Cirrus cloud properties derived from high spectral resolution infrared spectrometry during FIRE II .2. Aircraft HIS results

This paper presents analysis of cloud observations by the High-Resolution Interferometer Sounder made from the NASA ER-2 aircraft during FIRE II. Clear and cloudy sky radiance spectra are presented in terms of differences between observations and radiative transfer model simulations. Doubling/adding radiative transfer model simulations demonstrate that the magnitude of the brightness temperature differences (Delta BT) is a function of the cloud particle size distribution and the cloud ice water path. For effective radii greater than approximately 30 mu m (size parameter of 18) there is little spectral variation in the brightness temperature (BT). An analysis of brightness temperature differences indicates that cirrus clouds over the FIRE II central site possessed a small-particle mode. The cases analyzed had similar appearances in a plot of Delta BT between 11 and 12 mu m (BT11 - BT12) versus the observed Delta BT between 8 and 11 mu m (BT8 - BT11), suggesting similarity in the microphysical properties of nongray cirrus. Brightness temperature differences between cirrus cloud over the central site and the Gulf of Mexico are presented to illustrate differences in the cirrus microphysical properties at the two different locations. Cloud effective emissivities and effective radiative temperatures were derived for observations over the FIRE central site using complementary lidar and radiosonde data. Small variations in these effective properties were seen on 5 December and 22 November. Although they had similar effective temperatures, the emissivities were very different. Very few clouds were observed to have an emissivity near unity.