Intro to Weather SatellitesWeather satellites were first launched to observe hurricanes. Meteorologists needed some way to watch these storm systems form, and satellite observations offered the best solution for monitoring their track.There are two primary satellite orbits:Geostationary OrbitPrimary satellite orbit used for weather observationsLooks at the same point on the equator at the same longitude and sees nearly half of the earth at any given timeThe orbital period of the satellite matches the rotation of the EarthLow-Earth OrbitPrimarily used in researchMost orbit from pole to pole and let the Earth spin underneath themVery high spatial resolution (250 m to 15 m)Only see a small swath of the Earth at any given timePrimary Satellite ChannelsSatellite channel: a range in wavelength of light that the satellite’s cameras are sensitive toThree main channels1. Visible Satellite ImageryVisible channels measure reflected sunlight. They work just like a digital camera without a flash. Because of this, you cannot use a visible satellite image at night to observe the weather.Objects that are bright on a visible satellite image are those that have a high albedo. The albedo is the ratio of the reflected sunlight to the incident sunlight. Objects with high albedo are bright like snow, ice caps and thick clouds (like thunderstorms).Objects that are dark on a visible satellite image are those that have a low albedo like the ocean, land, vegetation and thin clouds (like cirrus)2. Infared Satellite ImageryInfrared satellite channels measure the emitted thermal infrared radiation from an object and can therefore measure the temperature of an object.All objects emit radiation according to their temperature. The hotter the object becomes, the more radiation it will emit.All infrared images are inverted so that warm objects are dark and cold objects are bright.Bright objects in an infrared satellite image are cold objects. Cold objects are high in the atmosphere, where the temperatures are very low. So high clouds like cirrus and the tops of deep thunderstorms are very cold so they appear bright on these images.Dark objects in an infrared image are warm. Warm objects are found close to the surface of the Earth. Therefore, low clouds, the Earth’s surface, and even snow appear dark on these images.3. Water Vapor ImageryThe water vapor channel measures emitted radiation from water vapor and tells us information about the moisture content of the air.Bright areas on a water vapor image, like the tops of thunderstorms, have very high water vapor content.Dark areas on a water vapor image have low water vapor content, like deserts or areas that are very cold.Meteorologists use water vapor as a tracer of atmospheric motionDoppler RadarRadar systems were first mass-produced during WWII. These radars were not designed for remotely sensing precipitation but rather for civil defense during the war. Precipitation was an annoyance when radar operators were searching for enemy aircraft and ships.First radar network in the United States was deployed in the late 1950s.The current radar network, called the NEXRAD Network, uses 158 WSR88-D radars placed all over the United States. "WSR88-D" stands for “Weather Surveillance Radar” made in 1988 with Doppler capabilities.Most of these radars are located at National Weather Service offices. They look like large white soccer balls sitting on top of a metal structure. They are protected from the weather, critters, and people by a large fiberglass sphere called a “raydome.”The best feature of the current Doppler radar network is its ability to measure the movement of the precipitation.Since this movement can only be caused by the wind, radar meteorologists can use this information to extract the wind speed and directionHow Radar WorksDoppler radar systems work by measuring the reflection of a microwave pulse off a raindrop. Essentially, the radar dish sends out a powerful pulse of electromagnetic radiation that travels away from the radar at the speed of light. When this radiation hits the raindrop, an echo is produced as the pulse reflects off the raindrop. The echo travels back to the dish, where it is collected.The strength of the return signal is proportional to two properties of the raindrops:The number of raindropsThe size of the raindropsThe more raindrops there are or the bigger those raindrops become, the more powerful the radar echo.Radar echoes are called “radar reflectivity.” The radar reflectivity provides us with…The presence and coverage of the precipitationThe location of the precipitationThe intensity of the precipitationThe amount of precipitationDoppler radar can measure all of the things above AND it can also measure the wind speed and direction.Data are viewed using color-coded maps of precipitation intensity.If the radar reflectivity values exceed 55 dBZ (dBZ = decibels of reflectivity), then the radar beam has hit hail in the storm. Therefore, when you look at a radar image and see dark red, pinks, purple or white, these colors indicate the presence of hail.The radar reflectivity values can be used to estimate how much rain will fall over a location. See the image below.The Doppler EffectIf a raindrop is moving toward or away from the radar, the frequency and phase of the return echo will change compared to the pulse that was sent out. Doppler radar can measure these shifts in the frequency and phase of the echo to find the wind speed and direction of the precipitation.When looking at a Doppler radar image, which is called a “radial velocity” image,Red colors indicate precipitation that is being blown away from the radarBlue and green colors indicate precipitation that is being blown toward the radarClear Air Mode and Storm Total PrecipitationWhen there is no precipitation nearby, the sensitivity of the radar can be increased to see some interesting thingsRadars can operate in two modes:Precipitation Mode: Less sensitive mode used for the detection of measurable precipitation.Clear Air Mode: Highly sensitivity mode used when there is no precipitation. This mode can be used to see birds, bugs, dust, turbulence in moisture content, and even light snow.Ground clutter is seen near the radar where echoes from non-meteorological targets appear on the radar image.Besides detecting severe thunderstorms and tornadoes, radars are excellent tools for monitoring a developing flood. The “storm total precipitation” image shows how