Alessio Lattanzio

Coupling Diffuse Sky Radiation and Surface Albedo

Abstract New satellite instruments have been delivering a wealth of information regarding land surface albedo. This basic quantity describes what fraction of solar radiation is reflected from the earth’s surface. However, its concept and measurements have some ambiguity resulting from its dependence on the incidence angles of both the direct and diffuse solar radiation. At any time of day, a surface receives direct radiation in the direction of the sun, and diffuse radiation from the various other directions in which it may have been scattered by air molecules, aerosols, and cloud droplets. This contribution proposes a complete description of the distribution of incident radiation with angles, and the implications in terms of surface albedo are given in a mathematical form, which is suitable for climate models that require evaluating surface albedo many times. The different definitions of observed albedos are explained in terms of the coupling between surface and atmospheric scattering properties. The ana...

Spectral conversion of surface albedo derived from meteosat first generation observations

Comparison of surface albedos derived from spaceborne radiometers with different spectral bands requires, first of all, the conversion of these quantities into common spectral intervals. This letter proposes a spectral conversion method specifically dedicated to surface albedo derived in a large-band instrument such as the solar channel onboard the Meteosat first-generation radiometer. This new method accounts for the retrieval algorithm assumptions and radiometer spectral limitations that might have an impact on the retrieved surface albedo in such a large band. It is also shown that the proposed approach has no impact when surface albedo is derived in narrow bands and confirms the results of previously published spectral conversion methods.

Land Surface Albedo from Geostationary Satelites: A Multiagency Collaboration within SCOPE-CM

Climate has been recognized to have direct and indirect impact on society and economy, both in the long term and daily life. The challenge of understanding the climate system, with its variability and changes, is enormous and requires a joint long-term international commitment from research and governmental institutions. An important international body to coordinate worldwide climate monitoring efforts is the World Meteorological Organization (WMO). The Global Climate Observing System (GCOS) has the mission to provide coordination and the requirements for global observations and essential climate variables (ECVs) to monitor climate changes. The WMO-led activity on Sustained, Coordinated Processing of Environmental Satellite Data for Climate Monitoring (SCOPE-CM) is responding to these requirements by ensuring a continuous and sustained generation of climate data records (CDRs) from satellite data in compliance with the principles and guidelines of GCOS. SCOPE-CM represents a new partnership between operat...

Surface albedo response to sahel precipitation changes

The Sahel is a narrow region in West Africa, lying between the Sahara desert at 18°N and savannah and equatorial forest at about 15°N. The climate and atmospheric dynamics of this region are determined by the West African monsoon resulting from the alternating southwesterly and northwesterly winds, corresponding to a north-south migration of the Intertropical convergence Zone (ITCZ). during the northward shift of the ITCZ, rainfall occurs over Sahel during a short but intense wet season, between late June and mid-September, which induces fast vegetation growth. The dry season that follows this rainy period produces a rapid drying the vegetation.

Intercalibration of Polar-Orbiting Spectral Radiometers Without Simultaneous Observations

A new intercalibration method for two polar-orbiting satellite instruments or two instrument constellations’ Fundamental Climate Data Records (FCDRs) is presented. It is based on statistical fitting of reflectance data from the two instruments covering the same area during the same period, but not simultaneously. A Deming regression with iterative weights is used. The accuracy of the intercalibration method itself was better than 0.5% for the Moderate Resolution Imaging Spectroradiometer (MODIS) versus MODIS and Advanced Very High Resolution Radiometer (AVHRR) versus AVHRR test data sets. The intercalibration of an AVHRR FCDR generated by NOAA versus a combined MODIS Terra and Aqua data set of red and near-infrared (NIR) channels was carried out and showed a difference in the reflectance values of about 2% (red) and 6% (NIR). The presented intercalibration method can be used for checking the calibration of two instruments or FCDRs in all viewing angles used separately.

Optimal estimation applied to the joint retrieval of aerosol optical depth and surface BRF using MSG/SEVIRI observations

Passive spaceborne imagers observe radiation that has interacted both with the atmosphere and the surface. Interactions with the atmosphere include gaseous absorption and scattering by molecules and particulate matter or aerosols. Their characterization from remote sensing observations relies essentially on their capacity to modify differently the amount of radiation observed as a function of the wavelength, the viewing directions or the polarization. One of the major issues when retrieving tropospheric aerosol properties using spaceborne imager observations is to discriminate the contribution of the observed signal reflected by the surface from the one scattered and absorbed by the aerosols. In particular, it is crucial when the retrieval occurs above land surfaces which might be responsible for a non-negligible part of the total signal. Conceptually, this is equivalent to solving a radiative system composed of minimum two layers, where the upper layers includes aerosols and the bottom ones represents the soil/vegetation strata. This problem is further complicated by the intrinsic anisotropic radiative behavior of natural surfaces and its coupling with atmospheric radiative processes. An increase in aerosol optical thickness is responsible for an increase of the fraction of diffuse sky radiation which, in turn, smooths the effects of surface anisotropy.

A Multisensor Approach to Global Retrievals of Land Surface Albedo

Satellite-based retrievals offer the most cost-effective way to comprehensively map the surface albedo of the Earth, a key variable for understanding the dynamics of radiative energy interactions in the atmosphere-surface system. Surface albedo retrievals have commonly been designed separately for each different spaceborne optical imager. Here, we introduce a novel type of processing framework that combines the data from two polar-orbiting optical imager families, the Advanced Very High-Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiometer (MODIS). The goal of the paper is to demonstrate that multisensor albedo retrievals can provide a significant reduction in the sampling time required for a robust and comprehensive surface albedo retrieval, without a major degradation in retrieval accuracy, as compared to state-of-the-art single-sensor retrievals. We evaluated the multisensor retrievals against reference in situ albedo measurements and compare them with existing datasets. The results show that global land surface albedo retrievals with a sampling period of 10 days can offer near-complete spatial coverage, with a retrieval bias mostly comparable to existing single sensor datasets, except for bright surfaces (deserts and snow) where the retrieval framework shows degraded performance because of atmospheric correction design compromises. A level difference is found between the single sensor datasets and the demonstrator developed here, pointing towards a need for further work in the atmospheric correction, particularly over bright surfaces, and inter-sensor radiance homogenization. The introduced framework is expandable to include other sensors in the future.