Satellite temperature measurements areinferences of thetemperature of theatmosphere at various altitudes as well as sea and land surface temperatures obtained fromradiometric measurements bysatellites. These measurements can be used to locateweather fronts, monitor theEl Niño-Southern Oscillation, determine the strength oftropical cyclones, studyurban heat islands and monitor the global climate.Wildfires,volcanos, and industrial hot spots can also be found via thermal imaging from weather satellites.
Weather satellites do not measure temperature directly. They measureradiances in variouswavelength bands. Since 1978microwave sounding units (MSUs) onNational Oceanic and Atmospheric Administrationpolar orbiting satellites have measured the intensity of upwelling microwave radiation from atmosphericoxygen, which is related to the temperature of broad vertical layers of the atmosphere. Measurements ofinfrared radiation pertaining to sea surface temperature have been collected since 1967.
Satellite datasets show that over the past four decades[timeframe?] thetroposphere has warmed and thestratosphere has cooled. Both of these trends are consistent with the influence of increasing atmospheric concentrations ofgreenhouse gases.
Satellites measure radiances in various wavelength bands, which must then be mathematically inverted to obtain indirect inferences of temperature.[1][2] The resulting temperature profiles depend on details of the methods that are used to obtain temperatures from radiances. As a result, different groups that have analyzed the satellite data have produced differing temperature datasets.
The satellite time series is not homogeneous. It is constructed from a series of satellites with similar but not identical sensors. The sensors also deteriorate over time, and corrections are necessary for orbital drift and decay.[3][4][5] Particularly large differences between reconstructed temperature series occur at the few times when there is little temporal overlap between successive satellites, making intercalibration difficult.[citation needed][6]
Infrared radiation can be used to measure both the temperature of the surface (using "window" wavelengths to which the atmosphere is transparent), and the temperature of the atmosphere (using wavelengths for which the atmosphere is not transparent, or measuring cloud top temperatures in infrared windows).
Satellites used to retrieve surface temperatures via measurement of thermal infrared in general require cloud-free conditions. Some of the instruments include theAdvanced Very High Resolution Radiometer (AVHRR),Along Track Scanning Radiometers (AASTR),Visible Infrared Imaging Radiometer Suite (VIIRS), theAtmospheric Infrared Sounder (AIRS), and the ACE Fourier Transform Spectrometer (ACE‐FTS) on the CanadianSCISAT-1 satellite.[9]
Weather satellites have been available to infersea surface temperature (SST) information since 1967, with the first global composites occurring during 1970.[10] Since 1982,[11]satellites have been increasingly utilized to measure SST and have allowed itsspatial andtemporal variation to be viewed more fully. For example, changes in SST monitored via satellite have been used to document the progression of theEl Niño-Southern Oscillation since the 1970s.[12]
Over land the retrieval of temperature from radiances is harder, because of inhomogeneities in the surface.[13] Studies have been conducted on theurban heat island effect via satellite imagery.[14] By using the fractal technique,Weng, Q. et al. characterized the spatial pattern of urban heat island.[15] Use ofadvanced very high resolution infrared satellite imagery can be used, in the absence of cloudiness, to detectdensity discontinuities (weather fronts) such ascold fronts at ground level.[16] Using theDvorak technique, infrared satellite imagery can be used to determine the temperature difference between theeye and thecloud top temperature of thecentral dense overcast of mature tropical cyclones to estimate theirmaximum sustained winds and their minimum centralpressures.[17]
Along Track Scanning Radiometers aboard weather satellites are able to detect wildfires, which show up at night as pixels with a greater temperature than 308 K (35 °C; 95 °F).[18] TheModerate-Resolution Imaging Spectroradiometer aboard theTerra satellite can detect thermal hot spots associated with wildfires, volcanoes, and industrial hot spots.[19]
TheAtmospheric Infrared Sounder on theAqua satellite, launched in 2002, uses infrared detection to measure near-surface temperature.[20]
Stratospheric temperature measurements are made from the Stratospheric Sounding Unit (SSU) instruments, which are three-channel infrared (IR) radiometers.[21] Since this measures infrared emission from carbon dioxide, the atmospheric opacity is higher and hence the temperature is measured at a higher altitude (stratosphere) than microwave measurements.
Since 1979 the Stratospheric sounding units (SSUs) on the NOAA operational satellites have provided near global stratospheric temperature data above the lower stratosphere.The SSU is afar-infrared spectrometer employing a pressure modulation technique to make measurement in three channels in the 15 μm carbon dioxide absorption band. The three channels use the same frequency but different carbon dioxide cell pressure, the corresponding weighting functions peaks at 29 km for channel 1, 37 km for channel 2 and 45 km for channel 3.[22][clarification needed]
The process of deriving trends from SSUs measurement has proved particularly difficult because of satellite drift, inter-calibration between different satellites with scant overlap and gas leaks in the instrument carbon dioxide pressure cells. Furthermore since the radiances measured by SSUs are due to emission bycarbon dioxide the weighting functions move to higher altitudes as the carbon dioxide concentration in the stratosphere increase.Mid to upper stratosphere temperatures shows a strong negative trend interspersed by transient volcanic warming after the explosive volcanic eruptions ofEl Chichón andMount Pinatubo, little temperature trend has been observed since 1995.The greatest cooling occurred in the tropical stratosphere consistent with enhancedBrewer-Dobson circulation under greenhouse gas concentrations increase.[23][non-primary source needed]
Lower stratospheric cooling is mainly caused by the effects ofozone depletion with a possible contribution from increased stratospheric water vapor and greenhouse gases increase.[24][25] There has been a decline in stratospheric temperatures, interspersed by warmings related to volcanic eruptions.Global Warming theory suggests that thestratosphere should cool while thetroposphere warms.[26]
The long term cooling in the lower stratosphere occurred in two downward steps in temperature both after the transient warming related to explosive volcanic eruptions ofEl Chichón andMount Pinatubo, this behavior of the global stratospheric temperature has been attributed to global ozone concentration variation in the two years following volcanic eruptions.[27]
Since 1996 the trend is slightly positive[28] due to ozone recovery juxtaposed to a cooling trend of 0.1K/decade that is consistent with the predicted impact of increased greenhouse gases.[27]
The table below shows the stratospheric temperature trend from the SSU measurements in the three different bands, where negative trend indicated cooling.
| Channel | Start | End Date | STAR v3.0 Global Trend |
|---|---|---|---|
| TMS | 1978-11 | 2017-01 | −0.583 |
| TUS | 1978-11 | 2017-01 | −0.649 |
| TTS | 1979-07 | 2017-01 | −0.728 |
From 1979 to 2005 themicrowave sounding units (MSUs) and since 1998 theAdvanced Microwave Sounding Units on NOAA polar orbitingweather satellites have measured the intensity of upwellingmicrowave radiation from atmosphericoxygen. The intensity is proportional to the temperature of broad vertical layers of theatmosphere. Upwelling radiance is measured at different frequencies; these different frequency bands sample a different weighted range of the atmosphere.[30]
Figure 3 (right) shows the atmospheric levels sampled by different wavelength reconstructions from the satellite measurements, where TLS, TTS, and TTT represent three different wavelengths.
A different technique is used by theAura spacecraft, theMicrowave Limb Sounder, which measure microwave emission horizontally, rather than aiming at the nadir.[9]
Temperature measurements are also made byGPS radio occultation.[31] This technique measures therefraction of theradio waves transmitted byGPS satellites as theypropagate in the Earth's atmosphere, thus allowing vertical temperature and moisture profiles to be measured.
Planetary science missions also make temperature measurements on other planets and moons of theSolar System, using both infrared techniques (typical of orbiter and flyby missions of planets with solid surfaces) and microwave techniques (more often used for planets with atmospheres). Infrared temperature measurement instruments used in planetary missions include surface temperature measurements taken by theThermal Emission Spectrometer (TES) instrument onMars Global Surveyor and theDiviner instrument on theLunar Reconnaissance Orbiter;[32] and atmospheric temperature measurements taken by the composite infrared spectrometer instrument on the NASACassini spacecraft.[33]
Microwave atmospheric temperature measurement instruments include theMicrowave Radiometer on theJuno mission to Jupiter.
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