(About the Author: Author Group Captain Biswanath Nandi (Retd) is alumnus of IIT, Delhi. He completed his PhD in the field of Numerical Weather Prediction (NWP) from Centre for Atmospheric Science. Served Meteorological branch of Indian Air force for 33 years in various capacity as teacher, forecaster and founder of Centre for Numerical Weather Prediction. Currently Senior consultant ( Meteorology ) with Indigo Airlines for 13 years)
Coastal Weather
- India has vast coast line. West coast next to Arabian Sea has Western Ghats along the coast line. East coast next to Bay of Bengal has Eastern Ghats mainly close to Visakhapatnam area. Coastal weather is influenced by the mesoscale environment to a large extent. The mesoscale system prominent to influence weather are
- Land-Sea breeze
- Anabatic-Katabatic winds of Ghats.
- Stress-differential along coast
- Upwelling-down welling
- Nocturnal jet
- Land breeze. This is prominent when contrast of temperature of land and ocean is highest. Land breeze occurs around late night, early morning and morning when land is cooler than ocean and wind blows from land to ocean that is easterly wind over west coast and westerly wind over east coast. When the prevailing flow is onshore, temperature differences across the shore line are small and land breeze is weak. The nocturnal land breeze is usually rather shallow, being only a few hundred meters deep. The seaward range of the land breeze is much smaller Influences of Land breeze can be categorised as follows.
- Descending motion over land. As wind blows out of land during land breeze phase, it is to be compensated by descending motion over land and ascending motion over sea. The effect of descending motion gets accentuated in case of narrower the peninsula like Malaysian Peninsula because land breeze blows both side east and west coast. So morning weather in general clears over land during this cool phase.
- Prevailing easterlies (trade or otherwise). Easterly Land breeze enhances over west coast and blows away to sea making the morning comparatively drier.
Over east coast land breeze confronts the prevailing easterlies away from the coast into sea. This convergence leads to convection cloud formation off east coastal region and this convection clouds are driven to land by prevailing easterlies of much larger depth. That’s the story behind the morning weather usually as showers over east coast, during prevailing easterlies.
- Prevailing westerlies. Westerlies may prevail as equatorial westerlies over southern parts of peninsula or as monsoon westerlies over entire west coast. Over west coast land breeze confronts the westerlies away from the coast into sea. This convergence leads to convection cloud formation off west coastal region and they are driven to land by prevailing westerlies of much larger depth. That’s the story behind the morning weather, usually as showers over west coast during westerlies. The return current of land breeze is westerlies on west coast and usually brings stratus cloud which often forms as fog over slightly highland over coast like Mangalore airfield.
Westerly Land breeze enhances over east coast and blows away to sea making the morning comparatively drier.
3. Sea Breeze. Primarily afternoon and evening phenomenon when land is much warmer than sea. And wind blows from sea to land giving rise to the name Sea Breeze. As the breeze blows from sea to land it is westerly for west coast and easterly for east coast. The sea breeze may extend inland up to 25 to 50 NM (50 to 100 km). The sea breeze reaches altitudes of 4300 to 4600 feet (1300 to 1400 m) in tropical coastal areas, and is strongest a few hundred meters above the surface (Edson and Condray, 1989).
- Ascending motion over land. As wind blows from Sea during Sea breeze phase, it is to be compensated by descending motion over Sea and ascending motion over land. The effect of ascending motion gets accentuated in case of narrower the peninsula like Malaysian Peninsula. So weather in general forms over land during this hot phase of land during afternoon/evening.
- Prevailing easterlies (trade or otherwise). Over west coast Sea breeze confronts the easterlies overland. This convergence leads to convection cloud formation interior of coastal region and this convection clouds are driven to sea by prevailing easterlies of much larger depth. That’s the story behind the afternoon-evening weather over west coast during easterlies.
Sea breeze enhances over east coast and blows into the land making afternoon-evening comparatively moist. On encountering eastern Ghats near Visakhapatnam convection cloud develops over the Ghats and do not come over Visakhapatnam due prevailing easterlies of greater depth, but on days of westerlies in mid tropospheric level It blows over to Visakhapatnam to give afternoon-evening thunderstorm.
- Prevailing westerlies. Westerly Sea Breeze enhances over west coast and blows into the land making afternoon/evening moist comparatively cool and moist. On encountering western Ghats, it climbs and give rise to clouds over Ghats.
Over east coast Sea Breeze confronts prevailing westerlies over land and this convergence give rise to moderate convection over land during afternoon/evening
4. Anabatic wind. During afternoon/evening The hill slope gets heated up due solar heating whereas the adjacent free air is not heated up that much. Thus the air in contact with the hill surface starts climbing the hill. This phenomenon is termed as Anabatic wind. This vertical motion of climbing the hills can be enhanced by the Sea breeze blowing over the Ghats. The combined effect of Sea Breeze and Anabatic wind give rise to deep convection cloud which can be driven over to coastal region if we have mid tropospheric favourable wind like easterlies for west coast and westerlies for east coast. The rise of deep monsoon westerlies over western Ghats usually causes deep convection of very tall cloud. But mid troposphere being a transition layer between monsoon westerlies in low level and easterlies in upper level they hardly move. Gives copious rainfall over Ghats. The downdraft of this cloud and monsoon westerlies form an opposing wind and convergence zone help regeneration of convective cloud, which eventually reaches the coastal region in later parts of evening. In many occasions the outflow of upper tropospheric cloud descends over coastal region to give rain. Thus during monsoon, we have two peak weather hours of morning and evening over west coast.
5. Katabatic wind. This is night time phenomenon when hill slopes become cooler faster than the free air at the same height. The air in contact with hill slope becomes cooler and heavier, starts sliding down the slope generating Katabatic wind. This more prominent cooler is the ground meaning that Katabatic wind is strongest during early morning hours. Katabatic wind reinforces the Land breeze over coast adding to increased probability of morning weather on either coast.
6. Stress-differential along coast. The friction is more over land due to the nature of underlying surface. This leads to four kind of effects.
- Wind is parallel to the coast and land is having lower pressure that is to say Northerly wind for west coat and southerly wind for east coast. According to Bryson and Kuhn (1961) due frictional effect surface wind over land cross isobars towards lower pressure at greater angle more than over sea. Hence, if pressure is lower over the land (as in all upwelling regions), winds diverge and air subsides at the coast. This is partly why vigorous upwelling borders coastal deserts.
- If pressure is higher over the land, surface winds converge at the coast, a condition often accompanying continental anticyclones (Fett and Bohan, 1986).
- If wind is blowing into the land slowdown at coast will lead to convergence.
- If wind is blowing into the sea will accelerate at coast will lead to divergence.
7. Upwelling- downwelling.The movement of surface currents of sea also plays a role in the vertical movements of deeper water, mixing the upper water column. is the process that brings deeper water to the surface, and its major significance is that it brings -rich deep water to the nutrient-deprived surface. Downwelling is where surface water is forced downwards, where it may deliver oxygen to deeper water. Downwelling leads to reduced productivity, as it extends the depth of the nutrient-limited layer. Easterly and Northerly wind along west coast bring upwelling and cooler ocean temperature. Westerly and Southerly wind along east coast bring upwelling and cooler ocean temperature. This upwelling enhances the marine inversion.
8. Nocturnal Jet. To quote (Persson 2002) – There is no end to the surprises the Coriolis force has in store for us. For example, after a calm and sunny day, when night falls, the wind can suddenly start to pick up some 100m above the ground. In a few hours’ time, the wind speed at this height might have doubled or trebled. These upper winds, known as nocturnal jet streams, are so common that they even show up in climatological averages. While the wind at 10 m has a maximum during the day, the wind at 200-1000m has a maximum in the early hours of the morning. Nocturnal jet streams can affect late-night thunderstorms, bird migration and even the spread of pollution When the sun rises they disappear like the trolls in the sagas so what is going on? Paradoxically, it is all about friction and, more paradoxically, it has much in common with the mathematics of drifting icebergs (Persson 2001). After sun set temperature in lowest 100m or so starts falling, more so on the surface of earth than air above it creating inversion. This inversion cuts off the air flow from frictional effect leaving behind to pressure gradient force and Coriolis force to get into balancing act and giving rise nocturnal jet. This is very common in many parts of India, especially, for the SWly flow over Orissa Bengal coastal area and over Gujrat, Rajasthan area. Many a time can be seen over noncoastal area also.
9. Conclusion. Leopold (1949) studied interaction of land-sea breezes with the prevailing easterly trade winds
(1) the size of the land area,
(2) the heights and shapes of the mountain ranges and their relation to the height of the subsidence inversion, and
(3) the aspect of the area over which the land-sea breeze develops (i.e., windward or leeward exposure to the trade winds.
To these should be added (4) the shape of the coastline, (5) the strength of the trade winds, and (6) the vertical profile of trade wind speed.
Similarly for monsoon westerly regime all those physiographic feature is important to make variations in rainfall and weather.
References
Bryson, R.A., and P.M. Kuhn, 1961: Stress-differential induced divergence with application to littoral precipitation. Erdkunde, 15, 287-294.
Edson, R.T., and P.M. Condray, 1989: The Caribbean basin. An electrooptical climatology for the 8- 12 micron band. Vol. III. Northern South America. USAFETAC/TN-89/006, 250 pp.
Fett, R.W., and W.A. Bohan, 1986: Navy Tactical Applications Guide, Volume 6, Part 1 — Tropics, Weather Analysis and Forecast Applications, Meteorological Satellite Systems. Bohan, Park Ridge, Illinois.
Leopold, L.B., 1949: The interaction of tradewind and sea breeze, Hawaii. J. Meteor., 6, 312-320.
Persson A. 2001c. The Coriolis force and drifting icebergs. Weather 56(12): 439– 444.
Persson A. 2002a. The Coriolis force and the nocturnal jet stream. Weather 57(1): 28– 33.