Validation of the GOMOS high-resolution temperature product (HRTP) using lidar Kerstin Stebel (1) , Georg Hansen (1) , Yasjka Meijer (2) , Daan P.J. Swart (2) ,Hans Claude (3) , Wolfgang Steinbrecht (3) , Roland Neuber (4) , Shiv Pal (5) , Hideaki Nakane (6) , Phillipe Keckhut (7) , Hassan Bencherif (8) , Iain S. McDermid (9) , Thierry Leblanc (9) , and Kevin Strawbridge (10) (1) Norwegian Institute for Air Research, Polar Environmental Centre, 9296 Tromsø, Norway (2) Nat. Inst. for Public Health and the Environment, Postbus 1, 3720 BA Bilthoven, Netherlands (3) German Meteorological Service, Albin-Schwaiger-Weg 10, 82383 Hohenpeissenberg, Germany (4) Alfred-Wegener Institute, Telegrafenberg A43, 14473 Potsdam, Germany (5) York University, 4850 Keele Street, North York, Ontario M3J 3K1, Canada (6) The National Institute for Environmental Studies, Ibaraki 305, 16-2 Onogawa, Tsukuba, Japan (7) Service d'Aeronomie du Centre National de la Reche, BP 3, 91371 Verrieres le Buisson Cedex, France (8) Universite de la Reunion, 15 Avenue Rene Cassin , La Reunion, France (9) Jet Propulsion Laboratory, P. O. Box 367, Wrightwood, CA 92397-0367, United States (10) Meteorological Service of Canada, 4905 Dufferin Street, Downsview, Ontario, M3H 5T4, Canada ABSTRACT At the beginning of 2006 the complete GOMOS mission data set, including the high-resolution temperature product (HRTP), became available. The time period covered is July 2002 to March 2005. Comparing high-resolution temperatures from the pre-processor (GOPR_LV2/6.0d) and the final processor (GOPV_LV/6.0f) show ca. 2% change in temperatures at the upper initialisation altitude and about 10-15 times increased temperature errors. Spurious data are still frequent, which significantly reduces the applicability of the data for studies of the atmospheric. For validation of the upper part of the HRTP profiles temperatures from 10 lidar sites, which contribute to the EQUAL (ENVISAT Quality Assessment with lidar) project, have been used. 1500 lidar profiles have been available for comparison with ca. 10000 GOMOS profiles taken within a 500 km radius from the lidar locations. Due to the high vertical resolution of the HRTP data product, validation needs to be performed with strict co-location criteria to account for possible atmospheric small-scale structures. We choose 3-hour time difference and 200 h spatial differences as co-location criteria. Unfortunately, this strongly reduces the number of co-located pairs. Therefore no statistically significant conclusions have been drawn so far. Here, we show the best correlated GOMOS HRTP/lidar pairs from several EQUAL stations. In many cases a negative bias of about 2% occurred at around 35 km altitude. This can be related to the choice of the HRTP initialisation value. The agreement between ‘realistic’ HRTP profiles and lidar temperatures below 30 km altitude is encouraging to continue validation work with an extended ground-based dataset (sonde data). 1. INTRODUCTION The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument (see [1], [2], [3], [10]) on ENVISAT was designed to perform altitude resolved global monitoring of ozone as well as other trace gases (NO 2 , NO 3 , O 2 , H 2 0), aerosols, air density, temperature and turbulence. ENVISAT was launched successfully on March 1 st , 2002 and all atmospheric instruments were operational in summer 2002. Compared to solar occultation measurements star occultation instruments, like GOMOS, gives higher measurement rate (30-50 stars per occultation), but with the drawback of lower signal-to-noise rate. Accuracy and precision depend on stellar magnitude and, to a lesser extend, star temperature, obliquity of the occultation trajectory and the limb illumination condition (dark, twilight/stray-light, bright). Originally two temperature products were planned by ESA: a low- and a high-resolution temperature product. Because of insufficient quality of the data product low-resolution temperatures are not available. High Resolution Temperatures (HRTP) is retrieved from the scintillation observed with two fast (1kHz) photometers. HRTP profiles are obtained from the time delay between the red (650-700 nm) and blue (470-520 nm) photometer signal. Due to variations in the index of refraction of air with wavelength the light beam of an occulted star is more bent in the blue part of the spectrum than in the red. Computing of the chromatic time delay gives information on the bending angle, which is related to the density, and thereby to the temperature profile of the atmosphere. The inversion is done downward, starting at the top at around 35 - 40 km. A model atmospheric profile at the tangent point is used for initialisation. Temperature profiles between 18 and 35 km altitudes are derived with a vertical resolution of about 200 meters [5].