Upscaling from Local Convection to the Global Jet Stream
Understanding and accurately simulating the interactions between small-scale and global-scale phenomena are a key aspect of global climate modeling. This study found that in a global variable-resolution model with regional refinement to resolve small-scale processes over areas of interest, scale interactions can influence simulations of the jet stream through multiple long-distance causal pathways.
Global variable-resolution modeling is an important tool for simulating regional climate, but complex interactions between small-scale and large-scale climate phenomena are extremely difficult to interpret and evaluate for their realism. This study is the first to document, in detail, the impact of local-scale convection on the large-scale jet stream (an “upscale effect”) in a global variable-resolution model. The study also demonstrated the variable-resolution model’s consistency with a reference high-resolution global model, implying that upscale effects that manifest in global high-resolution simulations can be achieved by regional refinement in a global variable-resolution model at a fraction of the computational cost.
In the climate system, small-scale processes often influence larger ones, and the effects ripple out to remote locations. The El Niño-Southern Oscillation and its remote influence on the U.S. regional climate is a notable example. Department of Energy researchers at Pacific Northwest National Laboratory and a collaborator at the Institute of Atmospheric Physics in China studied such “upscale effects” found in a global variable-resolution model (the Model for Prediction Across Scale, “MPAS”), with refined grid resolution over South America, North America, and Asia. The process was initiated with more vigorous atmospheric convection in the regions with a higher grid resolution. Although located thousands of miles away, the jet in the Southern Hemisphere responds to the change by shifting its position poleward. One pathway for this response is the Hadley cell, a large-scale overturning circulation connecting the tropics and subtropics. Another pathway involves large-scale atmospheric wave motion called the "Rossby Wave," which is stimulated by enormous convective energy from the Asian monsoon region before traveling across the equator to the Southern Hemisphere, where it interacts with the jet stream. The team found the latter pathway in a global high-resolution simulation indicating that a variable-resolution model can reproduce the scale interactions captured by the global high-resolution model. Consequently, simulations of large-scale circulations and climate over remote locations can be improved.