Satellite derived solar irradiance over Mexico



Global solar irradiance is determined for Mexico from GOES satellite data using, the statistical method of Tarpley (1979). Seven daily digital images taken from GOES satellite over Mexico for the period July 1982 - December 1984 are analysed. Monthly and seasonal means are evaluated by computing monthly averages of the daily totals at a 1º x 1º latitude-longitude grid points. Values of the monthly average daily totals are plotted on a physiographic map of Mexico. Tridimensional surfaces of the distribution of global solar irradiance on Mexico are obtained as a function of latitude and longitude. The standard error of the estimates with respect to ground-truth data is between 3 to 5. The results show clearly the existence of two-well defined patterns of solar irradiance over Mexico from October to April the isopleths are very regular on the monthly maps whereas the latitudinal and longitudinal energy surfaces are uniform in most of the country indicating a weak cloud modulation of the radiation field denoting the so-called "dry season". The maximum value is found during April. The exception is presented at the northern part of Mexico where maximum values are reached during June and July. The other pattern of solar irradiance flux is presented from May to September, coincident with the rainy season and frequent cloudiness; the isopleths show distorsions and nodes particularly over the sierras. September reaches the minimum values; these results indicate modulation of the radiation field by an enhanced clouds amount and atmospheric water vapor absorption. As a consequence of this distribution, it is found a latitudinal solar irradiance gradient such that E/ ≥ 0 occurs during the dry season. The condition E/ ≥ 0 is fulfiled during the wet season. These facts indicate polewards transport of energy during spring and summer through a fine combination of astronomical and geographical parameters, i.e, a longer sunshine duration and absence of clouds during summer at the northern part of the country, whereas the southern part is strongly modulated through the constant presence of clouds reducing the incoming radiation flux. Finally, it is shown that the geographical distribution of the solar radiation flux strongly depends on the irregular relief of the country.

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