Concept for a Climate Calibration Observatory: "NIST in orbit"

We have a major disconnect between high accuracy NASA research missions for short periods of time, and lower accuracy NOAA weather missions for long periods of time. Neither set of observations is designed to eliminate climate record gaps. Data continuity and stability over time is not likely to meet climate requirements (e.g. CCSP Strategic Plan). High accuracy and stability climate calibration is expensive and is completed as the last steps before delivery of a space based instrument that is typically behind schedule and over budget (e.g. EOS, NPOESS….). Further, for NPOESS in particular, climate stability metrics are included in the requirements but are not “critical” requirements. This means that they can (and most likely will) be sacrificed to save budget and time. The NPOESS VIIRS imager instrument early in its design eliminated the ability to observe the moon for stability checks used routinely by SEAWIFS and MODIS. NPOESS has a primary weather mission that cannot be sacrificed to a climate mission. NASA on the other hand cannot maintain long records of research satellite data unless mandated by Congress (e.g. TOMS ozone). As a result there is no designed climate observing system, but rather one of fortune and misfortune.

The cost to implement climate quality calibration and stability (typically 0.1% level) is much more stringent and expensive than weather quality calibration. For example: weather forecast initialization receives no benefit from data that is stable at 0.01K, versus data that slowly drifts by 0.1K per decade. But the entire global climate surface temperature signal is order 0.1K per decade. Radiative forcing of climate is 0.6 W/m-2 per decade (IPCC, 2001, nominal scenario). As a result, a change in global net cloud radiative effects of 0.15 Wm-2 per decade is a cloud feedback of 25%: increasing or decreasing climate sensitivity. But mean radiative fluxes are 100 Wm-2 for solar reflected shortwave (SW) and 240 Wm-2 for thermal emitted longwave (LW) radiative flux at the top of the atmosphere. The corresponding radiative effect of clouds is 50 Wm-2 for SW and 30 Wm-2 for LW. So the simple global mean stability requirement is 0.15 out of 50 = 0.3% per decade for SW flux and 0.15 out of 30 = 0.5% per decade for LW flux. This requirement is just to hold uncertainty in global cloud feedback to +/- 25%. Understanding the source of this cloud feedback will require similar accuracies in cloud fraction, height, optical depth, etc. A summary of such stability requirements, each tied to key climate forcing, feedback, or response can be found in the multi-agency report on Satellite Instrument Calibration for Measuring Global Climate Change (Ohring et al., BAMS Sept 2005). The report summarizes both the physical variable accuracy and stability, as well as the related instrument measurement value. Requirements for stability per decade in deg K for thermal infrared measurements, in equivalent instrument gain for the other measurements. So for example, 1% stability for vegetation is an instrument gain stability of 1%, so that vegetation with a global average spectral albedo of 0.10 could have change detected over a decade if its albedo changed from 0.100 to 0.101.

Variable Stability/Decade (Instrument gain change in % or deg K) TOA SW and LW Fluxes 0.3 to 0.5% Cloud Optical Thickness 1% Cloud Temperature 0.2K Surface albedo 1% Ocean color 1% Vegetation 1% Water Vapor 0.03K Tropospheric Temperature 0.04K

Space and time variability in temperature, humidity, clouds, and radiation put very stringent sampling requirements on contiguous global coverage, diurnal sampling, etc. The costs of marrying research quality with operational global sampling spiral out of control rapidly. We conclude that a new method is needed to achieve these types of calibration and stability for climate data records (CDRs) that are key to climate research.

We suggest an alternative solution called the Climate Calibration Observatory. The mission provides NIST-like transfer radiometer time series that underfly all orbiting weather and research satellites. The transfer radiometers cover the full solar and infrared spectrum to allow calibration of radiometers, spectrometers, and interferometers from 0.3 to 100 ?m: the full earth spectrum that drives climate change from solar scattering/absorption through thermal emission/absorption. This observatory is designed not to sample the earth but to calibrate other radiometers in orbit. In this mode field of view size is large (higher signal to noise, smaller optics, less mass and power), accurate pointing control is needed but not full earth scanning (less mass and power), and a dedicated small spacecraft is used to allow full control of spacecraft pointing modes for regular lunar, solar, and instrument intercalibrations (unlike NPOESS or even the large EOS Terra and Aqua missions). As a result, the instruments can be small, light, and fit on a small mission. The mission is clearly research quality focused, but would provide great direct benefit to NPOESS, geostationary satellites, and international missions for climate applications. Large cost savings would be obtained when compared to each individual instrument being required to reach climate accuracy.

Why is this possible now? Recent developments and experience have advanced many of the required areas including:

a) Interferometers and blackbody calibration (many examples) b) Long time series of earth viewing active cavities showing agreement with global ocean heat storage of 0.1% interannual over a decade. (ERBS) c) Long time series of overlapped solar irradiance (ERBS, SORCE, etc) d) New solar spectral irradiance designs (SORCE TIM and SIM) e) Improved broadband thermistor bolometers (CERES) f) Problems with current imagers for ocean color and improvements using lunar stability with current imagers (MODIS, SEAWIFS) g) Experience with multiple satellite platform intercalibration using pointable and programmable CERES instruments to align with other leo and geo instruments

We conclude that the “missing dimension” in climate research is to drive the accuracy of calibration and stability of the radiometers used for global climate observations. For climate, the “calibration” dimension is more important than achieving higher spatial resolution, angular resolution, or time resolution. The Climate Calibration Observatory is designed to attack this missing link in climate data records.