Present satellite Earth radiation budget (ERB) measurement programs have provided valuable observations that have advanced our understanding of the two science issues described in the introduction to this chapter, and further advances in this understanding are expected in the Earth Observing System (EOS) and post-EOS eras. However, understanding of the radiation budget of the planet is still largely confined to top-of-the-atmosphere (TOA) fluxes; we have not made significant progress in achieving the science goals stated above. Although TOA information is important for a number of reasons, it is unable to give direct insight into processes that influence the radiation budget of the atmosphere and surface.
The Clouds and the Earth's Radiant Energy System (CERES) developed for EOS (Wielicki et al., 1996) can meet the TOA requirements defined in the NOAA IORD-1 climate requirements document (IPO NPOESS, 1996). Surface flux requirements, however, cannot be met with these measurements alone and require significant ancillary information. The requirements for some of this additional information (such as cloud base) cannot be met entirely with planned National Polar-orbiting Operational Environmental Satellite System (NPOESS) observations, although the issue of how much cloud base information is contained in the NPOESS observing system is a topic of ongoing research.
Future ERB instruments should not be simply a copy of CERES but should have enhanced capabilities that CERES does not provide. The next advance in ERB measurements must come in the direction of providing a better and more direct way of determining the radiation budget at the surface, within the atmosphere, and at the top of the atmosphere. The challenge is to determine the most appropriate enhancement to achieve this goal (one example is an enhanced spectral flux measurement capability).
There is an ongoing debate on the climatic value of continuous ERB measurements. Closure on this debate is needed, and the extent to which the data expected from EOS will advance progress toward the stated goals of an ERB measurement program will have to be assessed. Therefore current planning activities should not preclude the incorporation of CERES-like or ideally enhanced ERB observations as part of the NPOESS climate-observing strategy.
This should include the NPOESS Preparatory Project Pathfinder mission being planned to bridge the gap between the end of the first EOS missions in 2006 and the start of NPOESS in 2009.
The next steps in ERB observations might include:
A calibrated spectral imager to fly alongside a CERES-like instrument. Imager data are required for scene identification and to define the appropriate angular model used to retrieve flux information. These data must be calibrated and contain more capable cloud channels beyond those channels used for ocean color. Current AVHRR-like channels are not sufficient for this purpose, but the channels of MODIS and those planned for VIIRS would provide an adequate means for scene identification.
Sampling requires measurements from multiple satellites. A minimum of two orbits is required to sample the diurnal cycle, but three are preferable.
A measurement strategy that provides a more direct way of determining the radiation budget at the surface and within the atmosphere. The challenge is to determine the most appropriate observing strategy to achieve this goal (such as more spectral flux measurement capability).
Correlative measurements of the principal atmospheric constituents governing the distribution and variability of the fluxes measured at the TOA. This ideally should include information on cloud water and ice path as well as aerosol optical depth.