Calibration and validation studies over the North Atlantic and UK for the Global Precipitation Mission

Understanding the changing global precipitation patterns that result from a changing climate represents one of the great research priorities of the next decades. In order to better define the future we need to understand the present and the inter-annual variability of the precipitation cycle, which mirrors, in short periods, the expected climate-change-induced long-term variability. However, while observational studies based on the past 30 year records suggest rises in precipitation and evaporation at a global rate of 6-7%/K, global and regional climate models predict a muted response of the hydrologic cycle (2-3%/K). Quantitative estimation and prediction of precipitation still remain grand challenges in the hydrological and atmospheric sciences, both carrying huge uncertainties and thus preventing us from solving the previous conundrum. Observation-wise surface precipitation varies on spatial scales ranging from tens of meters to hundreds of kilometers, thus inhibiting the determination of spatio-temporal structures of precipitation fields from pointwise measurements only (e.g. rain gauges). Active and passive space-borne microwave measurements (precipitation radars, multi-wavelength radiometers) are considered to be suitable for estimating day and night-time precipitation rates and distributions on a planetary scale. Thanks to a satellite constellation concept the NASA-JAXA Global Precipitation Mission, due to launch early in 2014, promises to produce a significant step forward in improving coverage and reducing uncertainties in global precipitation products at a level sufficient to critically challenge numerical models. The presence of a first-ever-in-space dual frequency radar in the core satellite, with coverage up to 65 degrees latitude, will allow not only to know how much rain falls at the surface but also the detailed three-dimensional knowledge of rain, snow, and other forms of precipitation within the atmosphere above the surface and, with it, the links and the transfer of latent heat energy between the Earth's surface and atmosphere. This is an unprecedented opportunity in the mid-latitudes.
In our effort, by specifically focusing on UK and on the North Atlantic region, we will address two scientific objectives:
1) To quantify, understand, and potentially mitigate regime dependent biases that are present in today's passive microwave rainfall retrieval over ocean; 2) to critically assess the potential of Global Precipitation Mission-era passive microwave rainfall over mid-latitude coastal and rural areas. The UKMO radar network will be used as ground-reference for the satellite products; as a result, improvements on radar-based rainfall estimates over UK are expected as well. In addition, through improved measurements of precipitation globally, the Global Precipitation Mission will help advancing our understanding of Earth's water and energy cycle, improving forecasting of extreme events that cause natural hazards and disasters, and extending current capabilities in using accurate and timely information of precipitation to directly benefit society. This project will foster and help UK scientists and UK society in taking full advantage of such a unique opportunity.

Life depends upon water. It impacts our lives and the natural environment through the every-day weather and the hydrological extremes of floods and drought. On a scientific level, water is one of the main drivers of the climate system, transferring and regulating energy within it. Despite the importance of water to us and the Earth system, our current knowledge of the occurrence and distribution of water across the planet and the mechanisms that drive the water cycle is very limited. Furthermore, our ability to accurately predict precipitation and its inherent spatio-temporal variability have not reached a mature level, partly due to the currently poor capability to accurately represent clouds and precipitation systems in cloud-resolving, regional and global models. The same applies to climate models and climate prediction. The GPM mission has a large potential towards advancing our knowledge of the global water cycle and its variability in a changing climate. It will provide, particularly over mid-latitude regions, improved measurements and predictions of precipitation that will lead to a greater understanding of water balances and processes of interest for water resources management, to a deeper insight in microphysics control on severe rainfall processes and to obvious consequential benefits for the UK society (e.g. improved capabilities in predicting flood and flash-flood producing storms).
Beneficiaries from this research will be (i) ESA and EUMETSAT, (ii) weather forecasting and climate prediction institutions like the UKMO, ECMWF, (iii) water management companies (especially in developing countries) , (iv) the International Panel on Climate Change, (v) UK government organizations and (vi) the general public.
The proposed research is extremely relevant and timely for upcoming ESA/EUMETSAT MetOp follow missions that will have three operational microwave instruments and will be part of the GMP constellation.
Weather forecasting and climate prediction institutions will benefit from an improved understanding of precipitation processes on all scales. GPM radiometer constellation will produce global estimates of precipitation within storms and our methodology will avoid or at least mitigate biases in such retrievals over the mid-latitudes. Our results will contribute to the final quality of the GPM mission products and, in the medium to long term, to improved micro-physical parameterizations and precipitation processes in weather and climate prediction models. Similarly the cross-calibration study between the GPM radar and the Met Office radars will contribute to improve precipitation products over UK with obvious benefit for organizations like DECC and DEFRA. Improved over-land precipitation products will be beneficial to water management companies, especially in developing countries where the ground-based measuring network is almost totally absent.
Climate change will have profound impact on the water cycle, the precipitation being one of the most unpredictable variables in a climate change scenario. While there is a general agreement on the fact that a warmer climate will also be wetter, there is no consensus on the amplitude of such an increase in precipitation and in the spatial patterns associated with it. The distribution of precipitation is likely to change as well, with frequency of heavy precipitation, polar precipitation and tropical cyclone intensity likely to increase, and precipitation over subtropical arid land regions likely to decrease (IPPC 2007 Report). Our research will improve understanding of precipitation events and their frequency of occurrence over the mid-latitudes and over UK, with great impact for government organizations and the wider public. The PI will act as GPM 'ambassador' in the UK and will convey the relevance of this mission both to the UK scientists and to the UK public with an articulated outreach program.