India is experiencing a warmer winter

FROM THE NEWS EDITION: http://www.downtoearth.org.in/news/india-is-experiencing-a-warmer-winter-52435

DATE OF UPLOAD: 13TH JANUARY 2016

Persistent above normal temperatures have been recorded in many regions of the country

After the monsoon, it's the winter season which has played truant with India. More than half of the winter season has passed, but the characteristic chill felt in many parts is missing due to persistent above normal temperatures.

Palam in New Delhi witnessed a maximum temperature of 25.7°C on January 12, 2016, which was 6°C above normal. Minimum temperature in this region on the morning of January 13 was 13.9°C, 7°C above normal. Srinagar, which is in the middle of Chilai Kalan (40-day harsh winter period), witnessed minimum temperature of -2.5°C on January 13, which is still 5°C above normal. Most places in Himachal Pradesh, Uttarakhand, Punjab, Haryana, Chandigarh, western Uttar Pradesh, Rajasthan, western Madhya Pradesh and Vidarbha in Maharashtra witnessed maximum temperatures which were 2-5°C above normal on January 12. 

This warm weather might be good news for those who don't like the winter, but such weather and temperature fluctuations adversely impact the growth of winter crops. Apples of Himachal Pradesh and strawberries of Maharashtra have been affected by the warm and dry weather.

Generally, during winter, temperatures often fluctuate but never stay above or below normal for extended periods. This means that a few days during winter may not experience cold conditions. People living in northern Indian states observe this rise in temperatures whenever the weather turns cloudy due to arrival of a western disturbance. But once the western disturbance passes, temperatures return to normal or many times fall below normal. 

But this winter season (October 2015 onwards), the number of such warm days appears to be high. More importantly, anomalies in the maximum temperatures have been higher than those in the minimum temperatures. This means the days have been quite warm. This weather data will give us an idea of how warm the winter has been.

Temperature anomalies in various cities 

Climate Prediction Centre (CPC) of National Centers for Environmental Prediction (NCEP) monitors city temperatures in many parts of the world, including India. Below data represents temperature anomalies between October 10, 2015 and January 10, 2016. For example, Srinagar 1.01°C means temperature in Srinagar during the period was 1.01°C above normal. It must be noted that this data is preliminary and official data from India Meteorological Department (IMD) may differ. 

Aurangabad (Maharashtra)  3.05°C

Bhuj (Gujarat)  2.88°C 

Solapur (Maharashtra) 2.86°C

Hyderabad 2.67°C

Kurnool (Andhra Pradesh) 2.27°C

Gwalior  2.16°C

Veraval (Gujarat) 2.09°C

Dehradun  2.04°C

Pune 1.85°C

Nagpur  1.68°C

Jaipur  1.6°C

Patiala  1.43°C

Panaji  1.41°C

Jodhpur  1.27°C

Patna  1.27°C

Surat  1.2°C

Bengaluru 1.18°C

Safdarjung (Delhi) 1.15°C

Ahmedabad  1.09°C

Dumdum (Kolkata) 1.08°C

Srinagar  1.01°C

Trivandrum  0.98°C

Amritsar  0.79°C

Mumbai  0.68°C

Chennai  0.64°C

The range -0.5°C to +0.5°C of normal is considered as the normal range. But anything more than +0.5°C is considered as a positive anomaly (warmer than normal). Numerically, the above figures might appear small (such as 0.7°C), but they are significant in the case of temperature anomalies, especially in the case of Aurangabad which was quite warm. 

Temperature departures from October-December 2015

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October to December maximum temperature departure from normal (Image Credit: World Ag Weather)

 

Monsoon withdraws from most of India in October and it's after that the temperatures start falling. That's why the period from October is important for analysis of temperatures. IMD calls October to December the post-monsoon period and January to February the winter period. Usually, meteorologists consider December to February as the winter period in the northern hemisphere, but it's better to consider temperature data of October to February for the purpose of analysis. 

Above image shows maximum temperature departures between October and December 2015. The scale used is in degree Celsius. Most of India has been shaded in yellow, orange and red colours which suggest above normal maximum temperatures. Grey areas represent normal temperatures whereas blue areas represent below normal temperatures. 

This image clearly shows that central India has been the warmest (highest anomaly) as compared to other parts of India. Parts of Maharashtra have seen significant above normal maximum temperatures (1.5-3°C above normal) with that in Marathwada being 2-3°C above normal. Eastern Madhya Pradesh is 1.5-2°C above normal, Gujarat 1-1.5°C, Rajasthan and Uttar Pradesh 0.5C-2°C, Delhi-NCR 1.5-2°C, Andhra Pradesh and Telangana 2-3°C. In northeastern states and Tamil Nadu, the temperature anomaly is 0.5-1°C. In most of Jammu and Kashmir, Punjab and some parts of Rajasthan, maximum temperatures have been in the normal zone. 

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October to December minimum temperature departures from normal (Image credit: World Ag Weather)

Talking about minimum temperature anomalies, once again the highest impact is seen in Maharashtra, especially Marathwada where minimum temperatures have been 2-3°C above normal in the crucial winter period. In southern India, anomalies have been lower, in the 0.5°C-1°C range. In parts of Delhi-NCR, they are in the 1°C-1.5°C range. Temperatures were below normal in eastern Sikkim, western Assam and adjoining West Bengal in this period.  

Many parts of India had witnessed mini-summer like conditions in October. The Marathwada-Telangana- northeastern Karnataka belt had received very poor monsoon rainfall and now, temperatures in this belt have been above normal. This is likely to cause a significant impact on agriculture in these areas.

Temperature departures in December 2015

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December maximum temperature departure from normal (Image credit: World Ag Weather)

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December minimum temperature departure from normal (Image credit: World Ag Weather)

An interesting temperature pattern can be observed in the above images. Most of the areas below the Tropic of Cancer were warmer than normal as compared to areas above it. Temperatures were sort of normal in most of Jammu and Kashmir, Himachal Pradesh, Punjab, Uttar Pradesh and Delhi. But maximum temperatures were above normal in many areas of Rajasthan whereas minimum temperatures were below normal (blue). Over eastern India, maximum temperatures were normal but minimum temperatures were slightly above normal in December 2015. 

What is causing this pattern? 

Strong El Niño conditions in the Pacific Ocean are being held responsible but the mechanism through which El Niño is impacting India's winter isn't clear yet. At first glance, the problem seems to be more local than global. 

After the southwest monsoon withdraws, north-westerly winds (winds blowing from the northwest direction) set over India in the lower levels of the troposphere. We know that winds blow from a high pressure region to a low pressure region. During post-monsoon months, the air pressure is higher in northern parts (higher latitudes) than that in southern parts (lower latitudes). This is mainly because of reduction in surface heating in the high latitude areas. 

As a result, India gets these north-westerly winds which are responsible for bringing northern cold air to various parts of India. 

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Normal v anomalous wind pattern at 5,000 feet above ground (Credit: NCEP)

While the left side of the image above represents the climatology (the usual pattern) of winds at an altitude of 5,000 feet between November and January, the right side shows anomaly (variations) in these winds observed between November 2015 and January 10, 2016. Anomalous winds from the western and southwestern directions were present during this period (shown by arrows). This implies that these anomalous winds didn't allow the penetration of cold air from higher latitudes which kept central and southern India warm (at least during the nights). 

Weaker and a lower number of western disturbances are responsible for less cold weather in north India and also in other parts of India. A western disturbance is not only a source of rain or snow but also of cold air. Frequent, stronger and lower latitude western disturbances keep on supplying cold air which is mandatory for proper winter conditions. 

The biggest question now is why these western disturbances have been weak and less in number this time. Some research has been done in the past on the relationship between western disturbances and El Niño Southern Oscillation. The research says that western disturbances tend to intensify over northwest India under El Niño conditions. However, we are seeing the opposite occur and, hence, need to understand the reasons behind this trend.

Key weather phenomena to follow in 2016

FROM THE BLOG EDITION: http://www.downtoearth.org.in/blog/key-weather-phenomena-to-follow-in-2016-52260

DATE OF PUBLISHING: 31ST DECEMBER 2015

From western disturbances to the monsoon, a look at possibilities of various weather phenomena in India

Right from unprecedented episodes of "unseasonal" rain and hailstorms in early summer to severe floods in the late monsoon, 2015's weather kept everyone on their toes. Most of these events didn't occur out of the blue but the way they impacted us (for instance the March floods of Kashmir) showed a lack of preparedness. The impact of 2015's weather phenomena like the poor monsoon is so severe that sustaining lives in parched areas like Marathwada of Maharashtra, Bundelkhand of Uttar Pradesh and numerous villages of Madhya Pradesh till the next monsoon is going to be very difficult. Agriculture is the worst-hit in these regions and water levels are critically low (especially in parts of Maharashtra). Eventhough summer is months away, a large number of tankers are already being used to provide water, which shows the severity of the drought. 

As 2015 was the second consecutive year of below normal monsoon rainfall, all eyes will be on the monsoon of 2016. But other meteorological phenomena will occur too, which should not be underestimated as they would have the potential to take a heavy toll. The interesting thing is that these annual weather phenomena are set to occur in India in 2016 but no one knows how strong or weak they would be. This is because of the inability of weather models to give precise information of the weather over a time scale of many months. The intensity of weather events generally doesn't repeat on yearly scales. For example it isn't necessary for 2016 to be a below-normal monsoon rain year just because 2014 and 2015 were. But learning from past weather events definitely helps in preparing better for the next impacts. 

El Niño nearing peak intensity

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IMAGE 1: Weekly Sea Surface Temperature anomalies showing domination of El Niño (black dotted circle). Red colour indicates significant above normal sea surface temperatures

Courtesy: NOAA/ESRL/PSD

Before we look at the aforementioned meteorological phenomena, it is important to take a look at the present status of El Niño in the Pacific Ocean. It is pretty strong and is already producing significant global impacts such as recent flooding in the United Kingdom and globally high temperatures. But data of Sea Surface Temperature (SST) anomalies in the equatorial Central Pacific Ocean suggests El Niño nearing its peak intensity or perhaps it has already peaked and henceforth, will start decaying. It is likely that this El Niño will be placed in the top three strong El Niño events (1997-98, 1982-83) ever recorded since 1950. El Niño conditions will definitely continue for some period in 2016. 

Behaviour of Western Disturbances till monsoon of 2016 

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IMAGE 2: Rainfall in India between October 1 and December 23, 2015. India has received 20 per cent below normal rainfall in this period. 

Courtesy: India Meteorological Department 

Frequency and intensity of western disturbances impacting North India have been lower in 2015 (i.e October onwards). Stronger western disturbances often produce effect (i.e rain) on the plains of North India which is beneficial for proper growth of winter crops. But, as India is yet to witness such a powerful system, the post-monsoon rainfall situation in the plains of North India remains grim. Between October 1 and December 23, Delhi got just 5 per cent of its normal rainfall. More observations suggest that between October 1 and December 29 2015, Palam has got just 2mm rain (27mm less than normal), Safdarjung just 1mm (39mm less than normal), Chandigarh 19mm rain (40mm less than normal). Interestingly, Jammu and Kashmir has got excess rainfall as the western disturbances have been highly localised until now.  

The first thing to look at in 2016 would be the frequency and intensity of western disturbances when they strike North India. Their strength in January and February will be a deciding factor for states like Jammu and Kashmir and Himachal Pradesh. Jammu and Kashmir's water reservoirs largely depend on snow streams and net snowfall/rainfall determines the state of agriculture, flora-fauna, tourism etc. Any prolonged dry spell or irregular precipitation patterns in these months will have the potential to affect these states. In the plains of North India (particularly Delhi-NCR), improper rainfall patterns or scanty rainfall in these two winter months will not be of any help in cleaning the air i.e reducing the air pollution. 

Mid-February onwards, temperatures in the hilly areas start increasing and rise above the freezing point in many regions. March witnesses warmer days and hence, snowfall gets converted to rainfall in these areas. Due to this rain, flooding potentials increase considerably in rivers like the Jhelum and subsequently in cities like Srinagar which was observed in March 2015. An increased and intense western disturbance activity in March 2015 flooded Srinagar and parts of the Kashmir valley, causing heavy damage. Thus, the behaviour of western disturbances in these months needs to be closely followed.  

Summer season thunderstorms

Here, summer refers to the months of March, April and May. In India, the transition from winter to summer takes places in March and that is why, this month is a bit unstable in terms of weather. The weather in these months is very vital for agriculture as Rabi crops mature and are harvested around this period. So, any widespread rainfall or hailstorm event damages these crops as well as horticulture. That is why, the best way to safeguard these crops would be effective implementation of the crop insurance scheme and making sure all varieties of crops (including horticulture) come under it. Farmers must keep harvested crops in dry places without waiting for any advisories. 

Since the past few years, India is witnessing multiple days of severe thunderstorms (including hailstorms) in these months. The hailstorm event of 2014 and "unseasonal" rain event of 2015 significantly impacted India's agriculture which broke the backbone of the farmers. Increased interference of western disturbance was the main reason behind these severe weather events. Thus, any such event in 2016 will prove to be catastrophic, especially in those areas where agriculture distress is at its peak. But if such thunderstorms occur in drought-hit villages such as those of Maharashtra or Madhya Pradesh, the associated rain would bring temporary relief. Under a strong thunderstorm season, effects of ongoing drought could also reduce. 

Not to forget, these thunderstorms, along with associated lightning strikes, have been a major killer in India. They have killed more than 2,500 people in India in 2014. Hence, the behaviour of western disturbances and thunderstorms in March-May 2016 also needs to be closely followed. 

Temperatures and duration of summer season 

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IMAGE 3: Temperature anomalies during March-May 2014 and 2015. Yellow to red shades indicate warmer than normal temperatures whereas bluish shades indicate cooler than normal temperatures. 

Courtesy: NASA GISS

We know that 2014 is the warmest year (so far) recorded since 1880 but 2015 is likely going to take this position as per World Meteorological Organisation. "The global average surface temperature in 2015 is likely to be the warmest on record and to reach the symbolic and significant milestone of 1° Celsius above the pre-industrial era. This is due to a combination of a strong El Niño and human-induced global warming" according to a release of World Meteorological Organization 

Despite some heatwaves and overall global heating, India didn't witness any significant heating in the March-May period of 2014 and 2015 as can be seen from above image. In fact, some cooling signs (white to light blue shade suggesting negative temperature anomalies) are visible in these periods, which are likely due to a plethora of clouds and rain. In any summer season, the skies need to remain clear for temperatures to increase but frequent clouding and rain episodes cause cooling and hence reduction of summer season. We may dislike the summer but proper heating of the Indian Subcontinent is vital for various ecosystems. It is believed that because of this summer heating, a low pressure establishes over Pakistan-North-West India which anchors the south-west monsoon winds. But, a milder summer season can be a boon for parched areas as lesser the temperatures, lesser will be the evaporation of water from water resources. Sustaining lives under such a scenario will be comparatively easy. 

Hence, in a background of global warming and devolving El Niño, temperature anomalies during 2016's summer will play a major role in India's weather. 

South-West and North-East Monsoon Season 

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IMAGE 4: Normal VS actual performance of South-West Monsoon of 2015. Red shading in right side image shows areas which have received very low monsoon rainfall. 

Courtesy: India Meteorological Department 

Undoubtedly, the South-West Monsoon of 2016 would be the most important and highly-followed weather event of 2016. This can be understood from the fact that it is many months away but guesswork on how it would be, has already started on the internet. Already, mixed reactions are coming from various people/agencies which usually start coming after April when the India Meteorological Department issues its first stage monsoon forecast. 

The key debate at present is on "El Niño or La Niña" during the next monsoon. Present estimates (of course all estimates are subjected to change) from Climate Prediction Centre of National Oceanic and Atmospheric Administration (NOAA) suggest that El Niño conditions will persist till late spring or early summer of 2016 (i.e around June-July) following which, a transition will take place to neutral conditions (known as ENSO Neutral). El Niño Southern Oscillation (ENSO) exists in two modes: El Niño (bad for monsoon) and La Niña (good for monsoon).  But, when both are absent (i.e Pacific Ocean has normal sea surface temperatures) then it is called ENSO neutral. 

The latest update of the second version of Climate Forecast System model (which is popularly used in ENSO forecasting) expects El Niño to continue through June-August 2016. The Bureau of Meteorology of Australia doesn't expect the end of El Niño to happen at least till autumn of 2016. Moreover, the present forecast of Indian Ocean Dipole (IOD) doesn't indicate any positive IOD event during next year's monsoon. A positive IOD tends to be favourable for monsoon. Thus, at present, there is a sort of disagreement between the models and agencies which will likely continue in the coming period given the mischievous nature of ENSO and IOD. 

So, if El Niño persists through the early monsoon of 2016, the million dollar question would be whether it will remain that strong to influence the monsoon. It is definitely going to be challenging for monsoon prediction models to give a precise forecast of India's monsoon in 2016. 

Amidst all this, we certainly know one thing, which is monsoon's intra-seasonal variability which affects the distribution of rain over space and time. The above image clearly shows monsoon rainfall variation. In 2015's monsoon, eastern Madhya Pradesh got very less rain as compared to the western part. An increase in extreme wet and dry spells of monsoon is very well documented in parts of India like Central India. So, authorities need to monitor such spells (particularly those occurring in sowing months like June) more carefully to avoid any 2015-like situation (in which sowing took place speedily but then, the rain disappeared). 

India badly needs a proper and well-distributed monsoon rainfall in 2016 but any irregularities will be capable of impacting the Kharif season and worsening of the drought scenario. Likewise, the North-East monsoon season will also be a thing to watch for South India. 

Tropical Cyclones 

2015's cyclone season for India was relatively quiet as compared to previous years. Excluding Cyclonic Storm Komen (July 26-August 2), no tropical cyclone hit the mainland of India. The most interesting thing was that this year, the Arabian Sea was more productive than the Bay of Bengal (the situation is usually reverse) and Komen was one of the few cyclones to have formed in the peak monsoon season. As a result of low cyclone activity in India, damage due to them has also been less. But, under a warming Indian Ocean scenario, it will be interesting to see how many cyclones form and where they head.  

What caused heavy rains in Tamil Nadu

FROM THE BLOG EDITION: http://www.downtoearth.org.in/blog/what-caused-heavy-rains-in-tamil-nadu-51998

DATE OF UPLOAD: 4th December 2015

An independent weather forecaster tries to explain the record-breaking December rains in Tamil Nadu

(NOTE: Views expressed in this blog are personal and they don't reflect the views of any organisation)

 

It wasn't a monsoon depression or a tropical cyclone. Then what was it? The 24-hour rainfall recorded in Chennai, parts of Tamil Nadu and Puducherry on December 1-2, 2015, raises this big question for researchers. Heavy rains and flooding potential in Chennai and other areas were anticipated, but the record-breaking rainfall figures have stumped everyone. 

 

In just 24 hours (till 08:30 am IST on December 2, 2015), Chembarambakkam, a lake southwest of Chennai, received a whopping rainfall of 475 mm. In the same period, Kattankulathur (near Chennai) received 445 mm rainfall, followed by 429 mm rainfall at Kattupakkam. The astonishing fact is that as per a report, 24-hour rainfall in at least three places in Tamil Nadu is more than maximum 24-hour rainfall of 380 mm recorded at Banki, Odisha during Tropical Cyclone Phailin. 

 

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24-hour rainfall in Tamil Nadu and Puducherry ending on 08: 30 am IST on December 2, 2015 (Courtesy: Vagaries of Weather)

 

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Daily mean rainfall in Tamil Nadu and Puducherry between October 1 and December 2, 2015. Fifty-eight per cent above normal rainfall has been recorded in this period (Courtesy: IMD RMC Chennai)

Chennai airport got 345 mm rain in a day whereas city-side got 294 mm rain. Rainfall statistics too present an unusually heavy rainfall picture with the star mark representing December's heavy rainfall event. 

 

Tropical cyclones are mostly responsible for bringing such heavy rainfall but Tamil Nadu's heavy rain wasn't caused by any tropical cyclone. It was caused by an ordinary low pressure system in the Bay of Bengal which performed extraordinarily. 

 

Monsoon low pressure systems 

 

India (in fact South Asia) experiences two monsoon seasons in a year. We are often familiar about the southwest monsoon season (June to September) which is a primary source of rain in most of India. For residents in parts of India (excluding southern) it's winter period right now. But, for southern parts of India such as Tamil Nadu, it's the real monsoon season. The state receives about 48 per cent of its annual rain in the northeast monsoon period (October to December). Coastal districts of the state get upto 60 per cent of their annual rain in this period. So, it's completely natural for rain in these areas at this time of the year and there isn't any "unseasonal" factor involved. 

 

Northeast monsoon (NEM) is known to be more variable (both in space and time) than the southwest monsoon (SWM). Technically, co-efficient of variation of NEM is 25 per cent as against SWM's 10 per cent which makes its forecasting even more challenging. The most important drivers of both these monsoon seasons are low pressures (known as monsoon low pressure systems), particularly those which form in the Bay of Bengal and move towards various parts of India. Every year, India witnesses many such monsoon low pressures some of which even intensify into monsoon depressions or even a tropical cyclone. Intensity-wise, tropical cyclones are the strongest systems followed by monsoon depressions and then monsoon low pressures. 

 

This year, around 11 low pressure systems were reported during the SWM season. Such monsoon low pressure systems usually have a life span of about 3-4 days and they bring heavy rain (often more than 100 mm in a day). This often inundates the areas through which they pass. But, when one compares Tamil Nadu low pressure system with other monsoon low pressure systems, some puzzling questions arise.  

 

How a weaker system brought so much rain? 

 

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Comparison of monsoon depression in central India (left) and Tamil Nadu monsoon low pressure (Image courtesy: University of Dundee)

 

 

To bring out the aforementioned question, I have compared it with one monsoon depression which was observed in central India during September 16-19, 2015. As per reports, more than 150 places in Maharashtra recorded more than 100 mm rain in a day because of this monsoon depression but there isn't any observation of rainfall as high as 475 mm of Chembarambakkam. Silod, a city in Aurangabad district of Maharashtra got 262 mm rain on September 17, while Vakadi (a place in Jalgaon district) got 220 mm rain. The low pressure system which brought rains in Tamil Nadu was very feeble (in terms of air pressure) as compared to the Septembersystem. The thumb rule is that the stronger a system is, the lower will be its air pressure and such a system will produce lot of rain under favourable conditions. From preliminary investigation, it looks like Tamil Nadu low pressure system had mean sea level pressure of around 1,008 millibar unlike around 997 millibar in the case of the September system. But naturally, maximum moisture content with the September system appears to be higher than that of Tamil Nadu system as reflected by total cloud water and total precipitable water data (which measures total atmospheric water vapour content in a vertical column of air between two levels). In the above image, compactness of clouds of monsoon depression (photographed on September 17) is another indicator of its strength over Tamil Nadu system (photographed on December 1). Thus, its definitely intriguing to see such a feeble system producing more rainfall than a powerful system. 

 

Internet is already flooded with several articles where without providing substantial evidences, people have linked this heavy rain event with factors like climate change or global warming. Below, we have a look at what could have made this low pressure system so powerful.

 

Variations in North-East Monsoon 

 

Dr. M Rajeevan, director of Indian Institute of Tropical Meteorology, Pune, and a senior monsoon researcher says that there is an increasing trend in the amount of northeast monsoon rainfall (NEMR) in the recent years and this rainfall posseses a lot of variability on inter-annual scale . But, he also agrees to the fact that it's not normal for a feeble low pressure (which impacted Tamil Nadu) to bring so much rain. He says that the total precipitable water for this system was about 1.4 times higher than the normal levels. According to him, even the weather models couldn't precisely capture the high rainfall associated with this system. He asserts that it will be too early to link this heavy rain event with climate change and more evidences will be needed to prove climate change's role. 

 

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21-year moving average of northeast monsoon (NEM) rainfall during 1901 to 2010. Dashed horizontal line indicates long-term average whereas recent increase in the rainfall is shown by red circle (Source: M Rajeevan et al 2012)

When asked about the influence of El Niño on Tamil Nadu rains, Dr. Rajeevan said that probability of getting a normal or above normal NEMR in an El Niño year is more than getting below normal NEMR. An analysis by Sreekala and others in 2011 states that there is 22 per cent chance of El Niño year being deficient NEMR as against 41 per cent chance of it being an excess NEMR year. So record El Niño in the Pacific is a sort of necessary but not sufficient condition to explain this year's excess NEMR. Between 2001 and 2010, there were four El Niño events (2002, 2004, 2006 and 2009). In 2002 and 2004, NEMR was below normal but in 2006 and 2009, it was slightly above normal. Also, in this decade, the statistical relationship between El Niño Southern Oscillation and NEMR has weakened as per Dr. Rajeevan. So El Niño's does have a role to play in this year's NEMR but its role isn't that straightforward. 

 

Role of vortex suspected 

 

Weather forecaster Rajesh Kapadia who writes a blog called "Vagaries of Weather" believes that Tamil Nadu heavy rains were caused by development of a vortex within the low pressure near surface levels. Vortex basically refers to rotating motion observed in a fluid (say air in this case). According to him, such vortex (or called vortices when in high number) developing within a low pressure can bring sudden heavy rain. Usually such vortex lasts till 3-4 hours after it's formed but in rare cases (like July 26, 2005, Mumbai rain), it can last even for a day which then brings such a huge rainfall. As such vortices have high unpredictability, weather models don't easily catch them which unnecessarily underestimates any rainfall forecast (say in the case of Chennai). 

 

A cocktail of weather conditions 

 

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GFS model map of winds at 20,000 feet above ground simulating the situation on December 1, 2015 (Courtesy: earth.nullschool.net)

 

The atmospheric setup on December 1, 2015, appears to be more complex than was anticipated. It's a cocktail of various atmospheric conditions. As can be seen from above image, a high pressure "H" (region of clockwise winds) anchored over eastern Bay of Bengal caused an inflow of moisture rich warm air from Bay of Bengal in Tamil Nadu. To represent this, I have used red dotted arrows. "C" refers to a cyclonic circulation (low pressure) which was observed east of Chennai. As monsoon has withdrawn from most parts of India, the air over central and northern parts of India is drier and colder than the warm moist air coming from southeast. Such an interaction of warm moist air from south-east and cold dry air from northeast crates a lot of instability in the atmosphere which is known to triggers strong thunderstorms. 

 

After a preliminary investigation, to me, Tamil Nadu's heavy rain event seems to be a result of a Mesoscale Convective System (MCS) which was pretty intense given the above unstable conditions. Mesoscale in meteorology refers to a scale or horizontal extent of about 1-100 km. We know about thunderstorms and some of which we come across are single cell thunderstorms (meaning their influence is restricted to a certain area only). Such thunderstorms mostly pop up during the afternoon hours, produce bad weather and die. But, when such individual thunderstorms cluster together, they become organised on a larger scale and can last for several hours. Such a large cluster of thunderstorms is known as MCS which is observed in many parts of world like United States of America. In satellite or doppler radar images, such MCS can be linear or round in shape (appearing sort of round in Tamil Nadu's case) and it often produces severe weather events such as heavy rains which lead to flooding. 

 

In Tamil Nadu's case, role of MCS can't be ruled out. In addition to this, development of vortices can't be ruled out as well which will be clear after investigation of doppler radar images. In the case of the September monsoon depression, such a large instability in the atmosphere or a cocktail of atmospheric conditions weren't observed. That's how Tamil Nadu low pressure system is different from other low pressure systems and perhaps this caused it to produce so much rain.

More rain for South India; storms return to central region

FROM THE NEWS EDITION: http://www.downtoearth.org.in/news/storms-to-return-to-central-india-rain-to-the-south-51919

(FIRST UPLOAD NOVEMBER 28TH 2015, UPDATED 30TH NOVEMBER 2015)

A new upper air trough has hit India, changing the weather conditions across the central and southern regions of the country

Thunderstorms are set to return in parts of Central India after they brought severe weather a month ago. A new upper air trough (see image below) is set to reach India, November 30 onwards, which will create conditions favourable for outbreak of storms. As the week progresses, this trough and associated thunderstorms will move eastwards, covering states like Rajasthan, Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Jharkhand and possibly Bihar. 

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Map showing winds in upper parts of troposphere on 1st December 2015. The axis of trough is shown as dashed red lines over Western India. (Courtesy- weatheronline.co.uk )

Troughs are elongated low pressure regions in Earth’s troposphere (lowermost part of atmosphere where weather systems exist) that are powerful enough to change a bright sunny morning into a stormy afternoon. 

The round of thunderstorms in Central India will begin from November 30, Monday. These will be experienced in the northern and western parts of Madhya Pradesh and South-eastern Rajasthan (Kota-Sawaimadhopur) for two days.

On December 2, a large region comprising of North-eastern Madhya Pradesh, Southern Uttar Pradesh and parts of Jharkhand and Bihar can get some rain. December 3 onwards, the weather will start normalising in Central India. These thunderstorms would be capable of bringing localised heavy downpours, lightning and some winds.

Possibility of hailstorms

The thunderstorms may turn severe, given the availability of required conditions. The freezing level (pressure level in troposphere with temperature of 0°C) has dropped below normal in Madhya Pradesh which will be ideal for occurrence of hailstorms. Hailstorms are anticipated in south-east Rajasthan (around Kota), Northern Madhya Pradesh (Guna-Gwalior belt) and neighbouring districts of Uttar Pradesh. They are expected to progress towards North-western Madhya Pradesh and adjoining parts of Uttar Pradesh on Wednesday.

As per agricultural experts, this round of rain will be beneficial to Rabi as it stands to improve the soil moisture. However, localised heavy downpours (or hailstorms, if they occur) will have a potential to damage the horticulture and harvested crops in these areas, especially Madhya Pradesh. After December 3, northerly winds will dominate in many parts of India including Central India and enable drop in temperature in these areas, which have been above normal in November.  

Wet week for Southern states

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District rainfall map of Tamil Nadu (1st October to 25th November period) showing districts with normal and excess rainfall. Courtesy- Indian Meteorological Department (Courtesy- Indian Meteorological Department)

Coastal and interior parts of Tamil Nadu recently witnessed heavy north-east monsoon rain which caused massive flooding. North-east monsoon period (October to December) is a major period of rain activity in Southern India and accounts for 48 per cent of Tamil Nadu’s annual rainfall as per a India Meteorological Department report.

Under the influence of a new low pressure system in southern Bay of Bengal, parts of Andhra Pradesh, Pondicherry and Tamil Nadu will witness rain in this week. Places in Tamil Nadu will get occasional moderate to heavy rain in this week that can create fresh water logging in cities like Chennai and add to existing problems due to flooding. There will be flooding potential in coastal areas of Andhra Pradesh as well.