GPS radio occultation data from the CDAAC: COSMIC Data Analysis and Archive Centre

The GPS RO technique has a number of advantages over the traditional RS such as its global coverage, high vertical resolution, 24 hour availability, high accuracy, all weather capability and lack of bias effects. The RO data contains high resolution height, temperature, pressure, refractivity, bending angle and water vapour content at the tangent point locations. There are a number of satellite constellations that are capable of deriving RO measurements. In this project we utilised data from the FORMOSAT-3 (Taiwan's Formosa Satellite Missions # 3)/ COSMIC (Constellation Observing System for Meteorology Ionosphere and Climate) and CHAMP (CHAllenging Minisatellite Payload) satellites. These constellations are capable of producing many measurements daily, the Cosmic Data Analysis and Archive Centre (CDAAC) provides around 1800 neutral atmospheric profiles per day. RS measurements have been the dominant method for the acquisition of upper air atmospheric information for the last 70 years. The RS monitoring technique measures atmospheric profiles of pressure, temperature and humidity using sensors attached to balloons. The data collected by the sensors is transmitted to the ground based weather station. The usual operational frequency is two times per day (0000 and 1200 UT). A global RS network of approximately 1500 stations is currently in operation. The RS monitoring method has a limited coverage, low spatial and temporal resolution and is normally restricted to land masses. In the Antarctic region there are only 16 weather stations mainly distributed along the coastal fringe due to the environmental harshness and costs involved. As such this RS network is far from ideal for studying the atmosphere, meteorology and climatology in the Antarctic region. It does however provide excellent reference stations to test and validate the RO technique as a suitable meteorological data type in the Antarctic region.These data were downloaded from the CDAAC: COSMIC Data Analysis and Archive Centre website.http://cdaac-www.cosmic.ucar.edu/cdaac/index.htmlThese data are freely available. We downloaded and used data from the CHAMP and COSMIC wetPrf, atmPrf and sonPrf data files.The GPS RO data was tested against co-located radiosonde measurements from 16 radiosonde weather stations located in Antarctica. We investigated the spatial and temporal buffer required for a large and accurate data set. We found that a spatial and temporal buffer set of 300km and 3 hours to be appropriate to test the RO data sets. The RO data sets were found to match well with the radiosonde measurements in the Antarctic region.We then used these data sets to investigate annual, bimonthly temperature trends at various heights (pressure levels) and at various locations.These data were collected by the CDAAC: COSMIC Data Analysis and Archive Centre.We used COSMIC data collected from 1st January 2007 to 31st December 2014.We used CHAMP data collected from 2003 to 2008.

GPS无线电掩星（GPS Radio Occultation，简称GPS RO）技术相较于传统遥感（Remote Sensing，简称RS）具备诸多优势，包括全球覆盖、高垂直分辨率、全天候可用、高精度、全天气作业能力以及无系统偏差效应。掩星数据包含切点位置处的高分辨率高度、温度、气压、折射率、弯曲角与水汽含量。

当前已有多款卫星星座可获取掩星观测数据。本项目采用了福尔摩沙卫星三号（FORMOSAT-3，中国台湾地区福尔摩沙卫星任务3号）/宇宙气象电离层与气候星座（COSMIC, Constellation Observing System for Meteorology Ionosphere and Climate）以及挑战小卫星有效载荷（CHAMP, CHAllenging Minisatellite Payload）卫星的数据。上述星座每日可生成大量观测数据，宇宙数据分析与存档中心（Cosmic Data Analysis and Archive Centre, CDAAC）每日可提供约1800份中性大气廓线数据。

遥感观测是过去70年来获取高空大气信息的主流手段。遥感监测技术通过搭载在探空气球上的传感器，测量气压、温度与湿度的大气廓线，传感器采集的数据会传输至地面气象站。常规业务观测频次为每日两次（世界时0000与1200时）。目前全球运行着约1500个站点组成的遥感监测网络。该遥感监测方法覆盖范围有限，时空分辨率较低，且通常仅局限于陆地区域。受极端环境与建设成本限制，南极地区仅设有16个气象站，且主要分布在沿海地带。因此，该遥感网络并不适合用于研究南极地区的大气、气象与气候学特征，但可为验证GPS掩星技术作为南极地区适宜的气象数据类型提供优质的参考基准。

本研究使用的数据均下载自宇宙数据分析与存档中心（CDAAC）官网：http://cdaac-www.cosmic.ucar.edu/cdaac/index.html，且均可免费获取。我们下载并使用了CHAMP与COSMIC的wetPrf、atmPrf以及sonPrf数据文件。

我们将GPS掩星数据与位于南极地区的16个探空气象站的探空仪（radiosonde）观测数据进行了比对。本研究探究了构建大规模高精度数据集所需的时空缓冲窗口，最终确定采用300km空间缓冲与3小时时间缓冲为合适参数。结果显示，南极地区的GPS掩星数据与探空观测数据匹配度良好。随后，我们利用上述数据集探究了不同高度（气压层）与不同位置处的年际、双月温度趋势。本研究使用的数据均由宇宙数据分析与存档中心（Cosmic Data Analysis and Archive Centre, CDAAC）采集，其中COSMIC数据的采集时段为2007年1月1日至2014年12月31日，CHAMP数据的采集时段为2003年至2008年。