3 Electromagnetic Waves Consist of an electric field and a magnetic fieldPolarization: describes the orientation of the electric field.
4 Remote Sensing Passive vs Active sensors Active sensors emit radiation and measure what is sent back to themRadars are active sensors.Passive sensors detect naturally occurring radiationMost satellites are passive sensors.
5 Satellite Basics Satellites are in many different types of orbits. Geosynchronous (36,000 km)LEO ( km)MEO (10,000 km)Polar (LEO orbit)SunsynchronousMolinyaConstellationsAscending vs Descending Nodes
6 Satellite BasicsHow many times a day a particular point is seen by a sunsynchronous satellite
7 Satellite BasicsSatellite constellations fly multiple satellites in varying orbits to provide maximum earth coverage.Satellite Formations fly multiple satellites in the same orbit.
8 Satellite Basics Most satellites carry multiple sensors Saves money Instrument designInstrument launchMultiple fundersAllows many earth/atmosphere parameters to be observed at one time
9 Satellite BasicsCurrent weather satellites include radiometers, radars, and soundersRadiometers sense emitted radiationRadars sense reflectivity in the vertical rather than horizontalSounders sense profiles of radiance from many layers of the atmosphere.Radiometers sense emitted radiation in the infrared and microwaveBrightness temperature (Tb) – The temperature of an equivalent blackbody emitting radiation.Infrared and microwave sensors measure the emitted radiation and report it as a brightness temperatureInsert LOTS of complicated math here.Tb = Ts
10 EmissivityEmissivity – amount of absorbed radiation that is emitted by a bodyHas values between 0 and 1Ocean has an emissivity of about 0.5Land and cloud cover have an emissivity > 0.8Over land, emissivity is dependent on soil moisture and vegetation cover, and is very difficult to determine.Over ocean, emissivity is dependent on wind speed and sea surface temperature
11 RetrievalsA retrieval is a method by which we infer information about the atmosphere based on measured brightness temperaturesRetrievals can be used to inferCloud liquid waterWater vapor contentConcentrations of atmospheric gases (CO, CH4, CO2)Land surface properties (Soil Moisture, vegetation)If y is the observed brightness temperature, x is the state of the Earth/Atmosphere system, and f is a forward model that predicts y based on x:y = f(x) x = f-1(y)
12 What is x?(So/4) (1-α)ơ TA4atmosphereơ TS4ơ TA4surface
14 Visible Satellite Imagery Visible satellites basically take pictures of the earth.Most visible satellites are in geosynchronous orbit.In the US, we use the GOES satellites.GOES: Geostationary Operational Environmental SatellitesGOES satellites carry 5 sensorsVisible (.6 microns)Shortwave IR (3.9 microns)Water Vapor (6.7 microns)Infrared (11 microns)Infrared (12 microns)
20 Visible Satellite Imagery AdvantagesAlways located in the same place (homogeneous sampling)View at an angle to give some information about vertical development of cloudsCan be used to monitor snow coverHigh spatial resolution (GOES Vis: 1km)DisadvantagesCannot be used at nightViews of the poles are limited due to the viewing angleNot very useful for retrievals
21 Infrared Satellite Imagery Displays the brightness temperature calculated based on the amount of sensed infrared radiation.Useful for determining whether clouds are convective or stratiform.Land surfaces appear warmer than cloud.Land surface emissivity is very close to 1 in the infrared.
23 Infrared Satellite Imagery AdvantagesCan infer cloud heights by temperatureCan be used at nightCan be combined with visible imagery to infer cloud typesCan be color enhancedCan be used in retrievals
25 Some satellites using IR frequencies MODIS (MODerate resolution Infrared Spectroradiometer)AIRS (Atmospheric Infrared Sounder)GOES (Geostationary Operational Environmental Satellite)AVHRR (Advanced Very High Resolution Radiometer)MOPITT (Measurements Of Pollution in The Troposphere)
26 Southwest Asian Dust Storm image from MODIS at 500m resolution
28 AIRS Retrieved Temperature Isotherms over Southern Europe viewed from the west, September 8, 2002
29 Water Vapor Imagery Retrieved water vapor based on sensed emissivity Shows where significant weather systems are locatedUses infrared wavelengthsRemember: water vapor is a strong absorber of IR radiation.
31 Microwave Satellite Imagery Senses emitted radiation in the microwave frequenciesAlso frequently used in retrievals.Land surface emissivity in the microwave averages 0.95.Most microwave sensors carry radiometers in multiple frequenciesAllows for varying resolutionsUseful in retrievalsAtmospheric water vapor absorbs at ~22 GHz
32 Microwave SensorsAMSR-E (Advanced Microwave Scanning Radiometer for EOS)TRMM (Tropical Rainfall Measuring Mission)TMI (TRMM Microwave Imager)TRMM-PR (Precipitation Radar)VIRS (Visible and Infrared Satellite)SSM/I – Special Sensor Microwave ImagerAMSU – Advanced Microwave Sounding Unit