Detection of clear air turbulence by water vapor emission anomalies: weighting functions and sensitivities.

It has been demonstrated by Kuhn et al. [Science 196, 1099 (1977)] that clear air turbulence (CAT) ahead of an aircraft can be sensed by a forward-looking IR radiometer tilted at a small elevation angle and detecting in the water vapor rotational emission band. An increase in the radiometer signal due to the upward transfer of moist air by turbulence provides several minutes warning of impending CAT. In this paper the radiance changes due to a model water vapor anomaly are calculated. The corresponding weighing function and its dependence on wavelength, altitude, and viewing angle are investigated. Consideration of the radiative transfer equation indicates that a weighting function which gives the change in transmittance with total water path is the most useful for the calculation of the water vapor emission changes due to CAT. The water vapor emission in the strongly absorbing rotational band and along a slant path in both the troposphere and stratosphere comes mainly from atomspheric layers close to the detector. It is thus possible to describe the water vapor emission in terms of an isothermal slant path to a good approximation.