Satellite Sensors Measuring Atmospheric Water Vapour

Because of its importance in the atmosphere (for surface weather, energy balance of the climate system, and atmospheric chemistry), water vapour has been measured from space since the early days of satellite Earth observation by a variety of sensors and different techniques covering the altitude range from the surface up to the lower thermosphere region around 100 km. Information on the water content of the troposphere, where the bulk of atmospheric water resides, is available from nadir sounding (down-looking) instruments, widely used since the 1960s in the operational meteorological observation systems. Measurements of the small water vapour amounts of only a few water molecules per million air molecules in the upper troposphere and throughout the entire middle atmosphere require use of the limb sounding observation geometry, in emission or (solar, stellar, satellite-to-satellite) occultation, providing high sensitivity owing to the long emission or absorption path along the line-of-sight. Generally, limb scanning techniques allow to obtain vertically well resolved profile measurements of water vapour throughout the middle atmosphere with resolution of typically only a few kilometres, since altitude information is obtained from the relative pointing information of the scanning satellite sensors. Water, cloud, or aerosol absorption are often limiting factors of limb observations in the troposphere and lowermost stratosphere, depending on wavelength. The vertical resolution of a nadir measurement is determined by the frequency dependence of the water absorption in the observed set of spectral channels, and retrievals provide the (Jacobian weighted) average humidity over wider layers of typically several kilometres thickness. The penetration depth of a nadir sounder into the troposphere depends on the absorption in the observed spectral band(s), which is often dominated by cloud absorption but also depends on the amount of water vapour itself as a dryer atmosphere allows deeper penetration.