .
In this way July 2020 has record number of low LPS days i.e.0 till today.
Mean number of LPS days in July is 13.9.( Maximum Number of LPS days occured in Central India. From 1888-1983 , there were 884 LPS days.
Although there is no off shore trough on West coast,I think there must be sufficient off shore cyclonic vorticity arising from across the flow shear of monsoon westerly. In the expression of vorticity in natural coordinate, we know there are two terms, shear vorticity and curvature vorticity. Generally curvature vorticity is attributed for the vorticity associated with off shore trough. I think we n the present situation, shear vorticity term is very much strong and curvature vorticity term is dormant.
July 2020 Rainfall features
We all know that Low Pressure System (LPS) over BoB form at the eastern end of Monsoon trough in its normal orientation. Monsoon trough being the locus of all points with maximum cyclonic vorticity, fulfills the criteria for Barotropic instability. Thus genesis of cyclonic vortex over Head BoB during southwest monsoon season is attributed to the barotropic instability. Non formation of LPS during this July 2020 may be due to absence or inadequate barotropic instability.
Of course it is my own hypothesis only, which needs to be negated or verified by comparing meridional shear of mean zonal wind, with climatological normal. Of course during July on most of the days, eastern end of Monsoon trough was not over head BoB, rather to the north. This synoptic feature itself demonstrate the inadequacy of Barotropic instability over head BoB.
July 2020 Rainfall features
a) no Low Pressure system
b) wetest Mumbai July
c) above normal rainfall over rain shadow subdivisions viz. Marathwada, North interior Karnstaks (NIK) , Telangana, Rayalaseema, etc.
d) below normal rainfall over central India
Absence of LPS
1) Absence of remnants from west equatorial Pacific, as cyclonic activity over Pacific was subdued.
2) subdued activity of off shore trough over west coast of India. Xang (2013) showed that diabatic heating in the west coast trough excites stationary Rossby wave with ridge over Peninsular India and trough over BoB giving cyclonic vorticity for formation of LPS
3) not dipping east end of monsoon trough but situated over head BoB.
Above points showed that no cyclonic vorticity prevailed over head BoB for formation of LPS.
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