Laila weakens, monsoon on track

New Delhi, May 20 (PTI) Monsoon watchers appear to have
heaved a sigh of relief on the weakening of cyclone 'Laila'
as they believe it would now have little impact on the annual
rainfall season.
"We are still going ahead with the initial prediction on
monsoon. This season, monsoon will be normal and is expected
to reach Kerala by May 30," Earth Sciences Minister Prithviraj
Chavan told reporters here.
He said all the weather parameters have been factored in
while arriving at the monsoon forecast.
A section of the meteorologists said that following the
landfall, the cyclone will weaken further as supply of
moisture to the weather system will be cut off after it hit
the land.
But how the cyclone dubbed as "severe" by the weather
office lose its sheen?
In fact, Laila began weakening significantly since last
night as it travelled very close to the coast for a long time
but did not make a landfall. This could have been because of a
sudden increase in the value of 'vertical wind shear' (VWR).
Wind shear relates to a difference in wind speed and
direction over a relatively short distance in the atmosphere.
Wind shears are of two types -- vertical and horizontal.
"A cyclone gets energy from sea through evaporation of
water from sea surface. This evaporated water is soaked by the
winds. The moisture-laden winds flow into the cyclone's centre
through a process called moisture convergence," said a top
operational forecaster.
"When half of the circulation came over land, this
process got cut off from western side leading to reduction of
moisture supply as well as moisture convergence. This
obviously has affected intensity of the cyclone," he said.
After churning across the Bay of Bengal, 'Laila'
virtually came to a halt about 200 km south of the coastal
city of Machilipatnam and remained there for quite a long
time.
Similarly, shift in the mid and upper air high pressure
area could have also played a major role as the intensity of a
cyclone depends on the 'vertical wind shear'.
"Low value of vertical wind shear creates ventilation in
the vertical column of cyclone so that the moisture laden flow
enters into cyclone inner core, rises up and then gets out of
the cyclone from top. This way a continuous supply of moisture
is ensured," the forecaster said.
Since its formation, 'Laila' had benefitted from a low
value of the vertical wind shear but suddenly Thursday, the
high pressure area shifted slightly and the value of the wind
shear started rising. MORE PTI SKU
RDM


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