Dr. Marty Ralph is the director of the new Center for Western Water and Water Extremes, as well as the director of the Center at Scripps Institution of Oceanography. In the fall of 2013 at the National Water Research Institute’s Drought Response Workshop, Mr. Ralph gave a presentation where he discussed drought, atmospheric rivers, and a new center dedicated to studying them.
Dr. Ralph began by saying that he is a meteorologist by training and he tries to understand how weather information might be used by others to help make better informed decisions in the face of a lot of uncertainty. He recalled that during his time at NOAA, when he would talk to the leadership in Washington DC, he found that they understood hurricanes and tornadoes, but when he would talk about water in the west, their eyes would glaze over. He pointed out that precipitation back east is essentially the same, year after year, relative to precipitation in the west, which can be highly variable. “Solutions developed for back east were applied out west, and frankly they just don’t work as well as they need to,” he said, “so the motivation behind creating the Center for Western Weather and Water Extremes is a bit of a call-out to a need for more action, more focus on research, and more focus on understanding and predicting the type of phenomena in meteorology that are vital out here.”
The new Center for Western Water and Weather Extremes will focus on not only atmospheric rivers, but other western weather events such as southwestern monsoons, upslope storms on the Front Range, and the Great Plains deep convection. The mission of the Center is to ‘provide 21st Century water cycle science, technology and outreach to support effective policies and practices that address the impacts of extreme weather and water events on the environment, people and the economy of Western North America.’ The goal of the Center is to revolutionize the physical understanding, observations, weather predictions, seasonal outlooks and climate projections of extreme events in the west, as well as their impacts on floods, droughts, hydropower, ecosystems and the economy.
The major activities of the center include:
Create a “Hydroclimate Testbed” (HCT)jointly with NOAA
Use and develop state-of-the-art observing systems to study the roles of atmospheric rivers, orographic precipitation, boundary layer, soil moisture and microphysical process in water cycle extreme events
Develop prototype techniques to improve extreme event predictions and projections – from short-term (days) to medium range (weeks) and beyond (e.g., seasonal)
He then presented a slide with a picture of an atmospheric river. He explained that the water vapor is measured by satellite with the orange colors representing the highest amounts of water vapor. “The water floating around up there has to get turned into rain somehow, so it flows up in an atmospheric river, or AR for short, and it will flow up into the mountains and then condense and fall out.” This was a brand new measurement developed 15 years ago, and before no one knew these looked like this, he said. “We can track these every day now, we have automated tools to monitor them and we’re looking at numerical weather predictions more in the context of these than we used to in the past,” he said.
We’ve been studying atmospheric rivers relative to flooding events in watersheds, and the Russian River is where we cut our teeth on this, said Dr. Ralph. “The big flooding events are a result of atmospheric rivers being strong and stalling, coming after a previous event has already moistened the watershed and made it prime for flooding.”
He noted that these events were not even recognized just six or eight years ago, but now it’s become a popular subject. He presented a slide of a graphic from a recent article in Scientific American, noting that the graphic was a beautiful depiction of what happens as the air stream in an atmospheric river flows up over the coast range and then descends over the Central Valley, which is why the Central Valley is dry, and the heavy rain is in the Sierra.
Atmospheric events are not stationary; they are very fluid and very mobile, he said, showing an animation of a series of three atmospheric river events which hit Northern California in 2012, producing 20 inches of rain in only three days. “What that rain does, it not only can produce problems but it can also produce benefits,” he said.
Dr. Ralph said that he, Mike Dettinger, and others published a study in 2011 that determined that 25-35% of the annual precipitation that falls in the Northwest and 35-45% of the precipitation that falls in California is associated with atmospheric river events. He noted that there was another study that showed that on average, about 40% of the Sierra Nevada snowpack comes in handful of atmospheric river events each year. An atmospheric river storm can carry 7.5 times the average discharge of liquid water in the Mississippi River, he said. “In one day, in one atmospheric river event, if you added it up, 10 million acre-feet equivalent of liquid water,” he said. “It’s all vapor, and somehow the atmosphere needs to turn part of that into rain and snow which ends up in our rivers and water systems.”
These storms are very potent, he said. “It turns out that 90% of the water vapor transported in the mid-latitudes happens in atmospheric rivers, so this is the engine for the water cycle in the global climate, and if we want to look at climate models and try to understand how climate change is going to impact water supply, we have to understand how climate change is going to impact atmospheric rivers.”
He then presented a graph of the Eight Station Index for 2012, noting that one event produced seven inches in four days, about 14% of the annual average, and a second one a little bit later that produced eleven inches or 22% of the annual average. “It’s really just a handful of events that do a lion’s share of the work,” he said.
He next presented a slide of a schematic of an atmospheric river observatory, noting that the light blue is the atmospheric river storm. The observatory has a wind profiling radar that measures winds about 1 kilometer or 3000 feet up, and it has a GPS MET receiver that measures the amount of water vapor. “What we do is we take that hourly measurement from the GPS MET receiver and the hourly measurement of winds and we multiply them together and it gives us an estimate of how much water vapor transport is going into the coastal mountains,” he said. “We can now, to the hour, determine objectively when an atmospheric river has reached a given location and when it ends. And we can add up how many hours that is and it turns out an average AR lasts about 20 hours on the west coast of California.”
He noted that this is new technology to have hourly measurements being made unattended. “In traditional meteorology, every 12 hours, someone walks out, fills up a balloon with helium, ties some sensors on it, and launches it. Phenomenal discoveries were made 80 years ago … nowadays it’s just not good enough. If we only use 12 hour sampling for something that lasts 20 hours, are chances of missing it are really high.” This means we can pin down atmospheric river phenomena much better than ever possible in the past, and the four new atmospheric river observatories on the west coast are going to change the game in meteorology, said Mr. Ralph.
Now that we can track the starting hour and ending hour, plus every hour in between, we now have a measurement of how much water vapor is flowing up the mountainside, he explained. “If we add up those hourly measurements and we get a storm total amount of water vapor going up the slope, and that’s what’s on this axis here,” he said, acknowledging that the units are sort of ‘weird’.
He noted that the correlation is color coded by how long the ARs lasted in hours and that the results show that 75% of the storm to storm variance in precipitation is explained by variance in how much water vapor is going up the slope. “This is not normal to see this kind of correlation,” he said. “I have ignored cold fronts, I have ignored low pressure centers, I have ignored microphysics, I have ignored thunderstorms, things that people have done their entire careers on are ignored and that you see this correlation, we’re seeing it time and again, this is over several years and many many events. This is the lynchpin for forecasting in the future in the west. We need to know when an atmospheric river starts, how long it lasts, how strong the transport is in the middle, and if it happened after a previous event or not.”
He said that in the past, the best correlation he’d ever seen was about 50%, and here was 75% correlation, so if there is that kind of correlation in the precipitation, what about the runoff, he wondered. Noting that he wasn’t a hydrologist, he simply added up how much water went down the stream during the same hours as the atmospheric river event was hitting. “This is the same axis as the top panel,” he said. “It’s how much water vapor going up the slope. Now we see about 60% of the variance and how much water is coming down the stream is simply the result of how much water vapor went up the slope. Now I’m sweeping all sorts of hydrology under the rug, but I think this is really going to change how we do predictions and we’re in the process of trying to figure out how to take advantage of it.”
Forecasters are working to predict the location and duration of atmospheric river conditions, and it is going to be a major focus for training and research and development in the coming years to get that right, he said. Using the latest weather prediction models, researchers looked at the past three winters, and using data, predicted where it would hit the coast, and then compared it to where it actually hit the coast. Right now, about 5 days out, there’s about 500 kilometer error in the predicted position of the landfall of an atmospheric river. “My watershed if I were a water manager is much smaller than that, so we’ve got a lot of work to do meteorologically to get better at this,” he said, noting that when NOAA started making hurricane forecasts, there were landfall position errors, but with concerted effort, that was improved.
“That I think is what we’re starting to see here in the West,” said Dr. Ralph. “We’ve got new tools, new concepts, and new methods to evaluate models; we have forecasters being trained on it; and we’ve got this new center to really focus on it, and pursue the research and hopefully develop applications associated with it.”
Dr. Ralph then briefly discussed Mike Dettinger’s latest research on atmospheric rivers as ‘drought busters. Using the Palmer Drought Severity Axis, he looked over the past 60 years at where the drought index went from below -2 (very dry) to above and stayed above, or in other words, the end of a drought, he said. “He found events that met those criteria and he averaged them and what you see there is the gradual approach into a drought, and then the rapid jump out of a drought,” he said. “So drought endings are very abrupt; drought beginnings are very gradual, or as Mike colorfully puts it, the average drought ends with a bang, the average drought begins with a whimper.”
Michael Dettinger then looked at how many of these events were associated with an atmospheric river. “He found that up in the Pacific Northwest, about two-thirds of the droughts were broken in months that had characteristics of atmospheric river conditions providing a big event, and in California, about one-third to 40% of the droughts were ended basically by atmospheric rivers coming in and dumping a lot of rain.”
Marty Ralph concluded by saying that the Center for Western Water and Weather Extremes will be studying atmospheric rivers and other weather phenomena that affect the western states. “These are the problems we are going after at the center, we really want to try and understand them physically and work with people like yourselves who are needing information on these types of events.”