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 local, regional and global climate models. Focusing on the diurnal cycle of precipitation, research has shown that current global 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 supports a number of research efforts to advance our understanding and modeling of the water cycle and improve simulations of severe weather events including winter storms); 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 and water resources over North America.

This effort was made possible through an 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). This paper already has already been cited 96 times (see Figure 1).  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. and other severe weather phenomena such as hurricanes.   Key recent 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, AF, RM Rasmussen, K Ikeda, C Liu, MP Clark, GJ Holland (2017) The future intensification of hourly precipitation extremes. Nature Climate Change, 7(1), DOI: 10.1038/nclimate3168

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 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

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, L. A. Prein, Y. Li, C. Liu, K. Ikeda and R. Rasmussen: 2019: Simulating the diurnal cycle of convective precipitation in North America's current and future climate with a convection-permitting model, Climate Dynamics, DOI: 10.1007/s00382-019-04754-9.

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, doi.org/10.1175/JAS-D-17-0166.1

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

Chen, L., Y. Li, F. Chen, M. Barlage, Z. Zhang, and Z. Li, 2019: Using 4-km WRF CONUS Simulations to diagnose surface coupling strength, Clim. Dyn., https://doi.org/10.1007/s00382-019-04932-9.

Zhang, Z., Y. Li, F. Chen, M. Barlage, and Z. Li, 2018: Evaluation of convection-permitting WRF CONUS simulation on the relationship between soil moisture and heatwaves. Climate Dynamics, pp.1-18, Clim. Dyn., http://doi.org/10.1007/s00382-018-4508-5.

Fig. 1: Annual number of papers mentioning convection-permitting climate modeling from 2000 through August 2019. Years in which NCAR’s Water System simulations over the Colorado Headwaters (2010) and CONUS1 (2014) were completed are highlighted in red. The inlay shows citation rates of key publications from the Water System group on convection-permitting climate modeling from 2011 to 2019. Blue bars show studies using the Colorado Headwaters simulations and warm colors show publications based on the CONUS simulations.
Figure 1. Annual number of papers mentioning convection-permitting climate modeling from 2000 through August 2019. Years in which NCAR’s Water System simulations over the Colorado Headwaters (2010) and CONUS1 (2014) were completed are highlighted in red. The inlay shows citation rates of key publications from the Water System group on convection-permitting climate modeling from 2011 to 2019. Blue bars show studies using the Colorado Headwaters simulations and warm colors show publications based on the CONUS simulations.

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), hurricanes, 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. Many of these studies build on the success and experience gained from the Colorado Headwater high-resolution climate modeling simulations (Rasmussen et al. 2011; see Figure 1) and allowed the expansion of the Water System’s research into new phenomena and regions. The impact of the Water System’s activities in high-resolution climate modeling can be seen in the high citation rates of literature from Water System scientists (Figure 1).

At this time only the thermodynamic future climate impacts can be addressed through the PGW approach. Future work will extend these results to include climate change impacts on the large-scale flow (internal dynamics of the flow).

Scientists at the University of Saskatchewan are using the model output to examine climate change and water in the Canadian prairies, including the effect of land-atmosphere coupling strength and the connection between soil moisture and heatwaves.  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. Many other researchers are using the CONUS data as well as can be seen from the more than 130 internet downloads form NCAR’s Research Data Archive (https://rda.ucar.edu/datasets/ds612.0/#!metrics).

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 (e.g., Lin et al. 2017).  A team was formed to investigate the cause of this bias and they determined it be due to the need to include groundwater at fine model grid spacings (< 4 km). The team found through a series of simulations with and without groundwater at varying spatial resolutions that at the convective-permitting scale (~4km) the resolved riparian regions in the central U.S. become an important local moisture source. This reduces the warm bias through direct effects of shifting surface energy budget components and also in feedbacks, such as increased cloudiness. A paper on this important result is currently in process.

The CONUS team is currently conducting a second set of current and future simulations at high resolution (4 km horizontal grid spacing) over North America (called “CONUS2”) 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 and will include the new ground water treatment developed by water system scientists to eliminate the warm, dry bias over the central U.S. (and likely over central Canada). 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. This effort enables to address systematic large-scale flow changes due to climate change, but more simulations are needed in order to address the known sensitivity of the flow dynamics to small changes in initial conditions revealed by the work by Clara Deser (e.g., Deser et al. 2014).

A two-day Water System retreat was held on January 29-30th 2019 with approximately 50 NCAR scientists participating. The goal of this workshop was to provide a forum for the discussion of NCAR water related science, to foster new collaborations and to refine goals for the next year of NCAR water-cycle research.  The workshop was well attended with excellent keynote talks by Andreas Prein and Flavio Lehner.   Two of the major outcomes of the workshop was: 1) a draft strategic plan for the water system program and 2) an agreement by participants to form an affinity group regarding research related to South America.  This region was chosen as it represents a challenge for climate simulations for both CESM and WRF/MPAS, especially considering that it spans a variety of climatic regions, from tropic convection, Amazon rainforest, and one of the tallest mountain ranges in the world, the Andes. The next Water System retreat will be held for two days on February 10 and 11, 2020 and will include topics such as activities supporting the movement towards global convective permitting simulations, linkages to the GEWEX Water for Foodbaskets Grand Challenge, and results from the developing South America initiative.  All three of these topics are highlighted in the Water System draft plan.


A major event for the Water System program was supporting the LATSIS international workshop in late 2019 at ETH, Zurich titled: LATSIS SYMPOSIUM 2019 High-Resolution Climate Modeling: Perspectives and Challenges August 21 – 23, 2019, ETH.  This workshop was primarily funded by the LATSIS Foundation and partially sponsored by GEWEX and the NCAR Water System program.   Christoph Schaer was the primary convener of this workshop, with help from Roy Rasmussen and Andreas Prein.  The workshop focused on both the scientific and computational changes to perform high resolution climate modeling (called storm resolved or kilometer-scale modeling at the workshop).  This workshop was the third in a series of workshops co-sponsored by the Water System program and GEWEX and the first outside the U.S.  This was an outstanding workshop with over 10 high profile invited speakers.  This scientific area continues to grow rapidly, with over 160 papers published in 2018 on this topic (see Figure 1).  Next year Japanese scientist have requested to host the workshop in Kyoto, Japan on September 2-4, 2020. Roy Rasmussen and Andreas Prein are actively working with the Japanese on the logistics and agenda for this workshop. 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.

Water System scientists worked with Aiguo Dai of the University of Albany to submit a proposal to the NSF call “Accelnet”.  This call is designed to provide funding to support networks of scientists working around the world.  While the proposal did not get funded, the discussions associated with creating the proposal ultimately helped engage the Japanese CPCM contingent to host a workshop next year.  This NSF call is expected to occur every year, and we are currently considering whether to re-apply.

In FY2020, the Water System program will support a major WRF simulation effort called CONUS2 expanding the CONUS thirteen year simulation both in size (extending into northern Canada collaborating with the Global Water Futures program at the University of Saskatchewan) and period of time considered (twenty years current and future).  The horizontal grid spacing will be 4-km, and the model will be forced with a bias corrected current and future climate 6 hourly weather output for the RCP8.5 scenario out to 2100.  This simulation will include the updated ground water treatment discussed above in order to significantly reduce the warm and dry central U.S. bias. 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. We anticipate that these simulations will be completed during FY2020 and scientific analysis to be well underway by the end of FY2020.


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.


In FY2019, 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 FY2020, 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 FY2019 was the publication of a number of papers using data from the CONUS1 simulations. In particular, a paper co-authored by Changhai Liu, Kyoko Ikeda and Roy Rasmussen (submitted to Climate Dynamics and currently under review) investigated high-impact Severe Convective Weather (SCW; such as tornadoes, thunderstorm winds, and large hail) in the central and eastern United States for the current and future warmed climate. It was shown that the spatial distributions and seasonal variations of the observed SCW events were reasonably well captured by the retrospective simulation. In a warmer-wetter future, most regions were projected to experience intensified SCW activity and severity most notably in the early-middle spring, with the largest percentage increase in the foothills and higher latitudes. In addition, in FY2019 the water system team conducted a series of 11-year test runs at 12-km grid spacing over North America to 1) quantify the value of ERA-Interim reanalysis based bias-correction to CESM data; 2) assess the impact of different lake water temperature and ice treatments, usage of a lake model, monthly versus daily sea ice, snow fraction treatment, and cloud fraction treatment; 3) evaluate the sensitivity to domain configuration and model physics; and 4) determine and mitigate the sources of the detected model deficiencies (in particular, the winter-spring cold bias in northern U.S. and Canada and year-round low-precipitation bias in Deep South). These test runs and significant efforts to improve the physics of the WRF model enabled us to come up with an optimal model setup for 4-km-resolution production runs of CONUS2.

The plans for FY2020 include continuing analyses of the PGW CONUS1 simulations, conducting and completing the new transient-climate high-resolution CONUS2 simulation, upgrading WRF-Hydro, further enhancement to NOAH-MP, and establishing an affinity group on South America research.


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 hCommunity 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. 

FY2019 accomplishments and FY2020 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. During 2019 this model was used to perform a number of fundamental scientific studies and has been the basis for a number of scientific proposals to various agencies.

FY2019 accomplishments and FY2020 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.

FY2019 accomplishments and FY2020 plans are described in the “Land Atmosphere Interactions” 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. FY19 accomplishments and FY2020 plans are described in the Water Resources Research portion of this report.

Regionally-refined CESM

Figure 2.  Spectral elements from an SE-dycore grid refined to a resolution of around 25km over South America from an outer global grid with Dx~100km.  This grid was generated on NCAR’s HPC cluster using a GUI-driven package.  The grid depicted here possesses around 0.14 as many grid points as a full global grid with Dx~25km.
Figure 2.  Spectral elements from an SE-dycore grid refined to a resolution of around 25km over South America from an outer global grid with Dx~100km.  This grid was generated on NCAR’s HPC cluster using a GUI-driven package.  The grid depicted here possesses around 0.14 as many grid points as a full global grid with Dx~25km.

Support from the NCAR Water System Program has led to the development of a flexible regional-refinement (RR) “tool chain” for use with the Community Earth System Model (CESM) Spectral Element dynamical core (SE-dycore).  A GUI-driven interface for the creation of RR grids has been implemented on NCAR’s HPC systems and will be generalized to run on a variety of systems, including Mac OS, during FY20.  The SE-dycore is a state-of-the-art core based on a high-order piecewise spectral representation on rectangular elements arranged on a cubed-sphere grid.  It is both highly-scalable and highly-accurate.  Refinement factors of 8 or more have been tested successfully in dry dynamical test suites.  The RR SE-dycore in CESM will be used in FY20 to perform high-resolution simulations over South America (Figure 2) for comparison with WRF simulations over the same region.  


Deser, C., Phillips, A.S., Alexander, M.A. and Smoliak, B.V., 2014. Projecting North American climate over the next 50 years: Uncertainty due to internal variability. Journal of Climate, 27(6), pp.2271-2296.

Lin, Y., Dong, W., Zhang, M., Xie, Y., Xue, W., Huang, J. and Luo, Y., 2017. Causes of model dry and warm bias over central US and impact on climate projections. Nature communications, 8(1), p.881.

Rasmussen, R., Liu, C., Ikeda, K., Gochis, D., Yates, D., Chen, F., Tewari, M., Barlage, M., Dudhia, J., Yu, W. and Miller, K., 2011. High-resolution coupled climate runoff simulations of seasonal snowfall over Colorado: a process study of current and warmer climate. Journal of Climate, 24(12), pp.3015-3048.