GRACES (G-band RAdar for Cloud and prEcipitation Studies)

尽管云和降水对气候的影响已得到公认，但我们观测云特性的能力仍存在重大差距，这些特性是测试和改进云过程在模型中的表现方式所必需的。据IPCC称，这导致云和气溶胶成为气候模型中最大的不确定性来源。此外，云过程的不确定性对我们预测天气的能力有重要影响，因为降水是由云产生的，云调节我们白天接收的阳光量和晚上损失的热量，云和降水中的潜热过程驱动风暴的动态变化。由液态水滴组成的低空云层覆盖了地球仪的大片区域，使地球气候变冷。然而，我们在气候模式中模拟这些云的能力很差，毛毛雨的产生被认为是一个关键的弱点。我们需要新的观测来解开这些云的过程，并改善它们在模拟中的表现。与此同时，冰云在任何时候都覆盖着地球的三分之一，平均来说提供了净变暖。然而，这种变暖的幅度是非常不确定的，它们对我们气候的影响对我们对它们的物理学的假设非常敏感。因此，我们迫切需要限制那些控制冰粒在自然云中演变的物理过程。最后，层状降水是水文循环和辐射收支的重要组成部分。典型地，这样的降水包括在高空的冰相和在较低海拔的液相。然而，这两个阶段的过程仍然是不确定的，需要新的观测来有力地约束它们。我们的新建议利用新的雷达技术，突破目前的限制，我们目前可以检索有关云的属性和过程，驱动水凝物的演变。在英国气象局和ECMWF的项目合作伙伴的帮助下，我们将利用这些信息来改进天气和气候预报中云过程的模拟。2018年，英国航天局和地球观测仪器中心同意资助开发一种新的200 GHz（G波段）多普勒雷达系统，称为GRaCE，由研究人员Huggard和Battaglia领导。这一突破性的演示仪器将于2020年初在奇尔博尔顿天文台收集其第一批数据，并将能够穿透多层云层，由于雷达波长为1.5毫米，对小型亚毫米粒子具有前所未有的灵敏度，这是全球任何云雷达系统中最小的。该雷达将与一套其他遥感仪器协同工作22个月。GRACES的科学家们将利用这一前所未有的数据集，他们是雷达遥感技术的领导者，并率先采用了多波长多普勒雷达的检索技术。将获得云的物理特性的垂直廓线，包括含水量以及毛毛雨和冰晶的大小分布，这些数据将被用来测试云过程在数值模式中比以前可能的更详细的表现。通过我们观测云的能力的这一飞跃，GRACES系统将成为未来发展一种新的地面流的先驱。这将大大加强目前的地球观测系统。雷达的高频率意味着它也将适合于发展成与云有关的研究的机载/星载仪器，事实上，考虑到美国航天局喷气推进实验室在较小频率（165至173千兆赫）上建造机载差分吸收雷达（用于测量水蒸气）的平行努力，该提案非常及时，并开发G波段的CubeSat雷达（见美国航天局喷气推进实验室的LoS）。

Despite the well-recognised influence of clouds and precipitation on our climate, there are still critical gaps in our ability to observe cloud properties that are needed to test and improve how cloud processes are represented in models. This leads to clouds and aerosols being the biggest source of uncertainty in climate models, according to the IPCC. In addition, uncertainties about cloud processes have important impacts on our ability to predict the weather, because precipitation is produced by clouds, clouds modulate the amount of sunlight we receive during the day and heat we lose at night, and latent heat processes in clouds and precipitation drive dynamical changes in storms. Low-altitude clouds of liquid water droplets cover large swathes of the globe, and cool the earth's climate. However our ability to simulate these clouds in climate models is poor, and the production of drizzle has been identified as a key weakness. We need new observations to unravel the processes in these clouds and improve their representation in simulations. Meanwhile ice clouds cover around one third of the earth at any one time, and provide a net warming on average. However the magnitude of this warming is very uncertain, and their impact on our climate is very sensitive to what we assume about their physics. Thus we urgently need to constrain those physical processes controlling how ice particles evolve in natural clouds. Finally, stratiform precipitation is an important component of the hydrological cycle and the radiation budget. Typically such precipitation include an ice phase aloft and a liquid phase at lower altitude. Yet there are processes in both phases which remain uncertain, and require new observations to robustly constrain them.Our novel proposal exploits new radar technology to break through the current limitations on the information we can currently retrieve about cloud properties and the processes that drive the evolution of the hydrometeors within them. With the help of our project partners at the Met Office and the ECMWF we will use this information to improve the simulation of cloud processes in weather and climate forecasts. In 2018 the UK Space Agency and Centre for Earth Observation Instrumentation agreed to fund the development of a new 200 GHz (G-band) Doppler radar system, called GRaCE, led by investigators Huggard and Battaglia. This ground-breaking demonstrator instrument will collect its first data at the Chilbolton Observatory early in 2020, and will be able to penetrate multiple layers of clouds with unprecedented sensitivity to small sub-millimetre particle thanks to the radar 1.5 mm wavelength, the smallest for any cloud radar system worldwide. The radar will be operated for 22 months in synergy with a suite of other remote sensing instruments. The unprecedented dataset will be exploited by GRACES scientists who are leaders in radar remote sensing techniques and have spearheaded retrieval techniques for multi-wavelength Doppler radars. Vertical profiles of cloud physical properties including water content as well as drizzle drop and ice crystal size distributions will be obtained and this data will be used to test the representation of cloud processes in numerical models in much greater detail than has been possible before.Through this leap forward in our ability to observe clouds the GRACES system will become the forerunner for future development of a new stream of ground-based remote sensing instruments, greatly strengthening the current Earth observing system. The high frequency of the radar means that it will also be suitable for development into air-borne/space-borne instruments for cloud related studies, and indeed the proposal is very timely given parallel efforts at NASA's JPL to build an airborne differential absorption radar (for measuring water vapour) at smaller frequencies (165 to 173 GHz), and to develop CubeSat radars in the G-band (see NASA-JPL's LoS).