Sensitivity of upwelling radiance in nimbus 6 HIRS channels to multilayered clouds


  • Robert G. Feddes,

  • Kuo-Nan Liou


An infrared radiative transfer model is developed that can simulate the effects of clouds on satellite upwelling radiances at the top of the atmosphere for a range of satellite scan angles. The model allows the calculations of the upwelling radiance for single layers of middle or high clouds and for multilayered clouds. Sensitivity analyses are carried out for the upwelling radiance in the presence of clouds in the Nimbus 6 high-resolution infrared sounder (HIRS) channels. Computational results show that thick cirrus cloud reduces the upwelling radiance most in both single and multicloud cases. Thin cirrus and thin middle clouds show a greater decrease in the upwelling radiance in comparison with thin cirrus and thick middle clouds in CO2 channels where the weighting functions peak in or above the thin middle cloud. More variability is noted between window and water vapor channel upwelling radiances than is present in the CO2 channels. Shortwave CO2 channels are further shown to have a greater sensitivity to changes in cloud scenes than longwave CO2 channels. We show that the effect of the atmospheric temperature profile is minimized by dividing the clear column radiance into the cloudy radiance. By utilizing the actual HIRS data a case study for the determination of the cloud thickness based on the theoretical sensitivity analyses has been carried out. When infrared and visible cloud pictures from the NOAA 4 satellite are compared, the case study shows some promise of cloud parameter determination using the combination of infrared channels.