SATELLITE USES FOR PURPOSE OF NOWCASTING Kedir, Mohammed National Meteorological Agency of Ethiopia Introduction The application(uses) of satellite sensing data deals to obtain information about the basic state of parameters of the atmosphere, moisture and winds.they also discus the use and identification of clouds from satellite in order to obtain meteorological information about synoptic weather patterns, tropical storms and weather system and small scale weather features. Application of satellite sensing data also deals with sever meso scale storms and rain fall estimation techniques, as well as with satellite application related to weather modification and cloud physics programs, air sea interaction and climatologically studies. This set of notes is designed as all introductions to the application of satellite sensing data it has been possible to cover most of the application devoted to weather forecasting. Satellite uses for the purpose of now casting 1. What are the most common meteorological parameters derived from satellite for now casting? 2. How does satellites used for the purpose of now casting? In order to answer the above questions and talk about the purpose of satellite uses for the purpose of now casting let me define what satellite means. Definition of what is satellite: satellite is anything that orbits something else, as, for example, the moon orbits the earth. In a communications context, a satellite is a specialized wireless receiver/transmitter that is launched by a rocket and placed in orbit around the earth. There are hundreds of satellites currently in operation. They are used for such diverse purposes as weather forecasting, television broadcast, and amateur radio communications, Internet communications, and the Global Positioning System (GPS).this is the general term for the definition of the satellite but the difference is that meteorological satellites are the satellites that observing and detecting weather elements for the purpose of weather forecasting and early warning for the daily life of human being. It is also termed as the eyes in the sky.
1. What are the most common meteorological parameters derived from satellite for now casting? The most common meteorological parameters derived from satellite are: The parameters of the condition of the atmosphere such as the temperature, pressure, and humidity of the air, wind speed and direction, cloud cover, precipitation, and visibility (the transparency of the atmosphere), as well as soil and surface water temperatures, solar radiation, and long wave terrestrial and atmospheric radiation. Meteorological elements also include weather phenomena such as thunderstorms and snowstorms. The variations in the meteorological elements are the result of atmospheric processes, and they determine the weather and the climate. The meteorological elements are observed at aerological and meteorological (weather) stations and at meteorological observatories by means of aerological and meteorological instruments as well as satellite are the main element to give any types of forecasting. 2. How does satellites used for the purpose of now casting? 1. How satellite detect temperature Infrared light lies between the visible and microwave portions of the electromagnetic spectrum. Infrared light has a range of wavelengths, just like visible light has wavelengths that range from red light to violet. "Near infrared" light is closest in wavelength to visible light and "far infrared" is closer to the microwave region of the electromagnetic spectrum. The longer, far infrared wavelengths are about the size of a pin head and the shorter, near infrared ones are the size of cells, or are microscopic. Far infrared waves are thermal. In other words, we experience this type of infrared radiation every day in the form of heat! The heat that we feel from sunlight, a fire, a radiator or a warm sidewalk is infrared. The temperature sensitive nerve endings in our skin can detect the difference between inside body temperature and outside skin temperature. 2. How satellite detect Moisture Water vapor imagery is used to analyze the presence and movement of water vapor moisture in the upper and middle levels of the atmosphere. The wavelength spectrum used to detect water vapor is in the 6.7 to 7.3 micrometer wavelength range. The upper and middle levels of the atmosphere are from about 650 mb to the top of the troposphere. Above the troposphere there is very little moisture. Why doesn't water vapor imagery detect moisture in the lower atmosphere and near the surface? Only the top most level of moisture is detected using these wavelengths. For example, think about clouds on visible imagery. If there is a thick layer of clouds in the upper levels then you will not be able to see the clouds that are closer to the surface. The same applies to water
vapor. When upper level moisture is present, this layer of moisture will prevent the detection of water vapor that is below this layer. If the upper levels are dry then water vapor can be detected in the middle layers of the atmosphere. There is usually though enough moisture in the middle and upper levels that moisture in the lower levels is not detected. Water vapor absorbs radiation in the wavelength from 6.7 to 7.3 microns. Suppose there is a thick layer of moisture in the upper levels. This layer of moisture will absorb the IR radiation in the 6.7 to 7.3 micron wavelengths that are being emitted from the lower troposphere up to this layer. Thus, the satellite receives less radiation in the 6.7 to 7.3 micron spectrum where there is a higher concentration of upper level moisture. The satellite will interpret this greater absorption as a colder temperature and a higher concentration of water vapor. If the upper levels are dry then less radiation is absorbed by moisture in the 6.7 to 7.3 micron spectrum in the upper levels. The satellite will interpret this lesser absorption as a warmer temperature and a lesser concentration of moisture. A high emission in the 6.7 to 7.3 micron spectrum indicates the emission is coming closer to the lower levels of the troposphere where it is warmer and more moisture is present. A dark color or warm color indicates a relative lack of upper level moisture. It does not mean though there is a lack of moisture in the lower levels or at the surface. It could be very moist at the surface or it could be fairly dry. A white or cold color indicates a high concentration of water vapor. This layer of water vapor is absorbing radiation in the 6.7 to 7.3 micron range. 3. How satellite dectecloud detection There are several pitfalls with using satellite images. This essay will explore many of these common pitfalls. 1. VIS (low sun angle and at night) Visible imagery is of no use at night since it requires the sun to reflect its rays off objects in order to detect them. In the winter VIS is less useful since the days are so short and the sun angle is low. When the sun angle is low (near the horizon) there is not as much reflection off the clouds. Thus, around the time of sunrise and sunset VIS is not as useable. 2. Water Vapor (vapor vs. clouds) There is an important difference between water vapor and clouds. Water vapor is in the gaseous state while cloud is in the liquid or solid state. On water vapor imagery it is important to distinguish between the vapor and the cloud. On black and white imagery the clouds will show up a fairly bright white. The water vapor will be wispy in appearance. Also remember the clouds and water vapor are mainly being detected in the upper levels of the troposphere, thus cloud and water vapor near the surface is not being detected as much.
3. Cloud location error / angle displacement Since the earth is a spherical type shape and the satellite images you look at are flat there will be some error in the cloud location. Since the satellite is over the equatorial region, the cloud displacement error will be greater toward the pole. The displacement also increases when moving east or west of where the satellite is located. As the latitude increases or when looking further east or west of the satellite position the satellite is looking at the clouds at more of an angle. The angle eventually gets so large that it makes the image fairly useless for example in the polarregions. 4. Can't see clouds under cloud The satellite can only see the highest cloud deck. If there is upper level cloud covering an area the satellite will be unable to detect clouds under this cloud deck. Because of this the satellite can be useless at times at detecting clouds that are influencing weather closer to the surface. Satellite can be used to sense surface features and the temperature profile of the troposphere. 5. Infrared is totally different from VIS Infrared detects clouds be determining temperature by the wavelength of emitted radiation from the cloud. This is a totally different concept of detection from what VIS uses. VIS uses reflection to detect cloud. Clouds that do not look significant on IR can be very significant. For example, clouds near the surface will not show up well on IR since they are close to the earth surface temperature. These clouds may in fact though have a significant influence on the surface weather. 6. Satellite images are not pictures and not video recordings VIS is closest to being a picture but of course it does not use a flash like a camera needs to sometimes. Satellite images don't require film and film developing. The satellite images are much more similar to digital pictures, except the entire picture is not taken all at once but rather is a line by line scan that when complete produces a complete image. It takes about 20 minutes to produce an image. When looking at the satellite images in motion there are significant time gaps between each frame. This is far more time than the multiple frames per second that video recordings have. Because of this delay in data, satellite images are not near as close to being instantaneous as compared to radar images. 4.Identifying rain cloud Precipitation call use indirectly measured from space mainly in two ways. One is way using microwave radiation measurements. The microwave region is also used by radar system. The other way to monitor rainfall from space is to use imagery of the operational satellites. This is a more indirect way to estimate the rainfall because only the radiation from the top or reflected by the top of cloud readers the satellite and this has to be related to the precipitation at the ground.
The mapping of precipitation intensities is providing by radioactive parameters related to thickness of the cloud system and from cloud top temperature. The deeper and more substantial a cloud and the colder the top the more intense is the precipitation. Results show that the resulting estimates of precipitation are consistent with actual surface observation problems arise mainly with the identification of drizzle falling from the shallow clouds, and with precipitation which is forced way mountain slopes. Here what I want to indicate is that as we receive data from the satellite we analyze and interpret the data with local area prediction system and give the very short range as well as now casting of the specified area. There fore satellite use for the purpose of now casting is great. 5.Solar radiation at the earth surface The other meteorological elements that detected by satellite is solar radiation at the earth surface. The solar radiation reaching the earth surface can be determined from the measurement mode by a satellite far out in space. There are three contributory sources to the total short wave energy, called the radiation available at the surface of the earth. A. the direct solar beam B. solar energy reflected to the surface by clouds C. solar energy reflected to the surface by air molecules and aerosols. The method for determine the solar radiation is first to calculated by means of theoretical model the solar radiation at the ground in cloud free condition in the next step the impact of the existing cloud on the solar radiation is determined using meteosat data to derive information on the extent, type and physical properties of the cloud. The result which produced is daily sum and monthly means of global radiation. Only satellite can provide date covering a whole area and show the horizontal difference that occurs over small distances. Detail data on the horizontal and temporal variation of global radiation are needed for the applications in solar energy utilization, agriculture, climatology and numerical weather forecasting. In addition to the above mentioned meteorological elements satellite can also detect and gives information to the forecasters to give their own forecast. There fore satellite is the main and most important meteorological instrument used to give now casting as well as any type of forecasting.
Summary for satellite uses for nowcasting. Meteorological satellite data are well suited to monitoring in a qualitative way the initiation and rapid development of precipitation generating systems both in space and time. Rapid imaging (on the order of minutes) is critical to now casting, but it is not yet provided by all geostationary satellites. With some satellite systems, the rapid scan of small areas competes with broader coverage requirements. Frequent images from geostationary satellites provide good to adequate horizontal resolution for identifying the initiation, evolution and movement of synoptic and mesoscale cloud systems or of local circulations over most of the tropics and temperate zones. Nowcasting can be applied to many phenomena including severe weather, but is most frequently used to forecast and monitor: convective storms with attendant phenomena (gust fronts, hail, intense precipitation, intense electrical activity, tornadoes); Wintertime severe weather (blizzards, snowstorms, ice, avalanches, etc.); mesoscale features associated with extra tropical and tropical storms; local flows or circulations (breeze, foehn, low level jets, convergence lines, dry lines, boundaries, intersection of boundaries, etc.); fog and low clouds; locally forced precipitation events; sand and dust storms; Wild fires. Key nowcasting parameters for which observational data are required are: clouds and precipitation; dust; surface variables: pressure, wind, temperature, humidity, present weather, visibility and precipitation accumulation, snow layer, land cover or structure; 3 D wind field; 3 D humidity field; 3 D temperature field. In the paper as I try to indicate meteorological satellites are the main instruments that receive and detect different weather parameters where surface observation is no available or remote areas as well as gives more timely information even where observation is good. using this information the meteorologists and meteorological forecasters given nowcasting weather forecast by applying different forecasting techniques. So meteorological satellites are important and that is why meteorologists said eye in the sky.