加州是极端天气的“故乡”

California isn't going to face a superstorm like Hurricane Sandy because the Pacific Ocean is too cold to feed that kind of weather system. But that doesn't mean California won't see extreme weather, say researchers from the University of California, Merced.

"We can see very big storms, and there are a couple of issues related to climate change to think about," said Roger Bales, director of the Sierra Nevada Research Institute. "Most of our biggest storms are snow storms, which builds up snowpack in the mountains. The snowpack is a reservoir, storing water that will be used throughout the year across the state.

"But if you warm the climate," he said, "those storms become rain events -- there's more immediate runoff（径流，决赛）, less water storage, and the rain will actually melt some of the existing snowpack."

The worst-case winter scenario would be a series of storms that cause flooding, said James Brotherton, warning-coordination meteorologist for the National Weather Service in Hanford.

"We definitely have the potential to be impacted by major winter storms, or a series of them," Brotherton said.

"It's not uncommon during the winter, at least once, that we will see storms coming off the Pacific and drop more than 100 inches of snow in the mountains over short durations," said project scientist and lecturer Robert Rice, with SNRI. "That could translate into 10 inches of precipitable water -- numbers similar to what they're measuring in Hurricane Sandy. Snow events, which we commonly see in the Sierra, and across the western U.S., are generally unheard of on the East Coast, even during Hurricane Sandy, or a Nor'easter."

There have been years when what's commonly called The Pineapple Express -- a persistent, strong flow of atmospheric moisture coming from the area near Hawaii -- has pummeled the West Coast. Those kinds of "atmospheric rivers" historically caused problems in California, flooding Sacramento and the Central Valley. But land use has changed dramatically from the last time that was a problem -- in the 1800s -- and California has much more control of its waterways, Brotherton said.

"We have very large storms that cross into California and affect our region -- not with the same widespread damage as Hurricane Sandy, but with water and wind that are comparable to hurricanes and tornados," Rice said.

Rice cited a storm over the last weekend of November 2011 that included a wind event similar to a Category 3 tornado or Category 4 hurricane, with sustained winds higher than 100 mph and gusts of more than 150 mph.

"These storms would be very destructive and costly to urban areas," Rice said, "and they are more frequent than most people imagine. Not much attention is focused on them because they rarely affect large urban populations, more often being restricted to the Sierra."