Xubin Zeng, Chair of GPEX Interim SSG
University of Arizona, Tucson, Arizona, USA
Introduction
Precipitation is a critical component of the water cycle, and it has a direct connection to society, the environment and natural hazards. Hence it has already received much attention. For instance, it has been emphasized by numerous national and international projects (e.g., the WCRP Global Energy and Water Cycle Experiment (GEWEX)), and precipitation as part of hydrology has received substantially increased attention in the World Meteorological Organization (WMO) Research Strategy for Hydrology. Nevertheless, there remain major challenges in accurate measurement and modeling of precipitation across scales.
In response, WCRP launched the Global Precipitation EXperiment (GPEX) as a new Lighthouse Activity in October 2023, with the Science Plan published in November 2023 (https://www.wcrp-climate.org/lighthouse-activities/gpex/GPEX-SciencePlan-Nov2023.pdf). This short essay is based on this document and its peer-reviewed publication (Zeng et al. 2024, and references therein). Figure 1 summarizes the key elements of the GPEX Science Plan: The four key points are related to the four science questions of GPEX; and the four key activities of GPEX will cover various storm types (as indicated by the icons in the center) using various approaches (as represented by the five icons outside the circle), including field measurements, satellites, modeling/computing, artificial intelligence, and capacity development.
Key science questions
GPEX’s mission is to accelerate advances in precipitation knowledge and prediction at different temporal and spatial scales, to enhance access to relevant datasets, and to benefit society, through the coordination of national and international activities. The key science questions include:
- Q1: What are the sources and magnitude of uncertainties in quantitative liquid and solid precipitation estimates over land and ocean, particularly in regions of vulnerable populations and limited observational capabilities, and how can we reduce these uncertainties?
- Q2: How is precipitation produced by complex moist processes and their interactions with atmospheric dynamics and other components of the Earth system?
- Q3: What are the sources of errors in model precipitation intensity, phase, and duration in meteorological and climate models and how can we reduce them to improve predictions and projections of precipitation at different temporal and spatial scales?
- Q4: How can we enhance regional and local capacity for precipitation observations, process understanding, prediction and projection services including those designed to aid drought and flood early warning systems and applications to climate change adaptation?
The overall strategy is to plan the WCRP Years of Precipitation (YoP) as the GPEX flagship activity and associated activities before and after it. The four key activities are discussed below.
WCRP Years of Precipitation
The YoP planning will take 2-3 years and its underpinning science focus is (1) understanding and stratifying precipitation predictability in the context of the large-scale forcing by connecting precipitation to storms and their environments; and (2) bringing the weather and climate communities together on a common challenge that has been addressed from different perspectives.
Several key phenomena responsible for extreme precipitation events and their interactions across scales will be the foci of the YoP. Atmospheric Rivers are long, narrow, and transient corridors of strong horizontal water vapor transport that fuel extreme precipitation often associated with mid- and high-latitude cyclones and can be enhanced by orographic effects. Mesoscale Convective Systems in the tropics and mid-latitudes are a fundamental feature for organizing convection and associated precipitation. Tropical cyclones are an additional major driver of extreme precipitation events in the tropics and can extend into the subtropics and mid-latitudes. Finally, monsoon circulations are vitally important in regulating seasonal precipitation patterns and connecting monsoon precipitation to different storm types in highly populated regions.
These globally coordinated field campaigns will also be designed to include comprehensive measurements in the atmosphere, in the upper ocean, at and below land surface, and within the planetary boundary layers where these Earth system components interact. Innovative technologies for observing systems and quantitative estimates of precipitation will be encouraged. Data from previous field campaigns related to the storm types should be studied to augment new campaign data. Satellite and surface-based observations are crucial in planning for GPEX field campaigns. For instance, ahead of the campaigns, particularly satellite-derived data can be used to identify potential locations, air-mass trajectories, and expected frequencies for events of interest.
Precipitation-Relevant Databases
A wide range of global and regional precipitation datasets already exist, and GPEX will contribute to the systematic evaluation of such data sets as well as aid the establishment and/or expansion of global and regional precipitation databases. The rescue of historical data is particularly key for extremes that require long records for robust estimates of statistics.
GPEX will work with other WCRP Projects to set up a baseline surface precipitation network over land, with comprehensive high temporal resolution precipitation hydrometeor size distribution and phase measurements. GPEX will seek to stimulate development and deployment of low-cost, easy-to-maintain instruments for enhancing the global precipitation measurement network. GPEX will also work with other projects and interact with data users on the further assessment and quantification of uncertainties of gridded precipitation products (including reanalysis) at different spatiotemporal scales.
Precipitation Modeling, Prediction, and Process Understanding
GPEX will seek to coordinate precipitation analyses and forecasts from different centers (particularly national meteorological and hydrological centers) for the GPEX period, and support the establishment of multi-model databases, along with common evaluation metrics for deterministic, probabilistic, and extreme forecasts of precipitation.
Kilometer-scale models are the workhorses for regional prediction and are also used in some global studies. To gain the most benefit from them, new observational products and model evaluation metrics will need to be developed. To address this challenge, GPEX will need to work with the km-scale global and regional modelling communities. GPEX also intends to support research on precipitation predictability, prediction techniques and applications at various time scales.
National/Regional Activities and Capacity Development
GPEX should support existing national/regional activities and/or the establishment of new activities, partly through capacity building. New tools need to be developed for data visualization and delivery. For these and other activities, funding mechanisms and opportunities need to be explored, coordinated and realized.
GPEX will seek to support capacity development by entraining early career scientists, students, and institutions into the YoP. GPEX should also explore ways to help make km-scale modeling available for resource-challenged scientists in the Global South.
Implementation, Timeline, and Path Forward
While the detailed implementation plan of GPEX remains to be developed, we can divide GPEX activities into three phases:
- Pre-YoP Phase (e.g., Years 1-3): Data synthesis and development, modeling, and capacity development. The priority is to seek and encourage large GPEX-endorsed projects as anchors for global field campaigns and then entrain additional projects from various countries.
- YoP Phase (e.g., Years 4-6): Field campaigns, data development, modeling, and capacity development.
- Post-YoP Phase (e.g., Years 7-9): Analyses of data collected during the field experiments and linking those to long-term records, modeling advances, and capacity development.
The timeline for the above activities is flexible, as financial, logistical/political, and technical issues often delay planned field campaigns. We envisage the GPEX activity to be completed and fully integrated into one or more of WCRP Core Projects in 2-3 years after the field campaigns are completed. This would happen within a decade.
GPEX should emphasize its unique role as the only international project with substantial globally coordinated field campaigns and draw on the expertise across all the WCRP projects and many other programs (including the World Weather Research Programme). It should focus on a small set of activities outlined above, with remaining tasks led by other programs.
Moving forward, the WCRP JSC approved in August 2024 the GPEX Scientific Steering Group (SSG; including its two Co-Chairs) that will start in January 2025. The GPEX Interim SSG will also approve the Co-Chairs and some members for each of the four Working Groups in September 2024, purposely leaving adequate Working Group membership slots for candidates in the near future who will actively participate in and coordinate field campaigns on each storm type, development of precipitation related datasets, and modeling and prediction experiments. Furthermore, we are seeking anchor projects for global field campaigns.
REFERENCES
Zeng, X., and Co-authors, 2024: Global Precipitation Experiment - A New World Climate Research Programme Lighthouse Activity, Bull. Amer. Meteor. Soc. 105, https://doi.org/10.1175/BAMS-D-23-0242.1