Search Results

You are looking at 1 - 6 of 6 items for :

  • Author or Editor: Robert Frouin x
  • Journal of Climate x
  • Refine by Access: Content accessible to me x
Clear All Modify Search
Catherine Gautier and Robert Frouin

Abstract

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.

Full access
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.

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

Abstract

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.

Full access
Mamoudou B. Ba, Robert Frouin, Sharon E. Nicholson, and Gérard Dedieu

Abstract

Downward surface solar irradiance and albedo of the African continent are estimated from Meteosat B2 data at 30-km spatial resolution. The algorithm, based on Dedieu et al.’s approach, is verified against other satellite estimates and ground-based measurements. In the computations, the International Satellite Cloud Climatology Project’s (ISCCP) radiometric calibration is adjusted using the Libyan desert as a reference target of constant reflectance properties. Surface albedo is corrected for sun zenith angle effects, allowing for better detection of seasonal changes due to the vegetation cycle. The estimates obtained with Meteosat B2 data agree generally well with other satellite estimates, although biases of 20 W m−2 (downward surface solar irradiance) and 0.15 (surface albedo) are obtained in some cases. There is evidence, from comparisons with surface measurements, that the clear-sky downward surface solar irradiance is overestimated over semiarid regions of Africa because of uncertainties in aerosol characteristics. In the Sahel region, where spatial albedo gradients are high, it is advantageous to use 30-km Meteosat B2 products instead of the current, coarser 280-km-resolution ISCCP products.

Full access
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.

Full access
Akiko Higurashi, Teruyuki Nakajima, Brent N. Holben, Alexander Smirnov, Robert Frouin, and Bernadette Chatenet

Abstract

Global distributions of the aerosol optical thickness and Ångström 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 Ångström 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 Ångström exponent values around the east coast of North America, Europe, and eastern Asia. Satellite-retrieved values of Ångström parameters are further compared with measured spectral optical thickness obtained by the National Aeronautics and Space Administration Aerosol Robotic Network sky radiometer network.

Full access