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

夜光云（NLC）或极地中层云（PMC）是出现在极地夏季中层顶附近的光学薄水冰云，纬度超过50度，高度为82 - 85公里。在过去的二十年中，NLC被广泛研究，因为它们作为全球变化的潜在指标。事实上，使用雨云7号和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显示了NLC的长期增加的证据（DeLand等人，2007）和NLC发生频率（Shettle等人，2009年）-除了一个明显的11年太阳周期签名。Stevens等人（2007年）最近的一项研究对这一长期趋势提出了质疑，他们使用BUV/SBUV/2数据集的一部分，仅限于固定的当地时间，调查了NLC冰量和冰量的长期趋势。令人惊讶的是，这一子集的选择导致NLC的长期趋势消失之前报道。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范围的变化可能会错误地引入SBUV/2 NLC观测的长期趋势。尽管这一问题对于我们科学地理解地球大气层上部中层区域的气候变化非常重要，但它仍然没有得到解决。本研究的主要目的是解决使用欧洲最低点观测仪器GOME进行卫星观测的SBUV/2观测的NLC分析中的这一差异（全球臭氧监测实验）、SCIAMACHY（扫描成像吸收光谱仪用于大气特征分析）和GOME-2。使用这些仪器进行的天底观测特别适合于研究上述问题，因为这些仪器所在的卫星（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，即跨赤道的当地太阳时。这是一个很大的优势相比，NOAA卫星托管SBUV/2仪器，并意味着可能的长期趋势，在派生的NLC参数将不会受到当地时间drifts.Because的GOME和SCIAMACHY纳迪尔观测尚未用于NLC检索的NLC检索算法将被实施，并将被应用到整个GOME和SCIAMACHY数据集的一部分，在这里提出的研究。到拟议项目结束时，将可从欧洲最低紫外线探测器获得一个20年的数据集，其中包括国家南极洲冰量、出现率和冰量的数据集，几乎涵盖两个太阳活动周期。将使用由此得出的NLC数据产品来量化太阳周期（11年和27天时标）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这一问题。

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.