The Hindu
The continuous topography of the Ghats in Karnataka serve as a barrier
to rain-bearing winds from crossing over. Photo: K. Murali Kumar
Due to the greater width, there is more time for drops to coalesce and precipitate
In a recent study of rainfall trends using remotely
sensed satellite data and actual field data from the Indian
Meteorological Department of the Western Ghats region over the past 14
years, it was found that during the monsoon months of June, July,
August, September, the average rainfall was more over Karnataka than
Maharashtra and Kerala.
The Western Ghats run
parallel to the Arabian Sea coast for approximately 1,600 km from the
Maharashtra-Gujarat border to the southern tip of Kerala.
There
are several reasons for this. First, the mountain topography in
Karnataka is broader than the narrow topography of the Ghats in
Maharashtra. Due to the greater width of the mountains, the rain bearing
winds have to necessarily travel a longer distance and have more time
for the drops to coalesce and precipitate as rainfall, resulting in
higher rainfall. In contrast, the narrow width of the Ghats in
Maharashtra allows the rain-bearing wind to cross over to the leeward
side rapidly before precipitation can occur. As for Kerala, the Ghats
there are in the form of isolated mountains, where the rain-bearing
winds can easily cross over to the leeward side through the gaps in
between without precipitation occurring.
Second, the
slope of the mountain has a direct bearing on the possibility of
precipitation. This is borne out by the Ghats of Karnataka where the
mountains are gently sloping, compared to the steep slopes of the Ghats
in Maharashtra and Kerala.
The air parcel will retain
its energy and speed for a longer time when the slope is gradual. This
will provide sufficient vertical motion to cloud droplets to grow by
collision–coalescence process and hence form precipitation.
Third,
the gentle slope provides a greater area for sunlight absorption and
heating leading to greater convection when compared with an abrupt slope
i.e. less Ghat area such as that of the Maharashtra and Kerala Ghats.
Fourth,
the continuous mountain range presents a greater barrier to
rain-bearing winds than a range comprising isolated mountains with gaps
in between where the winds can easily pass to the leeward side. Unlike
in the case of Kerala, the Ghats in Maharashtra and Karnataka are
continuous.
The study carried out by Sayli A. Tawde and Charu Singh was published recently in the International Journal of Climatology.
Ms. Tawde is pursuing her PhD in the Centre for Atmospheric & Ocean
Sciences, Indian Institute of Science (IISc), Bangalore, and Ms. Singh
is a scientist at the Marine and Atmospheric Sciences Department, Indian
Institute of Remote Sensing, ISRO, Dehradun, Uttarakhand.
Interestingly,
the study found that often areas of heavy rainfall were far away from
the summits of the mountains, as much as 50 km away.
“The
reason for this is that there is more chance of rainfall occurring at
the foot of the mountain as there is greater depth for the moisture in
the clouds to coalesce into big drops which finally reach the ground,”
notes Ms Tawde in an email to this correspondent.
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