Monsoon intra seasonal Rainfall oscillation over Gujarat state 2021.
Manorama Mohanty, F Vigin Lal
IMD, Ahmedabad
Indian summer monsoon which exhibits a wide spectrum of variability on Diurnal,
Daily, Seasonal, Inter annual and Decadal time scales over different region/state of the country.
Gujarat state is located in the extreme western part of India with Thar Desert in the north-east,
Rann of Kutch in the west and the mighty Arabian Sea which envelops the state towards the
south and south-west. The bulk of the rainfall activity over this region occurs during the months
of July and August under the influence of synoptic scale systems. However during monsoon
2021, major rainfall activity occurred during June and September due to the typical synoptic
situations. A study has been taken to analyse the intra seasonal variability of monsoon rainfall
and associated characteristics for Indian Summer Monsoon 2021 over Gujarat state for Month
of September has been carried out. Also occurrences of heavy rainfall to extremely heavy
rainfall due to various synoptic systems formed over land and oceans has been analysed.
Gujarat state has two meteorological subdivisions Gujarat region and Saurashtra-Kutch.
During monsoon season 2021, the subdivision Gujarat Region received normal rainfall of about
80cm (-12% departure), subdivision Saurashtra-Kutch received excess rainfall of about 62 cm
(24% departure) and Gujarat state as a whole received normal rainfall of about 70 cm rainfall
(2% departure).Though Gujarat state received normal rainfall during monsoon 2021 but there
is large variation in monthly rainfall over the state. The state received 88 percent rainfall during
June(-12 % departures), 60 percent during July(-40% departure), 30 percent during August(-
70% departure) and 368 percent during September(268 % departures). From the weekly rainfall
district rainfall data analysis it can be seen that over Saurashtra and Kutch region major spell
of rain was during month of September (Fig: 3a, 3b).
Due to a few spells of rain during September 2021 the seasonal rainfall of the state became
normal (Fig: 3a). This study has been carried out to analyse the variation of intra seasonal
rainfall and associated dynamical parameters for the month of September 2021. The time series
of daily average rainfall over Gujarat state is shown in the Fig: 2. The data sets used for the
study are u and v wind data from ERA5 reanalysis data set with 0.25*0.25 .deg horizontal
resolution with vertical levels 1000 to 500 hpa standard levels for the month September 2021
with daily time steps (00UTC, 06UTC, 12UTC and 18UTC) and for area 68 .deg E to 72 .deg
E and 20 .deg N to 24 .deg N; daily area averaged rainfall data from Rain gauges, AWS over
Gujarat state for the period September 2021.
In this study analysis is being done for the subdivision Saurashtra-Kutch during
September 2021 as there was excess rainfall over the subdivision. During the first week of
September the subdivision Saurashtra-Kutch received heavy to very Heavy rains (Fig 1a, 3b)
with maximum rainfall of 17cm at Junagadh and Kutch districts as monsoon trough was
running across Gujarat state.
Due to interaction of Low Pressure Area over southwest Madhya Pradesh &
neighbourhood up to 7.6 km above mean sea level and the shear zone along Latitude
20°N between 3.1 km & 5.8 km above mean sea level, the state received heavy rain over
Gujarat region and heavy to very heavy rain with isolated extremely heavy rain over
Saurashtra-Kutch 2nd week of September (Fig 1a, 3b). The highest rainfall recorded on 9th
September was 25 cm in Gir Somnath District. During 13th to 15th September 2021(Fig 1a, 3b)
Gujarat state received an active wet spell due to the oscillation of monsoon trough up to
Saurashtra coast and interaction with cyclonic circulation over Gujarat region. During the
period heavy to very heavy rainfall with extremely heavy rainfall occurred over the state with
highest rainfall recoded 52 cm recorded at Rajkot, 47 cm at Junagadh and 41 cm at Jamnagar
on 14th September 2021. On 24th September Gujarat state received a good spell of monsoon
rainfall (Fig 1a, 3b) with highest daily rainfall of 19 cm in Jamnagar district due to the cyclonic
circulation over West Rajasthan. Last rainfall spell for the month was from 28th to 30th
September 2021 (Fig 1a, 3b) due to intensification of Well-Marked Low Pressure Area into a
Depression over northeast Arabian Sea & adjoining Kutch due to which highest rainfall
recorded was 29 cm in Junagadh district on 30th September 2021.
The ERA5 data is used to compute relative vorticity and hovumuller plot is prepared for lower
level(1000-850mb) and higher level(700-500) vorticity for the month September 2021 are
shown in the Fig 1b. During the extremely heavy rainfall events, positive vorticity advection
is observed at higher level also. Thus vertical advection of positive vorticity up to higher level
can be considered as an indicator for extremely heavy rainfall forecast.
Fig1a: Daily average rainfall in mm over Saurashtra & Kutch during September
2021
Fig1b: Relative Vorticity Hovumuller plot September 2021
Daily average rainfall in mm over Gujarat State Monsoon 2021
45
40
AVERAGE DAILY RAINFALL IN MM
35
30
25
20
15
10
0
7th July
11th July
15th July
19th July
27th July
4th Aug
8th Aug
12th Aug
16th Aug
20th Aug
24th Aug
28th Aug
1st Sep
21st Sep
31st July
3rd July
23rd July
5th Sep
9th Sep
13th Sep
17th Sep
25th Sep
29th Sep
5th June
9th June
13th June
17th June
25th June
29th June
21st June
1st June
DATE
Actual Normals
Fig2: Daily average rainfall in mm over Gujarat State
Weekley Rainfall% Departures
500
433
400
322
300
278 269
200 209 216
212
202
142 144
100 101
95
89 104
0 -1 15
7
-3 9
-22 -23 -18
-27 -9
-18
-49 -31
-46 -34 -33
-60 -63
-73 -71 -55 -65
-70
-78 -79 -80
-91 -80 -91
-94 -87
-91
-95 -87
-90
-96
-100 -98
-200
GUJARAT STATE
SUBDIVISIONAL RAINFALL GUJARAT REGION
SUBDIVISION RAINFALL SAURASHTRA KUTCH SUBDIVISION
Fig3a: Weekly percentage departures of rainfall from long period average
Fig3b: Weekly rainfall (mm) over districts of Saurash-Kutch
