Impact of Sun-Synchronous Diurnal Sampling on Tropical TOA Flux Interannual Variability and Trends

Abstract

Abstract Satellite observations of the earth’s radiation budget (ERB) are a critical component of the climate observing system. Recent observations have been made from sun-synchronous orbits, which provide excellent spatial coverage with global measurements twice daily but do not resolve the full diurnal cycle. Previous investigations show that significant errors can occur in time-averaged energy budgets from sun-synchronous orbits if diurnal variations are ignored. However, the impact of incomplete diurnal sampling on top-of-atmosphere (TOA) flux variability and trends has received less attention. A total of 68 months of 3-hourly tropical outgoing longwave radiation (OLR) and reflected shortwave radiation (RSW) fluxes from the Clouds and the Earth’s Radiant Energy System (CERES) synoptic (SYN) data product is used to examine the impact of incomplete diurnal sampling on TOA flux variability. Tropical OLR and RSW interannual variability and trends derived from sun-synchronous time sampling consistent with the Terra satellite from 2000 to 2005 show no statistically significant differences at the 95% confidence level with those obtained at 3-hourly time sampling at both 1° × 1° and 10° × 10° regional scales, as well as for tropical means. Monthly, 3-hourly OLR composite anomalies are decomposed into diurnally uniform and diurnal cycle shape change contributions to explain the impact of sampling on observed TOA flux variability. Diurnally uniform contributions to OLR variability account for more than 80% of interannual OLR variability at 1° × 1° spatial scales. Diurnal cycle shape variations are most important in equatorial land regions, contributing up to 50% to OLR variability over Africa. At spatial scales of 10° × 10° or larger, OLR variance contributions from diurnal cycle shape changes remain smaller than 20%.