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Mountain topography of Western Ghats affects monsoon rainfall

The topography of the Western Ghats influences the rainfall over the mountain region, finds a recent study.

The Western Ghats,a mountain range that runs parallel to the western coast of India, is home to a diverse range of flora and fauna. Though a world heritage site, it also houses a sizeable human population. Rainfall from the summer monsoon plays a huge role for all inhabitants of the Ghats: the flora and fauna, and the people. Accurate prediction of rainfall in this region will be very useful.

Sayli Tawde and Charu Singh analysed rainfall trends in the Western Ghats for the last 14 years, using remotely sensed satellite data and actual field data from the Indian Metrological Department. “Over 6000 rain gauges placed far and wide  all over India help gather gridded information about the rainfall on a regular basis”, said first author Sayli Tawde in an interview.

Predictions made by the Indian Metrological Department and European Range Medium Range Forecasts could correctly predict when heavy rainfall was to occur, but they underestimated the magnitude of the rainfall. Publishing their findings in the International Journal of Climatology, the researchers write that according to earlier researches, the reason for underestimation of rainfall specially in mountainous terrain  could be that the current prediction  models do not take into account the influence of different topography parameters on the rainfall in the region.

When strong wind hits a mountain barrier, a strong upward draft is created;water droplets accumulate and clouds are formed. When these clouds rise to a specific height over the Western Ghats, rainfall occurs from the clouds with temperature greater than 0˚C. Right now, the remote sensing satellite method cannot accurately capture this kind of warm rainfall.

The authors studied spatial distribution of rain and magnitude of rainfall with respect to the topography, elevation and mountain slope. They also studied spatial distribution of the occurrence of extreme rain events. The data for elevation, slope and topography was obtained from the Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER) Digital Elevation Model (DEM).

According to the data collected in past 14 years, heavy rain events occurred mostly in June and July whereas low rain events occurred mostly in August and September. During summer, i.e. March-May, the land heats up and the difference in the sea and land temperatures is large. Due to this, moist winds from ocean to land surface get strengthen. This intensifies cloud formation  and heavier rains occur in June and July.

After the rains, the temperature difference is reduced and large cloud formation is slowed down. So rainfall in August and September is lesser. Satellite data over the Western Ghats reveals that Kerela had least amount of extreme rain events in the past 14 years. The average rainfall amount was greatest in Karnataka, but the most number of extreme rain events happened close to Ratnagiri, in Maharashtra. The areas most prone to heavy and extremely heavy rainfall are close to Mumbai in Maharashtra, and the area between 16.25-16.5°N and 73.25-73.5°E.

It was observed that rain on the “windward” side of cascaded mountains, the side of mountain ranges that are exposed to monsoon winds, is greater than that compared to isolated mountains. This is because, the barrier created by a cascaded mountain range restricts flow of clouds for a longer time, thus giving more time for rain formation. If the barrier is small, like an isolated mountain, the water droplets are carried away to the other side of the mountain and rain on windward side is less.

Since Karnataka has a cascade mountain range in the Western Ghats, more rainfall is observed on the windward side of Karnataka, as compared to Maharashtra.  In the rain shadow region, more rain is seen in Maharashtra than Karnataka. In addition to the length of the mountain, the width of the mountain also affected the rainfall. The mountains in Maharashtra are narrower as compared to mountains in Karnataka. This is also a contributing factor to more rain in Karnataka as compared to Maharashtra

On the windward side of the mountains, as the elevation increases, the rainfall also increases. But beyond the elevation of about 800 metres above sea level, the rainfall drops. So there is lower rainfall near the summits. The maximum rainfall is not at the highest points in the region, but about 50 km away from the summits. In general, taller mountains received higher rainfall.

But there were a few exceptions. The exceptions were due to the fact that rainfall also depends on the gradient or slope of the mountain.  It has been observed that rainfall increases for areas where there is a gradual slope as compared to areas where the slope is steeper.

These observations indicate a clear relationship between the topography of the Western Ghats region and spatial distribution of rainfall in the region. This information can be used to predict better rainfall estimations in future; this in turn will make disaster management easier. Author Sayli says “My research has given a path which needs to be explored more in terms of quantitative/ mathematical expressions, to include it in atmospheric models. Further research on this topic  may lead to an improvement of atmospheric models and predictions of rainfall in mountainous terrains.”

About the authors:

Sayli A. Tawde is a PhD student in Centre for Atmospheric & Ocean Sciences, Indian Institute of Science, Bangalore. Email: saylitawde@gmail.com.

Charu Singh is a scientist at Marine and Atmospheric Sciences Department, Indian Institute of Remote Sensing, ISRO, Dehradun, Uttarakhand, India

International Journal of Climatology; DOI: 10.1002/joc.4146