[1] Solar radiation assessment by satellite is constrained by physical limitations of imagery and by the accuracy of instantaneous local atmospheric parameters, suggesting that one should use simplified but physically consistent models for operational work. Such a model is presented for use with GOES 8 imagery applied to atmospheres with low aerosol optical depth. Fundamental satellite-derived parameters are reflectance and cloud cover. A classification method applied to a set of images shows that reflectance, usually defined as upper-threshold R max in algorithms assessing cloud cover, would amount $0.465, corresponding to the transition between a cumuliform and a stratiform cloud field. Ozone absorption is limited to the stratosphere. The model considers two spectral broadband intervals for tropospheric radiative transfer: ultraviolet and visible intervals are essentially nonabsorbing and can be processed as a single interval, while near-infrared intervals have negligible atmospheric scattering and very low cloud transmittance. Typical values of CO 2 and O 3 content and of precipitable water are considered. A comparison of daily values of modeled mean irradiance with data of three sites (in rural, urban industrial, and urban coastal environments), September-October 2002, exhibits a bias of +5 W m À2 and a standard deviation of $15 W m À2 (0.4 and 1.3 MJ m À2 for daily irradiation). A comparison with monthly means from about 80 automatic weather stations (covering a large area throughout the Brazilian territory) still shows a bias generally within ±10 W m À2 and a low standard deviation (<20 W m À2 ), but the bias has a trend in September-December 2002, suggesting an annual cycle of local R max values. Systematic (mean) errors in partial cloud cover and in nearly clear-sky situations may be enhanced using regional values for atmospheric and surface parameters, such as precipitable water, R max , and ground reflectance. The larger errors are observed in situations of high aerosol load (especially in regions with industrial activity or forest or agricultural fires). The last case is evident when sites in the Amazonian region or São Paulo city are selected. When considering daily values averaged within 2.5°Â 2.5°cells, the standard error is lower than 20 W m À2 ; present results suggest an annual cycle of mean bias ranging from +10 to À10 W m À2 , with an amplitude of $10 W m À2 . These values are close to the proposed requirements of 10 W m À2 for the mean deviation and 25 W m À2 for the standard deviation. It is expected that the introduction of a reference grid containing mean values of parameters within a cell could induce a decrease in the standard deviation of mean errors and the correction of their annual cycle. A model adaptation for assessing the effect of high aerosol loads is needed in order to extend improvements to the whole Brazilian area.

In this study, an empirical method proposed by Caselles et al. (1992a) is utilized to determine the potential evapotranspiration (ETP) on a regional scale. This method uses the global solar radiation data retrieved by the global radiation model GL1.0, which in turn utilizes data from the visible channel of the GOES-8 satellite. This method is applied to the northeast region of Brazil, using daily and monthly climatological data as the ground truth information to estimate the ETP and the estimated daily ETP data for September, 1997. The methodology involved three steps: 1) to perform a spatial regionalization of the ETP using the method of Ward, which is available in the Statistical Package for the Social Sciences (SPSS); 2) to obtain the correlation between the ETP as estimated by the methods of Jensen & Haise (1963) - MJH, Caselles (1992a) - MCA, and the Penman's combined method (1948) - MCP; 3) to test the sensibility of the empirical formulations proposed and to assess the estimates using the satellite-based global solar radiation provided by the GL1.0 model. The spatial regionalization shows two distinct regions in the Northeastern Brazil. The MCA yielded better results than the MJH. The ETP estimates using satellite data were satisfactory, showing a maximum error of 20% when compared with the ground truth data.

GOES-8 Imager radiances in water vapor and infrared channels 3, 4 and 5 were used for assessing outgoing longwave radiation (OLR) at the top of the atmosphere. Estimation by ITPP5 software applied to HIRS/NOAA 14 passes over Brazil was considered as true reference. Imagery from both satellites is currently acquired and processed at CPTEC/INPE. GOES full-resolution imagery allows assessment of mean irradiance for sets of GOES pixels contained within the area of a single HIRS pixel. Additional GOES variables were estimated, such as: an equivalent channel at 8 mm and a longwave tail for l>15 mm (this one is not detected neither by HIRS nor GOES sensors). Isotropic outgoing radiance was assumed. Multivariate regression of GOES irradiances on OLR ITPP estimates provided a GOES estimator with accuracy comparable with others already published and based on AVHRR/NOAA information. It was found that a regression based only on channel 4 and the longwave tail yielded estimates with the same accuracy: mean deviation of 3 W.m -2 and standard deviation of 11 W.m -2. The application for another period in the year, averaged over 1°´1° grid cells, yielded similar mean deviation and standard deviation of 7 W.m -2. These results suggest that the algorithm applied in this work has physical rather than purely statistical meaning and could be used for OLR monitoring in daily and seasonal scales.Keywords: Outgoing Longwave Radiation (OLR), GOES 8 Imager, NOAA 14, Terrestrial Radiation RESUMO Radiâncias obtidas do satélite GOES 8 nos canais de vapor dágua e infravermelho termal (2, 4 e 5) foram utilizadas para estimar Radiação de Onda Longa (ROL) emergente no topo da atmosfera. As estimativas de ROL pelo utilitário ITPP5, aplicadas à informação do sensor HIRS/NOAA, foram consideradas como verdade de referência. Imagens dos dois satélites são continuamente recebidas e processadas no CPTEC/INPE. As imagens GOES de alta resolução permitiram considerar grupos de pixels (e a irradiância média correspondente) contidos na área de um único pixel HIRS. Variáveis adicionais foram elaboradas, como por exemplo: um canal equivalente em 8 mm e uma cauda espectral de onda longa para l>15 mm (esta última, não detectada nem pelo HIRS nem pelo GOES). Assumiu-se a hipótese de isotropia para a radiância emergente. Regressão multivariada das irradiâncias GOES com relação à estimativa ITPP da ROL forneceu um estimador GOES com precisão comparável à de outros já publicados e baseados na informação de AVHRR/NOAA. Verificou-se que uma regressão baseada apenas nas irradiâncias do canal 4 e da cauda de onda longa fornece estimativas com a mesma precisão: erro médio de 3 W.m -2 e desvio padrão de 11 W.m -2. A aplicação do algoritmo para outra época do ano, avaliando médias em células de 1°´1°, produz um desvio médio similar e um desvio padrão de 7 W.m -2. Estes resultados sugerem que o algoritmo resgatou propriedades físicas e não simplesmente estatísticas da ROL, e poderia ser aplicado para monitoramento de ROL em escala diária e sazonal.

Daily data of solar irradiation were used for analysing spatial decrease of correlation coefficients within northeast Brazil (a region extending over more than l.5 million km 2 ). Available data of the regional climatological network correspond to typical distances between stations of 150 km and more. Two 3-month periods were chosen: March-April-May (MAM) and August-September-October (ASO) 1975. It was found that spatial distribution of correlation for daily irradiations does not allow linear interpolations based on network data. However, factor analysis of 5-day means (particularly useful for agricultural purposes) allowed identification of four regions with internal similarity of time series behaviour, which is induced by the influence of typical meteorological systems. For this time-scale, correlation coefficients may be 0.7 and higher at rather long distances (in some cases, more than 600 km), so that the climatological network can provide data for a detailed description of the regional distribution of mean irradiation. For shorter time and space scales, a too dense solarimetric network would be necessary, and the use of modern techniques using satellite monitoring over the region may be more appropriate.

Resumo O presente trabalho investiga o comportamento do monóxido de carbono (CO) troposférico sobre a região central da América do Sul e sua variabilidade espaço-temporal usando informações do sistema de observações da terra do satélite AQUA (EOS/AQUA, em inglês) no período de 2003 a 2012. Os resultados mostram um comportamento sazonal bem definido da concentração de CO, com redução na estação chuvosa e aumento na estação seca, devido ao aumento da queima de biomassa nesse período. Como a queima de biomassa está diretamente associada à variabilidade climática, ou seja, à diminuição/aumento de chuvas na parte central e leste da América do Sul, o CO possui uma maior variabilidade sobre o Brasil Central, região esta que apresenta o maior número de focos de queima. Os resultados mostram também que a variabilidade de CO na escala interanual está relacionada ao fenômeno El Niño/Oscilação Sul (ENOS), de modo que a diminuição (aumento) de chuvas na parte central e leste da América do Sul durante a fase inicial do evento de La Niña (El Niño) contribui para aumentar (diminuir) os focos de queimada e consequentemente, as emissões de CO nesta região. Por outro lado, durante a fase madura do ENOS, as maiores variabilidades de precipitação e concentração de CO acontecem nas regiões norte e nordeste da América do Sul. Os resultados apresentados neste trabalho podem ser úteis para atividades de monitoramento.