Aerosol Retrieval Using Synthetic Polder Multi-Angular Data

The POLarizations and Directionality of the Earth's Reflectances (POLDER) instrument onboard the Japanese ADEOS satellite offers unique possibilities for the retrieval of aerosol parameters with its polarization and multi-angular capability. In this study we examine a technique that simultaneously retrieve multiple aerosol parameters, namely asymmetry factor, single scattering albedo, surface albedo, and optical thickness. using simulated POLDER reflectances. It is found that. over dark or bright surfaces, simultaneous retrieval of multiple parameters is indeed possible, but not over surfaces with intermediate reflectivity. Among the four parameters, the single-scattering albedo is retrieved with the best accuracy and is the least vulnerable when the reflectance value is subjected to a 0.1% white noise

Evaluating Satellite-Based Aerosol Retrievals Over Mountainous Regions Of The U.S.

Satellite-based aerosol retrievals from NASA’s Moderate Resolution Imaging Spectrometer (MODIS) and NASA’s Multi-angle Imaging Spectrometer (MISR) are evaluated above four mountainous U.S. sites. We (1) examine the influence of spatial and temporal variability in aerosol and surface properties on satellite / sunphotometer agreement, (2) apply and assess an automated method for optimizing collocation window and radius in the context of variability in surface properties and aerosol optical depth (AOD), and (3) compare the performance of satellite AOD products above the four sites, and examine factors influencing their performance. Maps of the Normalized Differential Vegetation Index (NDVI), topography, and land cover are used to characterize the surface properties within a 50 km radius of each site. At the eastern sites, satellite-sunphotometer mean bias is primarily influenced by topography, urban regions, and water bodies. Collocations at the western sites are complicated by heterogeneous surface types and NDVI. The collocation window optimization algorithm is insensitive to temporal window size and spatial radius for the eastern sites but is less successful at optimization for the western sites. Averages performed at the selected collocation window size indicate little seasonal influence at the eastern sites and reduced collocation frequency during winter at the western sites

Sensitivity Study of the Effects of Mineral Dust Particle Nonsphericity and Thin Cirrus Clouds on MODIS Dust Optical Depth Retrievals and Direct Radiative Forcing Calculations

A special challenge posed by mineral dust aerosols is associated with their predominantly nonspherical particle shapes. In the present study, the scattering and radiative properties for nonspherical mineral dust aerosols at violet-to-blue (0.412, 0.441, and 0.470 μm) and red (0.650 μm) wavelengths are investigated. To account for the effect of particle nonsphericity on the optical properties of dust aerosols, the particle shapes for these particles are assumed to be spheroids. A combination of the T-matrix method and an improved geometric optics method is applied to the computation of the single-scattering properties of spheroidal particles with size parameters ranging from the Rayleigh to geometric optics regimes. For comparison, the Mie theory is employed to compute the optical properties of spherical dust particles that have the same volumes as their nonspherical counterparts. The differences between the phase functions of spheroidal and spherical particles lead to quite different lookup tables (LUTs) involved in retrieving dust aerosol properties. Moreover, the applicability of a hybrid approach based on the spheroid model for the phase function and the sphere model for the other phase matrix elements is demonstrated. The present sensitivity study, employing the Moderate Resolution Imaging Spectroradiometer (MODIS) observations and the fundamental principle of the Deep Blue algorithm, illustrates that neglecting the nonsphericity of dust particles leads to an underestimate of retrieved aerosol optical depth at most scattering angles, and an overestimate is noted in some cases. The sensitivity study of the effect of thin cirrus clouds on dust optical depth retrievals is also investigated and quantified from MODIS observations. The importance of identifying thin cirrus clouds in dust optical depth retrievals is demonstrated. This has been undertaken through the comparison of retrieved dust optical depths by using two different LUTs. One is for the dust only atmosphere, and the other is for the atmosphere with overlapping mineral dust and thin cirrus clouds. For simplicity, the optical depth and bulk scattering properties of thin cirrus clouds are prescribed a priori. Under heavy dusty conditions, the errors in the retrieved dust optical depths due to the effect of thin cirrus are comparable to the assumed optical depth of thin cirrus clouds. With the spheroidal and spherical particle shape assumptions for mineral dust aerosols, the effect of particle shapes on dust radiative forcing calculations is estimated based on Fu-Liou radiative transfer model. The effect of particle shapes on dust radiative forcing is illustrated in the following two aspects. First, the effect of particle shapes on the single-scattering properties of dust aerosols and associated dust direct radiative forcing is assessed, without considering the effect on dust optical depth retrievals. Second, the effect of particle shapes on dust direct radiative forcing is further discussed by including the effect of particle nonsphericity on dust optical depth retrievals

Aerosol Lightning Enhancement over Northern Alabama: Predictions, Mechanisms, and Simulations

Satellite aerosol retrievals, ground-based radar and lightning detections, and model simulations are used to study the impact of aerosols on lightning and the usefulness of knowing the aerosol state in predicting enhanced lightning over northern Alabama. The results show that the Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) retrievals are less useful in predicting enhanced lightning flash rate (FR) for lightning-producing storms than the forecasts of other meteorological variables that are more closely linked to the intensification of convective storms. However, when relatively weaker convective available potential energy (CAPE) is forecast, the probability of enhanced lightning FR increases in a more polluted environment, making the knowledge of aerosols more useful in lightning inference in such CAPE regimes. The FR shows a stronger correlation with the optical depth of absorbing aerosols than that of non-absorbing aerosols, particularly in a low CAPE regime, suggestive of a potentially stronger regulation of storms by absorbing aerosols. The presence of absorbing aerosols may lead to the accumulation of CAPE, as suggested by an increased correlation between AOD and CAPE in the presence of absorbing aerosols. The optical depth of absorbing aerosols shows a weak negative correlation with the planetary boundary layer height (PBLH), suggesting that the interaction between absorbing aerosols and turbulent mixing may contribute to the regulation of lightning-producing storms. Aerosol enhancement of lightning may be associated with enhanced convergence in the boundary layer and secondary convection, which appears to result from a synthesis of multiple mechanisms related to both microphysical and radiative effects of aerosols. The impact of absorbing aerosols on deep convection is sensitive to the height of aerosol layer. A sensitivity modeling study suggests that a daytime heating layer above the PBL suppresses deep convection in the early afternoon and enhances nighttime storms when the accumulated CAPE is released. A daytime heating layer within the PBL delays the onset of the enhanced nighttime storms and may result in a faster development of the storms at night; the enhanced evaporation of cloud and rain water droplets right before the onset of nighttime storms may contribute to the onset delay