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ACCURATE: Climate Benchmark Profiling of Greenhouse Gases, Thermodynamic Variables, and Wind from Space by Combined MW and IR-laser Occultation

Gottfried Kirchengast(1), Susanne Schweitzer(1), Peter Bernath(2) and ACCURATE ESA Science Studies Team(3)

(1) University of Graz, Leechgasse 25, A-8010 Graz, Austria
(2) University of York, Heslington, York YO10 5DD, United Kingdom
(3) U. Graz (AT), U. York (UK), DMI (DK), CNIT (IT), DLR (DE), FMI (FI), IAP (RU), details see Abstract entry


ACCURATE ESA Science Studies Team: G. Kirchengast, S. Schweitzer, V. Proschek, J. Fritzer (U. Graz, AT); P. Bernath, J. Harrison, B. Thomas (U. York, UK); G.B. Larsen, S. Syndergaard, H.-H. Benzon (DMI, DK); L. Facheris, F. Cuccoli, E. Martini (CNIT, IT); C. Emde, N. Perlot (DLR, DE); V. Sofieva, J. Tamminen (FMI, FI); M. Gorbunov (IAP, RU).

The ACCURATE (Atmospheric Climate and Chemistry in the UTLS Region And climate Trends Explorer) mission was conceived at the Wegener Center in late 2004 and subsequently proposed in 2005 by an international team of more than 20 scientific partners from more than 12 countries to an ESA Earth Explorer Mission call. While the mission (very new at that time) was not selected for formal pre-phase A study, it received very positive evaluation (shortlist of seven) 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 systematically combines use of highly stable signals in the MW 17-23/178-196 GHz bands (LEO-LEO MW crosslink occultation) with laser signals in the SWIR 2-2.5 micron band (LEO-LEO IR laser crosslink 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 MW occultation is an advanced and at the same time compact version of the LEO-LEO MW occultation concept, studied in 2002-2004 for the ACE+ mission project of ESA for frequencies including the 17-23 GHz band, complemented by U.S. study heritage for frequencies including the 178-196 GHz bands (R. Kursinski et al., Univ. of Arizona, Tucson). The core of ACCURATE is tight synergy of the IR laser crosslinks with the MW crosslinks.

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 (13CO2, C18OO, HDO, H218O) from the SWIR 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. On-going science studies in the framework of ESA that importantly contribute to the progress are particularly addressed (projects ACTLIMB and IRDAS). The focus will be on performance analysis results obtained so far, in particular regarding the profiles of GHGs and main isotopes, but also of wind and thermodynamic variables, all at ~1 km vertical resolution. The results provide evidence that the GHG and isotope profiles can generally be retrieved within 5-35 km with < 1-5% 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). Monthly mean climatological profiles, assuming 30-40 profiles per climatological grid box per month, are found unbiased (free of time-varying biases) and accurate to < 0.2-0.5% (GHGs; e.g., CO2 < 1 ppm), < 0.5-1 m/s (wind), and < 0.1-0.2 K (temperature). These encouraging results are 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.