Investigating long-term trends in noctilucent clouds observed with European Nadir-Viewing satellite instruments

• Applicant: Professor Dr. Christian von Savigny, Ph.D.
• Subject Area: Atmospheric Science
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 255787981

Noctilucent clouds (NLCs) or polar mesospheric clouds (PMCs) are optically thin water-ice clouds occurring near the polar summer mesopause at latitudes exceeding about 50 deg and altitudes of 82 - 85 km. NLCs were intensively studied in the past two decades, because of their role as potential indicators of global change. Long-term satellite observations of NLCs using the BUV, SBUV and SBUV/2 instruments on Nimbus-7 and the NOAA satellite series in fact show evidence for a long-term increase in NLC albedo (DeLand et al., 2007) and NLC occurrence frequency (Shettle et al., 2009) - apart from a pronounced 11-year solar cycle signature. This long-term trend was recently questioned by a study by Stevens et al. (2007), who investigated long-term trends in NLC albedo and ice mass using part of the BUV/SBUV/2 data set limited to a fixed local time. Astonishingly, the selection of this subset led to the disappearance of the long-term trends in NLC reported earlier. These results suggest that the changing local time coverage associated with the drifting orbits of the satellites hosting the SBUV/2 instruments may spuriously introduce the long-term trends in SBUV/2 NLC observations. This issue is still unresolved, despite its great importance in terms of our scientific understanding of climate change in the upper mesospheric region of the Earth's atmosphere.The main purpose of this study is to address this discrepancy in NLC analyses of the SBUV/2 observations using satellite observations with the European nadir-viewing instruments GOME (Global Ozone Monitoring Experiment), SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) and GOME-2. Nadir observations with these instruments are uniquely suited to investigate the issue discussed above, because the satellites hosting these instruments (ERS-2 (GOME), Envisat (SCIAMACHY), and MetOp-A (GOME-2)) orbit the Earth in sun-synchronous orbits with fixed descending nodes, i.e. equator-crossing local solar times. This is a great advantage compared to the NOAA satellites hosting the SBUV/2 instruments, and implies that possible long-term trends in the derived NLC parameters will not be affected by local time drifts.Because the GOME and SCIAMACHY Nadir observations were not yet used for NLC retrievals an NLC retrieval algorithm will be implemented and will be applied to the entire GOME and SCIAMACHY data sets as part of the research proposed here. By the end of the proposed project a 20-year data set - covering almost 2 solar cycles - of data sets of NLC albedo, occurrence rate and ice mass will be available from the European Nadir UV sounders. The derived NLC data products will be used to quantify solar cycle (both at the 11-year and the 27-day time scale) and long-term variations in NLC parameters, and to address the issue, whether the long-term trend in SBUV/2 NLC time series is affected by slow changes in the local time sampling associated with drifting orbits.

• DFG Programme: Research Grants