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Robert Frouin and Beth Chertock

Abstract

An algorithm based on radiative transfer theory is presented to generate the first accurate, long-term (84- month) climatology of net surface solar irradiance over the global oceans from Nimbus-7 earth radiation budget (ERB) wide-field-of-view planetary-albedo data. Net surface solar irradiance is computed as the difference between the top-of-atmosphere incident solar in-irradiance (known) and the sum of the solar irradiance reflected back to space by the earth-atmosphere system (observed) and the solar irradiance absorbed by atmospheric constituents (modeled). Apart from planetary albedo and sun zenith angle, the most important parameters governing net surface solar irradiance variability, the model input parameters (water vapor and ozone amounts, cloud absorptance, aerosol type, and surface visibility), are fixed at their climatological values. It is shown that the effects of clouds and clear-atmosphere constituents can be decoupled on a monthly time scale, which makes it possible to directly apply the algorithm with monthly averages of ERB planetary-albedo data. Compared theoretically with the algorithm of Gautier et al., the present algorithm yields higher solar irradiance values in clear and thin cloud conditions and lower values in thick cloud conditions. The agreement, however, remains within 10–20 W m−2.

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Beth Chertock and Y. C. Sud

Abstract

A global, 7-year satellite-based record of ocean surface solar irradiance (SSI) is used to assess the realism of ocean SSI simulated by the nine-layer Goddard Laboratory for Atmospheres (GLA) General Circulation Model (GCM). January and July climatologies of net SSI produced by the model are compared with corresponding satellite climatologies for the world oceans between 54°N and 54°S. This comparison of climatologies indicates areas of strengths and weaknesses in the GCM treatment of cloud-radiation interactions, the major source of model uncertainty. Realism of ocean SSI is also important for applications such as incorporating the GLA GCM into a coupled ocean-atmosphere GCM. The results show that the GLA GCM simulates too much SSI in the extratropies and too little in the tropics, especially in the summer hemisphere. These discrepancies reach magnitudes of 60 W m−2 and more. The discrepancies are particularly large in the July case off the western coast of North America. In this region of persistent marine stratus, the GCM climatological values exceed the satellite climatological values by as much as 131 W m−2. Positive and negative discrepancies in SSI are shown to be consistent with discrepancies in planetary albedo.

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Beth Chertock, Robert Frouin, and Catherine Gautier

Abstract

The present study constitutes the generation and validation of the first satellite-based, long-term record of surface solar irradiance over the global oceans. The record is generated using Nimbus-7 earth radiation budget (ERB) wide-field-of-view (WFOV) planetary-albedo data as input to a numerical algorithm designed and implemented for this study based on radiative transfer theory. Net surface solar irradiance is obtained by subtracting the solar radiation reflected by the ocean-atmosphere system (measured by satellite) and the solar radiation absorbed by atmospheric constituents (modeled theoretically) from the solar irradiance at the top of the atmosphere (a known quantity). The resulting monthly mean values are computed on a 9° latitude-longitude spatial grid for November 1978°October 1985.

Because direct measurements of surface solar irradiance are not available on the global spatial scales needed to validate the new approach, the ERB-based values cannot be verified directly against in situ pyranometer data. Although the ERB-based annual and monthly mean climatologies are compared with those obtained from ship observations and empirical formulas, a comparison with long-term mean climatologies does not provide an assessment of the month-to-month accuracies achieved using the new technique. Furthermore, the accuracy of the ship-based climatologies is questionable.

Therefore, the new dataset is validated in comparisons with short-term, regional, high-resolution, satellite- based records (which were generated using methods that in turn have been validated using in situ measurements). The ERB-based values of net surface solar irradiance are compared with corresponding values based on radiance measurements taken by the VISSR (Visible-Infrared Spin Scan Radiometer) aboard GOES (Geostationary Operational Environmental Satellite) series satellites during the TOGA (Tropical Ocean Global Atmosphere), Tropic Heat, and MONEX (Monsoon Experiment) field experiments. The rms differences are 14.5 W m−2 (i.e., 6.2% of the average VISSR-based value on monthly time scales) for the TOGA data comparison, 6.4 W m−2 (i.e., 2.5% of the average VISSR-based value on monthly time scales) for the Tropic Heat data comparison, and 16.8 W m−2 (i.e., 7.5% of the average VISSR-based value on monthly time scales) for the MONEX data comparison. The ERB-based record is also compared with an additional satellite-based dataset, focused primarily over the Atlantic Ocean, that was generated using radiance measurements from the Meteosat radiometer. On the basis of these validation studies, errors in the new dataset are estimated to lie between 10 and 20 W m−2 on monthly time scales.

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Beth Chertock, Robert Frouin, and Richard C. J. Somerville

Abstract

A new method has been used to generate the first satellite-based long-term climatology of surface solar irradiance over the world oceans. These monthly mean data cover the period November 1978 through October 1985 on a global, 9° latitude-longitude spatial grid. The large-scale variability of surface solar irradiance is assessed over the world oceans for the entire (84-month) record. The results demonstrate the ability of the method to reveal large-scale seasonal and interannual phenomena. The reduction in surface solar irradiance due to clouds is evaluated globally both on monthly and long-term climatological scales. Monthly cloud forcing anomalies are found to display eastward propagation over the course of the 1982–1983 El Niño event. The mean January climatology is found to he consistent with the climatology obtained from a general circulation model run in perpetual January mode. This study marks the first large-scale observation-based examination of cloud solar forcing at the ocean surface. In addition, empirical orthogonal function (EOF) analysis is employed to investigate modes of seasonal and nonseasonal variability. Nonseasonal EOF modes of surface solar irradiance are related to nonseasonal EOF modes of outgoing longwave radiation (OLR). The dominant modes during the 1982–1983 El Niño are associated with eastward propagation in both the shortwave and longwave fields. These dominant nonseasonal EOF modes of surface solar irradiance are found to display features and amplitude variations that are nearly identical to those of the corresponding nonseasonal EOF modes of OLR. The association of these modes with El Niño is quantified using the correlation of the mode amplitudes with the Southern Oscillation index (SOI). In each case modes 1 and 2 are positively correlated with the SOI, and mode I has a strong correlation of 0.75 for the shortwave and 0.76 for the longwave field. Finally, a study of the regionally averaged behavior of surface solar irradiance and sea surface temperature (SST) in a section of the tropical Pacific (9°N–9°5, 117°– 144°W) during this same period indicates that fluctuations of surface solar irradiance in the tropical Pacific are sometimes a regional response to underlying changes in SST (and associated changes in cloudiness), rather than a driving mechanism responsible for variations in SST.

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