Temperature and Humidity Profiles Sounding

Temperature and moisture structures of earth's atmosphere are some of the most significant factors that influence the weather and climate patterns on the local as well as on global scale. The vertical structure of temperature and humidity is retrieved by satellite "sounders" that operate in infrared and microwave frequencies. Sounders use the principle of gaseous absorption for the retrieval of temperature and humidity profiles. For the retrieval of temperature, absorption spectra of some uniformly mixed gas such as CO2, or, O2 is used (Fig. 3). These gases absorb earth's upwelling radiation. However, at some wavelengths (say, l1) their efficiency of absorption is very strong, while at some neighboring wavelengths (say, l2), it is very weak. Now, any radiation of wavelength l coming from lower layers of atmosphere has very little chance of reaching up to satellite, because it is getting strongly absorbed by the given gas. So, at this wavelength, most of the radiation will be coming from the upper layers of the atmosphere. Similarly, the radiation emitted from the ground at wavelength l will reach the satellite without much interruption, because the atmospheric gases absorb this radiation very weakly. It means that the radiation at wavelength l2 contains information about the atmospheric layers near the surface. Similarly, radiation at other wavelengths lying between l1 and l is sensitive to different atmospheric layers in vertical.

Since the radiance from these layers is highly sensitive to the temperature of these layers, the temperature information can be retrieved if we know the radiances reaching the satellite at different wavelengths. A sounder is designed to measure the upwelling radiances at different wavelengths, which are used for retrieving the temperature information. However the actual mathematical procedure of retrieval is quite complex.

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15^m CO2 Band as observed by the Nimbus - 4, IRIS

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Figure 3: (a) Curve showing the absorption of infrared radiation by different gases in the atmosphere. (b) Numbers shown in this curve (1,2,3,4.. .7) denote the central wavelengths of CO2 absorption band used for sounding of atmospheric temperature profiles

CO2 band

O2 band

Figure 4: Distribution of global temperature at four vertical levels by TOVS (Picture courtesy : rst.gsfc.nasa.gov)

The principle of the sounding of humidity profiles is similar to that of temperature. However, water vapor is not a uniformly mixed gas, and also it changes phase (i.e. ice, water, snow, or vapor) very frequently within the atmosphere. Satellite water vapor sounders use the water vapor absorption frequencies (~ 6 lm in IR sounder, and ~ 183 GHz in microwave sounders). However, the upwelling radiation at these wavelengths/frequencies not only depends upon the water vapor amount in different atmospheric layers, but also on the temperature of those layers. In this case a-priori information about the temperature structure of the atmosphere is crucial for the retrieval of humidity profiles.

An example of infrared sounders is TIROS Operational Vertical Sounder (TOVS), onboard NOAA series of satellites. This instrument is designed to profile temperature and water vapor. The TOVS is actually a three instrument complex: the High Resolution IR Sounder (HIRS-2), with 20 channels; the Stratospheric Sounding Unit (SSU), with three channels near 15 pm, and the Microwave Sounding Unit (MSU), a passive scanning microwave spectrometer with four channels in the 5.5 pm interval. Advance Microwave Sounding Units (AMSU-A, and AMSU-B onboard recent series of NOAA) are the examples of microwave sounders that are designed to sense temperature and humidity profiles respectively.

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