Monday, 10 February 2003

Sampling errors of monthly-mean radiative fluxes from the earth radiation budget satellite

The Earth Radiation Budget Satellite was placed into an orbit with an inclination of 57 degrees and initial altitude of 610 km. so that it precessed through all local times once every 72 days. This spacecraft carried a package of nonscanning radiometers which have provided broadband measurements of the solar radiation reflected from the Earth and Earth emitted radiation since November 1984. A major objective of this mission was to get measurements of shortwave and longwave radiative fluxes through all local times in order to define their diurnal cycles. In order to compute monthly-mean radiative fluxes from limited temporal sampling, it is necessary to account for these diurnal cycles. However, for some regions in some months, there are not adequate samples in terms of local time to compute a monthly-mean shortwave or longwave flux of sufficient accuracy as to be useful. The number of measurements required to get an accurate monthly mean depends on the local solar time at time of each measurement for the month. Techniques were developed for estimating the standard deviation of the temporal sampling errors for each region for each month for both the longwave and shortwave radiative fluxes and were applied to the data set. The shortwave or longwave monthly-mean flux of any region for which the estimated error has a standard deviation exceeding 12 Watts/m**2 is not recorded. This paper examines the results of applying these quality assurance algorithms to the 15-year data set which has been archived. Histograms are formed of the computed standard deviation of the temporal sampling errors in order to learn the effects of various parameters of the problem on the final accuracies of the radiant fluxes. The results are independent of longitude, but highly dependent on latitude. Most regions whose monthly means are rejected are in the most poleward zones due to the orbit geometry. Also, they tend to be in the winter hemisphere due to the shortened daylight there.

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