We present estimates of direct radiative effects (DRE) for background and smoke aerosol in the IR spectral region. The estimates are obtained using an original Monte Carlo algorithm and OPAC models...

We present model estimates of diurnally averaged direct radiative effects of smoke aerosol in the solar spectral range obtained using observations at the Tiksi station in the period of...

We present the first box model simulation results aimed at identification of possible effects of the atmospheric photochemical evolution of the organic component of biomass burning (BB) aerosol on the aerosol radiative forcing (ARF) and its efficiency (ARFE). The simulations of the dynamics of the optical characteristics of the organic aerosol (OA) were performed using a simple parameterization developed within the volatility basis set framework and adapted to simulate the multiday BB aerosol evolution in idealized isolated smoke plumes from Siberian fires (without dilution). Our results indicate that the aerosol optical depth can be used as a good proxy for studying the effect of the OA evolution on the ARF, but variations in the scattering and absorbing properties of BB aerosol can also affect its radiative effects, as evidenced by variations in the ARFE. Changes in the single scattering albedo (SSA) and asymmetry factor, which occur as a result of the BB OA photochemical evolution, may either reduce or enhance the ARFE as a result of their competing effects, depending on the initial concentration OA, the ratio of black carbon to OA mass concentrations and the aerosol photochemical age in a complex way. Our simulation results also reveal that (1) the ARFE at the top of the atmosphere is not significantly affected by the OA oxidation processes compared to the ARFE at the bottom of the atmosphere, and (2) the dependence of ARFE in the atmospheric column and on the BB aerosol photochemical ages almost mirrors the corresponding dependence of SSA.

We present the results from statistical simulation of the albedo and diffuse transmission of the atmosphere in the visible wavelength region in the presence of overcast and broken cirrus clouds. The main numerical experiments were performed using the third version of the model proposed by B.A. Baum, P. Yang, A.J. Heymsfield et al. (a mixture of particles of different shapes and sizes with a very rough surface). To estimate the effects of the random cloud geometry on the solar radiative transfer in the atmosphere, we used the method of closed equations, proposed by G.A. Titov, and developed within the model based on the Poisson point fluxes on straight lines. Analysis of how the microstructure of cirrus clouds influences the ensemble-averaged albedo and diffuse transmission at moderate cloud fractions showed that the average value of the uncertainty due to the lack of information on the particle shape and size is within ∼ ± 2%. This value is comparable to the effect of the random geometry in optically thin clouds; while in optically dense clouds the range of errors, caused by the neglect of the horizontal inhomogeneity, increases and is ∼ ± 5% in albedo calculations, with underestimation of diffuse transmission by ∼ 10–20%.

In this paper, we describe the results of simulating the bidirectional reflectance in three-dimensional (3D) cloud fields. For the calculations of reflectance, we use original statistical algorithms that ensure the effects of atmospheric sphericity and molecular absorption in the solar spectral range are accounted for. Cloud fields are simulated on the basis of a Poisson model of broken clouds; clouds are approximated by truncated paraboloids of rotation. The cloud heterogeneity effect on the averaging of reflection functions over an ensemble of cloud fields is estimated using numerical averaging of the stochastic radiative transfer equation, using a randomization. The simulation is performed for a mono-directional receiver with wavelength channels 0.55 and 2.15 µm, different realizations with small and moderate cloud fractions, and a set of sun-view geometries. With the appearance of an isolated cloud in the sky, the reflection function is determined by cloud presence/absence on the line of sight (LS), shading of LS by clouds/non-obscuration directed “toward the Sun,” and illumination of LS by cloud-reflected radiation. Passage to cloud fields gives rise to such additional factors as mutual shading and multiple scattering between clouds, which are mainly determined by cloud elements located near LS and directed “toward the Sun”. Strong fluctuations of reflectance as a function of the relative azimuth angle between sun and view directions in a specific realization are smoothed out after averaging over an ensemble of cloud fields. In interpreting the results of retrieving the cloud characteristics according to measurements of reflected radiation, it should be kept in mind that for fixed illumination conditions, the mean bidirectional reflectance may differ several-fold from bidirectional reflectance in a specific 3D cloud structure.

We consider the results of simulating the smoke aerosol radiative forcing (ARF) at the top and bottom of the atmosphere depending on the photochemical evolution of its organic component, illumination conditions, and underlying surface types (water, mixed forest, and snow/ice). At the top of the atmosphere, an increase in the surface albedo and a decrease in the aerosol optical depth can lead to the transformation of the cooling into a heating effect. An increase in absorptance of aerosol particles may cause the ARF to increase at the top of the atmosphere, which is manifested most significantly over snow/ice surfaces; while at the bottom of the atmosphere, ARF decreases. As the solar zenith angle grows, the ARF decreases in absolute value when the smoke plume propagates over weakly reflecting surfaces; however, with growing surface albedo, this dependence becomes nonmonotonic. It is shown that, when the transformations of the optical characteristics of organic aerosol are neglected, the aerosol radiative forcing at the top of the atmosphere can be overestimated or underestimated severalfold (by a few tens of W/m²), and this can also cause an error of determining the ARF sign.