Retrieval of Daytime Surface Net Longwave Flux over BOREAS from GOES Estimates of Surface Solar Flux and Surface Temperature

Abstract

RESUMEOn analyse la possibilite de deduire le flux net de rayonnement diurne de surface d grande longueur d'onde (L*) d partir d'information sur les flux solaires de surface et les temperatures de surface determines apartir des mesures du satellite GOES. Un algorithme statistique base sur un ensemble de donnees d'entrainement developpe apartir de mesures in situ acquises en 1995, au cours de la phase de terrain de BOREAS, est developpe pour l' ensemble de la region d'etude de BOREAS et valide pour le reseau de stations meteorologiques automatisees AMS (Automated Meteorological Station) du Conseil de recherches de ill Saskatchewan. Apartir des flux solaires de surface deduits des mesures realisees au cours de la seconde phase intensive de mesures de 1994, dans le canal visible du satellite GOES 7 et des temperatures de surface approximees d l'aide des mesures de temperature in situ du reseau AMS, nous avons deduit des flux nets diurnes de surface agrande longueur d'onde, a ill frequence d'une demi-heure et a une echelle spatiale de 8 x 8 km. Une discussion permet d'identifier des sources possibles d'erreur dans l'application de l'algorithme. Lorsque comparees avec des mesures in situ de L *, les erreurs quadratiques moyennes dans les flux deduits peuvent etre aussifaibles que 19 Wm-2 avec des correlations aussi elevees que 0,85 pour chacun des sites AMS. Quoique les erreurs de validation soient significativement plus elevees pour certains sites, nous montrons que ces resultats peuvent provenir d'une mauvaise mise a niveau des radiometres AMS. SUMMARY An investigation ofretrieving daytime surface net longwave radiation flux (L*) from information on surface solar flux and surface temperature determined from GOES satellite measurements is explored. A statistical algorithm based on a training dataset developed from in situ measurements taken during the 1995 BOREAS field phase is developed for the large scale BOREAS study area, and validated at the Saskatchewan Research Council'sAutomated Meteorological Station (AMS) networkfor summer conditions. Thus, at this juncture, the algorithm is only applicable for a snow-free boreal forest ecotone. Using surface solar fluxes retrieved from GOES 7 visible channel measurements obtained during the second intensive field phase of1994, and surface .. temperatures approximated with in situ AMS temperature measurements, we have retrieved daytime surface net longwave fluxes on a halfhourly time scale and an 8 x 8 km spatial scale. A discussion is included identifying possible sources of errors in applying the algorithm. When compared with in situ L * measurements, the rms errors in the retrieved fluxes are as low as 19 W m? with correlations as high as 0.85 at the individual AMS sites. Although validation errors are significantly greater at various of the sites, we show that these results may have resulted from mis-leveling of the AMS radiometers.

Modeling Carbon Sequestration over the Large-Scale Amazon Basin, Aided by Satellite Observations. Part I: Wet- and Dry-Season Surface Radiation Budget Flux and Precipitation Variability Based on GOES Retrievals

“…This result has also been found in the studies of Menzel et al (1990), Li et al (1995), and Lubin et al (1996). Second, the mean solar zenith angle in the Tropics is considerably smaller than at high latitudes, and therefore cloud albedo is lower because of the nonlinear behavior of directional reflectance from a cloud surface, which increases with increasing zenith angle (Gu et al 1997). Third, during the dry season, the presence of graphitic carbon from biomass burning tends to reduce cloud reflectance in the visible channel (Kaufman and Nakajima 1993).…”

“…The surface incoming solar radiation/PAR retrieval algorithm is based on a physical algorithm designed for a forest setting and first applied for a boreal forest ecosystem (Gu and Smith 1997). The design of the algorithm follows a method originally developed by Gautier et al (1980) and improved upon by Diak and Gautier (1983), Frouin et al (1989), and Gautier and Landsfeld (1997).…”

“…Modifications to the original algorithm developed by Gautier and her colleagues for calculations of surface albedo, aerosol attenuation, and cloud absorption had been made prior to LBA applications as discussed in Gu and Smith (1997). In moving to a tropical rain forest regime, further modifications were required for calculation of cloud absorption, modifications essential to reduce a positive bias error found in the K↓ and PAR↓ retrievals when using the initially modified parameterization.…”

“…In moving to a tropical rain forest regime, further modifications were required for calculation of cloud absorption, modifications essential to reduce a positive bias error found in the K↓ and PAR↓ retrievals when using the initially modified parameterization. In Gu and Smith (1997), cloud absorption for the total solar radiation and PAR bands had been calculated as 20% and 3% of cloud visible reflectance, respectively. We found that whereas such a parameterization is satisfactory for a high-latitude forest, it is too much of an oversimplification of the microphysics involved in the interaction between clouds and the radiation field for the more complex convective cloud structures found in the Tropics, leading to overestimations of both K↓ and PAR↓ at the surface.…”

“…1 for a schematic of the satellite study area). The radiation retrievals are based on previously developed algorithms designed for forest settings and were first applied to a boreal forest ecosystem (Gu and Smith 1997;Gu et al 1997). The rainrate retrievals are based on a GOES visible and infrared algorithm (King et al 1995) calibrated to Global Precipitation Climatology Project (GPCP) monthly analyzed rainfall (Huffman et al 1997(Huffman et al , 2001.…”