29 August 2013

Ahoy Aquaplanet: Identifying Model Resolution Shortcomings

Science

By putting models through their paces in an all-water world, scientists at Pacific Northwest National Laboratory found highly scale-sensitive issues in regional climate modeling. In the first of two studies, two approaches for high-resolution modeling produced uncertainties in circulation patterns due to the sensitivity of precipitation representations to model resolution. In the second study, they found that key model components that are supposed to simulate the upward transport of moisture important for modeling precipitation underestimate moisture transport across all scales.

"By simulating climate processes in a hypothetical ‘all-ocean' planet," said Dr. Samson Hagos, lead researcher and climate scientist at PNNL, "we are able to analyze the models without the complications of distracting and competing processes, such as land-surface interactions or topography."

Approach

In the first study, the research team from PNNL and Los Alamos National Laboratory used idealized global model simulations of the aquaplanet with Model for Prediction Across Scales-Atmosphere (MPAS-A) and Weather Research and Forecasting Model (WRF) to run at low, high and variable resolutions. The team examined the impacts of changing model resolution and using two techniques on the simulated tropical precipitation and circulations: variable resolution and nesting.

In the idealized aquaplanet setting, where the simulated climate should be zonally symmetric, their results showed that by introducing a high-resolution region in the tropics embedded in a global or very large domain with coarse resolution elsewhere produces zonal asymmetry in the simulated climate. This is due to amplified precipitation in the high-resolution region compared to the coarse-resolution region. Because the model parameterizations are not scale aware, increased precipitation produces zonally asymmetric climate circulation patterns that characterize the "errors" in the model simulations.

In the second study by PNNL researchers, they used two simulations with nested domains of higher spatial resolution to directly compare the moist processes in the nested domains with those in the low-resolution parent domains that are explicitly resolved instead of parameterized. Their analyses show that upward moisture transport by eddy fluxes dries the boundary layer and enhances evaporation and precipitation. This effect is better resolved with increasing spatial resolution. However, model physics process representations that are supposed to account for the eddy moisture transport effects on convection significantly underestimate them compared to simulations that explicitly resolved eddy moisture transport without using convective representations.

Impact

Although climate change is global, information on climate adaptation and mitigation are required by the local and regional decision-makers. High-resolution regional climate modeling can help inform that need, but it comes with a high computational cost. More computer resources are required to deliver these detailed accounts. So, scientists have developed different approaches to enable high-resolution simulations more efficiently. Although these techniques deliver the benefit of much smaller computing resources, the studies highlighted here show that certain cloud and precipitation processes are distorted. To support an informed assessment of climate change mitigation and adaptation strategies, model results for regional climate simulations must be robust at reasonable computational cost.

Summary

Regional Aquaplanet Model Simulations: Eddy Fluxes and Sensitivity of the Water Cycle to Spatial Resolution

Researchers at DOE’s Pacific Northwest National Laboratory found that in higher resolution simulations, moisture transported by eddy fluxes dry the boundary layer enhancing evaporation and precipitation. They found the effect of eddies, which is underestimated by the physics parameterizations in the low-resolution simulations, is responsible for the sensitivity of the simulated water cycle to model resolution. The research team used the Weather Research and Forecasting (WRF) model with the Community Atmospheric Model version (CAM4) physics package using an experimental design that allowed them to isolate the effect of moisture transport by eddy fluxes in the boundary layer. They performed a moisture budget analysis at multiple scales, and quantified the contribution of eddy fluxes to resolution differences. They found a correlation among moisture transport by eddies at adjacent ranges of scales that suggested a potential for reducing this sensitivity by representing the unresolved eddies by their marginally resolved counterparts. The issue of physics dependence on spatial resolution is an important area of research not only because of its implications for nested regional modeling, but also for variable and high resolution global modeling.

Reference: Hagos S, LR Leung, WI Gustafson Jr., and B Singh. 2013. “Eddy fluxes and sensitivity of the water cycle to spatial resolution in idealized regional aquaplanet model simulations.”Climate Dynamics, 0930-7575. DOI: 10.1007/s00382-013-1857-y.

Funding: RGCM

Contact: Samson Hagos, (509) 375-7395, Samson.Hagos@pnnl.gov

Contact
Siyu Chen
Acknowledgments

Sponsors: Both research studies were supported by the U.S. Department of Energy Office of Science Biological and Environmental Research Regional and Global Climate Modeling Program. Computing resources were provided by the National Energy Research Scientific Computing Center. The second study also used computing resources from the National Center for Computational Sciences through the INCITE Climate End Station project.

Research Teams: Samson HagosRuby LeungWilliam Gustafson and Balwinder Singh, PNNL; Sara A. Rauscher and Todd Ringler, Los Alamos National Laboratory.