Sampling for Climate Accuracy
Sampling the diurnal cycle differs for Sun-synchronous orbits (≈ 99° inclination), polar orbits, and low-inclination, precessing orbits. A polar-orbiting satellite remains in a fixed orbital plane, sampling at two times of day, 12 hours apart. Both local times cycle through 24 hours in the course of a year as the plane of the satellite orbit rotates with respect to the Earth-Sun line. A Sun-synchronous satellite orbits in a plane that corresponds to the Earth-Sun line, and samples two, fixed, discrete, local times only. A low-latitude orbiter can sample the diurnal cycle many times in a year (six times for an inclination of 33° and an altitude of 662 km), at the expense of limited coverage of the surface.
It is not practical to have all climate satellites in the same Sun-synchronous orbits with the same equator crossing times, nor would this be desirable, because the diurnal variation is not simple and varies over the globe. The only way to ensure benchmark quality is to adopt an observing strategy that leads to accurate climate means. This gives rise to another problem, namely aliasing the diurnal signal into the climate mean because of the periodicity of satellite measurements.
A theoretical study of aliasing errors for constellations of satellites in Sun-synchronous orbits has been made by Leroy (2001) for the case of large-amplitude diurnal variability in surface temperature. The following discussion is based on a numerical study of different orbits using the Salby Global Cloud Imagery data set (GCI, Salby and Callaghan, 1997). The GCI Data are regridded 11 μm radiances on a 612 × 512 grid of regions that cover 0.35° of latitude and 0.7° of longitude. The variance of 11 μm radiation is close to the maximum in the thermal spectrum, so that calculations using GCI data constitute a worst-case yield an upper limit to radiance temperature retrieval errors.
11μm brightness temperature is a measure of cloud-top temperature, or surface temperature if the skies are clear. As shown in Figure 5 brightness temperature is a minimum over Antarctica, a local minimum at the equator where deep convection is strongest, and a maximum over the subtropical deserts (Figure 5a). Variability (Figure 5b) is strongest in the equatorial belt, where brightness temperature varies between 190 K and 302 K, but weakest over the great stratocumulus fields of the subtropical oceans. There are secondary maxima in the mid-latitude storm tracks.
Figure 5c shows the diurnal variation of the brightness temperature. It dominates the total variance for clear skies in desert locations. In other regions, variations in cloud fraction dominate. Here we are concerned with the systematic errors caused by aliasing.
Figure 6: Retrieval errors for 11μ brightness temperature for single satellites, averaged for 1992. The grid boxes are 22.5° square. A sampling error less than 0.1 K is distinguished with an asterisk (*). Histograms of sampling errors are shown on the right. a. A single polar orbiter (inclination: 90°, altitude: 833 km). b. A single sun-synchronous orbiter (inclination: 98.765°, altitude: 833km). c. A single tropical orbiter (inclination: 33° altitude: 662km).
Figure 7. As for Figure 6 but for constellations of orbiters, equally spaced in longitude. a. Two polar orbiters. b. Three polar orbiters. c. Three sun-synchronous orbiters.