# Satellite Remote Sensing of Precipitation and the Terrestrial Water Cycle in a Changing Climate
## 5 Outlook  
Research on the terrestrial water cycle is in a very early stage, and no firm conclusions can be drawn on the closure of the cycle at regional or global levels. Climate models all agree on the DDWW paradigm characterizing the future of the Earth's climate. Storms (individual thunderstorms, extratropical rain or snow storms, or tropical cyclones) supplied with increased moisture produce more intense precipitation events. Moreover, warming induces more precipitation as rain instead of snow; snow, in turn, melts earlier, with increased runoff and risk of flooding in early spring and increased risk of drought in summer, especially over continental areas (307). This suggests that climate change must already be considered in flood risk management (308). Recent modeling results argue that concurrent soil drought and atmospheric aridity are greatly exacerbated by land-atmosphere feedback and soil moisture-precipitation feedback, amplifying precipitation and soil moisture deficits in most regions as derived from the Global Land-Atmosphere Climate Experiment (GLACE) Coupled Model Intercomparison Project phase 5 (CMIP5) (309-312). All these results rely upon the support of a global observing system that continuously monitors the atmosphere, the continents, and the oceans with a unified perspective, improving over existing  
Figure 11: Combined Drought Indicator in Europe for the first 10 days of September 2019. Courtesy of the European Drought Observatory ([[https://edo.jrc.ece.europa.eu/edov2/php/index.php?id=1000](https://edo.jrc.ece.europa.eu/edov2/php/index.php?id=1000)]([https://edo.jrc.ece.europa.eu/edov2/php/index.php?id=1000](https://edo.jrc.ece.europa.eu/edov2/php/index.php?id=1000)), last accessed 24 September 2019) (302). networks that were not originally designed for climate monitoring [61; 62; 66]. Satellite precipitation measurements have a key role in this scientific framework, and advances in the observing constellation are on their way as:  
* the better quantification of high-latitude precipitation including snowfall;
* the improved accuracy in precipitation detection and intensity retrievals;
* the definition of error models for each satellite product;
* the creation of multi-satellite and multi-source global precipitation products. At the same time, precipitation CDRs data need maintenance and a continuous launch of sensors that replace the old ones as they are decommissioned. This is crucial for climate observations which are based on long-time datasets. Smallsats may help with their low-cost strategy and light-weight payloads. The satellite precipitation community is at work to design the next generation of spacecrafts and sensors for clouds and precipitation monitoring [99]. Research efforts are devoted to solving the issues with retrieval algorithms that are discussed in Section 3.2. However, this strategy is not sufficient, as satellite-derived products need to be compared and integrated with ground-based products and model outputs to produce global comprehensive analyses for assimilation and model verification. Hydrological model ensembles will, more and more, represent a tool for global water resource evaluations [313] and for the quantification of uncertainties [314] and their propagation [315] in the analysis of climate change impact on water resources. The assimilation of satellite precipitation estimates in the models can significantly improve the quality of the single model [72], and thus, increase the value of ensembles. In fact, a good performance of hydrological models in the historical period increases confidence in projected impacts under climate change and decreases the uncertainty of projections related to the hydrological models themselves [316]. Note that studies are necessary on the performance of satellite precipitation products when used as main forcing in a grid-based distributed hydrological model to assess streamflow in medium to large-scale river basins [317].