Solar spectral absorption by marine stratus clouds: Measurements and modeling

Abstract

[1] The measurement of cloud absorption from aircraft has been a controversial subject largely because broadband measurements provide little insight into the physical mechanisms underlying the absorption. To partition and quantify the various mechanisms of cloud absorption, spectrally resolved measurements are required. Measurements of cloud solar spectral (400–2150 nm) absorption from airborne spectroradiometers are presented from two cases of extensive tropical marine stratus cloud fields. Radiative transfer modeling was used to retrieve the cloud optical thickness and cloud droplet effective radius from a best fit with the measured cloud spectral albedo. These values were used to estimate the cloud spectral absorptance. For the higher optical thickness case, the measurement-model agreement in absorptance across the spectrum is better than 0.05 and substantially better (within 0.01) at visible wavelengths unaffected by cloud absorption. For an optically thinner and more heterogeneous cloud field, the differences were higher, up to 0.07 in the near infrared. The standard deviations of cloud absorptance spectra show that the integrated absorbed irradiances, usually measured by broadband radiometers, are strongly affected by variations in the water vapor amount. Radiative transfer modeling is used to illustrate the spectral dependence of the absorption from radiatively important gases (e.g., water vapor), cloud liquid water, and absorbing aerosol particles. A novel sampling strategy, based on single aircraft measurements, is demonstrated, as is the value of spectrally resolved measurements in partitioning the various mechanisms of cloud absorption including the possible effects of absorbing aerosols embedded in clouds.