Modern meteorology provides essential scientific and technical services

For the fans of a particular brand of German chocolates, the critical limit is 27 degrees Celsius. The manufacturer in Frankfurt suspends its chocolate production if the temperature rises beyond its summer threshold. Otherwise, the company cannot guarantee the form, appearance and quality of its chocolates.

But how can the company be sure that the days with temperatures of more than 27 degrees are truly over? The German chocolate producer has an easy answer to this question working with the Deutscher Wetterdienst (DWD – National Meteorological Service), one of the top five services in the world. This aspect of its production is so important that every year in August the company sends a delegation to Offenbach to find out more about the current weather situation and to obtain forecasts for the various regions of Germany.

Generally speaking, weather reports are the media’s most popular service thanks to modern meteorology. The Deutscher Wetterdienst in Offenbach is an important source. The quality of a modern service depends on the data from measuring stations throughout the world, remote information such as satellite data and a high performance computer managing more than six billion mathematical operations per second, performance which could be compared to 30,000 connected home PCs. With this technology DWD really knows how to monitor weather developments and how to spread warnings and forecasts.

About one million new data day by day

Exact temperature forecasts are just one example of the different special services that modern National Meteorological Services (NMS) like DWD offer to their clients. For instance, the German National Railways requests forecasts of wind velocities to be able to adjust the speed of their fast trains that are affected by crosswinds. There is also close co-operation with airports and especially airline companies. They need to plan the best routes to “ride the jet stream” to save fuel and money.

Services are also provided to many other groups as well, including balloon pilots, shipping lines, farmers, hobby fishermen or insurance companies. Many now have the relevant data sent directly into their databases via the Internet.

There is hardly anyone who is not interested in weather and hardly any area of our lives not affected by weather and climate. The DWD records, analyses and monitors the physical and chemical processes in our atmosphere and in its capacity as reference for meteorology in Germany, is in the position to answer any questions concerned with weather and climate. The DWD holds information on all meteorological occurrences, offers an extensive range of services for the general public and operates the national climate archive.

The fact that DWD can supply such a wide variety of groups with information about the weather is due to the extensive amount of data that it collects. With nearly 2,500 measuring locations positioned throughout Germany – more than 100 of these are manned – the staff in Offenbach oversee one of the densest networks of data in the world.

The acquisition of reliable meteorological data provides the basis for the operational work of all NMS worldwide. So day by day everything starts with collecting current data from DWD’s high resolution meteorological network and the data from all over the world which are switched between all NMS via the Global Telecommunication System (GTS) within minutes. Most data are stored in the DWD archives for an unlimited period of time. The storage capacity is at present approx. 1,200 terabytes. The archive has to be recopied every 10 years or so, due to new read-write technology. This “treasure” is the everlasting memory of the DWD.

The communication tasks of a National Meteorological Service (collection of measured and observation data, data exchange, dissemination of the data) require reliable communication services with a high degree of availability and performance. The DWD has also taken over the task of network management centre for the German Federal Administration for Transport, Building and Urban Affairs, as it also operates an ATM backbone ring.

The observation and measurement of the current state of the atmosphere are the basis for solving all questions that have to do with weather and climate. The data are not only acquired by observing weather conditions at the weather stations, but also by means of other numerous methods and procedures, thus providing an almost complete monitoring of all weather events. The experts take what they can get: data from ships, drifting buoys and aircrafts. For over 30 years now, remote sensing methods like balloon-sondes, radar, sodar/lidar and satellites have been used in addition to conventional measurements.

Irreplaceable in detecting clouds and precipitation

Let’s have a closer look to the weather radar (RADAR = RAdio Detecting And Ranging) one of the most important tools. DWD maintains a network of 16 weather radar systems which cover Germany, the so-called Radar-Net. Conventional precipitation observations for example are only spot measurements in the spatial and temporal course of a precipitation event. However, radar information allows full-coverage weather monitoring, both locally and regionally. Modern systems supply data on the distribution of precipitation with high spatial and temporal resolution. When the radar images from various sites are combined to form an overall image, the so-called composite image, then further possibilities for weather analysis and forecasting are opened up.

Weather radar systems are the most important aid for meteorology and hydrology in measuring a real precipitation and observing its development and direction of motion. By calibrating the weather radar systems, it is possible to achieve quantitative precipitation measurements. DWD’s Radar-Net of course is part of a European wide composite.

One of the further possibilities is FeWIS, the weather information system for fire brigades. According to the Law DWD has the task of issuing warnings on weather phenomena that can become a danger to public safety and order. The DWD cannot, of course, prevent the dangers to life and property, but it can make sure that the organisations that can carry out preventive measures are informed in good time of exactly what to expect. Only in this way can it be guaranteed that they can plan and carry out their operations properly.

FeWIS was developed and tested by the DWD together with the fire brigade of Berlin in a pilot project in 2003. Since then the DWD has been providing a closed user group on the Internet with all the necessary information on weather and severe weather. The main feature of FeWIS is the warning summary which gives the user a quick and comprehensive survey of all relevant warnings. FeWIS provides a whole range of information tailored to the individual user’s warning requirements. From the report on the general warning situation, to the advance warning, up to the weather and severe weather warning, all the important data for their area can be seen at a glance.

In addition, FeWIS provides a wealth of information, which can be of help in decision-making in other areas apart from an emergency situation, such as information on the latest weather situation, the latest forecasts, access to weather radar, calculation of pollutant dispersion, forest fire danger index and round-the-clock advice from trained meteorologists via the DWD hotline.

Furthermore, there are lots of elaborate computer programs that ultimately process all the incoming data into useful information. Their simulations allow relatively reliable overview as well as forecasts. The simulation of the weather all over the world is not possible without high-performance computers. The German Meteorological Computing Centre in Offenbach is one of the world’s top performers. The system currently in use is the fifth generation of its kind. Supplying the many internal and external customers requires up-to-the-minute production and data dissemination via regional servers to the approximately 2,500 IT workstations. The operation of all systems is monitored round the clock.

With all that stuff DWD’s Computing Centre needs about one hour to develop a simulation of the actual atmospheric processes worldwide with the object of deriving a prognosis of the future development, based on the present conditions using a tremendous huge package of knowledge in physics and mathematics. The temporally and spatially irregularly distributed observation data are processed with the aid of an assimilation procedure and transformed on to more than 15 million grids of a virtual global three-dimensional net, reaching up 30 kilometre from earth’s surface. This is the base of the following process called “numerical weather forecasting”. With the exception of forecasts for extremely short periods of time, all modern weather forecasts nowadays are produced on this basis.

Simulating the “chaos”

Experts say weather is a “system of chaos”. Some say weather isn’t really predictable. The simulation of atmospheric processes on a computer with the aim of taking their latest state to derive a prognosis of the future development is called numerical weather forecasting. Nowadays all weather forecasts are compiled on this basis, with the exception of extremely short forecast periods. The basis for the practicability of such simulations is that the atmospheric events – and thus the development of the weather – can be described by natural physical laws. The mathematical formulation of these natural physical laws leads to a set of equations that describe the temporal change of the atmospheric state variables (e.g. air pressure, wind, temperature).

Unfortunately, the mathematical form of these equations is so complex (nonlinear partial differential equations) that an exact analytical solution for determining the future state of the atmosphere is not possible. This can, however, be solved approximately with the numerical procedure. For this purpose all necessary variables are shown in a grid spanning the atmosphere. The atmosphere is thus described by a finite number of these variable values. Based on the initial state, the temporal development of the distribution of the variables on the grid points can then be computed approximately with a numerical solution procedure for the set of equations.

Precondition for the stability of the solution procedure is a step by step computation of the temporal development over short time intervals called “time steps”. The best size of the time steps depends on the spatial distance between the grid points (grid length) and the solution procedure used. This procedure allows forecasts up to ten days. After that period the quality of the results is decreasing rapidly.

The numerical solution of the model equations requires therefore, as first step, the definition of the model grid. This is made difficult by the fact that the spatial and temporal structure of the weather-relevant processes in the atmosphere is very variable. In addition to the large scale distribution of the high and low pressure areas (with their characteristic dimensions of several 1000 km), which determine the general weather situation, there are also small-scale phenomena such as heat thunderstorms (with characteristic dimensions of a few kilometres) which within their scale can have a dramatic influence on weather events.

Consequently, the finer the resolution of the model grid, the better. Ideally the high resolution grid should span the whole of the Earth. This requirement results from the global interaction between the weather relevant processes occurring in the atmosphere and on the ground. A restriction of the model area to any one region not only limits the spatial validity of the numerical forecast, but also leads to incorrect simulations within the model area when atmospheric developments, which originate outside the area, penetrate the area during the course of the forecast period.

This causes great distortions particularly in longer term forecasts with a so-called limited area model. The demand for a global model area with simultaneous fine resolution of the grid stands as limiting factor in direct opposition to the finite power of the computers that are available. As computation increases with the number of grid points, but a forecast for a period of several days has to be completed within a few hours’ calculating time, the number of model grid points should be selected to correspond with the capacity of the computer. That’s why those meteorological computer systems in most cases are the world’s biggest. After seven or eight years it has to be replaced.

The models used at present at the DWD on a routine basis solve this problem by embedding a high resolution local model (LMK) with a grid length of 2.8 kilometre for the central European area in the global model (called GME) with its grid length of 40 kilometre. During the forecast the larger scale atmospheric state variables of the corresponding GME prognosis are given at the edge of the LMK. The DWD makes another one, its HRM model (High resolution Regional Model) available to other Meteorological Services, especially in developing and newly industrialised countries. The model is being used at present by 24 institutions worldwide for their regional operational weather forecasts.

More and more data in the future

Nowadays the flood of data from different sources is constantly growing. And the next, even bigger flood has arrived. In 2006 the new joint European satellite Meteosat-9 began regular operations, thus replacing its predecessor. The geostationary satellite, positioned some 36,000 kilometres above Africa, has been supplying images of the Northern Hemisphere ever since. The new type not only delivers new images at double rate, but it also registers data from 12 wave-length ranges of light, while the old Meteosat generation was only able to receive on three spectral channels.

Meteosat-9 is able to investigate stratospheric winds, ozone concentration on a continuous basis and more. But the experts in Offenbach realised they got 20 times more data. And the story continues. Early 2007 the new European polar orbiting satellite METOP-A has started its operational program. METOP A is using 2000 spectral channels which requires again much more computer performance to use all the advantages of METOP.

DWD’s experts often say “The better the data, the better the forecast.” In the last decade the quality of weather forecasts raised up again. The statistical probability of an exact forecast for the next 36 hours is today higher than 90 %. That is much better than all forecasts for stock trading. Of course the direct earnings of the DWD do not completely cover the costs. A lot of services are completed for free. However, international studies commissioned by weather services have calculated a cost-benefit ratio of 1:20. This means that each Euro invested in a weather service helps save 20 Euros somewhere else – for example, by avoiding storm damage thanks to early warnings. And sometimes it just will help you to be sure to get your beloved chocolate in good shape and quality.

Published: 01st Jun 2007 in AWE International