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--- instruments:hatpro:hatpro [2016/06/11 20:30] – susanne
+++ instruments:hatpro:hatpro [2016/06/11 21:14] – susanne
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 ===== Introduction =====
 Microwave radiometers are very sensitive receivers designed to measure thermal electromagnetic radiation emitted by material media like the atmosphere. They are usually equipped with multiple receiving channels in order to derive the characteristic emission spectrum of the atmosphere or extraterrestrial objects. Microwave radiometers are utilized in a variety of environmental and engineering applications, including weather forecasting, climate monitoring, [[https://en.wikipedia.org/wiki/Radio_astronomy|radio astronomy]] and radio propagation studies.
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 {{:instruments:sunhat:p1040071.jpg?400|}} \\
-[[http://www.radiometer-physics.de/rpg/html/Home.html|RPG]] HATPRO-SUNHAT at the [[http://barbados.zmaw.de|Barbados Clouds Observatory]].
+Fig. 1: [[http://www.radiometer-physics.de/rpg/html/Home.html|Humitity and Temperature Profiler]] (HATPRO-SUNHAT) at the [[http://barbados.zmaw.de|Barbados Clouds Observatory]].
 ===== History of microwave radiometer measurements =====
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 Here we could keep the graphic from the original article
 https://en.wikipedia.org/wiki/Microwave_radiometer#/media/File:Radiometer_227629main_ostm-AMR-RSA.jpg
+Fig. 2
 =====  Principle of operation =====
-Solids, liquids (e.g. the earth's surface, ocean, sea ice, snow, vegetation) but also gases emit and absorb MW radiation. Traditionally, the amount of radiation a microwave radiometer receives is expressed as the equivalent blackbody temperature also called [[https://en.wikipedia.org/wiki/Brightness_temperature|brightness temperature]] (TB). The atmospheric components emit and absorb MW radiation in different ways: Atmospheric gases provide specific absorption features (Fig. 1) which allow to derive information about their vertical profile. Examples for such absorption features are the oxygen absorption complex (caused by magnetic dipole transitions) around 60 GHz which is used to derive temperature profiles or the water vapor absorption line around 22.235 GHz (dipole rotational transition) which is used to observe the vertical profile of humidity. Other significant absorption lines are found at 118.75 GHz (oxygen absorption) and at 183.31 GHz (water vapor absorption, used for water vapor profiling under dry conditions or from satellite). Weak absorption features due to ozone are also used for stratospheric [[http://link.springer.com/article/10.1023%2FA%3A1010406601571|ozone density and temperature]] profiling [7].
+Solids, liquids (e.g. the earth's surface, ocean, sea ice, snow, vegetation) but also gases emit and absorb MW radiation. Traditionally, the amount of radiation a microwave radiometer receives is expressed as the equivalent blackbody temperature also called [[https://en.wikipedia.org/wiki/Brightness_temperature|brightness temperature]] (TB). In the microwave range several atmospheric gases inhibit [[https://en.wikipedia.org/wiki/Rotational_spectroscopy|rotational lines]]. They provide specific absorption features (Fig. 3) which allow to derive information about their abundance and vertical structure. Examples for such absorption features are the oxygen absorption complex (caused by magnetic dipole transitions) around 60 GHz which is used to derive temperature profiles or the water vapor absorption line around 22.235 GHz (dipole rotational transition) which is used to observe the vertical profile of humidity. Other significant absorption lines are found at 118.75 GHz (oxygen absorption) and at 183.31 GHz (water vapor absorption, used for water vapor profiling under dry conditions or from satellite). Weak absorption features due to ozone are also used for stratospheric [[http://link.springer.com/article/10.1023%2FA%3A1010406601571|ozone density and temperature]] profiling.
-The emission and absorption of hydrometeors does not provide characteristic absorption line features as found for atmospheric gases. Liquid hydrometeors (small cloud and rizzle drops) are efficient emitters in the microwave. Besides the distinct absorption features of molecular transistion lines, there are also contributions of so-called „continuum“ absorbers in the microwave region as the water vapor continuum and the liquid water continuum. The latter is only present if a cloud is present in the field of view of the MWR, hence, measuring at two frequencies, one close to the water absorption line (22.235 GHz) and one in the nearby window region (typically 31 GHz) provides information on both the columnar amount of water vapor and the columnar amount of liquid water separately (two-channel radiometer).
+Besides the distinct absorption features of molecular transistion lines, there are also non-resonant contributions by hydrometeors (liquid drops and frozen particles). Liquid water emission increases with frequency, hence, measuring at two frequencies, typically one close to the water absorption line (22.235 GHz) and one in the nearby window region (typically 31 GHz) dominated by liquid absorption provides information on both the columnar amount of water vapor and the columnar amount of liquid water separately (two-channel radiometer). The so-called „water vapor continuum“ is arises from the contribution of far away water vapor lines.
-Larger rain drops as well as larger frozen hydrometeors (snow, graupel, hail) also scatter microwave radiation especially at higher frequencies (>90 GHz). These scattering effects can be used to distinguish between rain and cloud water content [10] but also to constrain the columnar amount of snow and ice particles from space [11] and from the ground [12].
+Larger rain drops as well as larger frozen hydrometeors (snow, graupel, hail) also scatter microwave radiation especially at higher frequencies (>90 GHz). These scattering effects can be used to distinguish between rain and cloud water content exploitinh polarized measurements [5] but also to constrain the columnar amount of snow and ice particles from space [6] and from the ground [7].
+{{:instruments:hatpro:mwr_5.png?600|Microwave spectrum: The black lines show the simulated spectrum (in brightness temperatures TB) for a ground-based receiver; the colored lines are the spectrum obtained from a satellite instrument over the ocean measuring at horizontal (blue) and vertical (red) linear polarization. Solid lines indicate simulations for clear-sky (cloud-free) conditions, dotted lines show a clear-sky case with a single layer liquid cloud. The vertical lines indicate typical frequencies used by satellite sensors like the [[https://en.wikipedia.org/wiki/Advanced_Microwave_Sounding_Unit|AMSU]] radiometer.}} \\
-{{ :instruments:hatpro:mwr_5.png?600 |Microwave spectrum: The black lines show the simulated spectrum (in brightness temperatures TB) for a ground-based receiver; the colored lines are the spectrum obtained from a satellite instrument over the ocean measuring at horizontal (blue) and vertical (red) linear polarization. Solid lines indicate simulations for clear-sky (cloud-free) conditions, dotted lines show a clear-sky case with a single layer liquid cloud. The vertical lines indicate typical frequencies used by satellite sensors like the [[https://en.wikipedia.org/wiki/Advanced_Microwave_Sounding_Unit|AMSU]] radiometer.}} \\
+Fig. 3: Microwave spectrum: The black lines show the simulated spectrum (in brightness temperatures TB) for a ground-based receiver; the colored lines are the spectrum obtained from a satellite instrument over the ocean measuring at horizontal (blue) and vertical (red) linear polarization. Solid lines indicate simulations for clear-sky (cloud-free) conditions, dotted lines show a clear-sky case with a single layer liquid cloud. The vertical lines indicate typical frequencies used by satellite sensors like the [[https://en.wikipedia.org/wiki/Advanced_Microwave_Sounding_Unit|AMSU]] radiometer.
-Microwave spectrum: The black lines show the simulated spectrum (in brightness temperatures TB) for a ground-based receiver; the colored lines are the spectrum obtained from a satellite instrument over the ocean measuring at horizontal (blue) and vertical (red) linear polarization. Solid lines indicate simulations for clear-sky (cloud-free) conditions, dotted lines show a clear-sky case with a single layer liquid cloud. The vertical lines indicate typical frequencies used by satellite sensors like the [[https://en.wikipedia.org/wiki/Advanced_Microwave_Sounding_Unit|AMSU]] radiometer.
 ===== Design =====
 The principal components of a microwave radiometer often follow a similar design and can be grouped into: antenna system, microwave radio-thermal receiver, recording and storage devices and a final processing unit. Usually ground-based radiometers are also equipped with environmental sensors (rain, temperature, humidity) and GPS receivers (time and location reference). The antenna itself often measures through a window made of foam which is transparent in the MW (light blue material in Fig. 1) in order to keep the antenna clean of dust, liquid water and ice. Often, also a heated blower system is attached the radiometer which helps to keep the window free of liquid drops or dew (strong emitters in the MW) but also free of ice and snow.
-{{ :instruments:hatpro:mwr_4.png?400 |Schematic diagram of a microwave radiometer [8].}} \\
+{{:stuff:mwr_design.png?200|Schematic diagram of a microwave radiometer}} \\
-Schematic diagram of a microwave radiometer [8].
+Fig. 4: Schematic diagram of a microwave radiometer.
 ===== Calibration =====
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 [4] Passive Microwave Remote Sensing of the Earth, Physical Foundations, Eugene A. Sharkov, Springer-Praxis Books in Geophysical Sciences, Chapter 14: Passive microwave space missions
-  - Cimini et al., 2009
+[5] Czekala et al. (2001), Discrimination of cloud and rain liquid water path by groundbased polarized microwave radiometry, Geophy. Res. Lett., DOI: 10.1029/2000GL012247
-  - Klein and Gasiewski, 2000
-  - Eugene A. Sharkov, “Passive Microwave Remote Sensing of the Earth”, Physical Foundations, Springer-Praxis Books in Geophysical Sciences, Chapter 3: Microwave radiometers: functions, design, concepts, characteristics [http://www.iki.rssi.ru/asp/pub_sha1/pub_sha1.htm] (INAKTIVE LINK!)
+[6] Bennartz, R., and P. Bauer (2003), Sensitivity of microwave radiances at 85–183 GHz to precipitating ice particles, Radio Sci., 38(4), 8075, doi:10.1029/2002RS002626.
-  - http://fas.org/irp/imint/docs/rst/Sect14/Sect14_4.html
-  - Czekala et al., Discrimination of cloud and rain liquid water path by groundbased polarized microwave radiometry, GRL, 2001, DOI: 10.1029/2000GL012247
+[7| Kneifel et al. (2010), Snow scattering signals in ground-based passive microwave radiometer measurements, J. Geophys. Res., DOI: 10.1029/2010JD013856
-  - Bennartz, R., and P. Bauer (2003), Sensitivity of microwave radiances at 85–183 GHz to precipitating ice particles, Radio Sci., 38(4), 8075, doi:10.1029/2002RS002626.
-  - Kneifel et al., Snow scattering signals in ground-based passive microwave radiometer measurements, JGR, 2010, DOI: 10.1029/2010JD013856