Showing papers by "Andrew A. Lacis published in 1995"

Calculation of surface and top of atmosphere radiative fluxes from physical quantities based on ISCCP data sets: 1. Method and sensitivity to input data uncertainties

Abstract: The largest uncertainty in upwelling shortwave (SW) fluxes (approximately equal 10-15 W/m(exp 2), regional daily mean) is caused by uncertainties in land surface albedo, whereas the largest uncertainty in downwelling SW at the surface (approximately equal 5-10 W/m(exp 2), regional daily mean) is related to cloud detection errors. The uncertainty of upwelling longwave (LW) fluxes (approximately 10-20 W/m(exp 2), regional daily mean) depends on the accuracy of the surface temperature for the surface LW fluxes and the atmospheric temperature for the top of atmosphere LW fluxes. The dominant source of uncertainty is downwelling LW fluxes at the surface (approximately equal 10-15 W/m(exp 2)) is uncertainty in atmospheric temperature and, secondarily, atmospheric humidity; clouds play little role except in the polar regions. The uncertainties of the individual flux components and the total net fluxes are largest over land (15-20 W/m(exp 2)) because of uncertainties in surface albedo (especially its spectral dependence) and surface temperature and emissivity (including its spectral dependence). Clouds are the most important modulator of the SW fluxes, but over land areas, uncertainties in net SW at the surface depend almost as much on uncertainties in surface albedo. Although atmospheric and surface temperature variations cause larger LW flux variations, the most notable feature of the net LW fluxes is the changing relative importance of clouds and water vapor with latitude. Uncertainty in individual flux values is dominated by sampling effects because of large natrual variations, but uncertainty in monthly mean fluxes is dominated by bias errors in the input quantities.

Nonsphericity of dust‐like tropospheric aerosols: Implications for aerosol remote sensing and climate modeling

Abstract: T-matrix computations of light scattering by polydispersions of randomly oriented nonspherical aerosols and Mie computations for equivalent spheres are compared. Findings show that even moderate nonsphericity results in suubstantial errors in the retrieved aerosol optical thickness if satellite reflectance measurements are analyzed using Mie theory. On the other hand, the use of Mie theory for nonspherical aerosols produces negligible errors in the computation of albedo and flux related quantities, provided that the aerosol size distribution and optical thickness are known beforehand. The first result can be explained by large nonspherical-spherical differences in scattering phase function, while the second result follows from small nonspherical-spherical differences in single-scattering albedo and asymmetry parameter. No cancellation of errors occurs if one consistently uses Mie theory in the retrieval algorithm and then in computing the albedo for the retrieved aerosol optical thickness.

Low-Cost Long-Term Monitoring of Global Climate Forcings and Feedbacks

Abstract: We describe the rationale for long-term monitoring of global climate forcings and radiative feedbacks as a contribution to interpretation of long-term global temperature change. Our discussion is based on a more detailed study and workshop report (Hansen et al., 1993b). We focus on the potential contribution of a proposed series of inexpensive small satellites, but we discuss also the need for complementary climate process studies and ground-based measurements. Some of these measurements could be made inexpensively by students, providing both valuable climate data and science educational experience.

Satellite remote sensing of nonspherical tropospheric aerosols

Abstract: In this paper we discuss the possible effect of nonsphericity of solid tropospheric aerosols on the accuracy of aerosol thickness retrievals from reflectance measurements over the ocean surface. To model light-scattering properties of nonspherical aerosols, we use a shape mixture of moderately aspherical, randomly oriented polydisperse spheroids. We assume that the size distribution and refractive index of aerosols are known and use the aerosol optical thickness 0.2 to computer the reflectivity for an atmosphere-ocean model similar to that used in the AVHRR aerosol retrieval algorithms. We then use analogous computations for volume- equivalent spherical aerosols with varying optical thickness to invert the simulated nonspherical reflectance. Our computations demonstrate that the use of the spherical model to retrieve the optical thickness of actually nonspherical aerosols can result in errors which, depending on the scattering geometry, can well exceed 100%.

Incorporation of a rough ocean surface and semi-infinite water body in multiple scattering computations of polarized light in an atmosphere-ocean system

Abstract: The use of accurate space-born polarimetric measurements to retrieve tropospheric aerosol characteristics is a promising remote sensing tool, but also imposes strong requirements on the atmosphere-ocean model in terms of its adequacy and on computational techniques in terms of their accuracy and efficiency. The present work is concerned with computing the reflection matrix of an atmosphere-ocean system within this context. We use the Ambartsumyan non- linear integral equation to obtain the reflection matrix for a semi-infinite homogeneous ocean body containing hydrosols. The reflection and transmission matrices of a statistically rough ocean surface are obtained using the standard Kirchhoff formulation, with shadowing effects taken into account. The reflection properties of the combined ocean body and ocean surface are obtained employing the adding method. We use the Fourier decomposition of the scattering matrices and separation of the first-order scattering to substantially reduce the computational burden. An atmospheric model containing aerosols and molecules is computed and added on the top of the ocean system using the adding/doubling method. We report preliminary computational data and discuss the variation of the degree of linear polarization of singly and multiply scattered radiation as a function of scattering geometry, surface roughness, and aerosol and molecular optical depth.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.