Role of Low Level Jetstream in Intense Monsoon Rainfall episodes over the West Coast of India Dr. P.V. Joseph Professor Emeritus Department of Atmospheric Sciences Cochin University of Science and Technology Fine Arts Avenue, Kochi-682 016 e-mail: porathur@vsnl.com
Topics presented Low Level Jetstream during SW monsoon Factors associated with Heavy Monsoon Rainfall along the west coast of India a. Vorticity of Low Level Jet Stream b.vertically Integrated Moisture c. Convective cloud formation A hypothesis for intense monsoon rainfall episodes
Horizontal and Vertical Profiles of Wind in LLJ on 9 July 1961 (Joseph and Raman,1966)
Rainfall (OLR) and wind at 1.5km (850hPa) at MOK (Joseph and Sijikumar,Journal of Climate, 2004)
Rainfall (OLR) and wind at 1.5 km in Active Monsoon (Joseph and Sijikumar,Journal of Climate, 2004)
Rainfall (OLR) and wind at 1.5 km in Break Monsoon (Joseph and Sijikumar,Journal of Climate, 2004)
A recent study by Francis and Gadgil for the planning of ARMEX has shown the following, analysing data of 1951-1987 a. Heavy Monsoon Rainfall (HMR) of more than 15cm per day occur along west coast during monsoon causing considerable damage b. Such HMR events have maximum frequency near 19N and between 14N and 16N latitudes c. Most favorable time for HMR events is between 15 June and 15 August d. 62% of these events are associated with systems organised on the synoptic and larger scales (TCZ)
.Mumbai 19.0N,72.9E.Honavar 14.2N,74.4E
Cases of rainfall at Honavar of 20cms per day or more during monsoon seasons of 1975-1989 Observation at 0830 hrs of 08 June 1977 16 June 1977 17 July 1977 19 July 1977 15 July 1979 23 June 1982 03 August 1982 26 June 1985 16 June 1986 05 August 1986 13 June 1988 07 June 1989 Rainfall (cms) during previous 24 hours 22.12 25.66 21.42 29.44 20.64 21.27 25.92 21.00 30.38 22.87 46.63 43.22
Composite 850hPa wind for 12 cases of monsoon rain at Honavar (1975-1989) of 20cms per day and more.. Red lines are isotachs of wind speed in m/s at 1m/s interval
Composite of OLR for 12 cases of monsoon rain at Honavar (1975-1989) of 20cms per day and more. Honavar
Cases of rainfall at Colaba, Mumbai of 20cms per day or more during monsoon seasons of 1975-1990 Observation at 0830 hrs of 31 July 1975 01 August 1979 23 September 1981 02 July 1984 17 June 1985 25 June 1985 16 June 1990 15 August 1990 Rainfall (cms) during previous 24 hours 41.72 20.62 24.16 54.43 34.55 30.93 42.12 25.90
Cases of rainfall at Santacruz, Mumbai of 20cms per day or more during monsoon seasons of 1975-1990 Observation at 0830 hrs of 09 July 1975 05 August 1976 23 September 1981 19 July 1982 21 July 1982 17 July 1983 02 July 1984 13 September 1984 17 June 1985 Rainfall (cms) during previous 24 hours 22.34 26.47 31.82 27.56 20.85 25.34 24.01 21.52 22.36
Frequency of Heavy Monsoon Rain at Colaba and Santacruz 1950-1990 Colaba in cms 15 52 Santacruz in cms 15 72 20 19 20 28 30 06 30 06 40 04 40 NIL 50 02 50 NIL
Composite 850hPa wind for 8 cases of monsoon rain at Colaba,Mumbai of (1975-1990) 20cms per day and more.. Red lines are isotachs of wind speed in m/s at 1m/s interval
Composite of OLR for 8 cases of monsoon rain at Colaba, Mumbai (1975-1990) of 20cms per day and more. Mumbai
Composite 850hPa wind for 9 cases of monsoon rain at Santacruz,Mumbai of (1975-1990) 20cms per day and more..
Composite of OLR for 9 cases of monsoon rain at Santacruz, Mumbai (1975-1990) of 20cms per day and more. Mumbai
Case Studies for Heavy Rainfall at Honavar, Mumbai & S. Gujarat Heavy rain recorded at 03 GMT (0830 IST) Honavar 07 June 89 43.2cm Colaba 02 July 84.54.4cm Santacruz 27 July 05 94.4cm Colaba 01 Aug 79 20.6cm S. Gujarat 27 June 02 Pardi 61cm Valsad 54 cm
850 hpa wind and LLJ axis on 06 June 1989 Case of Honavar rainfall 43.22cm.
Zonal wind shear (m/s per 500km) at 850hPa - Honavar from 29May to 12 June 1989. Heavy rain on 6 June. -(δu/δy)
Vorticity (in box around Honavar) at 850hPa from 25May to 15 June 1989. Heavy rain on 6 June. (δv/δx-δu/δy)
Vertically Integrated Moisture (kg/m 2 ) - Honavar from 29May to 12 June 1989. Heavy rain on 6 June.
Vertically Integrated Moisture (kg/m 2 ) on 2 June 1989 (isolines more than 40kg/m 2 at 2kg/m 2 intervals).
Vertically Integrated Moisture (kg/m2) on 4 June 1989 (isolines more than 40kg/m2 at 2kg/m2 intervals).
Vertically Integrated Moisture (kg/m2) on 6 June 1989 (isolines more than 40kg/m2 at 2kg/m2 intervals).
24 hour Rainfall recorded at 0830IST on 02 July 1984 Colaba - 54cm Santacruz - 24cm. Colaba. Santacruz 24 hour Rainfall recorded at 0830IST on 27 July 2005 Colaba - 7cm Santacruz - 94cm
850 hpa wind and LLJ axis on 01 July 1984. Case of Colaba rainfall 54.43cm.
850 hpa wind and LLJ axis on 29 June 1984.
Zonal wind shear (m/s per 500km) at 850hPa - Mumbai from 20June to 10July 1984. Heavy rain on 1 July. -(δu/δy)
Vorticity (in box around Mumbai) at 850hPa from 20June to 10 July 1984. Heavy rain on 1 July. (δv/δx-δu/δy)
Vertically Integrated Moisture (kg/m 2 ) - Mumbai from 20June to 10 July 1984. Heavy rain on 1 July.
Vertically Integrated Moisture (kg/m2) on 1 July 1984 (isolines more than 40kg/m2 at 2kg/m2 intervals).
850 hpa wind and LLJ axis on 26 July 2005. Case of Santacruz rainfall 94.4cm.
850 hpa wind and LLJ axis on 24 July 2005..
Zonal wind shear (m/s per 500km) at 850hPa - Mumbai, from 18July to 03August 2005. Heavy rain on 26 July. -(δu/δy)
Vorticity (in box around Mumbai) at 850hPa from 15July to 05 August 2005. Heavy rain on 26 July. (δv/δx -δu/δy)
Vertically Integrated Moisture (kg/m 2 ) - Mumbai from 18July to 03 August 2005. Heavy rain on 26 July.
850 hpa wind and LLJ axis on 31 July 1979. Case of Colaba rainfall 20.62cm.
Zonal wind shear (m/s per 500km) at 850hPa - Mumbai, from 20July to 10August 1979. Heavy rain on 31 July -(δu/δy)
Vorticity (in box around Mumbai) at 850hPa from 20July to 10 August 1979. Heavy rain on 31 July. (δv/δx -δu/δy)
Vertically Integrated Moisture (kg/m 2 ) - Mumbai from 20July to 10 August 1979. Heavy rain on 31 July.
Nowcasting and warning for very heavy rain Step 1- Monitor growth of 850hPa zonal wind shear and vertically integrated moisture in a 500km x 500km box around Mumbai.. Colaba. Santacruz Step 2 When above parameters reach critical values begin radar watch for thunderstorm development and operate a mesonet for hourly rainfall and surface wind. A typical grid for mesonet of about 50 stations is marked on the map of Mumbai.
850 hpa wind and LLJ axis on 26 June 2002. Case of S.Gujarat rainfall 61cm and 54cm
Zonal wind shear (m/s per 500km) at 850hPa S.Gujarat, from 18June to 03July 2002. Heavy rain on 26 June. -(δu/δy)
Vertically Integrated Moisture (kg/m2) S. Gujarat from 18June to 3 July 2002. Heavy rain on 26 June.
A Hypothesis for the Heavy Rainfall along West Coast occurring in Mesoscale Systems There is intense cyclonic vorticity north of the LLJ axis (850hPa) The wind field below 850hPa is much weaker When cyclonic shear vorticity in the frictional boundary layer associated with the LLJ at a coastal station increases, cumulus / cumulonimbus convection increases Increased convection leads to pumping of moisture into the troposphere Thus as the shear vorticity at a station increases the Vertically Integrated Moisture in the atmosphere also increases and the rainfall potential increases
Hypothesis contd.. With shear and moisture increased & with favourable CAPE, large thunderstorms form with strong vertical upward velocities inside Around each thunderstorm there will be an environment with vertical downward motion which suppresses thunderstorm formation there. Each thunderstorm is a mesoscale system The downward motion field around thunderstorms (in an area about 9 times the size of the thunderstorm area) will transport the intense cyclonic vorticity of the LLJ at 850hPa to lower levels in the boundary layer
Hypothesis contd.. Increased cyclonic vorticity at levels like 950 and 900 hpa with greater frictional effects will lead to increased vertical motion in the atmosphere leading to more intense convective storms The increased cyclonic vorticity at levels below 850hPa where the existing wind field is weak can lead to formation of mesoscale cyclonic vortices as suggested in literature and for which ARMEX was conducted
Mechanism of Vorticity transfer to 900 and 950hPa from 850hPa 850hPa Level (Large Vorticity) Ground Level
Vertical mass flux (estimated) inside cloud (Mc) and in the environment outside cloud (M) in cloud clusters over Marshall - islands (Yanai et al 1973). M is the large scale mass flux. Note the large vertically downward flux outside cloud at low levels. From book Storm and Cloud Dynamics by Cotton and Anthes p 215.
At CUSAT Sijikumar in his PhD thesis looked at this problem. He performed MM5 simulations for heavy rainfall events at Honavar (latitude 14.2N) with initial conditions 48 hours before the reported heavy rainfall events. He used : Cumulus Parameterization : Betts Miller PBL scheme : Mellor Yamada Eta Moisture scheme : Simple ice (Dudhia)
The two-way nested domains (three) used in the experiment are shown below. Grid sizes 10km, 30km and 90km.
For the case of heavy rainfall of 43cm at Honavar at 03z of 07 June 1989 with full orography of the Western Ghats and initial data of 00z 05 June 1989, a mesoscale vortex formed at 950hPa by 18z of 05 June away from coast. It moved towards Honavar on the second day (06 June 1989).
850 hpa wind and LLJ axis on 05 June 1989. Case of Honavar rainfall of 6June 43.22cm.Honavar
Simulated 950hPa wind after (a)18 hrs to (f)48 hrs at 6 hour intervals (with Western Ghats orography in inner most domain)
Simulated 950hPa wind after (a)18 hrs to (f)48 hrs at 6 hour interval (with no Western Ghats orography in inner most domain)
It is found that the mesoscale vortex is only slightly weaker in the no-orography case showing that the most important factor is the shear vorticity of the LLJ at 850hPa. The experiment was repeated for a few more cases at Honavar. Similar mesoscale vortices were obtained. But similar experiments performed for Mumbai heavy rain yielded no mesoscale vortices
At CUSAT we are now trying to see whether LLJ vorticity is transferred to levels below 850hPa in cases of strong convection in the field of LLJ using MM5 and the recently acquired WRF model
Squally weather during monsoon along the west coast Squall : A sudden increase of wind speed by at least three stages on Beaufort s scale, reaching at least 22 knots and lasting at least for one minute
In a study of monsoon squalls at Goa, Dayakrishnan et al., 1977 found that on average 105 squalls occur during a monsoon season (extreme values : 139 squalls in 1971 and 74 squalls in 1969). At times more than 10 squalls occur in a single day. No well marked changes in surface temperature and pressure accompany these monsoon squalls.
Monsoon squalls at Goa on 2 and 3 June 1971 (Dayakrishnan et al., 1977) Monsoon onset over Kerala in 1971: 27 May
850 hpa wind and LLJ axis on 31 June 1971.
850 hpa wind and LLJ axis on 02 June 1971.
850 hpa wind and LLJ axis on 03 June 1971.
Monsoon squalls at Mangalore on 22 July 1989
Monsoon squalls at Mangalore on 23 July 1989
850 hpa wind and LLJ axis on 23 July 1989.
OLR (Watts/m 2 ) of 23 July 1989.
Dynes PT Anemogram at Mangalore on 10 July 1989
A mechanism proposed for monsoon squalls : The cyclonic shear north of the low level jet axis produces intense convective clouds and rainfall in a 3-4 degree latitude belt just north of the LLJ axis. The downdrafts of the convective clouds is hypothesised to carry the high momentum air at 850hPa level close to the LLJ axis to the surface level causing the monsoon rain squalls.
THREE STAGES OF AN ORDINARY THUNDERSTORM
Horizontal and Vertical Profiles of Wind in LLJ on 9 July 1961 (Joseph and Raman,1966)
VERTICAL SECTION OF A SEVERE THUNDERSTORM
15 13.5 12 10.5 HEIGHT IN KM 9 7.5 6 4.5 3 1.5 } VERTICAL WIND SHEAR 0 50 100 150 200 250 WIND SPEED IN KM PER HOUR TYPICAL VERTICAL WIND PROFILE IN JETSTREAM
Severe thunderstorms of north India during premonsoon season grow in an environment of strong vertical wind shear between surface and 500hPa. The dry air at levels 600 to 500hPa enter the storm and gets cooled by evaporation of rain falling into it which causes intense downdraft and surface wind squalls of 50 to 100 knots. Below these thunderstorms we get large variations in surface temperature and pressure. These storms are very much different from the monsoon convective storms.
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