Preconditions to GPS water vapour tomography with a resolution relevant for data assimilation in weather forecast models

提高对水汽场的认识对于各种气象和大气研究应用是必要的。湿度分布被用作大气模式的输入，并用于精确的数值天气预报。在过去十年中，gps衍生数据产品为提供综合水汽（IWV）做出了许多贡献。在已知干压场的假设下，通过分析由大气梯度引起的延迟和GPS信号来获取IWV特性的知识。几个台站的组合可以制作垂直IWV场的二维地图。直接地，沿着单一的GPS射线提取水蒸气含量是可能的，这是一个综合测量。考虑到在一个确定的区域内有足够数量的记录，就允许层析数据同化。这将为改进湿度和降水预报算法提供有益的工具。仅使用地面GPS接收器进行水蒸气重建的一个缺点是，仅使用小天顶角的链路可以获得较差的垂直分辨率。在最近的时代，低地球轨道卫星（LEO）已经发射携带GPS接收器。因此，增加GPS-LEO链路将显著提高垂直分辨率，前提是有大量的LEO卫星测量可用。这些三维场应提供的空间分辨率是本地/区域天气预报模式所使用的空间分辨率。本研究的目的是调查在数值模式中使用的必要规范下进行水蒸气含量层析重建所必需的GPS测量量和星座（包括地基和天基）。

Improved knowledge of water vapour fields are necessary for various meteorological and atmospheric research applications. Moisture distributions are used as input for atmospheric models and serve for precise numerical weather forecast. In the last decade, GPS-derived data products made many contributions to provide integrated water vapour (IWV). The knowledge of IWV characteristics is retrieved by analysing delays an the GPS signal caused by atmospheric gradients, using mapping functions and under the assumption of a known dry pressure field. The combination of several stations allow to produce two-dimensional maps of vertical IWV fields. Directly, it is possible to extract the water vapour content along single GPS rays, which state a integrated measurements. Considering a sufficient number of records in a defined area allows tomographic data assimilation. This would be a helpful tool to improve algorithms of moisture and precipitation forecast. A drawback of using only ground-based GPS receivers for water vapour reconstruction is the poor vertical resolution that can be achieved using links of small zenith angle only. In resent times, Low Earth Orbiter (LEO) satellites have been launched carrying an GPS receiver. Therefore, adding GPS-LEO links will significantly improve the vertical resolution, provided a large number of LEO satellite measurements are available. The spatial resolution that should be provided by such threedimensional fields are those used in local/regional weather forecast models. The aim of this study is to investigate the amount and constellation of GPS measurements - both groundbased and spacebased - necessary to perform tomographic reconstructions of water vapour content under the specifications requisite for use in numerical models.