Lessons from an All-Ocean World: Longstanding modeling challenges doused in simulations of important precipitation features
Being all wet was exactly the approach for scientists looking to piece together precipitation puzzles. New research led by scientists at Pacific Northwest National Laboratory investigated the reasons behind longstanding biases in climate models that depict a key climate feature known as the Intertropical Convergence Zone, or ITCZ. This feature is fueled by thermal energy in the tropics and forces warm, moist air to rise and dump rain over the tropics, and subsequently move toward the Earth’s poles, descend and dry the subtropics. The researchers teased out climate models’ inclinations using a simplified, all-ocean planet model to depict climate precipitation features without the complications of real-world factors such as land masses and mountains. In an all-wet world, researchers could focus on larger samples of data to establish the model’s statistical muscle in depicting the ITCZ and extreme precipitation.
“We found the feedbacks between convection and the atmospheric circulation amplify small differences in simulating atmospheric moisture,” said Dr. L. Ruby Leung, the PNNL atmospheric researcher and project lead of two papers on the topic published in the Journal of Climate. “This results in a single versus double ITCZ, and highlights how changes in model resolution and dynamical representation can produce very different results.”
The researchers also found that models’ extreme precipitation simulations are very sensitive to how well they depict the motion of rising air.
Researchers at three national laboratories, PNNL, Los Alamos National Laboratory and Sandia National Laboratories, sought to understand model biases in simulating extreme precipitation and the ITCZ’s structure. The team generated a suite of simulations for an all-ocean planet using the Community Atmosphere Model (CAM) and evaluated how the simulations are sensitive to the representations of the atmosphere’s dynamics and the model’s resolution. For extreme precipitation, they performed a moisture budget analysis to determine what processes are most sensitive to model representations and resolution. For the ITCZ structure, they analyzed processes associated with convection and tropical waves to narrow down why some models produce a double instead of a single ITCZ. They identified the primary source of moisture that feeds extreme precipitation: convergence from vertical transport of moisture. They showed that models running at different spatial resolutions can produce drastically different vertical motion, leading to significant differences in simulating heavy precipitation.
Precipitation—the main source of fresh water over land—has both scientific and societal significance. Uneven distribution of precipitation in space and time can lead to floods and droughts that challenge society. Understanding how precipitation is intimately tied to the energy that drives weather and climate is the scientific challenge. Enormous, complex systems such as the climate are studied through simulations that represent our understanding of the climate system as a whole. Scientists in this study are working to understand model biases in simulating different characteristics of precipitation. Climate models have long had difficulties simulating some of the most basic precipitation features, such as the tropical rain belt or ITCZ, and extreme precipitation generated by intense storms. By simulating in an all-water world, the researchers are able to better understand how well the model represents these features, and apply those lessons to whole-system models. Uncovering these simulation issues will ultimately improve scientific climate predictions and inform societies where and when the rain will fall.
Yang, et al. Increasing numbers of record-breaking and devastating hydrological extremes in the past decade have raised concerns that such extremes may be intensified in a warmer world. Researchers use climate models to attribute the causes of precipitation extremes and to project the frequency and intensity of these events in the future. In new research led by DOE scientists at Pacific Northwest National Laboratory, they quantified the moisture budgets associated with extreme precipitation and investigated the resolution dependency of the simulated precipitation extremes. They analyzed model outputs from a suite of aqua-planet simulations generated by two dynamical cores–Model for Prediction Across Scales-Atmospheres and High Order Method Modeling Environment–of the Community Atmosphere Model (CAM4). These were run at multiple resolutions ranging from about 0.25 to 2 degrees. Their moisture budget analysis quantified the contributions of moisture sources and sinks during extreme precipitation events. They found that vertical advective convergence is an important mechanism supplying moisture for extreme precipitation and its sensitivity to model resolution results in the scale dependency of extremes. The simulated extreme precipitation does not converge with increasing resolution over the tropics, although convergence occurs at a wide range of latitudes over the extra-tropics. This study provides an important metric for assessing the sensitivity of cloud parameterizations to spatial resolution as well as dynamical cores under extreme conditions.
Leung, et al.
Simulation of the Inter-tropical convergence zone (ITCZ), a belt of east-west-oriented high precipitation region over the tropics, in general circulation models is one of the most challenging aspects of modeling. Simulations of the ITCZ show two prevalent structures: a single ITCZ as observed that peaks to the north of the equator, or double ITCZs with peaks of precipitation on either side of the equator. Most climate models show a second precipitation belt over the southern hemisphere especially in the Pacific, showing a double peak structure when zonally averaged. Research led by U.S. Department of Energy scientists at Pacific Northwest National Laboratory examined the structure of the ITCZ in two sets of aqua-planet simulations produced by two different dynamical cores within the Community Atmosphere Model version 4 (CAM4): Model for Prediction Across Scales-Atmosphere (MPAS-A) and High-Order Method Modeling Environment (HOMME). The team found the structure of ITCZ is dependent on the feedbacks between convection and large-scale circulation. The dominance of anti-symmetric waves in the model is not enough to cause a double ITCZ, and the lateral extent of equatorial waves does not play an important role in determining the width of the ITCZ but rather the latter may influence the former. Despite similar model configurations, a change in one of the basic states, e.g. humidity, from differences in the dynamical cores alone can lead to differences in climate processes amplified by positive feedbacks that result in different structures of the salient circulation patterns.
The two studies were supported by the U.S. Department of Energy Office of Science as part of the Regional and Global Climate Modeling Program.