Optical Parameters of extended Cloud from Airborne Observation

Here data from past airborne experiments with NASAŒs Cloud Absorption Radiometer (CAR) and old data (1970-89) from airborne observations obtained in the USSR is be analyzed. Observations are spectral and contain a lot of useful information about the interaction between solar radiation, atmosphere and clouds. In the past we have proposed different approaches for processing and interpretation of experimental data of different origin (satellite, ground and airborne). Mainly all approaches are based on simulations and comparing results with observational data by choosing optical parameters that best satisfy all the measured radiative characteristics. Observations in one or in several spectral channels were used. In the later case the average droplet size was obtained. In many studies the approximation of conservative atmosphere (single scattering albedo equal to 1) in the shortwave spectral region was assumed. Some authors accept the semi-infinite optical thickness. Often certain links between optical parameters in different wavelength are suggested. However, these assumptions and a priori restrictions on desired parameters prevent the realization of true values. In this study the analytical approach of inverse asymptotic formulas of the transfer theory will be used to retrieve cloud layer optical parameters from airborne NASA data. The method is free from a priori restrictions and no links to parameters. We propose to construct the cloud optical model on the basis of obtained result from NASA and Russian data and to calculate radiative cloud characteristics: reflection, transmission and radiative divergence. NASA observations are made in viewing angle ranges 0 - 180° at 1° intervals; thus, there is a possibility for integrating data over the solid angle to obtain irradiance. The method allows mutual validation of different approaches. Observations were carried out at latitude 21.73304°S and longitude 13.70252°E. The solar incident angle was 37.59740°. The flight altitude range from 354 to 1170 m Duration of the experiment was around 1 hour. Measurements include 8 spectral channels: 340, 381, 472, 682, 870, 1,035, 1,219, 1,273 nm. The analytical method of inverse asymptotic formulas elaborated by authors is used to retrieve cloud optical parameters (optical thickness and single scattering albedo). The method has been applied to data set of different origin and geometry of observations. The realization of the method includes a set of formulas, which does not cited here and is possible to find elsewhere. Earlier results of the optical parameters retrieval from data of satellite (instrument POLDER on ADEOS-1) of reflected solar radiation and from airborne observation were published. Spectral dependencies of stratus cloud optical parameters in terms of the volume scattering and absorption coefficients were retrieved from airborne observational data of hemispherical fluxes of the solar diffuse radiation, which were carried out in USSR from 1970-80. Flights were accomplished in ranges of 8 radiation experiments in different geographical regions in 70-80th of last century on the URSS territory and above Atlantic near the North-West Africa. Observations were accomplished only in extended stratus cloudiness and above homogeneous surface (sand of desert, sea, ice and snow surfaces). It gives the possibility to analyse the radiative regime of cloudy atmosphere in different regions. The accurate analysis of the used technique is accomplished. The set of loop numerical experiments is accomplished for testing the effectiveness of considered approach. Several numerical radiative models of cloud with NASA computer codes of DISORT are used for testing the approach of the optical parameters retrieval. Obtained results are compared with initial parameters used for model intensity calculation. Thus uncertainties and restrictions of the analytical method of the inverse problem solution are elucidated.