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

夜光云(NLCS)或极地中间层云(PMC)是发生在极地夏季中层顶附近的光学薄水冰云，其纬度超过50度，高度82-85公里。在过去的二十年里，由于NLCs作为潜在的全球变化指示器的作用，人们对其进行了深入的研究。在Nimbus-7和NOAA卫星系列上使用BUV、SBUV和SBUV/2仪器对近地天体进行的长期卫星观测实际上表明，除了明显的11年太阳周期特征外，近地天体反照率(Deland等人，2007年)和近地天体出现频率(Shettle等人，2009年)长期增加的证据。史蒂文斯等人最近的一项研究对这一长期趋势提出了质疑。(2007)，世卫组织利用限于当地固定时间的BUV/SBUV/2数据集的一部分，调查了NLC反照率和冰量的长期趋势。令人惊讶的是，这个子集的选择导致了早些时候报道的NLC的长期趋势的消失。这些结果表明，与托管SBUV/2仪器的卫星的漂移轨道有关的当地时间覆盖范围的变化可能会错误地引入SBUV/2 NLC观测的长期趋势。这个问题仍然悬而未决，尽管它对于我们对地球大气层中层上层气候变化的科学理解非常重要。本研究的主要目的是解决使用欧洲最低点观测仪器GOME(全球臭氧监测实验)、SCIAMACHY(大气制图扫描成像吸收光谱仪)和GOME-2卫星观测对SBUV/2观测进行NLC分析时的这种差异。使用这些仪器进行的低谷观测特别适合于调查上述问题，因为托管这些仪器的卫星(ERS-2(GOME)、Envisat(SCIAMACHY)和MetOp-A(GOME-2))在太阳同步轨道上绕地球运行，具有固定的下降节点，即跨越赤道的当地太阳时间。与托管SBUV/2仪器的NOAA卫星相比，这是一个巨大的优势，这意味着推导出的NLC参数可能的长期趋势不会受到当地时间漂移的影响。由于GOME和SCIAMACHY NOIR观测尚未用于NLC反演，因此将实施NLC反演算法，并将应用于整个GOME和SCIAMACHY数据集，作为这里提出的研究的一部分。到拟议项目结束时，欧洲Nadir UV探测仪将提供一个20年的数据集--涵盖近2个太阳周期--NLC反照率、出现率和冰量的数据集。得到的NLC数据产品将用于量化太阳周期(11年和27天时间尺度)和NLC参数的长期变化，并解决SBUV/2 NLC时间序列的长期趋势是否受到与轨道漂移有关的当地时间采样缓慢变化的影响的问题。

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.