Overview

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 are listed under "Resources".

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.

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.