Remote Sensing in Meteorology: Satellites and Radar AT 351 Lab 10 April 2, 2008 Remote Sensing Remote sensing is gathering information about something without being in physical contact with it typically using electromagnetic radiation Passive sensors simply detect the radiation emitted by objects in certain frequency bands Active sensors send out a pulse of radiation and them measure what comes back to them Useful for seeing the whole picture of the current weather, and seeing where there are no other types of observations 1
Satellites A satellite, by definition, is something that revolves around a larger body we of course are talking about man-made satellites Multiple weather satellites (in addition to satellites for things like communication and navigation) are in orbit around the Earth as we speak Most instruments carried on weather satellites are passive sensors they receive and measure radiation from the earth at specific frequencies and use this information to calculate a brightness temperature Most satellites carry several instruments, and many instruments look at multiple frequencies Brightness Temperature The brightness temperature is basically the temperature that a satellite sees Represents what the temperature of the object would be if it emitted perfectly according to a black body curve 2
Visible Satellite Measures radiation in the visible wavelengths Basically takes a picture of the earth sees what our eyes would see Measures reflected solar energy so higher albedo looks brighter white Advantages: Can be used to keep track of snow cover High spatial resolution Disadvantages: Can t distinguish between high and low clouds by albedo alone Like us, the satellite can t see at night Visible Satellite 3
Infrared Satellite Measures radiance in the infrared, typically in the atmospheric window Calculates a brightness temperature based on the amount of radiance The ground will show up as a higher brightness temperature Typically, higher clouds have lower brightness temperatures, so cloud types are more easily distinguished Doesn t need daylight to work, and can sometimes see clouds that visible satellites miss Infrared Satellite 4
Infrared Satellite Other Types of Satellite Imagery Water vapor Water vapor imagery looks at a specific wavelength in the infrared which is very sensitive to the presence of water vapor Microwave Longer wavelengths in the microwave are scattered by raindrops, but not by smaller cloud drops and aerosols 5
Geostationary Orbit In orbit directly above the equator Moves at the exact speed of the earth Always over the same patch of land on earth High orbit is used to keep a balance of earth s gravity and centripetal force Sun Synchronous Orbit Rotates the earth at an angle Passes over the same points on the Earth's surface at the same local times each day Each orbit covers a different strip of the earth Orbit is much lower than that of a geostationary satellite 6
Some Satellites Currently in Use Geostationary Operational Environmental Satellite (GOES) Typically two of these are in orbit at a time, in order to provide a fuller picture of the earth (currently GOES 10 and GOES 12) Carries a 5-channel imager (1 visible and 4 IR) Most satellite images you find online will be from GOES Some Satellites Currently in Use Tropical Rainfall Measuring Mission (TRMM) Orbits at a low altitude and stays over the tropics Carries several useful instruments Precipitation Radar (PR) TRMM Microwave Imager (TMI) Visible Infrared Scanner (VIRS) Clouds and the Earths Radiant Energy System (CERES) Lightning Imaging Sensor (LIS) 7
Some Satellites Currently in Use Radar Radio Detection And Ranging Typically uses frequencies sensitive to precipitation Transmits a pulse and measures how much backscatter returns Time it takes to return tells how far away a storm is The more backscatter is returned, the more precipitation was in the storm to scatter the wave Measures reflectivity (Z for some reason) Depends on the 6 th power of diameter Shown in dbz (decibels of Z increases logarithmically: 30dBZ is ten times as much return as 20dBZ) Different wavelengths can be used Smaller wavelengths can see smaller drops (drizzle and cloud droplets), but can also be attenuated more easily Larger wavelengths aren t attenuated as easily, but must have a very large (and hence expensive) antenna to transmit Pulse travels at an angle, so we learn something about the vertical structure of storms as well 8
Typical Reflectivity Values <15 dbz: Probably not rain, probably ground clutter 15-25 dbz: Light rain or drizzle 30-40 dbz: Moderate rain 50 dbz: Heavy rain 55-60+: Very heavy rain or hail 9
Ground Clutter Hook Echo Supercells (rotating storms) often produce tornadoes, so it s important to be able to tell that they are rotating A clear signature of rotation on radar imagery is the hook echo 10
Doppler Effect This works the same with radar If the wind is moving toward the radar, the frequency of the radar pulse will be increased more so with higher velocities (green colors on a velocity image) If the wind is moving away from the radar, the opposite will occur (red colors) Doppler Examples 11
Rotation To better look at the details of a storm, we subtract out the velocity of the storm and make a picture of the storm relative velocity Like a hook echo in a reflectivity image, we can look for a tornado vortex signature (TVS) in an image of storm relative velocity A TVS, or velocity couplet, will show strong winds changing direction very quickly in other words, you will see a strong red right next to a strong green Tornado Vortex Signature 12