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ACCURATE is a climate satellite mission initially conceived at the Wegener Center in late 2004 and subsequently proposed in 2005 by an international science team as Atmospheric Climate and Chemistry in the UTLS Region And climate Trends Explorer (coining its name) to an ESA Earth Explorer Core Mission call. While the mission (very new at that time and therefore in part immature) was not selected for pre-phase A study, it received very positive evaluation and was recommended for further development and demonstration.
ACCURATE employs the occultation measurement principle, known for its unique combination of high vertical resolution, accuracy and long-term stability, in a novel way. It combines use of highly stable signals in the 17-23/178-196 GHz microwave bands (MW occultation) with laser signals in the 2-2.5 μm short-wave IR band (IR-laser occultation) for exploring and monitoring climate and chemistry in the atmosphere with focus on the UTLS region (upper troposphere/lower stratosphere, 5-35 km). The core of ACCURATE is tight synergy of the IR-laser and MW occultation measurements, which are performed along intersatellite cross-links between counter-rotating Low Earth Orbit (LEO) satellites.
The observed parameters, obtained simultaneously and in a self-calibrated manner based on Doppler shift and differential log-transmission profiles, comprise the fundamental thermodynamic variables of the atmosphere (temperature, pressure/geopotential height, humidity) retrieved from the MW bands, complemented by line-of-sight wind, six greenhouse gases (GHGs) and key species of UTLS chemistry (H2O, CO2, CH4, N2O, O3, CO) and four CO2 and H2O isotopes (HDO, H218O, 13CO2, C18OO) from the IR band. Furthermore, profiles of aerosol extinction, cloud layering, and turbulence are obtained. All profiles come with accurate height knowledge (< 10 m uncertainty), since measuring height as a function of time is intrinsic to the MW occultation part of ACCURATE.
The presentation will introduce ACCURATE along the lines above, with emphasis on the climate science value and the new IR-laser occultation capability. Retrieval performance results obtained are promising as they provide evidence that GHG and isotope profiles can generally be retrieved within 5-35 km with < 1-4% rms error (outside clouds), meridional wind with < 2 m/s rms error (outside clouds), and temperature/ pressure/ humidity from MW with < 0.5 K/ 0.2%/ 10% rms error (incl. in clouds), all at ~1 km vertical resolution. Monthly mean climatological profiles, assuming 30-40 profiles per climatologic grid box per month, are found unbiased (free of time-varying biases) and accurate to < 0.15-0.5% (GHGs, e.g., CO2 < 1 ppm), < 0.5-1 m/s (wind), and < 0.1-0.2 K (temperature).
This encouraging performance is discussed in light of the potential of the ACCURATE technique to provide benchmark data for future monitoring of climate, GHGs, and chemistry variability and change. Also on-going work towards ground-based field demonstration of the IR-laser links, for CO2 and CH4 as two key species, is briefly addressed.
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