Back to basics: From Sputnik to Envisat, and beyond: The use of satellite measurements in weather forecasting and research: Part 1 A history

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1 Back to basics: From Sputnik to Envisat, and beyond: The use of satellite measurements in weather forecasting and research: Part 1 A history Roger Brugge 1 and Matthew Stuttard 2 1 NERC Data Assimilation Research Centre, University of Reading 2 Logica UK Ltd On 28 February 2002 the European Space Agency (ESA) launched Envisat, a research satellite designed to monitor the earth s environment on a global scale. Data from Envisat will support earth science research and allow the monitoring of the evolution of environmental and climatic changes, providing a continuity that will continue the measurements of previous research satellites. Furthermore, they will facilitate the development of operational and commercial applications. The current operational Meteosat series is nearing the end of its expected life, and a replacement satellite series, the Meteosat Second Generation (MSG), is due to commence operating soon. MSG-1 was launched in August 2002, with a second satellite, MSG-2, due to follow in 18 months. Each satellite will have a nominal 7-year lifetime. During 2005, ESA will be launching METOP, an operational meteorological satellite capable of high-resolution measurements for use in weather forecast models. It seems timely, therefore, to assess the role of satellites in meteorology and to examine some of the information that is, and will become, available to forecasters and researchers from meteorological satellites. In this first article, we will describe some of the history of meteorological satellites and the current practical use of operational satellites. Subsequent articles will describe some of the data that can be obtained currently from research satellites, and also the instrumentation on board Envisat. On 4 October 1957 Russia launched the world s first satellite, Sputnik 1. Apart from achieving the aim of being a world `first, Sputnik 1 was launched with the aim of studying the ionosphere, and was the first in a long line of satellites that have had, as part of their mission, the objective of monitoring the atmos- 107

2 Table 1 Operational satellites currently (May 2002) used to provide meteorological data for numerical weather prediction models Satellite programme Current satellite Longitude View Geostationary satellites Meteosat Meteosat-7 08 Eastern Atlantic, Europe, Africa INDOEX Meteosat-5 638E Asia, Indian Ocean, Africa GOMS/INSAT INSAT-1D 748E Asia, Indian Ocean, eastern Africa INSAT INSAT-2E 838E Asia, Indian Ocean Feng-Yun Feng-Yun-2B 1058E Asia, Indian Ocean, Australia GMS GMS E East Asia, western Pacific, Australia GOES (WEST) GOES W Eastern Pacific, North America GOES (EAST) GOES-8 758W North and South America, western Atlantic Polar-orbiting satellites NOAA-14, NOAA-15, NOAA-16, Meteor-2, Meteor-3, Feng-Yun-1C Since the continuity of data from these satellites is important, many of them have back-up satellites, which are not listed here. Further details can be found at and phere. Two years later, the USA launched Explorer 7, the first satellite with an instrument payload designed for studying the meteorology of the upper atmosphere. Since those early days, a series of satellites has been used to monitor the earth s atmosphere, of which the Geostationary Operational Environmental Satellite (GOES), Meteosat, and the National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites are, perhaps, the most well known of the satellite systems in current operational use. Operational meteorologists rely upon information from a suite of meteorological satellites that provide both imagery and numerical data that can be used as input to weather forecast models. These satellites are shown in Table 1. As the table shows, there are two basic orbits that are used for meteorological satellites, namely geostationary and polar-orbiting. These papers discussed the difference between these two types of satellite (and how their data are used in weather forecast models), and also highlight the uses of research satellites (with emphasis on Envisat). Polar-orbiting satellites The world s first purpose-built meteorological satellite, a polar-orbiting satellite, was launched on 1 April Named TIROS (Television InfraRed Observation Satellite), it demonstrated the advantage of mapping the 108 earth s cloud cover from satellite altitudes. TIROS showed that clouds banded and clustered in unexpected ways (see Fig. 1). Sightings from the surface had not prepared meteorologists for the insight into cloud patterns that the view from an orbiting satellite would give, and that over the following years would become a familiar sight to everyone with a television (and later with Internet access). Ten TIROS satellites were launched, followed by the Environmental Science Services Administration (ESSA) and Improved TIROS Operational Satellite (ITOS) series. From October 1978 to July 1981, satellites in the TIROS-N series were launched. The `N represented the next generation of operational satellites including NOAA-6 and NOAA-7, which were launched during this time. Flight of the Advanced Very High Resolution Radiometer (AVHRR) and TIROS Operational Vertical Sounder (TOVS) instruments started on TIROS-N. On 28 March 1983, the first of the Advanced TIROS-N (ATN) satellites, designated NOAA-8, was launched. These satellites are physically larger and have more power than their predecessors. NOAA continues to operate the ATN series of satellites today with improved instruments, including NOAA-14 and NOAA-15 currently in orbit. NOAA-15 is the first in a series of five satellites with improved imaging and sounding capabilities that will operate over the next decade. Polar-orbiting satellites offer the advantage

Fig. 1 The first television picture of the earth s atmosphere from space, produced by TIROS, 1 April 1960 (courtesy of NOAA and the National Climate Data Center ± obtained from http://www5.ncdc.noaa.

This is achieved by making nearly-polar orbits, circling the earth approximately 14.1 times daily.

The orbits of these satellites are circular, with an altitude between 830 and 870 km, and are sun-synchronous.

3 Fig. 1 The first television picture of the earth s atmosphere from space, produced by TIROS, 1 April 1960 (courtesy of NOAA and the National Climate Data Center ± obtained from directive=quick_results&pop=yes) of daily near-global coverage from an almost constant altitude while keeping solar illumination as constant as possible. This is achieved by making nearly-polar orbits, circling the earth approximately 14.1 times daily. Since the number of orbits per day is not an integer, the sub-orbital tracks do not coincide from one day to the next. The orbits of these satellites are circular, with an altitude between 830 and 870 km, and are sun-synchronous. The sun-synchronous capability means that the descending node of each orbit views the earth at almost the same local time during the day, and the ascending node views the earth at the same time of night. An example of the high resolution available from the polar orbiters is shown in Fig. 2. A suite of instruments on board each NOAA ATN satellite is able to measure many parameters of the earth s atmosphere, surface and cloud cover. As a part of the mission, the satellites can receive, process and retransmit data from search and rescue beacon transmitters, and automatic data-collection platforms on Fig. 2 High-resolution imagery available from the polar-orbiter satellites. The resolution is approximately 1.1 km per pixel; the image was received on 24 February 1998 (courtesy of the NERC Satellite Station, University of Dundee, and taken from their website 109

Note that the term `surface can mean the surface of the earth, the upper surfaces of clouds, or the surface of a body of water.

4 land, on ocean buoys, or aboard free-floating balloons. The primary instrument on board the polar-orbiting satellite nowadays is the AVHRR. The AVHRR is a radiation-detection imager that can be used for remotely determining cloud cover and the surface temperature. Note that the term `surface can mean the surface of the earth, the upper surfaces of clouds, or the surface of a body of water. This scanning radiometer uses six detectors that collect information in different wavelength bands, enabling measurements of daytime cloud and surface mapping (0.58± 0.68 mm), land± water boundaries (0.725± 1.00 mm), snow and ice presence (1.58± 1.64 mm), night-time cloud mapping and sea surface temperature (3.55± 3.93 and 10.3± 11.3mm), and sea surface temperature (11.5± 12.5 mm). As a result of the characteristics of a polar orbit and because their sensors have a wide field of view, these satellites are able to collect global data on a daily basis for a variety of land, ocean, and atmospheric applications. Data from the NOAA series of polar-orbiting satellites support a broad range of environmental monitoring applications including weather analysis and forecasting, climate research and prediction, global sea surface temperature measurements, atmospheric soundings of temperature and humidity, ocean dynamics research, monitoring volcanic eruptions, forest fire detection, and global vegetation analysis. Geosynchronous satellites Images of whole earth discs are taken routinely by, amongst others, EUMETSAT s Meteosat, and NOAA s GOES series of satellites from geostationary orbits about km above the equator ± see Fig. 3. `Whole earth images are composited on a half-hourly basis. A geosynchronous orbit may be defined as one with an orbital period matching the rotation rate of the earth. This period is a sidereal day, which is 23 hours 56 minutes 4 seconds in length, and represents the time taken for the earth to rotate once about its polar axis relative to a distant fixed point. This time is about 4 minutes shorter than the civil day length, which is relative to the sun. A satellite is in a geostationary orbit when it appears stationary 110 Fig. 3 The GOES-2 meteorological satellite (courtesy of NOAA Photo Library, at index.html) from the point of view of an observer on the earth s surface. This can only occur when: (i) (ii) (iii) the orbit is geosynchronous; the orbit is a circle; and the orbit lies in the plane of the earth s equator. Note that a geostationary orbit is a special case of a geosynchronous orbit. Unfortunately, because of the altitude of geostationary satellites their imaging is of fairly low quality towards the poles (see Fig. 4), and polar-orbiting satellites generally produce better resolution data. However, polar orbiters suffer from the disadvantage that their orbit tracks are continually changing, so making them less useful in monitoring a given location continuously in comparison with geostationary satellites. Geostationary satellites provide the kind of continuous monitoring necessary for intensive data analysis, by maintaining station over one position on the surface. The geosynchronous plane is high enough to allow the satellites a full-disc view of the earth. Because they stay above a fixed spot on the surface, they provide a constant vigil for the atmospheric `triggers of severe weather conditions such as tornadoes, flash floods, hailstorms, and hurri-

Fig. 4 Meteosat global infrared image, 1200 GMTon 7 June 2002 (courtesy of EUMETSATand NOAA, and taken from http://www.goes.noaa. gov/f_meteo.html).

The warmer land masses are shown as black in the image. canes. When these conditions develop, the satellites are able to monitor storm development and track their movements.

5 Fig. 4 Meteosat global infrared image, 1200 GMTon 7 June 2002 (courtesy of EUMETSATand NOAA, and taken from gov/f_meteo.html). Note how the image shows the presence of deep (cold) convective cloud (white) in equatorial areas, and deep frontal cloud in midlatitude regions. The warmer land masses are shown as black in the image. canes. When these conditions develop, the satellites are able to monitor storm development and track their movements. Geostationary satellite imagery is also used to estimate rainfall during thunderstorms and hurricanes for flash flood warnings, as well as estimating snowfall accumulations and the overall extent of snow-cover. Such data help meteorologists to issue winter storm warnings and spring snowmelt advisories. Satellite sensors also detect ice fields and map the movements of sea- and lake-ice. Visible and near-infrared images show the sunlight that is reflected off clouds and the surface of the earth, using the 0.5± 1.0 mm band. They show all types of cloud and are the best type of image for seeing low-level weather systems, which do not show up well on far-infrared imagery. Thus fog may be invisible on a farinfrared image because of the lack of temperature contrast between the fog and surrounding land or sea. Visible imagery can only be used for this purpose when the area of interest is in daylight. An example of visible imagery is shown in Fig. 2. Types of image Satellite imagery is, nowadays, available widely on the Internet and it is worth reviewing, briefly, the main types of imagery available there. Examples of all types of imagery can also be found from time to time in Weather. Visible and near-infrared Far-infrared Far-infrared (commonly called thermal) imagery shows the amount of heat emitted by the different cloud features and the surface of the earth. Thermal images show clouds at higher levels better because they are colder. These images are usually presented such that lower temperatures appear as white parts of the image, and warmer parts appear dark (e.g. Fig. 4). 111

Fig. 5 NOAA) Water-vapour image over the western coast of North America at 1800GMT on 14 February 2002 (courtesy of The far-infrared images are derived from emissions in the 10± 12 mm waveband, and

However, since the emitted radiation must traverse the top of the earth s atmosphere before reaching the satellite, this upper atmosphere will modify the emitted radiation.

6 Fig. 5 NOAA) Water-vapour image over the western coast of North America at 1800GMT on 14 February 2002 (courtesy of The far-infrared images are derived from emissions in the 10± 12 mm waveband, and provide information to the satellite on the underlying surface or cloud. However, since the emitted radiation must traverse the top of the earth s atmosphere before reaching the satellite, this upper atmosphere will modify the emitted radiation. It is worth noting, however, that in the tropics even cloudless air may show up as a shade of grey in the image, due to the presence of high levels of humidity ± this impacts upon the radiative processes at the infrared wavelengths. Water vapour 112 Water-vapour images are images that show water-vapour content in the troposphere (Fig. 5), using measurements made of emissions around the 6± 7 mm band. This is an area of the electromagnetic spectrum where the water vapour is the major absorbing gas. If the upper troposphere is moist, then the radiation reaching the satellite instrument will mostly originate from this region, and will usually be displayed in white ± mirroring the convention for far-infrared images denoting this altitude. If the upper troposphere is relatively dry then the emissions will originate from water vapour at warmer, lower levels and the imagery will be shown as black by convention. In a normally moist atmosphere, most of the watervapour radiation measured by the satellite radiometer arrives from the 300± 600 mbar layer, but if the air is dry some may come from layers as low as 800 mbar (2± 3 km above the earth s surface). Part 2 of this article will describe how satellite data can be used in modern weather forecasting, and how data from research satellites are also being used in numerical models of the atmosphere. Correspondence to: Dr R. Brugge, NERC Data Assimilation Research Centre, Department of Meteorology, University of Reading, PO Box 243, Earley Gate, Reading, Berkshire RG6 6BB. brugge@met.rdg.ac.uk # Royal Meteorological Society, doi: /wea A

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