Cirrus cloud transmittance and backscatter in the infrared measured with a CO_2 lidar

Abstract

Two independent methods of measuring the transmittance of cirrus clouds are compared. Both used a CO(2) pulsed Doppler lidar at a wavelength of 10.59 microm. The first method used backscatter from the calibration target El Chichon stratospheric cloud that was present over Boulder in 1982 and 1983. The second method used conical lidar scans at different zenith angles when uniform cirrus decks were present. Extinction coefficients measured from both methods average 0.1 km(-1) for tenuous cirrus 1.0 km thick to 0.78 km(-1) for cirrus several kilometers thick. There is a wide standard deviation in extinction values. Extinction-tobackscatter ratios S vary from <1000 sr for tenuous clouds to 2600 sr for dense clouds. Mie scattering and extinction calculations for spherical ice particles of 10-50 microm in radius lead to ratios S > 2000 sr, so long as the ice absorption is entered into the calculations. The backscattering ratio for ice cylinders is 1 order of magnitude lower than for spheres. Backscatter in the IR may, therefore, be reasonably well modeled by some combination of spheres and cylinders. Cloud thickness statistics from lidar returns show that cirrus decks average ~500 m thick. Clouds thinner than 300 m were often overlooked by the unaided surface-based observer. These preliminary results are in rather close agreement with the LOWTRAN 6 cirrus cloud model predictions.

Lidar-Measured Winds from Space: A Key Component for Weather and Climate Prediction

“…Values of the volume backscatter coefficient p (rrr 1 sr 1 ) for atmospheric aerosols and clouds can easily range over six to eight orders of magnitude (Kent et al 1983a;Kent et al 1983b;Yue et al 1983;Hall et al 1988) depending on the wavelength. During the early conceptual studies on the precursor to LAWS, referred to as WINDSAT (wind satellite) (Huffaker 1978;Huffaker etal.…”

Atmospheric Radiation

“…Furthermore, the role of cirrus anvils in deep tropical convection is a subject of current study [ Ackerman et al , 1988]. Apart from FIRE, there have been other extensive efforts at elucidating the radiative properties of cirrus [ Platt et al , 1987, 1989; Hall et al , 1988; Sassen et al , 1989; Liou et al , 1990; Stone et al , 1990]. Recent studies in the theoretical modeling of radiative transfer in cirrus clouds include the work of Takano and Liou [1989], Liou et al [1990] and Detwiler and Ramaswamy [1990].…”