Satellite temperature measurements

Satellite temperature measurements

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The temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 of the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 at various altitudes as well as sea and land surface temperatures can be inferred from satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

 measurements. Weather satellite
Weather satellite
The weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be either polar orbiting, seeing the same swath of the Earth every 12 hours, or geostationary, hovering over the same spot on Earth by orbiting over the equator while...

s do not measure temperature directly but measure radiance
Radiance
Radiance and spectral radiance are radiometric measures that describe the amount of radiation such as light or radiant heat that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from...

s in various wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 bands. These measurements can be used to locate weather fronts, monitor the El Niño-Southern Oscillation
El Niño-Southern Oscillation
El Niño/La Niña-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years...

, determine the strength of tropical cyclones, study urban heat island
Urban heat island
An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night...

s and monitor the global climate. Wildfires, volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

s, and industrial hot spots can also be found via thermal imaging from weather satellites.

Since 1978 Microwave sounding unit
Microwave sounding unit
The Microwave sounding unit was the predecessor to the Advanced Microwave Sounding Unit.The MSU was first launched aboard the TIROS-N satellitein late 1978 and provided global coverage...

s (MSUs) on National Oceanic and Atmospheric Administration
National Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration , pronounced , like "noah", is a scientific agency within the United States Department of Commerce focused on the conditions of the oceans and the atmosphere...

 polar orbit
Polar orbit
A polar orbit is an orbit in which a satellite passes above or nearly above both poles of the body being orbited on each revolution. It therefore has an inclination of 90 degrees to the equator...

ing satellites have measured the intensity of upwelling microwave radiation from atmospheric oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, which is proportional to the temperature of broad vertical layers of the atmosphere. Measurements of infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 radiation pertaining to sea surface temperature have been collected since 1967.

Satellite datasets show that over the past four decades the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 has warmed and the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

 has cooled. Both of these trends are consistent with the influence of increasing atmospheric concentrations of greenhouse gases.

Measurement


Satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

s do not measure temperature. They measure radiances in various wavelength bands, which must then be mathematically inverted to obtain indirect inferences of temperature. 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. Among these are the UAH dataset
UAH satellite temperature dataset
The UAH satellite temperature dataset, developed at the University of Alabama in Huntsville, attempts to infer the temperature of the atmosphere at various levels from satellite measurements of radiance....

 prepared at the University of Alabama in Huntsville
University of Alabama in Huntsville
The University of Alabama in Huntsville is a state-supported, public, coeducational research university, located in Huntsville, Alabama, United States, is accredited by the Southern Association of Colleges and Schools to award baccalaureate, master's and doctoral degrees, and is organized in five...

 and the RSS dataset prepared by Remote Sensing Systems
Remote Sensing Systems
Remote Sensing Systems is a private research company founded in 1974 by Frank Wentz. It processes microwave data from a variety of NASA satellites. Most of their research is supported by the Earth Science Enterprise program. The company is based in Santa Rosa, California.They are a widely cited...

. The satellite series is not fully homogeneous - it is constructed from a series of satellites with similar but not identical instrumentation. The sensors deteriorate over time, and corrections are necessary for orbital drift and decay. Particularly large differences between reconstructed temperature series occur at the few times when there is little temporal overlap between successive satellites, making intercalibration difficult.

Surface measurements



Satellites may also be used to retrieve surface temperatures in cloud-free conditions, generally via measurement of thermal infrared from AVHRR. Weather satellites have been available to infer sea surface temperature
Sea surface temperature
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...

 (SST) information since 1967, with the first global composites occurring during 1970. Since 1982, satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

s have been increasingly utilized to measure SST and have allowed its spatial and temporal
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

 variation to be viewed more fully. For example, changes in SST monitored via satellite have been used to document the progression of the El Niño-Southern Oscillation since the 1970s. Over the land the retrieval of temperature from radiances is harder, because of the inhomogeneities in the surface. Studies have been conducted on the urban heat island effect via satellite imagery. Use of advanced very high resolution infrared satellite imagery
Advanced Very High Resolution Radiometer
The Advanced Very High Resolution Radiometer is a space-borne sensor embarked on the National Oceanic and Atmospheric Administration family of polar orbiting platforms . AVHRR instruments measure the reflectance of the Earth in 5 relatively wide spectral bands...

 can be used, in the absence of cloudiness, to detect density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 discontinuities (weather front
Weather front
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front...

s) such as cold front
Cold front
A cold front is defined as the leading edge of a cooler mass of air, replacing a warmer mass of air.-Development of cold front:The cooler and denser air wedges under the less-dense warmer air, lifting it...

s at ground level. Using the Dvorak technique
Dvorak technique
The Dvorak technique is a widely used system to subjectively estimate tropical cyclone intensity based solely on visible and infrared satellite images. Several agencies issue Dvorak intensity numbers for cyclones of sufficient intensity...

, infrared satellite imagery can used to determine the temperature difference between the eye
Eye (cyclone)
The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is a roughly circular area and typically 30–65 km in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the second most severe weather of a cyclone...

 and the cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

 top temperature of the central dense overcast of mature tropical cyclones to estimate their maximum sustained wind
Maximum sustained wind
The maximum sustained winds associated with a tropical cyclone are a common indicator of the intensity of the storm. Within a mature tropical cyclone, they are found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are...

s and their minimum central pressures
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

. Along Track Scanning Radiometers
AATSR
The Advanced Along Track Scanning Radiometer is one of the Announcement of Opportunity instruments on board the European Space Agency 's Envisat satellite....

 aboard weather satellites are able to detect wildfires, which show up at night as pixels with a greater temperature than 308 kelvins (94.7 °F). The Moderate-Resolution Imaging Spectroradiometer aboard the Terra satellite
Terra (satellite)
Terra is a multi-national NASA scientific research satellite in a sun-synchronous orbit around the Earth. It is the flagship of the Earth Observing System...

 can detect thermal hot spots associated with wildfires, volcanos, and industrial hot spots.

Tropospheric and stratospheric measurements



Since 1979, microwave sounding unit
Microwave sounding unit
The Microwave sounding unit was the predecessor to the Advanced Microwave Sounding Unit.The MSU was first launched aboard the TIROS-N satellitein late 1978 and provided global coverage...

s (MSUs) on NOAA polar orbiting satellites have measured the intensity of upwelling microwave radiation from atmospheric oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

. The intensity is proportional to the temperature of broad vertical layers of the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

, as demonstrated by theory and direct comparisons with atmospheric temperatures from radiosonde (balloon) profiles. Upwelling radiance is measured at different frequencies; these different frequency bands sample a different weighted range of the atmosphere. Channel 2 is broadly representative of the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

, albeit with a significant overlap with the lower stratosphere (the weighting function has its maximum at 350 hPa and half-power at about 40 and 800 hPa). In an attempt to remove the stratospheric
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

 influence, Spencer and Christy
John Christy
John R. Christy is a climate scientist at the University of Alabama in Huntsville whose chief interests are satellite remote sensing of global climate and global climate change. He is best known, jointly with Roy Spencer, for the first successful development of a satellite temperature...

 developed the synthetic "2LT" product by subtracting signals at different view angles; this has a maximum at about 650 hPa. However this amplifies noise, increases inter-satellite calibration biases and enhances surface contamination. The 2LT product has gone through numerous versions as various corrections have been applied.

Trends from the record


Year UAH Trend
1991 0.087
1992 0.024
1993 -0.013
1994 -0.003
1995 0.033
1996 0.036
1997 0.040
1998 0.112
1999 0.105
2000 0.095
2001 0.103
2002 0.121
2003 0.129
2004 0.130
2005 0.139
2006 0.140
2007 0.143

Records have been created by merging data from nine different MSUs, each with peculiarities (e.g., time drift of the spacecraft relative to the local solar time) that must be calculated and removed because they can have substantial impacts on the resulting trend. The satellite record is short, which means adding a few years on to the record or picking a particular time frame can change the trends considerably. The problems with the length of the MSU record is shown by the table to the right, which shows the UAH TLT (lower tropospheric) global trend
UAH satellite temperature dataset
The UAH satellite temperature dataset, developed at the University of Alabama in Huntsville, attempts to infer the temperature of the atmosphere at various levels from satellite measurements of radiance....

 (°C/decade) beginning with Dec 1978 and ending with December of the year shown.

The process of constructing a temperature record from a radiance record is difficult. The satellite temperature record comes from a succession of different satellites and problems with inter-calibration between the satellites are important, especially NOAA-9, which accounts for most of the difference between various analyses. NOAA-11 played a significant role in a 2005 study by Mears et al. identifying an error in the diurnal correction that leads to the 40% jump in Spencer and Christy's trend from version 5.1 to 5.2. There are ongoing efforts to resolve differences in satellite temperature datasets.

Christy et al. (2007) find that the tropical temperature trends from radiosondes matches closest with his v5.2 UAH dataset. Furthermore, they assert there is a growing discrepancy between RSS and sonde trends beginning in 1992, when the NOAA-12 satellite was launched. This research found that the tropics were warming, from the balloon data, +0.09 (corrected to UAH) or +0.12 (corrected to RSS) or 0.05 K (from UAH MSU; ±0.07 K room for error) a decade.

Using the T2 channel (which include significant contributions from the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

, which has cooled), Mears et al. of Remote Sensing Systems (RSS) find (through January 2011) a trend of +0.091 °C/decade. Spencer and Christy of the University of Alabama in Huntsville (UAH), find a smaller trend of +0.052 °C/decade.
Channel Start End Date RSS Global Trend
(K/decade)
UAH Global Trend
(K/decade)
STAR v2.0 Global Trend
(K/decade)
TLT 1979 2011-01 0.148 0.140
TMT 1979 2011-01 0.091 0.052 0.137
TTS 1987 2011-01 0.001
TLS 1979 2011-01 -0.306 -0.391

A no longer updated analysis of Vinnikov and Grody found +0.20°C per decade (1978–2005). Another satellite temperature analysis is provided by NOAA/NESDIS STAR Center for Satellite Application and Research and use simultaneous nadir overpasses (SNO) to remove satellite intercalibration biases yielding more accurate temperature trends. The SNO analysis finds a 1979-2010 trend of +0.140°C/decade for T2 channel.

Lower stratospheric cooling is mainly caused by the effects of ozone depletion
Ozone depletion
Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere , and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon...

 with a possible contribution from increased stratospheric water vapor and greenhouse gases increase. There is a decline in stratospheric temperatures, interspersed by warmings related to volcanic eruptions. Global Warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

 theory suggests that the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

 should cool while the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 warms 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 of El Chichón
El Chichón
El Chichón, also known as El Chichonal is an active volcano in Francisco León Municipality in northwestern Chiapas, Mexico. Its only recorded eruptive activity was on March 29, April 3 and April 4, 1982 , when it produced a one km-wide caldera that then filled with an acidic crater lake...

 and Mount Pinatubo
Mount Pinatubo
Mount Pinatubo is an active stratovolcano located on the island of Luzon, near the tripoint of the Philippine provinces of Zambales, Tarlac, and Pampanga. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is west of the dormant and...

, this behavior of the global stratospheric temperature has been attributed to global ozone concentration variation in the two years following volcanic eruptions.
Since 1996 the trend is slightly positive due to ozone recover juxtaposed to a cooling trend of 0.1K/decade that is consistent with the predicted impact of increased greenhouse gases.

Comparison to instrumental record


The satellite records have the advantage of global coverage, whereas the radiosonde record is longer. There have been complaints of data problems with both records.

To compare to the trend from the surface temperature record
Instrumental temperature record
The instrumental temperature record shows fluctuations of the temperature of the global land surface and oceans. This data is collected from several thousand meteorological stations, Antarctic research stations and satellite observations of sea-surface temperature. Currently, the longest-running...

 (approximately +0.07 °C/decade over the past century and +0.17 °C/decade since 1979) it is most appropriate to derive trends for the part of the atmosphere nearest the surface, i.e., the lower troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

. Doing this, through January 2011:
  • RSS v3.3 finds a trend of +0.148 °C/decade.
  • UAH v5.4 finds a trend of +0.140°C/decade.


An alternative adjustment introduced by Fu et al. (2004) finds trends (1979–2001) of +0.19 °C/decade when applied to the RSS data set.

Reconciliation with climate models



Climate model results summarized by the IPCC in their third assessment show overall good agreement with the satellite temperature record. In particular both models and satellite record show a global average warming trend for the troposphere (models range for TLT/T2LT 0.6 - 0.39°C/decade; avg 0.2°C/decade) and a cooling of the stratosphere (models range for TLS/T4 -0.7 - 0.08°C/decade; avg -0.25°C/decade).

There remain, however, differences in detail between the satellite data and the climate models used.

Globally, the troposphere is predicted by models to warm about 1.2 times more than the surface; in the tropics, the troposphere should warm about 1.5 times more than the surface. Most climate models used by the IPCC in preparation of their third assessment show a slightly greater warming at the TLT level than at the surface (0.03°C/decade difference) for 1979-1999 while GISS and Hadley Centre surface station network trends are +0.161 and +0.160 °C/decade respectively, the lower troposphere trends calculated from satellite data by UAH and RSS are +0.140 °C/decade and +0.148 °C/decade. The expected trend in the lower troposphere, given the surface data, would be around 0.194 °C/decade.

This greater global average warming in the troposphere compared to the surface (present in the models but not observed data) is most marked in the tropics. CCSP SAP 1.1 chapter 5 says:

"In the tropics, surface temperature changes are amplified in the free troposphere. Models and observations
show similar amplification behavior for monthly and interannual temperature variations, but not for decadal
temperature changes. Tropospheric amplification of surface temperature anomalies is due to the release of latent heat by moist, rising
air in regions experiencing convection."


Although all the datasets show the expected tropospheric amplification at seasonal and annual timescales it is still debated whether or not the long term trends are consistent with the expected moist adiabatic lapse rate amplification due to difficulty of producing homogenized datasets, some satellite temperature reconstruction are consistent with the expected amplification while others are not.

Historic differences



For some time the only available satellite record was the UAH version, which (with early versions of the processing algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...

) showed a global cooling trend for its first decade. Since then, a longer record and a number of corrections to the processing have revised this picture: the UAH dataset has shown an overall warming trend since 1998, though less than the RSS version. In 2001, an extensive comparison and discussion of trends from different data sources and periods was given in the Third Assessment Report of the Intergovernmental Panel on Climate Change
Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change is a scientific intergovernmental body which provides comprehensive assessments of current scientific, technical and socio-economic information worldwide about the risk of climate change caused by human activity, its potential environmental and...

(IPCC) (section 2.2.4).

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