Atmospheric Rivers

Some new information about these monster storms, from Weather West.

An excerpt.

“Atmospheric rivers in a California climate context

“Atmospheric rivers (ARs)—long, sinuous corridors of water vapor in motion in the lower half of Earth’s atmosphere—are a key aspect of California’s cool-season climate. ARs are in many cases pretty unremarkable when they’re out over the open ocean, visible on satellite as a narrow ribbon of cloudiness and (usually) light precipitation. But when these moisture plumes attach themselves to wintertime low pressure systems and make landfall along the California coast, the consequences can be dramatic—prolonged heavy rain and mountain snow often result, along with sometimes powerful winds. AR-related precipitation is strongly orographic, meaning that it tends to be strongly modulated by local topography. As a result, the most dramatic AR-related precipitation totals usually occur on the windward (south and west-facing) slopes of California’s coastal and inland mountain ranges. Precipitation accumulations in such regions during the most extreme AR events are comparable to that received along the Gulf Coast or Eastern Seaboard during landfalling hurricane events—and are occasionally measured in feet, rather than inches. As such, ARs can be either blessing or a curse, depending on context and antecedent conditions: they are responsible for up to 50% of California’s overall water supply, but also the vast majority of the region’s flood risk.

“The large ensemble approach: generating multiple plausible pasts and futures

“Since ARs are such a fundamental aspect of California’s historical climate, it’s critically important to understand how such events are changing in a warming world. Existing research on climate change and ARs has previously suggested that the strongest events may intensify considerably in some regions. But nearly all such studies to date have relied exclusively upon global climate models—which are excellent tools for understanding how global and regional climate is changing in a broader sense, but are often not granular enough to assess changes in fine-scale phenomena like ARs. In the study I describe in this blog post, my colleagues and I have attempted to resolve this challenge by using a high-resolution atmospheric model (the Weather Research and Forecasting Model, or WRF—commonly used in making day-to-day weather forecasts) to simulate the “storms of the future” in great spatial and temporal detail. We do so by drawing initial large-scale atmospheric conditions from traditional, coarse-resolution climate model simulations and providing these to the weather model. In this way, we are able to systematically pick out individual extreme AR storm events from these extensive climate model simulations and simulate them using a tool that much better represents the small-scale variations topographical quirks that characterize California climate. We focus mainly on extreme ARs that have large impacts in the Sierra Nevada, so the study is centered primarily across central and northern California.

“Importantly, this approach also lets us consider multiple “plausible pasts and plausible futures.” The climate model is part of a large ensemble of simulations, meaning that the simulations are repeated 40 times for the historical past and 40 times for a warmer future. This allows us to capture a wider range of possible storm sequences in both the present-era and future climate conditions—since we essentially have created a record of many dozens of synthetic but physically plausible extreme storms during each period. This means we have a large sample size to compare differences in storm characteristics between these two periods, which is much larger (by a factor of 40!) than would be available by looking at the historical record alone.

“It’s worth noting that the future climate scenario we use in this work is one characterized by a continued increase in greenhouse gas emissions through most of the 21st century (RCP8.5, for the climate wonks out there)—a trajectory that is (hopefully) worse than we’ll actually take in the coming decades. Still, most of the impacts we consider in this work will emerge even on lower emissions trajectories—but more slowly and to a lesser degree, dependent entirely on our eventual success in limiting and eventually bringing to zero net global carbon emissions.

“Stronger, wetter atmospheric rivers as the climate warms

“We report substantial (20-30%) increases in AR-associated integrated water vapor transport, IVT. (IVT is an aggregate measure of AR strength that takes into account both the level of atmospheric water vapor and the strength of transport-level winds). It’s important to keep in mind that we’re assessing events that are quite rare and extreme—and it’s interesting to note that the composite IVT pattern for both present era and future extreme ARs looks a lot like the canonical “Pineapple Express” subset of ARs. That suggests that the largest plausible AR storms in California are likely to be events with a deep subtropical moisture tap, coinciding with a relatively amplified meridional flow pattern over the North Pacific with a strong upstream blocking ridge over the Bering Sea and a deep low pressure center just northwest of California.

“We find a substantial increase in the amount of precipitation that falls during extreme ARs in a warmer climate just about everywhere we look—but the largest relative changes are not always where you might expect. These increases range from 15-30% on the orographically favored western slopes of the Sierra Nevada and the coastal mountains, 25-40% in the flatlands and rain-shadowed areas of the Central Valley and smaller coastal valleys, and locally 50% or greater in the strongly rain shadowed lee side valleys in far eastern California and western Nevada. (Absolute increases, of course, are still greater in the orographically favored areas). This spatial pattern of extreme AR precipitation increase suggests that events may become somewhat less strongly orographic in nature—meaning that physical processes other than simple orographic uplift may become more important in a warmer future. This portends potentially outsized increases in precipitation in places not historically accustomed to seeing large volumes of AR-associated precipitation.”

Retrieved July 16, 2020 from https://weatherwest.com/archives/7364

Be well everyone!

About David H Lukenbill

I am a native of Sacramento, as are my wife and daughter. I am a consultant to nonprofit organizations, and have a Bachelor of Science degree in Organizational Behavior and a Master of Public Administration degree, both from the University of San Francisco. We live along the American River with two cats and all the wild critters we can feed. I am the founding president of the American River Parkway Preservation Society and currently serve as the CFO and Senior Policy Director. I also volunteer as the President of The Lampstand Foundation, a nonprofit organization I founded in 2003.
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