Water System Program


The Water System Program is an NSF base-funded effort involving scientists from RAL, CGD, MMM, and EOL.  The program conducts research aimed at improving the representation of the water cycle in regional and global climate models. Focusing on the diurnal cycle of precipitation, research has shown that current climate models do not accurately simulate the frequency, intensity, and timing of summertime 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 society under global warming, including agriculture and water resources.  Water System funding at RAL supports a number of research efforts to advance our understanding and modeling of the water cycle and improve simulations of severe weather events; several of these efforts are described below and links to projects described more fully elsewhere in this report links are provided.

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 influenced 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 2017 in the journal Climate Dynamics (Liu et al. 2017).  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:

Musselman, K.N., F. LehnerK. IkedaM. ClarkA. PreinC. LiuM. Barlage and R. Rasmussen, Projected increases and shifts in rain-on-snow flood risk over western North America (2018), Nature Climate Change, 8, pp. 808–812.

Gutmann, E., R.M. Rasmussen, C. Liu, K. Ikeda, C.. Bruyere, J. Done, L. Garre, P. Friis-Hansen, V. Veldore (2018): Changes in Hurricanes from a 13 Year Convection Permitting Pseudo-Global Warming Simulation, J. Climate, D-17-0291.

Eidhammer, T., V. Grubišić, R.Rasmussen, & K. Ikeda (2018). Winter precipitation efficiency of mountain ranges in the Colorado Rockies under climate change. Journal of Geophysical Research: Atmospheres, 123. https://doi.org/10.1002/2017JD027995

Liu, C., K. Ikeda., R. Rasmussen, M. Barlage, A. J. Newman, A. F. Prein, F. Chen, L. Chen, M. Clark, A. Dai, J. Dudhia, T. Eidhammer, D. Gochis, E. Gutmann, S. Kurkute, Y. Li, G. Thompson, D. Yates, 2017:  Continental‑scale convection‑permitting modeling of the current and future climate of North America, Climate Dynamics, DOI 10.1007/s00382-016-3327-9.

Prein, A.F., C. Liu, K. IkedaS. TrierR. Rasmussen, G. Holland, M. Clark, Increased rainfall volume from future convective storms in the US, ,,,2017:  Nature Climate Change, 2017, 7, 880–884, , doi:10

Rasmussen, K. L., A. F. Prein, R. M. Rasmussen, K. Ikeda, and C. Liu, 2017: Changes in the convective population and thermodynamic environments in convection-permitting regional climate simulations over the United States. Climate Dynamics, https://doi.org/10.1007/s00382-017-4000-7.1038/s41558-017-0007-7

Prein, A. F., G. J. Holland, R. M. Rasmussen and M. P. Clark, 2017: The future intensification of hourly precipitation extremes. Nature Climate Change, 7, 48-52.

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. (2017), Continental-scale convection-permitting modeling of the current and future climate of North America. Climate Dynamics, doi:10.1007/s00382-016-3327-9

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. 2018: 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, 2017: The simulated impact of the snow albedo feedback on the large-scale mountain-plain circulation east of the Colorado Rocky Mountains. Journal of the Atmospheric Sciences, (Accepted for publication)

Minder, J.R., T.W.* Letcher, C. Liu, 2017: The character and causes of elevation-dependent warming in high-resolution simulations of Rocky Mountain climate change. Journal 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 element throughout all of these papers is a comparison of the model simulation to observations during the 13-year historical period.  For the most part, the comparison shows excellent agreement down to the hourly and 4-km horizontal scales (see below for a notable discrepancy).  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 and hail.

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 paper is being written describing the solution. 

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 6-hourly output of the NCAR CESM model 6-hourly output from the CMIP5 archive (Dai et al. 2017). The model domain will be expanded northward to include Canada and the Canadian Arctic.  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 in collaboration with scientists in the NCAR Water Systems program.

A two-day Water System retreat was held on January 16-17th 2018 with approximately 50 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 and to refine goals for the next year of NCAR water-cycle research.  The workshop was well attended and a highlight was a keynote talks by Mitch Moncreiff and Vanda Grubisic, and participation by the attendees was very energized. The next Water System retreat will be held on January 29-30th, 2019.  This workshop will include keynote talks by Andreas Prein and Flavio Lehner and will focus on further refining future research directions for the water system program including a discussion on the Water System program co-leadership of the GEWEX Water for Foodbaskets Grand Challenge.  

FY2018 Accomplishments

A major event for the Water System program was an international workshop in September 2018 at NCAR titled: “Convective Permitting Climate Modeling Workshop II”.  This workshop was co-sponsored by GEWEX and followed the very successful format used in the 2016 workshop.  Roy Rasmussen, Andreas Prein and Graeme Stephens (GEWEX co-chair) organized both the 2016 and 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 pursuit.  In addition, the NCAR Water System program has been hosting AGU sessions on this topic for the past three years and will host another session this year.

In FY2019, 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 and a significant extension of the domain to the north. Both current and future climate simulations will be performed for 20 years.


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 4-km grids over the contiguous U.S. (CONUS). One particular study area examines 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 FY2018 and Plans for FY2019:

In FY2018, Aiguo Dai 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 more 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 related to precipitation, drought during the review period as well as other aspects of the water cycle. These include two first-author papers by Dai et al. in Climate Dynamics directly related to the WRF-based CONUS simulations. In FY2019, the Water System program, working with Aiguo Dai, will focus on Phase II CONUS WRF simulations to downscale the CMIP5 model projections onto a 4-km grid over a large domain covering most of North America.

The major accomplishment in FY2018 was the publication of a number of papers using data from the CONUSI simulations. The plans for FY2018 include continued analyses of the PGW CONUSI simulations, performance of the new transient-climate high-resolution CONUS2 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 with 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 Version 2 of the National Weather Service National Water Model based on WRF-Hydro. 

FY2018 accomplishments and FY2019 plans are described in the “WRF-Hydro Community Modeling” section of this report.


Global populations have 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). A coupled Noah-MP LSM and WRF-Urban was released in WRF in 2018.  It is also being used to assess the degree to which a detailed urban modeling approach can improve real-time weather prediction for cities.

FY2018 accomplishments and FY2019 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 and to support the GEWEX Water for Foodbaskets effort co-led by the Water System program. Croplands cover 12.6% of the global land surface 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.

FY2018 accomplishments and FY2019 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 from 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 and of the CTSM effort at NCAR.

FY2018 accomplishments and FY2019 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 1,000 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.