Meteorological Satellites Systems - Essay Example

Meteorological Satellites Systems Introduction Meteorology manly concerns itself with observing theoretical physics and earth science. Being one of the branches of observational sciences, meteorology is charged with the task of giving accurate information about the atmosphere’s state. This can only be achieved through simultaneous and regular observations of the entire globe covering the earth surface going up to the upper atmosphere. Such observational progress at such great heights were in the past limited to balloon station networks that covered the entire globe although sparsely. The data harvested using this network helped in the discovery of some characteristics about the atmosphere that were previously not known. These characteristics formed the basis on which most of the theoreticians worked on to provide information about the atmosphere. However, these characteristics ware not enough to provide more precise information about the atmosphere yet such information were of great significance in predicting meteorological parameters and conditions. The introduction of computers and mathematical models resulted to an increased demand for observational data that was adequately sampled and reliable both in time and space. This was due to the fact that the atmosphere forecast models depended strongly on the initial atmosphere state that had been reached at through assumptions. Better data was needed as even after this initial information, there were still many areas that were left without atmospheric soundings that were conventional. Before the discovery of weather satellites, the weathermen work was almost impossible as they were handicapped with very limited information before them about the atmospheres state at any time (Kidder and VonderHarr, 1995). Even after the world war period when observational networks were expanded by most of meteorological services from different nations, the global vast land areas that were sparsely populated as well as the large areas covered by oceans remained virtually blank with regards to conventional meteorological observations. The introduction of metrological satellites was a major step towards overcoming all these deficiency. Satellite imagery has since its discovery proved to be a vital information source in forecasting operations. The images serve various important functions in the metrological field including; a tool used for analysis mostly when dealing with data from the tropics, a vital aid during forecast conducted in short periods before rainfall, cloud, and floods, serves as an input to weather prediction models mostly dealing with numbers so as to define initial conditions, and used to monitor forecast models. In addition, the images can also be used as a prized indicator of both physical and dynamic process offering the weathermen useful information on the structure of the atmosphere and its evolution. Some of the valuable numerical models inputs provided by satellite images include; vectors on cloud motions, temperature of sea-surfaces, and winds moving across sea surfaces and upper air. Other important data that are normally utilized by monsoon weather predictors include; geostationary satellite’s rainfall, microwave sensor’s rain rate and Outgoing Long-wave Radiation gotten from satellites launched on polar orbits (Krishna, 2000). Meteorological Satellite Requirements Meteorological satellites are expected to meet the following requirements (National Research Council, 1997); 1. To serve as a data collection platforms where they act as collectors of data relevant to meteorology from instruments that are based on unmanned ocean and land. 2. To act as the main platform for observation when equipped with relevant sensors and serve to transmit both sounding and imaging information to the receiving stations that are situated on the surface of the earth. 3. To act as a communication satellite to enable meteorological data to be rapidly exchanged among centers as well as rapid broadcasting of weather warnings and forecast to agencies using the satellites Types of Meteorological Satellites Meteorological satellites can be classified into two main categories; Geostationary and Polar orbiting. The polar orbiting type of meteorological satellite navigates over the poles at distances approximated to be of around 850 kms. This type of satellites have the ability to observe the entire earth surface as they move along fixed orbits in space as the earth rotates below them. The areas observed on each swath are normally near the equator as well as other regions that overlap the equator moving towards the pole. A single swath covers approximately 2600km wide. Such satellites cover 14 orbits each day and hence offer global coverage two times in two days. Some of the satellites orbiting the polar include; IRS, NOAA, ERS-2, and ERS-1, Oceans and DMSP (Rao, 1990). The Geostationary type of meteorological satellites provides meteorological images by moving around the earth surface but over the equator at a distance of approximately 36000kms above the surface. They take 24hours to go around one orbit as they are normally synchronized with the earth as it rotates about its axis. This synchronization with the earth makes them to remain in the same location with reference to the equator. This type of satellite has the advantage of having a high resolution of its data that is time-scaled. The satellite provides new images of the whole disc of the earth after every 30 minutes. However, the spatial resolution of this satellite is limited compared to that of the polar orbiting with respect to the earth’s distance. Useful information from the satellite is normally restricted to the region between 70deg. S and north latitudes. Some geostationary satellite examples include GEOS-E, GEOS-W, GMS, METEOSAT-6, METEOSAT-5, and INSAT-1(Rao, 1990). Satellite Sensor System Remote sensing instruments commonly known as the sensors are basically meant to measure photons. The sensor operation fundamental principle is mainly centered at the detector which remains a critical component of the sensors. The concept makes use of the photo electric effect. Sensors used by meteorological satellite can be classified further into; active and passive. The passive type of sensors depends on the radiation reflected or emitted through photoelectric effect thus do not make use of their own electromagnetic illumination source. The active sensors on the other hand use their electromagnetic radiation source for target illumination. They further use reflected radiation properties such as polarization, intensity and time delay to analyze the targets information. The sensors can also be subdivided into; categories and subcategories as well as being categorized according to their functions (Houghton, 1995). Sensors used by meteorological satellites can serve to obtain the following land-ocean characteristics of the atmospheres system (Houghton, 1995).; 1. Spatial Information: examples of such information include the temperature and extent of cloud cover, sea surface, soil moisture and vegetation. The objective of such kind of information is to come up with information required over a plane that is 2-dmentional. Sensors that are best suited for this type of information are the imaging radiometers which operate in microwave, infrared or visible frequencies. Sensors that are active such as the Synthetic Aperture Radar (SAR) are at times used effectively in imaging applications. 2. Spectral Information; for some given types of applications, the electromagnetic signal spectral details are very significant. One similar object in particular is the surface of the ocean or a layer of the atmosphere, interacting differently with electromagnetic spectra wavelengths. The interaction may arise due to the objects chemical composition. Processes such as reflection, absorption and emission of EM radiations from any object are as a result of the functions of the objects temperatures and EM radiation’s wavelength. Therefore the information about the spectrum can be the source of information about the chemical composition or the object’s temperatures. Sensors used in meteorological satellites make use of such information applications like sounding where the temperature’s vertical structure, atmospheric gases and humidity are retrieved. A good example of such sensors is the High Resolution IR Sounder (HIRS). Satellites that will be made in future will have sensors that are more advanced in that they will be able to provide imaging spectrometer (Djuri, 1994). 3. Intensity Information: The EM radiation intensity can be the source of a number of clues about the object under study. In most scenarios, the sensors measure the reflected radiation intensity from the object under study in order to find out its roughness and its dielectric properties. With the use of appropriate algorithms, such parameters can be easily converted to geophysical parameters properties such as the speed of wind on the surface of the ocean, the moisture of the soil, the direction of wind, and the roughness of the surface of the ocean. Sensors making use of such information include; polarimeters, radar, and scatterometter. Principles of Satellite Remote Sensing All objects are believed to be emitters of electromagnetic radiation. As the source gets hotter, the more intense the emission gets. Black bodies are substances that take in the entire radiation incident to them. The absorption coefficient of such objects is unity. These black bodies have emissivity that is unity as well. At any particular wavelength, such bodies emit their maximum radiation which is most suitable to their temperatures. However, most substances in meteorology are not black bodies that are perfect hence their emissivity happens to be less than unity. Solar radiation wavelengths are shorter while the terrestrial radiations wavelengths are longer. Gases cannot be black bodies as they emit or absorb at specific wavelengths. Gases with infrared as well as visible wave bands include; carbon dioxide, water vapor, and ozone which are of great significant in meteorology. Each of the above gases happens to be active in specific absorption bands (Djuri, 1994). Radiometers provide satellite imagery through the measurement of electromagnetic radiation that is scattered coming from the atmosphere, the earth and the sun. Satellite imageries commonly used include; 1. Infrared imagery consequent of atmosphere and earths emissions at infrared wavelengths that are thermal. 2. Visible imagery resulting from sunlight reflections at near infrared and visible wavelength. 3. Water vapour imagery that results from emissions from water vapour. 4. Microwave radiometer images like Special Sensor Microwave and 5. Channel 3 imagery resulting from the region overlapping terrestrial and solar radiation. Weather Satellites Data The initial satellites made use of visual imageries from polar orbiting satellites. Later, some new developments in the meteorology led to new avenues. Such developments include, the move to launch US GOES satellite on the latitudes above India following the MONEX experiment and the Japanese GMS satellites launch. Later in the eighties, meteorological satellites INSAT series were introduced and they ensured that more information about the atmosphere covering the Asian continent was readily available. In addition to satellites launched mainly for experimental purposes like the ERS-1, other satellites such as the TRMM have been based on to provide information about water vapour, monsoon features, rainfall, wind and SST (Burroughs, 1991). Such information is of great significant as tropical rain has had diverse effects in the economies and lives of most populations. Systems resulting from tropical rain such as typhoons, hurricane and monsoons are a threat to the lives of people in the tropics. Excess floods can lead to crop failure and drought. The TRMM is normally inclined at low degrees of about 35 degrees on orbits with high processors and are not synchronous with the sun so as to enable the satellite move over every position of the surface of the earth at different times. The TRMM satellite is equipped with precipitation radar, infrared and visible scanner, and microwave imager. The NOAA satellites have significant meteorological payloads which include (Burroughs, 1991); 1. TIROS Operational Vertical Sounder 2. Advanced Very High Resolution Radiometer 3. Earth Radiation Budget. AVHRR on the other hand has a radiometer that contains five scanning channels of infra-red and visible wavelengths which are used to analyze meteorological, hydrological and oceanographic parameters like snow, ice, clouds and vegetation index. Data obtained from the five channels is received with a 1km resolution. The data from the digital AVHRR is conveyed from the satellite when it is in form of High Resolution Picture Transmission in real-time and recorded selectively when need arises for successive playback in the event that the satellite happens to be in a range where it can communicate with the control station on the ground. Such data that is highly revolutionized is referred to as local Area Coverage (LAC). The data provided by AVHRR is later appraised on real time so as to give Global Area Coverage data of a lower resolution. The GAC’s data effective resolution is approximately 4kms (Chen, 1995). The vertical sounder of the TIROS makes use of a sounder with infrared radiation of high resolution (HIRS), a unit of microwave sounding (MSU) and a unit of stratospheric sounding (SSU). The HIRS purpose is to sample the radiation from the atmosphere in approximately 20 IR channels and serves the main role of obtaining the troposphere’s moisture distribution and the vertical temperature. In its operations, the HIRS utilizes double carbon dioxide bands in the process of temperature sounding. Seven of these channels are suited in the 15m band while the 4.3m channels are included so as to improve on the sensors sensitivity (Chen, 1995). The moisture in the atmosphere is detected by the three channels located at the 6.3m water vapor band. Ozone is sensed by the designed 9.7 m channel. Three of these channels are left in the atmospheric windows while the 3.76 and 11.1m channels are reserved to clouds detection. SSU is meant to sample the stratospheric radiation found in channels 3 IR. MSU on the other side samples atmospheric radiation found in channels 4 of the microwave region and helps to obtain the temperatures vertical distribution in the region below the clouds (Gurney and Parkinson, 1993) Significant Microwave Payloads and Their Applications Microwave sensors have been so instrumental in providing information that is valuable in many meteorological applications. This is with reference to both passive and active sensors. Active microwave sensors come in different forms including; altimeter, wind scatter meter, and radar for measuring precipitation. Scatterometers that operate with frequencies whose range lies in the c-Band or K-Band is regarded as a crucial tool used mainly to monitor the speed of wind on the ocean surface and its direction as well as global coverage. Winds on the surface of the ocean can be used for many applications including; act as a computation factor of mass exchange and energy from the air and the sea. They are also used as an input for models used to forecast global wave and oceans trends. The first radar to be used to observe precipitation in space was the Tropical Satellite Measuring Mission (TRMM). The instrument at its introduction was being operated at 13.6 GHz and was capable of observing global tropic’s rainfall on vertical profiles. Considering the passive type of microwave meteorological systems, the most successful of this type of sensors is the Special Program (DMSP) satellite. There are different versions of this type of sensor but they all offer observational information that are valuable to meteorology. These radiometers operate on frequencies of 19.36, 22.23, 37.0 and 85.5 GHz (Gurney and Parkinson, 1993). Of all the channels used by this sensor, it is only the 22.23 GHZ that operates in both V and H polarization while the rest of the channels operate with either of the polarization (Gurney and Parkinson, 1993). SSM/1 is used for observing global water vapour that is vertically integrated, rainfall rates, the speed of wind on the surface of the sea, and clod liquid water (Stephens, 1994). However, most of the observations made using the SSM/1 are only possible over the surface of the ocean due to limited microwave observations. SSM/1 observations cover a wide swath. The characteristics of the TMI are similar to those of the SSM/1. However, several significant differences stand out. TMI has one more channel operating at 10 GHz and on V and H polarization. This additional sensor enables the TMI to sense the temperatures of the sea surface. When the observations from TRMM sensor and TMI are combined, the result is used to measure the SST of each day with an accuracy that has been significantly improved. The additional channel also serves to provide rainfall rates that are improved. The latest NOAA satellite series is the Advanced Microwave Sounding Unit (AMSU). It is advanced in such away that it provides both the humidity and temperature soundings even when there is cloud cover (Stephens, 1994). Conclusion There are various types of meteorological satellites categories including, microwave and optical, imaging and non-imaging, and passive and active. Despite the several categories, all of these satellites are used to retrieve and to provide information about meteorological parameters. The information can also be used in weather forecasting which has proved to be a significant meteorological process in preventing climatic disasters such as hurricanes, floods and monsoons. More sophisticated meteorological satellites are being developed so as to provide more accurate observations and to get to regions on the atmosphere where current satellites cannot reach. Bibliography Burroughs, W. (1991). Watching the World’s Weather. Cambridge University Press: New York Chen, S. (1995). Space Remote Sensing Systems: An Introduction. Academic Press: Orlando Djuri, D. (1994). Weather Analysis. Prentice-Hall, Inc.: New York. Gurney, R. & Parkinson, L. (1993). Atlas of Satellite observations Related to Global Change. Cambridge University Press: New York. Houghton, D. (1995). Handbook of Applied Meteorology. John Wiley and Sons: New York Kidder, S. & VonderHarr, T. (1995). Meteorology: An Introduction. Academy Press: Orlando Krishna, P. (2000). Weather Satellites System Data and Environmental Application. American Meteorological Society: London National Research Council. (1997). Continuity of NOAA Satellites. National Academy Press Rao, P. (1990). Weather Satellites: Systems, Data and Environmental Applications. American Meteorological Society: Boston. Stephens, G. (1994). Remote Sensing of the Lower Atmosphere: An Introduction. Oxford University Press: New York. Read More