Book Review of “Satellite Meteorology” by R. R. Kelkar in WMO Bulletin

The April 2009 issue (Vol 58, No. 2) of the WMO Bulletin published by the World Meteorological Organization, Geneva,  carries this review of the book “Satellite Meteorology” by Prof. R. R. Kelkar:

Satellite Meteorology

R. R. Kelkar. BS Publications (2007).
ISBN 81-7800-137-3.
xix + 251 pp.
Price: US$ 26

Observing our weather from space—referred to here as satellite meteorology (SM)—has revolutionized our understanding of how the atmosphere, ocean, land and cryosphere operate and interact as part of a system. By the unique virtue of being global, satellite data have radically transformed the way meteorologists perform numerical weather prediction (NWP). Today, gigabytes of radiances from satellites are routinely assimilated into weather forecast models.

This book traces the fascinating history of satellite meteorology and its application to NWP, starting from the beginning of the space era up to the current state-of-the-art sensors, providing the reader with a comprehensive introduction to remote-sensing, climate monitoring and weather forecasting, with a particular focus on Indian meteorology. In particular, the book covers (and illustrates with nice colour figures) a variety of remote-sensing topics ranging from the orbits of the satellites, the types of radiation they sense, the physical understanding of their measurement and the retrieval of ocean, land and atmospheric parameters, up to the exploitation of their data to study tropical weather systems and constrain, validate and initialize NWP models.

The book constitutes very good material for university students planning a career in physics or Earth sciences, as well as a reference for scientists involved in Earth system research or operational weather prediction, in particular over tropical regions.

The author, R. R. Kelkar, who [was] ISRO Space Chair Professor at the University of Pune, India, has a long experience in satellite meteorology at the India Meteorological Department, and has done a very good job in synthesizing this fast growing field, highlighting its potential,as well as the related challenges and opportunities.

Reviewed by Pierre-Philippe Mathieu

Marathi Article on Artificial Rain

An article in Marathi by Prof R R Kelkar on Artificial Rain was published in the Marathi newspaper “Sakal” from Pune on 13 August 2008. Click here to read

Forecasting Hurricanes and Tropical Cyclones

The American Meteorological Society, on 5 June 2007, issued a policy statement that outlines the state of science of hurricane forecasting in the United States, including hurricane-related hazards, observations, forecasting skills, and continuing challenges. The AMS is a professional body that is not under the U. S. National Weather Service, and it has an independent and authoritative voice. The AMS statement on U. S. hurricanes makes several important points, some of which are applicable to tropical cyclones in general, and are therefore noteworthy for countries like India, which are regularly visited by such storms.

1. Track forecasts: The most heartening aspect of the AMS statement is that there has been a significant improvement in U. S. hurricane track forecasting in recent years. During the 5-year period 2001-2005, the hurricane track forecasts issued by the National Hurricane Center (NHC) of the U. S. National Weather Service had an average error of 65 nautical miles (120 km) for the 24-hour forecast and 118 nautical miles (269 km) for the 48-hour forecast. These errors are about half of what they used to be in 1990.

2. Intensity forecasts: The AMS statement, however, admits that forecasting of hurricane intensity still remains a challenge to forecasters. During the past 30 years, there has not been any noticeable improvement in the forecasts of storm intensity. Large errors typically occur when storms strengthen or weaken rapidly.

3. Warning and over-warning: The statement notes that although the forecasting of hurricane tracks has improved, hurricane warnings continue to be issued for large coastal areas of the U. S. However, the average length of the coastline warned has come down to 510 km in the current decade from 730 km in the preceding one. While only one-fourth of the warning area may actually experience hurricane conditions, some over-warning is justified in the interest of safeguarding life and property. Over-warning ensures that unexpected rapid increases in storm strength prior to landfall or unanticipated changes in the distribution of damaging winds, do not take under-prepared populations by surprise.

4. Rainfall: Prediction of rainfall from landfalling hurricanes is yet another illusive factor, particularly because of the effects of terrain.

5. Storm surge: The highest loss of life due to a hurricane continues to be attributed to the storm surge, which can be as high as 6 m when a strong hurricane strikes a coastline with shallow water offshore. The statement notes that in recent decades, large losses of life due to storm surge had become less frequent in the U. S. However, the rapid growth of the coastal population and related infrastructure, and the increasing complexity of evacuation have led to a greater vulnerability of the coastal communities. This became evident in the case of Hurricane Katrina in 2005, in which the loss of life in Louisiana and Mississippi was estimated to be 1700.

6. Storm surge modelling: Given a sufficiently accurate forecast of the hurricane’s track and surface wind structure, and the required topographic and bathymetric data, numerical models should be able to make a precise prediction of the inundation by storm surge. However, the AMS statement cautions that because of the uncertainty in hurricane forecasts, evacuation decisions should not be made on the basis of individual runs of a single storm surge model.

7. Seasonal prediction: The statement acknowledges the low confidence associated with seasonal predictions of hurricane activity in the Atlantic basin, particularly when applied to smaller areas of the basin.

8. Preparedness measures: The statement says that in order to build up a higher resilience to the hurricanes, there is a need for greater involvement of other disciplines including engineering, ecology, biology, the social, behavioral, and economic sciences, and public policy. A comprehensive framework is also essential for ensuring public understanding of the hurricane threat and the ability to take appropriate action to mitigate the loss of life and property that links the entire process — from data collection to forecast to communication of the societal impact.

9. New technology and modelling: According to the statement, the forecasts would benefit from continuing improvements in the observational systems such as Doppler radars and satellites that can effectively observe the details of the storm core, data assimilation techniques and modelling capabilities on both the global and regional scale, particularly in ocean–atmosphere interactions. More effective remote sensing of ocean surface winds can also improve the initial detection of storms and their further analysis and forecasting.

The complete text of the AMS statement is available online at http://www.ametsoc.org/POLICY/2007hurricaneforecasting.html.  

A Rational View of Climate Change: (1) Tropical Cyclones

One of the pre-conditions for the formation of a tropical cyclone is that the ocean should be warm enough (27 deg C or higher). Tropical storms therefore tend to form only over certain ocean basins and in certain preferred seasons where and when this condition could get satisfied. An increase in sea surface temperature brought about by global warming should, logically speaking, lead to enhanced cyclonic activity. However, it should be remembered here that a warm ocean is only one of the pre-conditions for the formation of a tropical cyclone, and not the only one. Tropical storms do not exist by themselves but are embedded in the general atmospheric flow which does influence them a great deal. And it is not only the number of tropical storms that is important, but also the peak intensity that they reach, and the length and orientation of the tracks that they follow. Thus statistical correlations between global warming and the frequency of occurrence of tropical storms cannot be viewed in isolation and without due regard to these other aspects.

Nowadays, whenever there is a major tropical cyclone or hurricane, heightened scientific and media attention gets focused upon global warming as a cause behind the extreme event. Thus Atlantic hurricanes like Katrina, Rita, or the recent Dean, or major typhoons elsewhere, at once result in a “we-told-you-so” stand being taken by climate change enthusiasts. In this context, it is worthwhile to take a look at what tropical cyclone experts from around the world have to say about it. An International Workshop on Tropical Cyclones (IWTC-VI) was organized by the World Meteorological Organization at San Jose, Costa Rica, in November 2006. At the end of the Workshop, the participants issued a statement on the linkage between anthropogenic (human-induced) climate change and tropical cyclones. Since there were 125 delegates from 34 different countries and regions, and since the process was overseen by a committee of the WMO Tropical Meteorology Research Programme TMRP Committee TC2, the statement can be regarded as an authoritative and consensus view of the global community of tropical cyclone researchers and forecasters. The statement is remarkably balanced in its approach and findings and it is very categorical in what it says.

First of all, the consensus statement makes it very clear that no individual tropical cyclone can be directly attributed to climate change. The increasing socio-economic impact that tropical cyclones have been making in recent years is largely because of rising concentrations of population and infrastructure in coastal regions.

Another important point that the statement makes is that as of now, no firm conclusions can be drawn about the influence of global warming on tropical cyclones as there is equal evidence both for and against it.

The statement draws attention to the various difficulties in determining accurate long-term trends in the characteristics of tropical cyclones. The observed multi-decadal variability of tropical cyclones in some regions could be natural or anthropogenic or both. Methods of estimating wind speeds associated with tropical cyclone have undergone changes in recent years and different practices are followed in different regions. In most regions there are no observations from instrumented aircraft flying into tropical cyclones.

The statement accepts that if the climate continues to warm, some increase in tropical cyclone peak wind speed and rainfall is likely to occur. There is, however, an inconsistency between models which project small changes in wind speed and some observational studies which suggest large changes. Also, how tropical cyclone tracks or areas of impact may change in the future cannot be foreseen now.

The statement also refers to the increased vulnerability of coastal areas due to cyclone-related storm surge, if the sea level were also to rise because of global warming.

The text of the Summary Statement on Tropical Cyclones and Climate Change can be read on the web site of the World Meteorological Organization at http://www.wmo.ch/pages/prog/arep/tmrp/documents/iwtc_summary.pdf and the text of the complete statement is available at http://www.wmo.ch/pages/prog/arep/tmrp/documents/iwtc_statement.pdf.

Tropical Cyclones

An article entitled “’Eye’ing the Cyclone” by Prof. R. R. Kelkar was published in the March 2006 issue of Geospatial Today. It covers various aspects like satellite monitoring of tropical cyclones, advances in satellite technology, nomenclature of weather systems, disaster management, cyclone track prediction and influence of global warming. Click here to read.

Stormfury

The weather over north India in the winter season is largely controlled by what are known as ‘western disturbances’, so named because they approach the country from the west. Although these disturbances keep coming throughout the year, they are more predominant during the winter season. They are the primary source of winter rains over the country, which sustain the winter or rabi crops, the southwest monsoon having withdrawn by October. Western disturbances also produce heavy snowfall over the slopes of the Himalayan mountains, at times giving rise to avalanches and causing a disruption of normal life. As these disturbances move away eastwards and weaken, temperatures drop in their wake, leading to cold waves, frost and fog across large parts of India.

The path of the western disturbances can be traced from India to as far back as the Mediterranean Sea, where they originate as low pressure areas. If they can maintain their strength all the way to India, it is easy to imagine what fury they would have over the regions close to the Mediterranean Sea. One such region is what is presently known as Lake Tiberias or Lake Galilee, and described in the Bible as the Sea of Galilee. This is in fact not a sea but a fresh water lake about 166 sq km in size and situated 40 km to the east of the Mediterranean Sea. It is about 200 m below sea level and has steep slopes on all sides.

It was on the western shore of the Sea of Galilee, in a town named Capernaum, that Jesus had delivered the Sermon on the Mount, which was in a sense his inaugural address, containing the most significant of His teachings. On one of the early days of His mission in Galilee, Jesus had narrated the first few of His parables, and gone ahead with His work of teaching and healing. He was being followed by crowds who were gathering in large numbers. At the end of this busy day, Jesus wanted to have a time of rest and seclusion, and decided to go along with His disciples to the other side of the lake or the Sea of Galilee in a boat.

In the mean time, a low pressure area had perhaps been brewing over the Mediterranean Sea and had been moving eastwards on its long journey towards India, passing over the Sea of Galilee on its way. In the Bible, there are three parallel accounts of the storm that caught the disciples of Jesus by surprise later in the night. (Matthew 8:23-27, Mark 4:35-41 and Luke 8:22-25). Compared to what we now call a hurricane, the storm described in the Bible was a very insignificant one. It could have been just a squall. Yet it had left the disciples shaken and made them feel helpless. They felt all the more deserted because Jesus who was with them, had been sleeping soundly in the midst of all the chaos as if unconcerned with their plight. When they woke Him up and sought His help, Jesus got up and rebuked the winds and the waves, and commanded, “Quiet! Be still!”. That was enough for calm to be re-established. The disciples were astonished and said, “What kind of man is this? Even the winds and the waves obey him!”

There is always a tendency to view the events of this type, or miracles, narrated in the Bible, with a degree of skepticism, because they go against the laws of nature as we understand them. But in fact, the Son of Man was just doing, and succeeded in doing, something that Man has forever been trying to do. The most cherished human ambition is to have control over nature.

It is worth recalling that in the aftermath of several devastating hurricanes, the U. S. had launched Project Stormfury in 1960 with the specific aim of taming hurricanes. This experiment was based upon the concept that hurricanes could be weakened by dropping silver iodide into their wall clouds. The project continued for 20 years, but was eventually terminated after the results were found to be inconclusive.

God’s ways and man’s ways are not the same, and there is no reason why they should be the same. “For my thoughts are not your thoughts, neither are your ways my ways, declares the Lord” (Isaiah 55:8).

Pioneer British Meteorologists in India: (1) Henry Piddington

We are quite familiar with how a tropical cyclone looks like from space, through the images that weather satellites send down. Television news channels often present animated sequences of the images that clearly show the cloud bands swirling around the centre or the eye of the cyclone, anticlockwise in the northern hemisphere and clockwise in the southern hemisphere.

In the early nineteenth century, little was known about these violent tropical storms, except that they wrecked ships on the high seas and caused untold destruction and loss of life while crossing the coastline. But there was one man who had unravelled their structure and visualized their rotating nature. He was Henry Piddington, first a British sea captain, and then the President of the Marine Courts of Inquiry at Calcutta (now Kolkata). He had made a thorough investigation of a storm that had struck disaster on the east coast of India in December 1789, killing over 20,000 people. He presented his results before the Asiatic Society of Bengal at Calcutta in 1840, and described the storm as a ‘cyclone’, a name derived from the Greek word ‘kuklos’ meaning going around, or encircling, like the coil of a snake.

Piddington introduced this newly coined word in the books that he wrote soon thereafter on the laws that governed the tropical storms. In 1844, Piddington published a book entitled “The Horn-Book for the Law of Storms for the Indian and China Seas”. In 1848, he published an enlarged version of this book, with the title “The Sailor’s Horn-Book for the Law of Storms”. He is said to have published yet another book in 1852, entitled “Conversations about Hurricanes: for the Use of Plain Sailors”. This was written in the style of a ship’s captain training an apprentice sailor, about how to deal with storms, how to know that they are approaching, and how to take advantage of them. The book included transparent storm cards with wind arrows that could help the captain of a ship caught in a storm to sail with the wind into safer waters.  

Piddington was engaged in scientific research in varied disciplines like botany, geology, mineralogy and soil chemistry, besides of course meteorology. He published numerous papers in the Journal of the Asiatic Society of Bengal. He was also the Secretary of the Agri-Horticultural Society of India at Calcutta. At the suggestion of Rev. Dr. William Carey, of the Serampore Mission, who had similar scientific interests, he compiled the English Index to the Plants of India.

It is said that in 1854, Piddington wrote an open letter to Lord Dalhousie that the new port that he was building at Calcutta would not be able to withstand the fury of a tropical cyclone if hit by one.  Dalhousie, however, did not heed this advice. Port Canning was built, but it was indeed destroyed by a cyclone in 1867 and later abandoned.

By 1875, the name cyclone had gained the official acceptance of the international meteorological community. Today, however, tropical cyclones are called by this original name only in India and the adjoining seas, Arabian Sea, Bay of Bengal, and the Indian Ocean. Over other oceanic basins, they are now commonly known as hurricanes or typhoons.

Henry Piddington, who was born in 1797, died at Calcutta in 1858.

(This post is based upon information derived from various sources, printed and web-based, and some of the details may need to be corrected. – R. R. Kelkar, June 2007)   

Understanding the Language of the Meteorologist: (4) Low Pressure Systems

On a weather chart, the lines joining values of equal pressure are called isobars. Sometimes the isobars form a wave-like pattern, showing “troughs” of low pressure and “ridges” of high pressure. Meteorologists follow the movement of the trough from one region to another and also monitor the pressure to see if it lowers further as time goes by.

If an isobar on the weather chart assumes the shape of a circle or an ellipse, with pressure within it less than what is outside it, a “low pressure area” is said to have “formed”. This is just the beginning of a sequence of developments.

If the pressure falls further, two concentric isobars can be drawn, and the low is said to have “become well-marked”. The isobars are normally drawn at an interval of 2 hPa. Low pressure areas usually do not have a clearly defined centre. Winds around a low pressure area are light, of the order of 30 km per hour or less.

A well-marked low may “organize” into a “depression”. The depression could “concentrate” into a “deep depression”. Depressions and deep depressions could be over land as well as sea. In the monsoon season, the northern parts of the Bay of Bengal, called the Head Bay, are the area of formation of what are called “monsoon depressions”. These low pressure systems move inland and normally follow a west-north-west track all the way up to Rajasthan. Some systems may, however, take a more southerly track and go towards Gujarat, while others may recurve and end up on the Himalayan foothills. During their 4 to 5 days of journey, monsoon depressions give copious rains, particularly in their southwest sector. They may linger on over catchment areas, and cause heavy flooding of peninsular rivers. On an average, 4 to 6 monsoon depressions form during the monsoon season, but this number may vary greatly from one year to another. The number of depressions and their paths have an important bearing on the monsoon rainfall distribution over India.

The centre of a depression can be identified from the isobar patterns and satellite images and given in terms of its latitude and longitude over the ocean. Over land, however, it suffices to talk in terms of sub-divisions, saying for example, that a monsoon depression over Chhatisgarh is likely to move westwards into east Madhya Pradesh. Depressions over land are essentially heavy rain systems and do not cause damage due to winds, which are of the order of 30-50 km per hour in a depression and 50-60 km per hour in a deep depression.

Over the ocean, there is a good chance for a deep depression to “intensify” into a “tropical cyclone”. This does not happen over land, as only a warm ocean (sea surface temperature 27 deg C or more) is capable of providing the storm with the required energy to sustain it. As pressure falls, winds build up in strength to 60-90 km per hour and blow anticlockwise around the cyclone centre. If atmospheric and oceanic conditions remain favourable, a marginal tropical cyclone can grow into a furious and deadly storm. Meteorologists maintain a constant vigil on all tropical cyclones because of their destructive potential and use certain terms to describe their current and predicted states. A tropical cyclone is said to “intensify further” into a “severe cyclonic storm” with winds of 90-120 km per hour. The next stage to which it can “intensify still further” is called a “very severe cyclonic storm” in which the winds blow at a speed of about 120-220 km per hour. The term “supercyclone” is used when the winds exceed 220 km per hour. Supercyclones such as the one which crossed the Orissa coast on 29 October 1999, are rare phenomena.

Centres of cyclonic storms can be defined accurately from satellite and radar imagery, especially when an eye is seen. The centre is given in terms of the latitude and longitude. However, it is also the practice to give its distance from a well-known place on the coast, usually the place towards which the storm seems to be heading, so that people on the coast can perceive how far away it is from them. What is called a “very severe cyclonic storm” over the Arabian Sea and the Bay of Bengal, is called a “hurricane” over the north Atlantic Ocean and east and central Pacific Oceans, and a “typhoon” over the northwest Pacific Ocean. But for the names, all these systems have similar meteorological characteristics.

When tropical cyclones cross the coast, the process of weakening can be very rapid, but they do go through all the above stages in reverse order and finally dissipate.