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

夜光云（nlc）或极地中间层云（pmc）是发生在纬度超过50度，海拔82 - 85千米的极地夏季中间层顶附近的光学薄水冰云。在过去的二十年中，由于它们作为全球变化的潜在指标的作用，对非自然气候变化进行了深入研究。利用Nimbus-7和NOAA卫星系列上的BUV、SBUV和SBUV/2仪器对NLC进行的长期卫星观测实际上显示了NLC反照率（DeLand等人，2007年）和NLC发生频率（shuttle等人，2009年）长期增加的证据——除了一个明显的11年太阳周期特征。Stevens等人（2007）最近的一项研究对这种长期趋势提出了质疑，他们使用限于固定当地时间的部分BUV/SBUV/2数据集调查了NLC反照率和冰质量的长期趋势。令人惊讶的是，这一子集的选择导致了NLC长期趋势的消失。这些结果表明，与SBUV/2仪器所在卫星的漂移轨道相关的当地时间覆盖变化可能虚假地引入SBUV/2 NLC观测的长期趋势。这个问题仍然没有得到解决，尽管它对我们对地球大气层上层中层区域气候变化的科学认识非常重要。本研究的主要目的是解决SBUV/2观测数据在NLC分析中的差异，NLC分析使用欧洲最低点观测仪器GOME（全球臭氧监测实验）、SCIAMACHY（扫描成像吸收光谱仪用于大气制图）和GOME-2进行卫星观测。这些仪器的最低点观测特别适合研究上面讨论的问题，因为承载这些仪器的卫星(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，即穿越赤道的当地太阳时。与搭载SBUV/2仪器的NOAA卫星相比，这是一个很大的优势，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 Nadir观测数据尚未用于NLC检索，因此将实现NLC检索算法，并将其应用于整个GOME和SCIAMACHY数据集，作为本文提出的研究的一部分。在计划项目结束时，欧洲纳迪尔紫外线探测仪将提供20年的NLC反照率、发生率和冰质量数据集（涵盖近2个太阳周期）。所得的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漂移轨道相关的当地时间采样缓慢变化的影响。

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.