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

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).

National weather services need accurate parameterisations of cloud processes in their numerical weather forecast and climate models. This project will evaluate the performance of current operational models and produce retrievals which will help their scientists to improve components of their parameterisations (e.g. drizzle drop size distributions and auto conversion rates). This outcome is facilitated by having collaborators from the Met Office and the European Centre for Medium-range Weather Forecasting within the project. This may of course have a huge societal impact.

Radar and microwave component manufacturers will benefit. The project will demonstrate the potential of G-band radar for weather and climate monitoring, and hence create a market for this new technology. With the STFC-RAL expertise UK is now in the position of spearheading this field, ahead of other international competitors like companies in the US (e.g. ProSensing, http://www.prosensing.com/) and in Germany (e.g. Radiometer Physics, http://www.radiometer-physics.de/) that are planning the construction of similar systems.

Space agencies planning satellite missions to monitor clouds and precipitation will benefit. G-band radar system are ideal for space deployment because they could be made very compact and generally require low amount of power to be operated. NASA has recently develop a differential absorption radar (VIPR, see Tanelli's LoS) operated at frequencies within the 183 GHz water vapour absorption band in order to prove water vapour profiling from space. It is now conducting technology studies to prove the feasibility of deploying a Ka-W-G band system in a CubeSat. ESA is also considering G-band for EarthCARE follow-up mission (ESA studies led by the PI, [PI-1]). EUMETSAT also will benefit indirectly via improved understanding of scattering at millimetre wavelengths and ice microphysics which underpin retrievals from the sub-millimetre ICI radiometer observations.

Two major societal applications are foreseen in the long-term for this kind of radar systems.
1) Fog is a major problem at ports, airports and for ground transportation. If we can demonstrate the potential for G-band radars to monitor fog depth and liquid water content then the operators of these facilities could benefit from that capability e.g. by improving now-casting capabilities.

2) Regions of very high ice water content, which nevertheless have relatively low radar reflectivities (i.e. they are composed of large numbers of small crystals) in proximity of convective cores have been recognised as a significant aviation hazard, potentially causing jet engine power losses or damage. These conditions are difficult to detect with current observing systems. Our work will develop methods to detect and profile clouds with small ice particles, and therefore will provide pioneering tools in the detection of such hazardous areas with obvious benefits for the aviation industry.