Robert Frouin

An emerging ground‐based aerosol climatology: Aerosol optical depth from AERONET

Long-term measurements by the AERONET program of spectral aerosol optical depth, precipitable water, and derived Angstrom exponent were analyzed and compiled into an aerosol optical properties climatology. Quality assured monthly means are presented and described for 9 primary sites and 21 additional multiyear sites with distinct aerosol regimes representing tropical biomass burning, boreal forests, midlatitude humid climates, midlatitude dry climates, oceanic sites, desert sites, and background sites. Seasonal trends for each of these nine sites are discussed and climatic averages presented.

Asian dust events of April 1998

On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the surface along the mountain ranges between British Columbia and California, and impacted severely the optical and the concentration environments of the region. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10–20%, but it reduced the near-UV cloud reflectance, causing a yellow coloration of all surfaces. The yellow colored backscattering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30–40%, doubled the diffuse radiation, and caused a whitish discoloration of the blue sky. On April 29 the average excess surface-level dust aerosol concentration over the valleys of the West Coast was about 20–50 μg/m3 with local peaks >100 μg/m3. The dust mass mean diameter was 2–3 μm, and the dust chemical fingerprints were evident throughout the West Coast and extended to Minnesota. The April 1998 dust event has impacted the surface aerosol concentration 2–4 times more than any other dust event since 1988. The dust events were observed and interpreted by an ad hoc international web-based virtual community. It would be useful to set up a community-supported web-based infrastructure to monitor the global aerosol pattern for such extreme aerosol events, to alert and to inform the interested communities, and to facilitate collaborative analysis for improved air quality and disaster management.

Passive remote sensing of tropospheric aerosol and atmospheric correction for the aerosol effect

The launch of ADEOS in August 1996 with POLDER, TOMS, and OCTS instruments on board and the future launch of EOS-AM 1 in mid-1998 with MODIS and MISR instruments on board start a new era in remote sensing of aerosol as part of a new remote sensing of the whole Earth system (see a list of the acronyms in the Notation section of the paper). These platforms will be followed by other international platforms with unique aerosol sensing capability, some still in this century (e.g., ENVISAT in 1999). These international spaceborne multispectral, multiangular, and polarization measurements, combined for the first time with international automatic, routine monitoring of aerosol from the ground, are expected to form a quantum leap in our ability to observe the highly variable global aerosol. This new capability is contrasted with present single-channel techniques for AVHRR, Meteosat, and GOES that although poorly calibrated and poorly characterized already generated important aerosol global maps and regional transport assessments. The new data will improve significantly atmospheric corrections for the aerosol effect on remote sensing of the oceans and be used to generate first real-time atmospheric corrections over the land. This special issue summarizes the science behind this change in remote sensing, and the sensitivity studies and applications of the new algorithms to data from present satellite and aircraft instruments. Background information and a summary of a critical discussion that took place in a workshop devoted to this topic is given in this introductory paper. In the discussion it was concluded that the anticipated remote sensing of aerosol simultaneously from several space platforms with different observation strategies, together with continuous validations around the world, is expected to be of significant importance to test remote sensing approaches to characterize the complex and highly variable aerosol field. So far, we have only partial understanding of the information content and accuracy of the radiative transfer inversion of aerosol information from the satellite data, due to lack of sufficient theoretical analysis and applications to proper field data. This limitation will make the anticipated new data even more interesting and challenging. A main concern is the present inadequate ability to sense aerosol absorption, from space or from the ground. Absorption is a critical parameter for climate studies and atmospheric corrections. Over oceans, main concerns are the effects of white caps and dust on the correction scheme. Future improvement in aerosol retrieval and atmospheric corrections will require better climatology of the aerosol properties and understanding of the effects of mixed composition and shape of the particles. The main ingredient missing in the planned remote sensing of aerosol are spaceborne and ground-based lidar observations of the aerosol profiles.

A review of satellite methods to derive surface shortwave irradiance

Abstract Shortwave radiative fluxes at the earths surface are of primary interest in climate research because they control the total energy exchange between the atmosphere and the land/ocean surface. Information on these fluxes is needed on a global scale, and therefore, has to be obtained by methods of remote sensing from observations made with instruments carried on satellites. The primary objective of this paper is to review current capabilities and activities to infer these forcing functions from satellite observations and to discuss future needs. Discussed will be fluxes of downward surface shortwave radiation (DSSR) and net surface shortwave radiation (NSSR). Methods for deriving DSSR fluxes on a global scale are becoming operational. They are being used increasingly to address climate issues, such as in determining the role of solar forcing in oceanic and atmospheric processes, hydrological modeling, and in carbon cycling. Based on extensive comparisons with ground-truth it is believed that estimates of DSSR fluxes can be obtained within 20 Wm−2 or better on monthly time scales, for areas of an average climate model grid size. Methods for deriving NSSR fluxes seem promising, but need to be further evaluated.

Spectral reflectance of sea foam in the visible and near‐infrared: In situ measurements and remote sensing implications

The spectral reflectance of sea foam was measured at the Scripps Institution of Oceanography Pier, La Jolla, California, by viewing the sea surface radiometrically in a region of breaking waves. Foam reflectance was found to decrease substantially with wavelength in the near-infrared, contrary to the findings of previous studies, theoretical as well as experimental. Values in the visible (0.44 μm) were reduced by typically 40% at 0.85 μm, 50% at 1.02 μm, and 85% at 1.65 μm. The spectral effect was explained by the nature of the foam, which is composed of large bubbles of air separated by a thin layer of water (foam stricto sensu) and of bubbles of air injected in the underlayer. The presence of bubbles in the underlayer enhances water absorption and thus reduces reflectance in the near-infrared. For ocean color remote sensing, affected by the presence of foam and aerosols, the consequences of neglecting the spectral dependence of foam are dramatic. With only a small amount of foam, in the presence of aerosols or not and thus irrespective of aerosol type, the errors in the retrieved water reflectance at 0.44 μm are above 0.01, which does not meet the accuracy goal of 0.001 for biological applications. Since under normal conditions the effect of foam may have the same magnitude as the effect of aerosols, atmospheric corrections will be inaccurate (and useless) in many cases, even taking into account the spectral dependence of the foam reflectance. Space observations potentially contaminated by an effective foam reflectance (product of reflectance and fractional coverage) above 0.001, i.e., corresponding to wind speeds above 8 m s−1, should be eliminated systematically. Utilization of near-infrared wavelengths above 0.9 μm for atmospheric corrections of ocean color, possible with the moderate-resolution imaging spectrometer (MODIS), would aggravate the problem. The measurements also indicated that foam significantly affects the retrieval of aerosol turbidity at 0.85 and 1.02 μm for wind speeds above 10 m s−1 but impacts minimally turbidity estimates at 1.65 μm. Over the oceans the spectral range above 1 μm is definitely recommended for remote sensing of tropospheric aerosol load and type from space.

Estimating Photosynthetically Active Radiation (PAR) at the earth's surface from satellite observations

Abstract Current satellite algorithms to estimate photosynthetically active radiation (PAR) at the Earths surface are reviewed, and selected results are presented. PAR can be obtained directly from top-of-atmosphere solar radiance, which is used to determine the transmissivity of the atmosphere. Since clouds do not absorb significantly at PAR wavelengths, the radiative transfer modeling is generally simplified compared to that for total insolation. The accuracies reported, about 10% and 6% on daily and monthly time scales, respectively, are useful for modeling oceanic and terrestrial primary productivity. The large short-term variability in the ratio of PAR and insolation, essentially due to clouds, is reduced at those time scales, suggesting that reasonably accurate PAR climatologies may be obtained from available insolation climatologies (satellite or other).

April 1998 Asian dust event: A southern California perspective

In late April 1998 an extreme Asian dust episode reached the U.S. western seaboard. This event was observed by several in situ and remote sensing atmospheric measurement stations. Dramatic reductions in boundary layer visibility were recorded and the resultant peak backscatter coefficients exceeded prevailing upper tropospheric background conditions by at least 2 orders of magnitude. An analysis of this event is given using lidar vertical backscatter profilometry, concurrent Sun photometer opacity data, and transport modeling. At San Nicolas Island the measured and modeled aerosol optical thickness at 500 nm increased dramatically from 0.15 on April 25 to 0.52 on April 26-27. Volume size distribution on April 27 exhibited a prominent coarse mode at 1-2 μm radius, and single-scattering albedo was observed to increase from 0.90 in the blue to 0.93 in the near infrared. Concurrent lidar observations tracked the evolution of the plume vertical structure, which consisted of up to three well-defined layers distributed throughout the free troposphere.

Determination from space of atmospheric total water vapor amounts by differential absorption near 940 nm : theory and airborne verification

Abstract A new technique is proposed to estimate atmospheric total water vapor amounts from space. The technique consists of viewing the Earths surface in two spectral channels, one narrow, the other wide, centered on the same wavelength at the water vapor absorption maximum near 940 nm. With these characteristics, the ratio of the solar radiance measured in the two channels is independent of the surface reflectance and yields a direct estimate of the water vapor amount integrated along the optical path. To test the technique, we designed and built a two-channel radiometer based on the above concept. Airborne experiments carried out with the new device demonstrate the techniques feasibility under clear sky conditions over both sea and land. Over the ocean and in the presence of thick aerosol layers, however, total water vapor amounts may be underestimated by as much as 20%. Compared to satellite microwave techniques, which are applicable under most weather conditions, the proposed technique has the adva...

Response of the equatorial Pacific to chlorophyll pigment in a mixed layer isopycnal ocean general circulation model

The influence of phytoplankton on the upper ocean dynamics and thermodynamics in the equatorial Pacific is investigated using an isopycnal ocean general circulation model (OPYC) coupled with a mixed layer model and remotely sensed chlorophyll pigment concentration from the Coastal Zone Color Scanner (CZCS). In the equatorial Pacific heat accumulation due to a higher abundance of chlorophyll pigments in the equatorial Pacific leads to a decrease of the mixed layer thickness. This generates anomalous westward geostrophic currents north and south of the equator. In the western equatorial Pacific, these anomalous geostrophic currents merge into and strengthen the equatorial undercurrent (EUC), supplying water mass from the 200 m depth to the eastern equatorial Pacific. This chlorophyll-induced response of the undercurrent enhances upwelling around 110W, resulting in a lower sea surface temperature (SST) than without chlorophyll. Thus, thermal gradients due to absorption of solar radiation by phytoplankton may contribute remotely to equatorial upwelling in the eastern Pacific.

A Study of Global Aerosol Optical Climatology with Two-Channel AVHRR Remote Sensing

Global distributions of the aerosol optical thickness and Angstrom exponent are estimated from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer channel-1 and -2 radiances for four months in 1990. Global distributions of those Angstrom parameters are consistent with present knowledge on the distributions of desert-derived, biomass-burning, and anthropogenic pollutant aerosols obtained by ground- based and aircraft measurements. Especially, it is found that thin anthropogenic aerosols can be identified with large Angstrom exponent values around the east coast of North America, Europe, and eastern Asia. Satellite- retrieved values of Angstrom parameters are further compared with measured spectral optical thickness obtained by the National Aeronautics and Space Administration Aerosol Robotic Network sky radiometer network.