Water System Program


The Water System Program is an NSF base-funded effort involving scientists from RAL, CGD, MMM, and EOL.  Since 2001 the program has conducted research aimed at improving the representation of the water cycle in regional and global climate models. Using the diurnal cycle of precipitation as a focus, research has shown that current climate models do not accurately simulate the frequency, intensity, and timing of summer time convection over much of the globe, including continental regions, despite reasonable simulations of precipitation amount. This model deficiency greatly hampers climate models' ability to predict future changes in intense storms, flash floods, tornados, hurricanes, and other severe weather events that likely have the largest impact on the society under global warming.  Water System funding at RAL supports a number of research efforts to advance our understanding and modeling of the water cycle and to improve simulations of severe weather events; several of these efforts are described below and links provided to projects described more fully elsewhere in this report.

Contiguous United States (CONUS) High-Resolution Climate Modeling

The primary goals of the “CONUS project” are to: 1) examine how key physical processes such as precipitation, snowfall, snowpack, runoff and evapotranspiration are impacted by climate change over a significant part of North America using a model with sufficient resolution to capture them (4 km horizontal grid size), and 2) to examine the impact of climate change on severe weather over North America. This effort was made possible through the award of 27.5 M core hours on the NCAR Yellowstone computer from the CISL Advanced Science Discovery grant process.  The first year of the project tested and evaluated the model configuration and parameterizations necessary to produce a faithful simulation of the current climate.  During the second and third years of the project, 13 years of the current and future climate simulation at 4 km resolution (Oct. 2000 – March 2013) were completed.  The simulations for the future climate were forced by a modified ERA-Interim reanalysis achieved by adding the CMIP5 climate model monthly mean perturbations of temperature, humidity, winds, and geopotential height to the re-analysis.

 A paper describing the simulation and verification as well as some preliminary results was published in September 2016 in the journal Climate Dynamics (Liu et al. 2016).  The dataset has been hosted by CISL on its web site and is available to the community through the following link:

DOI : https://rda.ucar.edu/datasets/ds612.0/

Info on the DOI is at https://ezid.cdlib.org/id/doi:10.5065/D6V40SXP

The output of the model runs is being used by NCAR Water System and university scientists to examine western snowfall and snowpack changes in a future climate, as well as convection in the central U.S.  Key papers include:

Rasmussen KL, AF Prein, RM Rasmussen, K Ikeda, C Liu (2017): Changes in the convective population and thermodynamic environments in convection-permitting regional climate simulations over the United States. Climate Dynamics (In Press)

Prein AF, C Liu, K Ikeda, S Trier, RM Rasmussen, GJ Holland, MP Clark (2017): Increasing rainfall volume from future severe convective storms. Nature Climate Change. doi:10.1038/s41558-017-0007-7

Prein AF, C Liu, K Ikeda, R Bullock, RM Rasmussen, GJ Holland, M Clark (2017) Simulating North American Mesoscale Convective Systems with a Convection Permitting Climate Model. Climate Dynamics. doi:10.1007/s00382-017-3947-8

Dai, A., RM Rasmussen, C Liu , K Ikeda , AF Prein (2017) A new mechanism for warm-season precipitation response to global warming based on convection-permitting simulations. Climate Dynamics, DOI 10.1007/s00382-017-3787-6.

Dai, A., R.M. Rasmussen, K. Ikeda, and C. Liu (2017) A new approach to construct representative future forcing data for dynamic downscaling. Climate Dynamics, DOI 10.1007/s00382-017-3708-8

Prein AF, RM Rasmussen, G Stephens (2017) Challenges and Advances in Convection-Permitting Climate Modeling. BAMS; doi:10.1175/BAMS-D-16-0263.12/5

Prein AF, RM Rasmussen, K Ikeda, C Liu, M Clark, GJ Holland (2017) The future intensification of hourly precipitation extremes. Nature Climate Change; 7(1):48–52; doi:10.1038/nclimate3168

Liu C, K Ikeda, RM Rasmussen, M Barlage, AJ Newman, AF Prein et al. (2016), Continental-scale convection-permitting modeling of the current and future climate of North America. Climate Dynamics, doi:10.1007/s00382-016-3327-9

Prein AF, GJ Holland, RM Rasmussen, MP Clark, MR Tye (2016) Running dry: The US Southwest’s drift into a drier climate state. Geophysical Research Letters, 43 (3), 1272-1279.

Prein AF, C Liu, K Ikeda, R Bullock, RM Rasmussen, GJ Holland, M Clark (2017) Simulating Convective Storms: A New Benchmark for Climate Modeling. BAMS (submitted)

Musselman, K.N., F. Lehner, K. Ikeda, M.P. Clark, A. Prein, C. Liu, M. Barlage and R. Rasmussen, Projected increases and regime shifts in rain-on-snow flood potential over western North America. Nature Climate Change, in review.

Musselman, K.N., M. P. Clark, C. Liu, K. Ikeda and R. Rasmussen (2017), Slower snowmelt in a warmer world. Nature Climate Change. 7(3), 214-219. DOI: 10.1038/nclimate3225

Scafe et al. 2017: Simulating the diurnal cycle of convective precipitation in North America's current and future climate with a convection-permitting model, In review at Climate Dynamics.

Letcher, T.W., J.R. Minder, 2017The simulated impact of the snow albedo feedback on the large-scale mountain-plain circulation east of the Colorado Rocky MountainsJournal of the Atmospheric Sciences, (Accepted for publication)

Minder, J.R., T.W.* Letcher, C. Liu, 2017The character and causes of elevation-dependent warming in high-resolution simulations of Rocky Mountain climate changeJournal of Climate (Accepted for publication)

Rasmussen, R.M., K. Ikeda, M. Clark, C. Liu, F. Chen, M. Barlage, A. Newman, E. Gutmann, J. Dudhia, D. Gochis, A. Dai and K. Musselman, 2018: Snowfall and Snowpack Future Trends in the Western U.S. as Revealed by a Convection Resolving Climate Simulation. (To be submitted to J. Climate)

A key aspect of all these papers is a comparison of the model simulation to observations during the 13 year historical period.  For the most part, the comparison showed excellent agreement down to the hourly and 4 km horizontal scales (see below for a notable descrepancy).  Given this confirmation, the papers go on to address the impact of future climate on convection, extreme convection (i.e., Mesoscale Convective Complexes), snowfall and snowpack in the western U.S., behavior of rain on snow events, and changes in the frequency and intensity of rainfall.

Scientists at the University of Saskatchewan used the model output to examine climate change and water in the Canadian prairies.  University of Quebec at Montreal scientists are examining climate change impacts on extreme winter storms, while University of Albany scientists are examining the impact of future climate change on the water cycle in the Northeast U.S. and snow albedo.

The notable deficiency of the simulation was a significant warm and dry bias in the central U.S. in the summertime.  This is a well-known problem with many weather and climate models and is an active research area for the Water System program and for the community. A team was formed to investigate the cause of this bias and a potential solution is currently being tested.

The CONUS team is preparing for a second set of current and future simulations at high-resolution over North America that will be forced by the CMIP5 ensemble mean using a novel way to force the future climate simulations using output of the NCAR CESM model 6-hourly output of one of the ensemble members from the CMIP5 archive (Dai et al. 2017). The domain will be expanded northward to include most of Canada and the length of both the current and future simulations will be twenty years. This effort is being led by Professor Aiguo Dai from the State University of New York, Albany.

A two-day Water System retreat was held on January 17-18 2017 with approximately 40 NCAR scientists participating.  The goal of this workshop was to provide a forum in which NCAR water related science can be discussed and new collaborations formed as well as refining goals for the next year of NCAR water cycle research.  The workshop was well attended and a highlight was a keynote talk by Kevin Trenberth and active participation by the attendees. The next Water System retreat will be held on January 16-17th, 2018.  This workshop will include keynote talks by Mitch Moncrieff and Vanda Grubisic and will focus on refining future research directions for the water system program.  


A major event for the Water System program next year will be an international workshop in September 2018 at NCAR entitled: “Convective Permitting Climate Modeling Workshop II”.  This workshop will be co-sponsored by GEWEX and will follow the very successful format used in the 2016 workshop.  (A summary of the previous workshop which was attended by 75 scientists from around the world, can be found at the following web site: https://ral.ucar.edu/events/2016/cpcm).  Roy Rasmussen, Andreas Prein and Graeme Stephens (GEWEX co-chair) will organize the 2018 workshop. This scientific area is growing rapidly and the participants are eager to come together and discuss the successes and challenges related to this new scientific endeavor.  In addition, the NCAR Water System program has been hosting AGU sessions on this topic the past two years and will have another session this year.

In FY2018, the Water System program will support a second major current and future CONUS simulation at 4 km resolution based on CMIP5 forcing that will include an improved simulation of the central U.S. surface temperatures and moisture. The primary difference from the previous simulation will be the use of transient current and future weather from one select Global Climate Model instead of the weather from current re-analysis. 


Global water cycle studies conducted by Aiguo Dai focused on historical and future changes in precipitation, drought, streamflow and continental discharge by analyzing observations and model simulations, including WRF-based downscaling of future climate projections on 4km grids over the contiguous U.S. (CONUS). One particular area is how precipitation frequency for light, moderate and heavy precipitation events may respond to future GHG changes. Another focus area is the separation of natural variations associated with the Pacific Decadal Oscillation (PDO) or the Inter-decadal Pacific Oscillation (IPO) on decadal to multi-decadal time scales from GHG-forced long-term changes in observational records and model simulations for precipitation and other fields. How the rising air temperature may affect surface aridity and drought is another research area of Dai’s group.

Accomplishments in FY2017 and Plans for FY2018:

In FY2017, Aiguo Dai has helped with the analyses of the PGW CONUS simulations and played a key role in preparing for the Phase II of the CONUS simulations, including obtaining new computer time on the new Cheyenne supercomputer, preparing the boundary forcing data, and choosing the updated domain. In addition, he has published over 10 journal articles during the review period that are related to precipitation, drought and other aspects of the water cycle. These include two first-author papers by Dai et al. in Climate Dynamics that are directly related to the WRF-based CONUS simulations. In FY2018, a major work will be the Phase II CONUS WRF simulations to downscale the CMIP5 model projections onto a 4km grid over a large domain covering most of North America.

The major accomplishment in FY2017 was the completion of the high-resolution PGW CONUS simulations, including publication of 5 papers describing the simulations and the key results. The plans for FY2018 include continued analysis of the PGW CONUS simulations, performance of the new transient climate high-resolution CONUS simulation, upgrading WRF-Hydro and SUMMA, further enhancement to NOAH-MP, and progress on the new Community Terrestrial Systems Model (CTSM).


The following research areas leveraged Water System funding with external funds from NOAA, the Bureau of Reclamation, and the Army Corps of Engineers.


A cornerstone of the NCAR/RAL Water Systems program is the development and support of community modeling tools for both process-based research and hydrometeorological forecasting applications.  These tools are co-developed by NCAR in close collaboration with University researchers and government agencies in the U.S. and around the world.  NCAR/RAL and the Water Systems program serve as focal points for training and collaboration in the hydrometeorological community.  The Community WRF-Hydro System provides scientists and forecasters extensible modeling tools to engage in process-based research into land-atmosphere coupling, hillslope routing processes, surface water-groundwater interactions and multi-scale hydrologic evaluations.  As a forecasting tool the WRF-Hydro System can run coupled or uncoupled to atmospheric prediction models and provide so-called ‘hyper-resolution’ forecasts of terrestrial hydrologic conditions such as soil moisture, snowpack, shallow groundwater, soil ice, streamflow, evapotranspiration and inundating waters.  A major accomplishment is the implementation of a version of WRF-Hydro as the National Weather Service National Water Model. 

FY2017 accomplishments and FY2018 plans are described in the “WRF-Hydro Community Modeling” section of this report.


Global population has become increasingly urbanized: Currently 52% of the world’s population live in cities, and this proportion is projected to increase to 67% by 2050. Urbanization modifies surface energy and water budgets and has significant impacts on local and regional hydroclimate. In recent decades, a number of urban canopy models (UCM) have been developed and implemented into the WRF model to capture urban land-surface processes, but those UCMs were coupled to the simple Noah land surface model (LSM). We recently coupled the more advanced Noah-MP LSM to WRF-Urban as well as to the urbanized high-resolution land data assimilation system (u-HRLDAS). This new modeling capability was tested over Phoenix and Beijing metro areas and will be released in WRF in 2017.  It is also being used to assess the degree to which a detailed urban modeling approach can improve real-time weather prediction for cities.

FY2017 accomplishments and FY2017 plans are described in the “Land Atmosphere Interactions” section of this report.


This project aims to improve the representation of cropland-atmosphere interactions in the community Noah-MP LSM with the ultimate goal to integrate it in a coupled model to improve seasonal weather forecasts and regional climate simulations for the NCAR Water System Program. Croplands cover 12.6% of the global land and 19.5% of the continental United States. Through seasonal change in phenology and transpiration, crops can efficiently transfer water vapor from the crop root zones to the atmosphere. Crops have a detectable influence on regional distributions of atmospheric water vapor and temperature, and can affect convective triggering by modifying mesoscale boundaries. Therefore, croplands can significantly influence land-atmosphere coupling, surface exchanges of heat, water vapor, and momentum, which in turn can impact boundary layer growth and mesoscale convergence/convection.

FY2017 accomplishments and FY2018 plans are described in the “Land Atmosphere Interactions” section of this report.


The Structure for Unifying Multiple Modeling Alternatives (SUMMA) has been developed as a next-generation hydrologic model, providing multiple options to simulate all dominant biophysical and hydrologic processes from the treetops to the stream. The SUMMA framework is centered on the structural core, which comprises the conservation equations for the hydrologic and thermodynamic states within the model domain, and general algorithms for their numerical solution. Different process representations and different spatial configurations are integrated into the structural model core, which enables users to decompose the modeling problem into the individual decisions made as part of model development and evaluate different “fine grain” model development decisions in a systematic and controlled way. The overall intent of SUMMA is to help modelers select among modeling alternatives (to improve model fidelity) and pinpoint specific reasons for model weaknesses (to better characterize model uncertainty and prioritize areas needing more research and development). SUMMA is beginning to see widespread use and is a core component of many new projects within RAL.

FY2017 accomplishments and FY2018 plans are described in the “Computational Hydrology” section of this report.


A joint project between the NCAR Water System program and the U.S. Army Corps of Engineers has led to the development of the Intermediate Complexity Atmospheric Research model (ICAR).  ICAR combines a simplified representation of atmospheric dynamics with physical parameterizations including microphysical and land surface processes.  The model simplifications permit ICAR to perform high-resolution simulations 100 to 1000 times faster than a traditional atmospheric model such as the Weather Research and Forecasting model (WRF). This is particularly important for climate downscaling applications.  Such applications are computationally constrained because end-users desire large ensembles of simulations to adequately represent the uncertainty in future climate projections. A key challenge is to adequately capture convection in this hybrid downscaling system.