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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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Catherine Gautier and Robert Frouin

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This paper examines the evolution of the net surface solar irradiance from March 1982 to October 1985 in an important region of the equatorial Pacific where the TROPIC HEAT Experiment took place (4.6°N to 7.4°S, 142.6° to 117.17°W). The investigation, which focuses on the dramatic modification of radiative processes as a consequence of the 1982-83 El Niño episode, analyzes annual and monthly net surface solar irradiance fields computed from geostationary satellite observations. An annual mean for the year October 1984-September 1985, the year most distant from the El Niño event during the study, is computed and compared with existing climatologies. It is found that, while our values compare well with those of Weare et al., they are significantly higher than those of Esbensen and Kushnir and those of Chou. In the case of Esbensen and Kushnir, a climatology often used to force numerical ocean circulation models, the discrepancy reaches 40 W m−2. Among the three annual fields computed, the two non-El Niño years are relatively similar, exhibiting features analogous to those found in all the climatologies. As expected, the El Niño year's annual mean field is remarkably different and is characterized by a complete disappearance of the zonal orientation and much smaller values along the equator, particularly in the western part of the studied region. The monthly mean fields confirm the El Niño's marked effect on net surface solar irradiance, especially during January, February, and March 1983, when the solar irradiance at the equator is reduced by more than 150 Wm−2. An empirical orthogonal function analysis of the monthly fields containing the annual cycle quantifies the annual and interannual variations of the net surface solar irradiance and demonstrates that the two main forcing periods for the study area are both 12 months. The one explaining most of the variance (62%), however, exhibits two pronounced maxima at six-month intervals. This mode is much more perturbed during the El Niño than the other and confirms that the effects of the El Niño on solar irradiance are much stronger in the equatorial region than in higher latitudes; it further indicates that the solar irradiance deviations from the annual mean are more positive (less cloudiness) than during normal years al the end of the event. Although the study's results are limited to the area and the period examined, they are strongly indicative of the radiative processes occurring near the equator during a strong El Niño.

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François-Marie Breon, Robert Frouin, and Catherine Gautier

Abstract

A method is proposed to compute the net solar (shortwave) irradiance at the earth's surface from Earth Radiation Budget Experiment (ERBE) data in the S4 format. The S4 data are monthly averaged broadband planetary albedo collected at selected times during the day. Net surface shortwave irradiance is obtained from the shortwave irradiance incident at the top of the atmosphere (known) by subtracting both the shortwave energy flux reflected by the earth-atmosphere system (measured) and the energy flux absorbed by the atmosphere (modeled). Precalculated atmospheric- and surface-dependent functions that characterize scattering and absorption in the atmosphere are used, which makes the method easily applicable and computationally efficient. Four surface types are distinguished, namely, ocean, vegetation, desert, and snow/ice. Over the tropical Pacific Ocean, the estimates based on ERBE data compare well with those obtained from International Satellite Cloud Climatology Project (ISCCP) B3 data. For the 9 months analyzed the linear correlation coefficient and the standard difference between the two datasets are 0.95 and 14 W m−2 (about 6% of the average shortwave irradiance), respectively, and the bias is 15 W m−2 (higher ERBE values). The bias, a strong function of ISCCP satellite viewing zenith angle, is mostly in the ISCCP-based estimates. Over snow/ice, vegetation, and desert no comparison is made with other satellite-based estimates, but theoretical calculations using the discrete ordinate method suggest that over highly reflective surfaces (snow/ice, desert) the model, which accounts crudely for multiple reflection between the surface and clouds, may substantially overestimate the absorbed solar energy flux at the surface, especially when clouds are optically thick. The monthly surface shortwave irradiance fields produced for 1986 exhibit the main features characteristic of the earth's climate. As found in other studies, our values are generally higher than Esbensen and Kushnir's by as much as 80 W m−2 in the tropical oceans. A cloud parameter, defined as the difference between clear-sky and actual irradiances normalized to top-of-atmosphere clear-sky irradiance, is also examined. This parameter, minimally affected by sun zenith angle, is higher in the midlatitude regions of storm tracks than in the intertropical convergence zone (ITCZ), suggesting that, on average, the higher cloud coverage in midlatitudes is more effective at reducing surface shortwave irradiance than opaque, convective, yet sparser clouds in the ITCZ. Surface albedo estimates are realistic, generally not exceeding 0.06 in the ocean, as high as 0.9 in polar regions, and reaching 0.5 in the Sahara and Arabian deserts.

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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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Jean-Yves Lojou, Robert Frouin, and René Bernard

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Vertically integrated atmospheric liquid water content derived from Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) brightness temperatures and from GOES-1 Visible and Infrared Spin-Scan Radiometer (VISSR) radiances in the visible are compared over the Indian Ocean during MONEX (monsoon experiment). In the retrieval procedure, Wilheit and Chang&apos algorithm and Stephens' parameterization schemes are applied to the SMMR and VISSR data, respectively. The results indicate that in the 0–100 mg cm−2 range of liquid water content considered, the correlation coefficient between the two types of estimates is 0.83 (0.81– 0.85 at the 99 percent confidence level). The Wilheit and Chang algorithm, however, yields values lower than those obtained with Stephens's schemes by 24.5 mg cm−2 on the average, and occasionally the SMMR-based values are negative. Alternative algorithms are proposed for use with SMMR data, which eliminate the bias, augment the correlation coefficient, and reduce the rms difference. These algorithms include using the Witheit and Chang formula with modified coefficients (multilinear regression), the Wilheit and Chang formula with the same coefficients but different equivalent atmospheric temperatures for each channel (temperature bias adjustment), and a second-order polynomial in brightness temperatures at 18, 21, and 37 GHz (polynomial development). When applied to a dataset excluded from the regressionn dataset, the multilinear regression algorithm provides the best results, namely a 0.91 correlation coefficient, a 5.2 mg cm−2 (residual) difference, and a −2.9 mg cm−2 bias. Simply shifting the liquid water content predicted by the Wilheit and Chang algorithm does not yield as good comparison statistics, indicating that the occasional negative values are not due only to a bias. The more accurate SMMR-derived liquid water content allows one to better evaluate cloud transmittance in the solar spectrum, at least in the area and during the period analyzed. Combining this cloud transmittance with a clear sky model would provide ocean surface insulation estimates from SMMR data alone.

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Robert Frouin, Pierre-Yves Deschamps, and Pierre Lecomte

Abstract

A new technique is proposed to estimate atmospheric total water vapor amounts from space. The technique consists of viewing the Earth's 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 technique's 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 advantage of simplicity and constitutes a promising alternative over land, where microwave radiometry is inappropriate.

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Francois-Marie Bréon, Robert Frouin, and Catherine Gautier

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Two sets of ocean surface longwave irradiance measurements collected during the FASINEX and MILDEX experiments are analyzed for quality and variability studies. Using concomitant radiosonde data, the clear-sky contribution to the downward flux at the surface is computed and, subsequently, the effect of clouds from the surface measurements is deduced. The longwave irradiance computations are performed using a broadband model, a simpler parameterization, and an empirical formula. The three schemes are chosen because they represent different, possible approaches for computing surface longwave irradiance. They are intercompared, separating clear and cloud components, and verified against in situ measurements.

During both experiments, which took place in midlatitudes during different seasons, variations in the downward longwave flux associated with clear-sky variations (air temperature, humidity changes) and cloud effects are found to be of the same order of magnitude (≈70 W m−2). Applied to radiosonde profiles, the two more physical schemes provide consistent results with a 4 W m−2 the standard deviation of the differences is only 4 W m−2. These schemes, however, exhibit a 4 W m−2 relative bias, which is comparable to the desired accuracy for monthly-mean longwave flux estimates. Intercomparisons with the empirical formula yield larger standard deviations, demonstrating the formula's inability to reproduce adequately the clear-sky flux variability. For all three schemes, the scatter around the measured values is rather large (20–25 W m−2); surprisingly, the empirical formula gives the best results. This is explained by the large uncertainty in the cloud parameters used as input to the schemes; when no reliable estimates of these critical variables can be made, it may be more accurate to take a simple parameterization for the cloud effect on the longwave flux.

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Mamoudou B. Ba, Sharon E. Nicholson, and Robert Frouin

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The temporal and spatial variabilities of the surface radiation budget over the African continent are examined using Meteosat data acquired during 1983–88. Continental maps of land surface albedo, downward solar irradiance, and net radiation are presented for the midseasonal months of January, April, July, and October. Surface albedo is further compared with Special Sensor Microwave Imager polarization difference of brightness temperature at 19 GHz and with the normalized difference vegetation index to assess the results and to test proposed explanations for some of the unanticipated results. An example of the latter is the finding that albedo increases throughout most of the Southern Hemisphere and in the lower latitudes of the Northern Hemisphere during the wet season. Overall, the study demonstrates the complexity of the relationships among surface albedo, vegetation, and soils and underscores a strong interhemispheric contrast in radiation regimes.

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Mamoudou B. Ba, Robert Frouin, and Sharon E. Nicholson

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Two satellite algorithms for rain estimation are used to study the interannual variability of West African rainfall during contrasting years of the period 1983–88. The first algorithm uses a frequency of occurrence index quantifying the number of times Meteosat thermal infrared radiance below 2.107 W m−2 sr−1 µm−1 (−40°C) occurs during the rainy season. The second algorithm uses the average Meteosat thermal infrared radiance over the period of interest. Appropriate calibrations are performed using these satellite parameters and ground-based rainfall observations. Separate calibration and equations are considered for each of three suggested subrainfall zones in West Africa: two Sahelian zones located just north of 9°N (one cast and one west of 5°W) and the region extending south from 9°N to the coast. Over 80% of the variance in the ground-based rainfall data is explained by both algorithms in regions located north of 9°N, but poor correlations between observed and estimated rainfall exist south of 9°N. The interannual variability of rainfall in the Sahel is well described by that of cold clouds and average radiances. The satellite estimates also reveal substantial longitudinal variability in the anomaly fields, indicating that some Sahelo–Soudanian areas may receive above average rainfall during a year cataloged as dry. The latitudinal displacement and the extent of the cloud band associated with the intertropical convergence zone (ITCZ), as derived from cold cloud indices, indicate a northward displacement of the ITCZ in some, but not all, wet years in the Sahel. No systematic anomalous southward displacement of the ITCZ is evident in dry years. Drought in the Sahel appears to be more closely linked to the latitudinal extent and the intensity of the convection within the ITCZ.

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Robert Frouin, Catherine Gautier, Kristina B. Katsaros, and Richard J. Lind

Abstract

Surface insulation data collected during the Mixed Layer Dynamiccs Experiment are used to intercompare the satellite technique of Gautier et al. (1980) and five commonly referenced empirical formulas for estimating daily insulation over the oceans. The results demonstrate the superiority of the satellite technique, which exhibits a 0.97 correlation coefficient, a 12.0 W m M−2 error of estimate, and a −4.9 W m−2 bias error, and which is also able to account for water vapor, ozone, and dust amount variations in the atmosphere and monitor quasi-instantaneously vast extents of ocean. Among the empirical formulas, Mosby's (1936) yields the best predictions with a 0.84 correlation coefficient, a 19.1 W m−2 standard error of estimate, and a 3.4 W m−2 bias. Kimball'(1928) and Reed's (1977) formulas however, perform nearly as well. The largest biases are obtained with Berliand's (1960) and Laevastu' (1960) formulas, which overestimate insolation by 15.2 and 24.5 W m−2, respectively. It is suggested the empirical formulas, even though established from visual cloud cover observations, would provide useful insolation estimates if employed with satellite-derived cloud cover.

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