Early Validation of Vertical Profiles from the Envisat Atmospheric Instruments Gomos and Mipas with the University of Bonn Lidar at the Esrange in July and August 2002

The vertical structure of the terrestrial atmosphere is governed by hydrostatic equilibrium, which relates altitude, temperature, and pressure (or density). The mixing ratios derived for the chemically important trace gases depend on a precise knowledge of this basic atmospheric structure. The Bonn University backscatter lidar on the Esrange (68N, 21E) near Kiruna, Sweden, was operated from mid-July to the end of August 2002 to obtain atmospheric structure data, i.e. altitude profiles of temperature and relative density for Envisat validation. In all we could perform 36 measurement runs with the lidar, of which 34 yielded usable density and temperature profiles as well as several observations of tropospheric and mesospheric clouds. The three Envisat atmospheric instruments Gomos, Mipas, and Sciamachy measure altitude profiles of temperature and density or pressure. This validation report reflects the state of the operational inversion software for these three instruments up to mid-November 2002. Comparisons of 38 Gomos profiles with lidar data in the altitude range 30 to 70 km show the shape of the density profiles to agree to better than 2 % and the mean deviation for temperatures does not exceed 1.4 % or 3-4 K. The Gomos altitude registration agrees with the lidar altitudes to better than 300 m. Independently Envisat instrument profiles were compared to an empirical model of the middle atmosphere in Arctic summer above northern Scandinavia, which is based on a decade with falling sphere measurements from the Andøya Rocket Range in northern Norway. Comparing 48 Gomos profiles to the falling sphere statistics in the altitude range 35 to 93 km shows on average an excellent agreement with mode values of deviation histograms being -0.6 % (i.e. 1.2-1.5 K) for the temperature and -1.8 % for the density. In the Gomos product files are in addition given pressure and temperature values from an internal model, which agree even better with the Gomos optimum profiles of density and temperature. The corresponding comparison of 47 Mipas profiles to the falling sphere model shows good agreement for the temperatures with a mode value of the deviation histogram of 2 %, whereas the Mipas densities were low on average by 20 %. Using hydrostatic equilibrium this mean density deviation can be interpreted as a downward shift in the Mipas altitude registration by 1.6 km. At this early stage of the Envisat data processing level 2 profiles from Sciamachy were not yet available. In the few nadir data presently available for Sciamachy we found one close coincidence for which we could compare cloud top pressure: the lidar detected the cloud top at an altitude 3 km higher than given in the nadir data product file. Working with the Envisat data products revealed several unexpected properties for some of the data generated by the early software versions GOPR LV2 5.3, MIPAS/4.53, and SCIA/3.51. Future Envisat level-2 products should unambigiously identify results from Envisat instruments measurements as opposed to climatological data or a priori model profiles which were used to facilitate data processing. The quality control should include consistency checks with physical laws and bounds on the range of values of atmospheric properties as they occur in the terrestrial atmosphere.