The radiative impact of the diurnal signal in monitoring decadal climate change

The NRC Decadal Survey has called for SI traceability of long-term hyper-spectral flux measurements in order to monitor climate variability. This mission is called the Climate Absolute Radiance and Refractivity Observatory (CLARREO) and is currently defining its mission requirements. The requirements are focused on the ability to measure decadal change of key climate variables at very high accuracy. The accuracy goals are set using anticipated climate change magnitudes, but the accuracy achieved for any given climate variable depends on the temporal and spatial sampling, calibration accuracy and orbital requirements. Some of the key climate changes include the anthropogenic radiative forcing by carbon dioxide and aerosols; climate feedbacks including cloud, temperature lapse rate and humidity; and climate system temperature response. The largest uncertainty in climate feedbacks remains the effect of changing clouds on planetary energy balance. Although the diurnal cycle of maritime stratus and land afternoon convection regions are considerable, the diurnal cycle might be very consistent inter-annually. If climate change has a diurnal cycle component then the CLARREO orbital constellation will require multiple satellite platforms, significantly increasing the cost of the mission. This study will examine the inter-annual variability of the radiative impact of the diurnal cycle. Since the study is global, and since it includes both reflected solar fluxes and emitted thermal infrared fluxes, it will include diurnal cycle effects of temperature, water vapor, clouds, aerosols, and surface reflectance. The study will incorporate Clouds and the Earth's Radiant Energy System (CERES) (Monthly TOA/Surface Averages) SRBAVG product TOA LW and SW climate quality fluxes. The fluxes are derived either from CERES measurements in a sun-synchronous orbit (called non-GEO) or by combining CERES data with 3-hourly 5- geostationary satellite estimated broadband fluxes, which are normalized using the CERES fluxes (called GEO). Normalization to CERES ensures that the GEO derived broadband fluxes do not alter the CERES calibration. This provides the ability to compare monthly fluxes based on a single measurement during the day with those with a complete diurnal cycle sampled every 3 hours. The normalization to CERES provides a way to eliminate many of the calibration and systematic angular viewing challenges in the geostationary data. Differences in inter-annual anomalies will be evaluated between the non-GEO and GEO products to determine the impact of the diurnal cycle in the inter-annual signals during 2000 through 2005. The evaluation will look at interannual anomalies in global, zonal as well as regional spatial scales. In particular, it will compare the magnitude of anomalies from a fixed sun-synchronous non-GEO orbits (Terra at 10:30, Aqua at 1:30) to the full 3-hourly sampled diurnal cycle. This will allow a separation of diurnal cycle effects in absolute accuracy of the time average mean, from diurnal cycle effects on the accuracy of decadal change of the mean. These magnitude of these diurnal cycle effects can then be compared to the accuracy goals for the CLARREO mission.