Information about the wind storms that are now more threatening to Gujarat

Information about the wind storms that are now more threatening to Gujarat


Onset of monsoon over the mainland of India, (Kerala Coast) known as MOK
has been much studied since the first work of Ananthakrishnan et al (1967). Regular
and false onset of monsoon has been investiaged (Fazallo & Webster 2003).
Monsoon onset is also connected with the 30-50 day oscillation (Joseph et al 1994,
2006). Formation of monsoon onset vorex (Krishnamurti et al 1981) in the Arabian
Sea was investigated with SMONEX-1979 aircraft data. Models have been used to
understand its dynamics (Krishnamurti and Ramanathan 1982) and dynamical
instability associated with it was investigated (Mishra et al 1985). Pearce and
Mohanty (1984) showed that the major precursor for monsoon onset is the build-up
of moisture up to mid-troposphere about 2 weeks in advance of MOK. Because of
the importance of MOK in the annual calendar, many observational investigations
have been undertaken in India as well as by the international scientific community to
unravel features and variability of MOK. Atmospheric general circulation models


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Have been also used to diagnose MOK and its IA variability (Ratnam et al 2005 and
Sikka et al 2010).

Most of these indicators or precursors related to rainfall, strengthening of
winds and seasonal reversal in temperature distribution between southern India and
Tibetan region are dynamically related to each other. These changes occur in a
slow manner within upper troposphere, mid-troposphere and lower troposphere at
different rates beginning from mid-March to get fully set or organized by mid-May
when the big event of Monsoon Onset Over Kerala (MOK) is about to take place
(Ananthakrishnan 1977). Several investigators have suggested objective criteria for
MOK. Recently Goswami and Xavier (2005) have emphasized the change in mid /
upper tropospheric meridional temperature gradient as an important parameter for
objectively defining MOK. There are other studies on this subject too (Ramesh et al
1996, Joseph et al 2006 and others). Pasch (1983) introduced global onset of
summer monsoon and Zhang and Lu (2004) have advocated globally unified
monsoon onset and retreat indices. Even energetic of monsoon onset were
examined by George and Mishra (1993). However, from the point of MOK, formation
of a synoptic scale disturbance (low pressure area, onset vortex, mid-tropospheric
vortex or trough along 8-100 N between 70-900 E) brings about the burst of rains over
Kerala for which the operational meteorologists in India wait to declare that the MOK
has indeed taken place. This is quite needed because the public perception of
MOK, traditionally, is based on the jump in rainfall and onset of strong winds. As
such IMD has recently developed multi-parameters criteria which include jump in
rainfall over Kerala Coast, satellite observed cloud band, strengthening of winds at
925 hpa within 5-100
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 N and 70-750 E, to define the MOK. There are several other
indices proposed by different authors to objectively define the MOK from different
parameters. However, most of the years objective indices-based MOK and the
IMD’s operational MOK date agree within 2-3 days of each other. The mean date of
MOK is 1 June with a standard deviation of one week.
Understanding of processes (such as burst of rainfall over Kerala, enhanced
moistening of lower and middle troposphere over Peninsular India / West Coast,
building up of the meridional temperature gradients in the middle and upper
troposphere, appearance of the tropical easterly jet over southern India, shift in the

Large scale trough patterns in mid-tropspherical global scale and disappearance (Yin
1949) of sub-tropical (westerly jet stream from India etc. and several others are
interlinked dynamically and take place within about a week of each other and
coinciding with the operational declaration of MOK.
Slingo et al (1988) investigated the role of physical parametrization schemes
in predicting monsoon onset by dynamical models. The high-resolution model being
used currently in NCMRWF has shown the capability to predict monsoon onset
within about 2-3 days of the observed one. Satellite photographs and OLR data
have also been used for monsoon onset in several studies (Lau and Chan 1986,
Simon et al 2006 and others).

SW Monsoon rains begin to advance northward after the MOK has taken
place. The advance process is not smooth but is punctuated by fast propagation for
3-5 days and then stops for a week or so to progress in steps along the West Coast,
Peninsular India, Central India, NE India, Indo-Gangetic plain and finally reaching the
borders of West Rajasthan by 15 July. On the average it takes nearly 45 days for
the advance process to be completed with extremes lying between 30 days to 70
days (Ananthakrishnan and Soman 1980, Soman & Krishnakumar 1989, Li an Yanai
1996 and several others). Biswas et al (1998) have found that the hiastus or
temporary cessation in the monsoon advance occurs due to increased activity of the
mid-tropospheric westerly trough along 65-800 E and temporary cessation of the
formation of synoptic scale disturbances over the Bay of Bengal. Flatau et al (2002)
studied the delayed monsoon onset in 2002 season and Prasad and Hayashi (2005)
have also examined the onset and withdrawal phases of monsoon over India. There
is need to further investigate the causes for cessation in the advance process and
determine the credibility of dynamical models to predict this cessation and its revival
on the medium-range scale.
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The performance of the seasonal rains over India, however, do not depend on
MOK (Dhar et al 1980) but on the period taken for the monsoon from MOK to its full
advance reaching upto west Rajasthan. Thus researchers may also focus their
efforts to find whether the advance period can be forecast on long-range scale. All
said and done the key role of formation and propagation of synoptic scale

Disturbances in the onset and advance processes of monsoon is dominant and
hence its diagnosis through dynamical models would provide guidance on mediumrange scale. On the long-range scale, it is also worthwhile to examine the interannual variability in the period of monsoon advance based on multi-decadal runs of
AGCMs or CGCMs available with different modeling centers in India and in other
countries, using ensemble runs, to determine their utility to forecast the duration of
monsoon advance just like these models are used to forecast monsoon seasonal
rainfall. An attempt to understand this aspect of variability in the period of monsoon
advance is made by Sikka et al (2010)

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