## 1 Introduction  
Climate change presents a threat to human societies. Understanding and prediction of the Earth's water distribution and its time evolution is essential for life and for a sustainable development of the Earth system. The study and simulation of the water and energy cycles are pivotal to address two present-day scientific questions that are urgent for the environment and society:  
1. Are extreme weather events predictable, and how can our forecasting skills be improved? 2. To what extent is today's regional and global water and energy cycles' change due to natural variability versus anthropogenic influences?Over the last decade, the successful launch of several satellite missions [1; 2; 3; 4], directly or indirectly devoted to studying global climate change, such as CHAllenging Minisatellite Payload for geophysical research and application (CHAMP), Gravity Recovery and Climate Experiment (GRACE) and its follow-on projects (GRACE-FO), Constellation observing system for meteorology, Ionosphere and climate (FORMOSAT-3/COSMIC), and Gravity field and steady-state Ocean Circulation Explorer (GOCE), has driven significant advances in the remote sensing of both atmospheric and terrestrial water storage (TWS). TWS and tropospheric water are key variables of the Earth's climate, impacting both surface water and atmospheric energy budgets. These satellite observations are providing a breakthrough for studying the global water cycle under current and future climate conditions [5]. Water vapour (WV) is a key component in the atmosphere's thermodynamics, and it is the most variable greenhouse gas in the troposphere as well, so understanding its spatiotemporal distribution is of paramount relevance in meteorological and climatological studies. Due to its high temporal and spatial variations, the WV content in the atmosphere has to be regularly and accurately determined for meteorological and climatological purposes. Since the pioneering paper by [PERSON] et al. [6] GPS has been applyied in meteorology, mainly for precipitable water vapour (PWV) retrieval, such that the atmospheric effect is being transformed from a noise source, degrading the accuracy of the geodetic observations, into a valuable meteorological probe. The global navigation satellite system (GNSS) has widely demonstrated its potential to probe the Earth's atmosphere (troposphere and ionosphere), ocean and soil moisture, as well as its cryosphere. Refracted, reflected and scattered GNSS signals have been successfully used to remotely sense the water content on the Earth's surface and atmosphere with an accuracy comparable to other techniques (e.g., radio soundings, microwave radiometers), with, even, good time resolution and under all meteorological conditions [7; 8]. Today's extensive use of permanent GPS stations, operating for geodetic purposes, offers a tool for a dense and reliable remote sensing of atmospheric water vapor. A number of major worldwide space projects have established GPS meteorology as an operational atmospheric sounding technique (see for instance [3] for the European area). GNSS data, collected at terrestrial and space-borne stations, have been widely applied for atmospheric sounding, including the detection of tropospheric PWV, ionospheric total electron content (TEC) and atmospheric profile information (e.g., pressure, temperature, humidity, tropopause and ionospheric electron density). These observations have facilitated greater advances in meteorology, climatology, numerical weather models, atmospheric science and space weather. GPS radio occultation (RO) from low Earth-orbiting satellites has widely improved the resolution of atmospheric profiles in making high vertical resolution and global atmospheric measurements [1]. Severe meteorological events, such as a hurricanes, emphasize the need to monitor and track weather using additional observational tools to mitigate these risks. Traditionally, water vapour data has been acquired primarily through the use of water vapour radiometers (WVR) and radiosondes.