Water Cycle

How will more realistic portrayals of features important to the water cycle (resolution, clouds, aerosols, snowpack, river routing, land use) affect river flow and associated freshwater supplies at the watershed scale?

We hypothesize that changes in river flow over the last 40 years have been dominated primarily by land management, water management, and climate change associated with aerosol forcing. During the next 40 years, greenhouse gas (GHG) emissions following a Representative Concentration Pathways (RCP) 4.5 or 8.5 scenario will produce changes to river flow with signatures that dominate those other forcing agents in at least one of the domains our experimental framework examines below.

The goal is to simulate the changes in the hydrological cycle, with a specific focus on precipitation and surface water in orographically complex regions such as the western United States and the headwaters of the Amazon. The experiment will exploit the combination of 10-km resolution atmosphere with a similarly highresolution land surface that is further enhanced with subgrid orographic parameterizations in the v1 configuration.

The combination of high land-surface resolution with the subgrid orographic treatment will enable much more realistic representations of upslope and orographically forced precipitation, high-altitude snowpacks, rainshadows on the downwind slopes of mountain ranges, and other interactions of precipitation and the landscape critical for the target river systems. Improved resolution, and improved parameterizations of clouds, aerosols, and their interactions, should produce a more realistic portrayal of the precipitation location, frequency and intensity, and aerosol deposits on snow and surface ice—all factors that influence runoff, snowpack, and snowmelt.

In this study, we will explore the role of various physical processes and their treatment in climate models in influencing river flow and fresh water supply, with a goal of producing an accurate portrayal of present-day river flow for major river basins on the planet. The Mackenzie and the Mississippi in North America, the Amazon and La Plata in South America, and the Ganges and Yangtze in Asia, are archetypes of major river basins dominated by very different climate and hydrologic regimes. Sea-surface temperature (SST) is a primary driver of regional precipitation patterns and, thus, the regional water cycle. As a result, the fidelity of the simulated regional water cycle will likely depend strongly on fidelity of near- and far-field simulated SST.

We will systematically explore the sensitivity of atmospheric and surface water budgets simulated in these river basins to ACME model improvements in resolution and the treatments of clouds, aerosols, subgrid orographic effects, and surface/subsurface hydrology. We will explore how water availability in the major river basins responds to anthropogenic forcing, including emissions, land use, and water use. A preliminary simulation plan is as follows:

  1. Using prescribed SSTs from the last 40 years, the effects of atmospheric and land-surface resolution can be explored by comparing the global 1° resolution version of the model to companion experiments with the global 0.25° (or possibly 0.125°) atmospheric resolution/1-km land resolution model. We will also compare the global high-resolution results with (a) coarse-resolution simulations with regional refinement over the river basins only, and (b) versions with and without the new subgrid orography scheme.
  2. For the same 40-year period, we will run fully coupled atmosphere–ocean–land–sea-ice simulations at both high and low resolutions to identify the feedback mechanisms prevalent in the two different dynamical regimes and their impacts on systematic model biases, particularly in the SST fields.
  3. Given the results of (2) above, we will continue the experiments for 40 additional years using the RCP 4.5 forcing scenario to test the hypothesis. This set of simulations is specifically identified in the letter requesting the ACME proposal.