The trouble with the weather

ADM-Aeolus will investigate the global wind profile
 
14 August 2002

ESA INFO 6-2002. Last year was characterised by extremes of weather all over the globe, making it the second-hottest year on record, beaten only by 1988. This year is set to follow that pattern, beginning with a major heat wave: during the first quarter of the year, temperatures were 0.71 degrees Celsius (1.3 degrees Fahrenheit) warmer than the mean for temperatures between 1961 and 1990. What's behind the apparent increase in weather extremes?

To answer this increasingly urgent question, we need precise and detailed information about the physical phenomena that determine our weather and climate. To meet this challenge, a new series of European geostationary meteorological satellites dubbed MSG ("Meteosat Second Generation") has been developed. This has been has been achieved through the close cooperation between the European Space Agency (ESA) and EUMETSAT, the European Organisation for the Exploitation of Meteorological Satellites.

With ESA's expertise in development of space technology and EUMETSAT's meteorological expertise and long-term operational perspective, the MSG system is set to provide an essential service for at least the next 12 years. The satellites will be operated by EUMETSAT and are expected to lead to a major leap in the accuracy that can be achieved in meteorological science and applications.

Humanity's age-old concern with the weather remains a perennial topic of conversation, particularly when the skies are in a capricious mood. A scientific definition of "weather" might be: "atmospheric conditions at a given time and a particular location". Forecasting the weather remains notoriously difficult because the atmosphere is not easy to predict, being affected by such factors as air pressure and temperature, air movements, the distribution of water in its various states in the atmosphere, and static electricity stored in the air. The continuous interplay of all these factors leads to the complex, and in some parts still imperfectly understood processes that underlie what we call the weather.

The two payload fairing half-shells are moved into position
MSG will help meteorologists produce increasingly accurate long-range forecasts

When a barometer will not do

Gone are the days when people used rising and falling barometer readings to predict the weather. Modern meteorology is very data-intensive. A continuous flow of meteorological data is provided by a global network of ten thousand ground stations, both land and sea-based, plus balloon-borne instrument soundings supplemented by mobile stations embarked in ships and aircraft.

In this way 80 million discrete data items are registered every day, feeding powerful computers that generate the global weather models. All of this happens in the troposphere, the lower layer of the Earth's atmosphere; this is where weather happens and most air circulates. The depth of the troposphere varies from 8 km (5 miles) at the poles to 17 km (11 miles) at the equator; in relative terms, given the size of the Earth, it is thinner than the skin of an apple.

What the impressive numbers don't show is that weather stations actually monitor only some 20% of the surface of our planet. Thus oceans, forming two-thirds of the Earth's surface, are covered either sparsely or not at all. With improved satellite observation the "blanks" in the weather map have progressively been filled in. The fact remains, though, that the global weather machine is vastly more complex than was thought just a few decades ago.

D+10

Meteorologists are under pressure to produce increasingly accurate long-range forecasts. Not one or two, but ten-day forecasts are being demanded by industry, agriculture, government and the public. Now the value of a weather forecast is a direct function of input data resolution, both spatial and temporal, and the computing power available. Forecasting is also based on analysis of historical trends; so the necessary statistical work provides both the basis for climatological studies and a long-term framework for examining weather and weather trends. As a result, weather forecasting has largely shaken off its historical hit-and-miss character.

Medium-range forecasts rely on a combination of practical meteorology, atmospheric physics and applied mathematics. Qualitative improvements require continuous measurement of large amounts of atmospheric data at the best possible spatial and temporal resolution. Most ground stations continue to measure weather conditions at a height of two metres (six feet) or less above the ground. Also, twice a day instrument packages attached to lighter than air gas balloons are released from stations around the world to measures profiles of atmospheric temperature, humidity, pressure plus wind direction and speed from the surface up to the stratosphere. But the instruments measure conditions only along a path where the balloon is taken by the winds at various levels in the atmosphere.

The sheer size of the remaining, unexplored portion of the troposphere gives some idea of the potential for improving forecasting models.

Europe viewed by Meteosat
Europe viewed by Meteosat

The weather seen from space

To tap this potential, satellites equipped with meteorological cameras were launched into orbit beginning in the 1960s. It became possible to assemble a complete global picture of the Earth's atmosphere. In 1977 the European Space Agency launched its own weather satellite, the first generation Meteosat, into geostationary orbit, marking the start of a success story that continues to this day.

Most weather satellites rely on specialised instruments called radiometers, which allow the Earth's surface to be viewed in discrete bands of the spectrum. Meteosat provides full images in three bands twice every hour, for continuous updating of weather forecasts: the visible band shows cloud formations, the infrared band provides temperature information, and a third band registers atmospheric water content. "Everyone knows Meteosat from the weather picture on the evening news," says ESA meteorologist Andreas Ottenbacher. "But far more valuable are the inputs to databases on wind strength, direction and formation. This kind of information is only possible with geostationary satellites, because they can provide uninterrupted coverage of a region."

ESA started planning an operational system of geostationary Meteosat spacecraft for meteorological observations in 1983. As part of those plans, a separate operational agency called EUMETSAT was created, which today counts eighteen European countries as members. Since 1995 EUMETSAT has been operating the Meteosat system on its own, while ESA is responsible for design, development and construction of new satellites.

At its Darmstadt centre, EUMETSAT processes raw satellite data, for use by national meteorological services, scientists, educational users, commercial customers and amateurs. Such a system does not come cheap: provision of the services from Meteosat costs some 10 million Euro per year (roughly 10 million US dollars). But the economic benefits are even greater: they have been estimated at an annual 140 million euros in Europe alone.

Artist's view of MSG launch and early orbit phase
Artist's view: MSG's critical launch and early orbit phase

The second generation: Meteosat's eagle eye

This is still not enough, say meteorologists. They want more information, and they want it faster. ESA and EUMETSAT have been preparing a new group of three advanced meteorological satellites known as Meteosat Second Generation, or MSG. The first, MSG 1, is planned for launch to geostationary orbit this coming August. It will be the most advanced satellite of its kind.

MSG's eagle eye is the Spinning Enhanced Visible and Infrared Imager, or SEVIRI. This is a state-of-the-art radiometer that 'looks' at the Earth's surface and cloud cover in twelve different channels (rather than three channels as before). That means an enormous improvement in the level of detail in the data that feeds the mathematical models which generate weather forecasts. This is a boon for meteorological research and day-to-day predictions alike. The SEVIRI also provides very high resolutions (down to 1 km from previously 2.5 km in the broad band), and it is fast: new image data is transmitted every fifteen minutes, a 100% improvement over the existing Meteosat system.

Ottenbacher is enthusiastic about the powerful new machine: "Having high-resolution images from the SEVIRI every fifteen minutes will greatly enhance nowcasting and short-range forecasting. Around-the-clock coverage of cloud and fog formation will translate into earlier and more detailed predictions about what frontal systems are doing."

Thomas Böhm, an expert in satellite data analysis employed by the German weather service, explains: "The twelve spectral channels were selected to maximise the information we will be able to extract. For example, one of the channels allows us to discriminate between ordinary clouds, composed of water droplets, and clouds formed of frozen ice crystals. Using this information we can provide detailed predictions of thunderstorms and hail for users of road, rail and air transport."

MSG will provide a ten-fold boost in the quantity of raw data; but before the data can be used, it will need to be interpreted with the aid of specially developed software. "Analysis of the data is highly automated," says Böhm. "The different types of cloud have been very precisely coded so we have a continuous overview of the global weather, even at night."

Indispensable weather

The daily weather reports familiar to us from radio and television are only the tip of the iceberg in forecasting services. In the public and private sector alike there is an enormous demand for specialised forecasts. For example, airline pilots need detailed information about high-altitude weather along the flight path, while project managers on large construction sites need reliable advance warning of storms for safety reasons. To meet these needs, national meteorological services provide forecasting services for maritime and air traffic, for agriculture, and so on. Detailed climate information is compiled for the benefit of architects and engineers drawing up plans for manufacturing facilities; climatological studies are performed for official certification of resorts and recreational areas; and the list goes on.

In some cases, only a yes-or-no answer will do. Will the cumulus formations result in a thunderstorm later today, or will they dissipate harmlessly? Will precipitation over the next two hours exceed runoff capacity around my construction site?

Stormy weather

26 December 1999: the holiday spirit is rudely dispelled in Europe, as powerful storms sweep across France, Germany and Switzerland. In the Zurich area, they topple two large construction cranes, seriously damaging surrounding buildings. Insurance companies start to ask tough questions like: "Were suitable precautions taken to secure the cranes against the risk of strong winds?" The builders considered the likelihood of such a storm to be negligibly small; were they negligent in doing so? Were forecasters at fault?

In the construction industry, the accuracy of weather forecasts can translate directly into major profits or losses. Rain and frost can seriously hamper work in the open, and for some areas of work a guaranteed period of good weather is a must. Construction schedules are thus highly forecast-driven, the objective being to make maximum use of every hour of favourable weather available, even if this means working overtime. Decisions on dispatching road construction crews are taken on the basis of the morning's weather bulletin; depending on the prospects, concrete-pouring operations may have to be postponed, because quality is affected by weather conditions. Customers in the construction industry rely on accurate, highly localised forecasts for a precise date and time. This is one example of the kind of service that will benefit from the meteorological information provided by MSG.

Budgeting for the Christmas goose in Berlin

The natural gas industry is another customer with a keen interest in the weather. In dry sunny weather the providers of natural gas can reduce the pressure in the gas mains; conversely, demand rises as soon as the temperatures fall. Utility companies need an analytical forecast that is tailored for predicting energy consumption patterns. The spokesperson for Berlin's energy utility Gasag, Josiette Hennef, explains: "We know from experience what the demand peak is going to be at Christmas, when households all around the region bake the traditional Christmas goose. What we don't know, and this is where we rely on the forecast, is how the demand for heating gas is going to change from one day to the next. We receive natural gas from Russia and from Norway, and we have some storage capacity to handle limited peaks; but our system is part of a complex European grid in which the member utilities are all interdependent."

Accurate forecasts are vital for the financial planning of utilities like Gasag. The grid, composed of thousands of miles of pipeline and a network of pressure-boost stations, reacts to change sluggishly, so operators have to budget their requirements carefully. If gas utilisation increases, the line pressure has to be raised; but a miscalculation may lead to an excess gas supply, which has to be liquefied in an involved and expensive procedure so it can be stored temporarily.

How much sun?

Photoelectric arrays, converting solar energy to electrical power, are an expensive but increasingly popular investment. The attraction is more than a simple matter of economics. Energy and meteorology are combined in a new discipline, as Detlev Heinemann, professor at Carl von Ossietzky University in Oldenburg, Germany, explains. The basic idea is to use precise statistical information about solar irradiance and wind strengths at different times of the day and of the year for a given location to get the maximum benefit from wind and solar power sources. Meteosat is a valuable tool, according to the scientist: "MSG's spectral resolution will allow us to determine the composition of the atmosphere in terms of water vapour, aerosols and clouds, so we can calculate the average distribution of solar energy without actually measuring it on the ground."

Irradiation data obtained from satellite imagery is already being used to verify the efficiency of solar power systems and recommend improvements. Home owners who have fitted a photovoltaic array to the roof of their houses can subscribe to have an assessment performed, for a modest annual fee. This is eminently sensible in countries such as Germany and France, which subsidise solar power. "When the German government introduced its subsidised solar power scheme, builders and homeowners frequently made mistakes," reports Heinemann. "Satellite data allowed us to recommend remedies, but in fact that information was available from the very beginning..."

Scientific farming

Gone are the days when weather forecasting for farmers essentially meant relying on folklore. The intense dependence on weather that has characterised rural life through the ages has not changed. However, many types of agricultural work are already being optimised with the use of accurate short-term forecasts. For example, pesticide use can be minimised by applying the chemicals at the start of a protracted dry spell, lest they be washed off the plants by rain. This reduces the need for repeat spraying.

Machinery syndicates formed to share expensive agricultural equipment also benefit from accurate weather forecasts, since demand for harvesters, for example, is strongly weather-dependent.

Two of the SEVIRI's spectral channels are used to track another interesting parameter: the chlorophyll content in vegetable matter, closely correlated with harvest yields. Since the chlorophyll index can be used to predict crop failures as well as successes, it is of great interest not only for farmers but also for crop auctions, traders and banks. The value of traded commodities can be sharply affected by predictions about crop yields, which may signal that a particular country has a crop surplus to export, rather than importing in a given year.

Performance under pressure

Major sporting events rely heavily on reliable weather forecasting information. The prime example is Formula One racing: the weather can be the crucial factor, bringing an exciting element of science - and chance - to the race. Racing teams are therefore generally advised by a meteorologist. For example, the Swiss Sauber-Petronas team begins receiving regular specialised weather bulletins on the Wednesday preceding a racing weekend, so they can prepare for any eventuality, rain or shine. The main significance of the weather is that racing tyres are specially designed for a specific race-course, a specific temperature range and, most crucially, for a wet or a dry surface. The temperature forecast is also used to fine-tune the engine cooling systems of the powerful F1 racers.

In the hours preceding the start of the race, refined short-term forecasts are prepared at shorter intervals, and continued after the race has actually started. Technicians keep one eye on the forecast, ready to change tyres to adapt to the unfolding situation. The Swiss team manager, Beat Zehnder, says that his team's forecasts achieve 90% reliability. "For the residual 10% it's between us and lady fortune," sighs Zehnder.

Sowing the wind...

Weather phenomena of all types influence our lives and our welfare. Weather extremes are on the rise, this much appears to be certain. Apart from more frequent storms, hurricanes and tornadoes, there are the statistical anomalies: heat waves, heavy rains, hail, drought and cold spells. Annually, 40,000 storms are estimated to take place, world-wide. Europe's largest re-insurer, Munich Re, registered the most costly hailstorm ever in Europe, over Munich on 12 July 1984: the damage was estimated at €1.54 billion. At the end of 1999 the winter storm "Lothar" outdid even that, causing a total of €6.14 billion in damage.

Insurers around the world are increasingly concerned at the growth in economic damage arising from natural disasters such as storms and floods. Munich Re have calculated that such damages increased almost ten-fold from the 1960's to the 1990's, reaching a total of €614 billion during the period from 1990 to 1999. The trend, says Gerhard Berz, head of Munich Re's earth sciences risk assessment group, is for total damages from natural disasters to double every ten years or so. Will this worrying trend continue? Only time will tell. But MSG will be a valuable ally in the fight to predict and help mitigate or prevent damages from natural disasters.

... reaping the benefits

The enormous volume of data that MSG will feed into existing statistical models also holds the potential for improving predictions of long-term climate change and its effects.

Meteorology will reap enormous benefits from this order-of-magnitude increase. But there are other potential benefits that we cannot quantify at this stage. Hendrik Stark, a scientist who is system and operations manager for the project, explains that the sheer complexity of MSG means it is just "the start of an evolutionary process of elaborating new weather forecasting and climate modelling techniques," and adds that Europe stands to benefit directly from those improvements. ESA's Ottenbacher points to the potential inherent in the sophisticated European system: "Once research centres and university departments get their hands on the MSG data, we can expect entirely new domains of meteorology science to emerge. I firmly believe that within two to three years, we will see new types of products and services being designed that we can't even imagine at present."

A promising outlook. But in the meantime, the weather remains, as always, difficult to predict. MSG is a joint programme of the European Space Agency, ESA, and the European Organisation for the Exploitation of Meteorological Satellites, EUMETSAT. ESA brings to the programme its expertise in satellite technology and in programme management for major space missions. EUMETSAT, with its meteorological expertise and operations experience, defined the user requirements for the new system. It has also developed a completely new ground processing and satellite control system which it will operate for at least the next 12 years. Data from MSG satellites will be processed by EUMETSAT in Darmstadt and at specialised Satellite Application Facilities throughout Europe. Users will receive data and processed meteorological products via the communications package on board MSG satellites and the Global Telecommunication System of the World Meteorological Organization.

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