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

对于各种气象和大气研究应用来说，必须增进对水汽场的了解。湿度分布被用作大气模式的输入，并为精确的数值天气预报服务。在过去十年中，全球定位系统衍生的数据产品为提供综合水蒸气作出了许多贡献。IWV特性的知识检索通过分析大气梯度引起的GPS信号的延迟，使用映射函数和一个已知的干压力场的假设下。几个站的组合允许产生垂直IWV场的二维图。直接地，它是可能的提取水蒸气含量沿沿着单一的GPS射线，其状态的综合测量。考虑到足够数量的记录在一个定义的区域允许层析数据同化。这将是一个有用的工具，以改善算法的水分和降水预报。仅使用地基全球定位系统接收器进行水汽重建的一个缺点是垂直分辨率很差，仅使用小天顶角的链路就可以实现这一点。近年来，低地球轨道（LEO）卫星已发射携带GPS接收机。因此，如果有大量的低地球轨道卫星测量数据，增加全球定位系统与低地球轨道的联系将大大提高垂直分辨率。这种三维场应提供的空间分辨率是用于局部/区域天气预报模型的空间分辨率。本研究的目的是调查的数量和星座的全球定位系统测量-地基和天基-必要的进行层析重建的水蒸气含量的规格下使用的数值模型。

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.