Tuesday, December 6, 2016

Snow data from satellites improves temperature predictions, researchers show

Researchers with The University of Texas at Austin have found that incorporating snow data collected from space into computer climate models can significantly improve seasonal temperature predictions.
The findings, published in November in Geophysical Research Letters, a publication of the American Geophysical Union, could help farmers, water providers, power companies and others that use seasonal climate predictions -- forecasts of conditions months in the future -- to make decisions. Snow influences the amount of heat that is absorbed by the ground and the amount of water available for evaporation into the atmosphere, which plays an important role in influencing regional climate.
"We're interested in providing more accurate climate forecasts because the seasonal timescale is quite important for water resource management and people who are interested in next season's weather," said Peirong Lin, the lead author of the study and a graduate student at the UT Jackson School of Geosciences.
Seasonal forecasts are influenced by factors that are significantly more difficult to account for than the variables for daily to weekly weather forecasts or long-term climate change, said Zong-Liang Yang, a co-author of the study and a professor at the Jackson School of Geosciences Department of Geological Sciences.
"Between the short and very long time scale there's a seasonal time scale that's a very chaotic system," Yang said. "But there is some evidence that slowly varying surface conditions, like snow cover, will have a signature in the seasonal timescale."
The researchers found that incorporating snow data collected by NASA satellites into climate models improved regional temperature predictions by 5 to 25 percent. These findings are the first to go beyond general associations and break down how much snow can impact the temperature of a region months into the future. Improving temperature predictions is a key element to improving the computer models that provide climate predictions months in advance.
The researchers analyzed how data on snow cover and depth collected from two NASA satellites -- MODIS and GRACE -- affected temperature predictions of the Northern Hemisphere in a climate model. The study examined seasonal data from 2003 through 2009, so the researchers could compare the model's predictions to recorded temperatures. The model ran predictions in three-month intervals, with January, February and March each used as starting months.
The computer model's temperature improvement changed depending on the region and time, with the biggest improvements happening in regions where ground-based measurements are sparse such as Siberia and the Tibetan Plateau. Climatic conditions of both these areas can influence the Indian Monsoon -- seasonal rains that are vital to agriculture in India, a fact that shows the far-reaching applicability of seasonal climate prediction, Yang said.
"This correlation between snow and future monsoon has been established for several decades, but here we are developing a predictive framework where you can run the model forward and get a quantity, not just a correlation," Yang said.
In the future the researchers plan to expand their research to predict other climatic factors, such as snowfall and rainfall. For the time being, they hope that their findings can be useful to national organizations that make climate predictions, such as the U.S. National Oceanic and Atmospheric Administration and the European Forecasting Center.
Randal Koster, a scientist at NASA's Goddard Space Flight Center who studies land-atmosphere interactions using computer models, said that the study is an example of how satellites can improve climate forecasts by providing more accurate data to inform the starting conditions of the model.
"In the future such use of satellite data will be standard," said Koster, who was not involved with the study. "Pioneering studies like this are absolutely critical to seeing this happen."

How lightning strikes can improve storm forecasts

Humans have always been frightened and fascinated by lightning. This month, NASA is scheduled to launch a new satellite that will provide the first nonstop, high-tech eye on lightning over the North American section of the planet.
University of Washington researchers have been tracking global lightning from the ground for more than a decade. Lightning is not only about public safety -- lightning strike data have recently been introduced into weather prediction, and a new UW study shows ways to apply them in storm forecasts.
"When you see lots of lightning you know where the convection, or heat-driven upward motion, is the strongest, and that's where the storm is the most intense," said co-author Robert Holzworth, a UW professor of Earth and space sciences. "Almost all lightning occurs in clouds that have ice, and where there's a strong updraft."
The recent paper, published in the American Meteorological Society's Journal of Atmospheric and Oceanic Technology, presents a new way to transform lightning strikes into weather-relevant information. The U.S. National Weather Service has begun to use lightning in its most sophisticated forecasts. This method, however, is more general and could be used in a wide variety of forecasting systems, anywhere in the world. The authors tested their method on two cases: the summer 2012 derecho thunderstorm system that swept across the U.S., and a 2013 tornado that killed several people in the Midwest. "Using lightning data to modify the air moisture was enough to dramatically improve the short-term forecast for a strong rain, wind and storm event," said first author Ken Dixon, a former UW graduate student who now works for The Weather Company. His simple method might also improve medium-range forecasts, for more than a few days out, in parts of the world that have little or no ground-level observations.
The study used data from the UW-based WorldWide Lightning Location Network, which has a global record of lightning strikes going back to 2004. Director Holzworth is a plasma physicist who is interested in what happens in the outer edges of the atmosphere. But the network also sells its data to commercial and government agencies, and works with scientists at the UW and elsewhere.
A few years ago Holzworth joined forces with colleagues in the UW Department of Atmospheric Sciences to use lightning to improve forecasts for convective storms, the big storms that produce thunderstorms and tornadoes.
Apart from ground stations, weather forecasts are heavily dependent on weather satellites for information to start or "initialize" the numerical weather prediction models that are the foundation of modern weather prediction.
What's missing is accurate, real-time information about air moisture content, temperature and wind speed in places where there are no ground stations.
"We have less skill for thunderstorms than for almost any other meteorological phenomenon," said co-author Cliff Mass, a UW professor of atmospheric sciences. "This paper shows the promise of lightning information. The results show that lightning data has potential to improve high-resolution forecasts of thunderstorms and convection."
The new method could be helpful in forecasting storms over the ocean, where no ground instruments exist. Better knowledge of lightning-heavy tropical ocean storms could improve weather forecasts far from the equator, Mass said, since many global weather systems originate in the tropics.
The study was funded by NASA and the National Oceanic and Atmospheric Administration. Greg Hakim, a UW professor of atmospheric sciences, is the other co-author. The Worldwide Lightning Location Network began in 2003 with 25 detection sites. It now includes some 80 host sites at universities or government institutions around the world, from Finland to Antarctica.
The latest thinking on how lightning occurs is that ice particles within clouds separate into lighter and heavier pieces, and this creates charged regions within the cloud. If strong updrafts of wind make that altitude separation big enough, an electric current flows to cancel out the difference in charge.
A bolt of lightning creates an electromagnetic pulse that can travel a quarter way around the planet in a fraction of a second. Each lightning network site hosts an 8- to 12-foot antenna that registers frequencies in the 10 kilohertz band, and sends that information to a sound card on an Internet-connected laptop. When at least five stations record a pulse, computers at the UW register a lightning strike, and then triangulate the arrival times at different stations to pinpoint the location.
The network's online map shows lightning strikes for the most recent 30 minutes in Google Earth. An alternate display shows the last 40 minutes of lightning in different parts of the world on top of NASA cloud maps, which are updated from satellites every 30 minutes. The program is the longest-running real-time global lightning location network, and it is operated by the research community as a global collaboration.
Lightning already kills hundreds of people every year. That threat may be growing -- a recent study projected that lightning will become more frequent with climate change.
"The jury's still out on any long-term changes until we have more data," Holzworth said. "But there is anecdotal evidence that we're seeing lightning strikes in places where people are not expecting it, which makes it more deadly."
On Nov. 19, NASA is scheduled to launch the new GOES-R satellite that will be the first geostationary satellite to include an instrument to continuously watch for lightning pulses. Holzworth will help calibrate the new instrument, which uses brightness to identify lightning, against network data. NASA also funded the recent research as one of the potential applications for lightning observations.
"GOES-R will offer more precise, complete lightning observations over North and South America, which will supplement our global data," Holzworth said. "This launch has been long anticipated in the lightning research community. It has the potential to improve our understanding of lightning, both as a hazard and as a forecasting tool."

Hurricane risk to Northeast USA coast increasing, research warns

The Northeastern coast of the USA could be struck by more frequent and more powerful hurricanes in the future due to shifting weather patterns, according to new research.
Hurricanes have gradually moved northwards from the western Caribbean towards northern North America over the past few hundred years, the study led by Durham University, UK, found.
The researchers suggest that this change in hurricane track was caused by the expansion of atmospheric circulation belts driven by increasing carbon dioxide emissions.
New York and other major cities along the Northeast coast of the USA could come under increased threat from these severe storms and need to be better prepared for their potential impact, the researchers said.
The findings are published in the journal Scientific Reports. Researchers reconstructed hurricane rainfall for the western Caribbean dating back 450 years by analyzing the chemical composition of a stalagmite collected from a cave in southern Belize, Central America.
They found that the average number of hurricanes at the Belize site decreased over time. When the hurricane history of Belize was compared with documentary hurricane records from places such as Bermuda and Florida, this information showed that Atlantic (Cape Verde) hurricanes were moving to the north rather than decreasing in total numbers.
Although natural warming over the centuries has had some impact on shifting hurricane tracks, the researchers found a marked decrease in hurricane activity in the western Caribbean coinciding with the late 19th Century industrial boom associated with increasing carbon dioxide and sulphate aerosol emissions to the atmosphere.
The researchers said that initial regional cooling of the Northern Hemisphere due to increased industrial aerosol emissions should have pushed the hurricane tracks southward since Industrialization.
But they added that rising amounts of atmospheric carbon dioxide had overridden this effect by expanding the Hadley cell -- a pattern of circulating air in Earth's tropical belt -- pushing hurricane tracks further north, away from the western Caribbean towards the Northeastern USA.
This suggests that from the late 19th Century, humanmade emissions have become the main driver behind shifting hurricane tracks by altering the position of global weather systems, the researchers said.
If future trends in carbon dioxide and industrial aerosol emissions continue as expected, hurricanes could shift even further northward, exacerbating the risk to the Northeast coast of the USA, they added.
In 2012, Hurricane Sandy struck the Caribbean and much of the eastern seaboard of the United States, stretching as far north as Canada. At least 233 people died as a result of the storm.
A large number of US states were affected by Hurricane Sandy with New York and New Jersey suffering the greatest impacts. The estimated cost of the damage caused by Hurricane Sandy in the USA is said to have run into tens of billions of dollars.
The study's lead author, Dr Lisa Baldini, in the Department of Geography, Durham University, said: "Our research shows that the hurricane risk to the Northeastern coast of the United States is increasing as hurricanes track further north.
"Since the 19th Century this shift was largely driven by humanmade emissions and if these emissions continue as expected this will result in more frequent and powerful storms affecting the financial and population centres of the Northeastern United States.
"Given the devastation caused by Hurricane Sandy it is important that plans are put in place to protect against the effects of similarly destructive storms which could potentially occur more often in the future."
Co-author Dr Amy Frappier, of the Geosciences Department, Skidmore College, USA, said the research showed Atlantic hurricanes were responding to warming. Dr Frappier said: "Aerosols from volcanoes and industrialisation in the Northern Hemisphere have a cooling effect, which tend to shift moisture belts and hurricane tracks southward, closer to the equator.
"On the other hand, warming from more carbon dioxide in the air tends to expand Earth's tropical belt, pushing hurricane tracks further north away from the western Caribbean and towards the Northeastern US.
"This suggests that the tracks of Atlantic hurricanes have responded more to warming than to regional cooling." The researchers added that the northward shift in hurricane tracks may not reduce the risk of tropical cyclones in the Caribbean.
Co-author Dr James Baldini, in Durham University's Department of Earth Sciences, said: "Although hurricane tracks have gradually moved northwards away from the western Caribbean, rising sea surface temperatures could promote the development of cyclonic storms within the western Caribbean.
"Consequently tropical cyclone activity across the western Caribbean may remain essentially stable over the current century, which has important implications for water availability in this region.
"However, increased sea surface temperatures also provide extra energy, potentially fueling larger storms. We therefore need to prepare for the effects of more frequent landfalls of larger storms along the Northeast coast of the United States and stronger storms impacting the Caribbean."

Climate change will drive stronger, smaller storms in U.S., new modeling approach forecasts

The effects of climate change will likely cause smaller but stronger storms in the United States, according to a new framework for modeling storm behavior developed at the University of Chicago and Argonne National Laboratory. Though storm intensity is expected to increase over today's levels, the predicted reduction in storm size may alleviate some fears of widespread severe flooding in the future.
The new approach, published today in Journal of Climate, uses new statistical methods to identify and track storm features in both observational weather data and new high-resolution climate modeling simulations. When applied to one simulation of the future effects of elevated atmospheric carbon dioxide, the framework helped clarify a common discrepancy in model forecasts of precipitation changes.
"Climate models all predict that storms will grow significantly more intense in the future, but that total precipitation will increase more mildly over what we see today," said senior author Elisabeth Moyer, associate professor of geophysical sciences at the University of Chicago and co-PI of the Center for Robust Decision-Making on Climate and Energy Policy (RDCEP). "By developing new statistical methods that study the properties of individual rainstorms, we were able to detect changes in storm frequency, size, and duration that explain this mismatch."
While many concerns about the global impact of climate change focus on increased temperatures, shifts in precipitation patterns could also incur severe social, economic, and human costs. Increased droughts in some regions and increased flooding in others would dramatically affect world food and water supplies, as well as place extreme strain on infrastructure and government services.
Most climate models agree that high levels of atmospheric carbon will increase precipitation intensity, by an average of approximately 6 percent per degree temperature rise. These models also predict an increase in total precipitation; however, this growth is smaller, only 1 to 2 percent per degree temperature rise.
Understanding changes in storm behavior that might explain this gap have remained elusive. In the past, climate simulations were too coarse in resolution (100s of kilometers) to accurately capture individual rainstorms. More recently, high-resolution simulations have begun to approach weather-scale, but analytic approaches had not yet evolved to make use of that information and evaluated only aggregate shifts in precipitation patterns instead of individual storms.
To address this discrepancy, postdoctoral scholar Won Chang (now an assistant professor at the University of Cincinnati) and co-authors Michael Stein, Jiali Wang, V. Rao Kotamarthi, and Moyer developed new methods to analyze rainstorms in observational data or high-resolution model projections. First, the team adapted morphological approaches from computational image analysis to develop new statistical algorithms for detecting and analyzing individual rainstorms over space and time. The researchers then analyzed results of new ultra-high-resolution (12 km) simulations of U.S. climate performed with the Weather Research and Forecasting Model (WRF) at Argonne National Laboratory.
Analyzing simulations of precipitation in the present (2002-2011) and future (years 2085-2094), the researchers detected changes in storm features that explained why the stronger storms predicted didn't increase overall rainfall as much as expected. Individual storms become smaller in terms of the land area covered, especially in the summer. (In winter, storms become smaller as well, but also less frequent and shorter.)
"It's an exciting time when climate models are starting to look more like weather models," Chang said. "We hope that these new methods become the standard for model evaluation going forward."
The team also found several important differences between model output and present-day weather. The model tended to predict storms that were both weaker and larger than those actually observed, and in winter, model-forecast storms were also fewer and longer than observations. Assessing these model "biases" is critical for making reliable forecasts of future storms.
"While our results apply to only one model simulation," Moyer said, "we do know that the amount-intensity discrepancy is driven by pretty basic physics. Rainstorms in every model, and in the real world, will adjust in some way to let intensity grow by more than total rainfall does. Most people would have guessed that storms would change in frequency, not in size. We now have the tools at hand to evaluate these results across models and to check them against real-world changes, as well as to evaluate the performance of the models themselves."
New precipitation forecasts that include these changes in storm characteristics will add important details that help assess future flood risk under climate change. These results suggest that concerns about higher-intensity storms causing severe floods may be tempered by reductions in storm size, and that the tools developed at UChicago and Argonne can help further clarify future risk.

Extreme downpours could increase fivefold across parts of the US

At century's end, the number of summertime storms that produce extreme downpours could increase by more than 400 percent across parts of the United States -- including sections of the Gulf Coast, Atlantic Coast, and the Southwest -- according to a new study by scientists at the National Center for Atmospheric Research (NCAR).
The study, published in the journal Nature Climate Change, also finds that the intensity of individual extreme rainfall events could increase by as much as 70 percent in some areas. That would mean that a storm that drops about 2 inches of rainfall today would be likely to drop nearly 3.5 inches in the future.
"These are huge increases," said NCAR scientist Andreas Prein, lead author of the study. "Imagine the most intense thunderstorm you typically experience in a single season. Our study finds that, in the future, parts of the U.S. could expect to experience five of those storms in a season, each with an intensity as strong or stronger than current storms."
The study was funded by the National Science Foundation (NSF), NCAR's sponsor, and the Research Partnership to Secure Energy for America.
"Extreme precipitation events affect our infrastructure through flooding, landslides and debris flows," said Anjuli Bamzai, program director in NSF's Directorate for Geosciences, which funded the research. "We need to better understand how these extreme events are changing. By supporting this research, NSF is working to foster a safer environment for all of us."
A year of supercomputing time
An increase in extreme precipitation is one of the expected impacts of climate change because scientists know that as the atmosphere warms, it can hold more water, and a wetter atmosphere can produce heavier rain. In fact, an increase in precipitation intensity has already been measured across all regions of the U.S. However, climate models are generally not able to simulate these downpours because of their coarse resolution, which has made it difficult for researchers to assess future changes in storm frequency and intensity.
For the new study, the research team used a new dataset that was created when NCAR scientists and study co-authors Roy Rasmussen, Changhai Liu, and Kyoko Ikeda ran the NCAR-based Weather Research and Forecasting (WRF) model at a resolution of 4 kilometers, fine enough to simulate individual storms. The simulations, which required a year to run, were performed on the Yellowstone system at the NCAR-Wyoming Supercomputing Center.
Prein and his co-authors used the new dataset to investigate changes in downpours over North America in detail. The researchers looked at how storms that occurred between 2000 and 2013 might change if they occurred instead in a climate that was 5 degrees Celsius (9 degrees Fahrenheit) warmer -- the temperature increase expected by the end of the century if greenhouse gas emissions continue unabated.
Prein cautioned that this approach is a simplified way of comparing present and future climate. It doesn't reflect possible changes to storm tracks or weather systems associated with climate change. The advantage, however, is that scientists can more easily isolate the impact of additional heat and associated moisture on future storm formation.
"The ability to simulate realistic downpours is a quantum leap in climate modeling. This enables us to investigate changes in hourly rainfall extremes that are related to flash flooding for the very first time," Prein said. "To do this took a tremendous amount of computational resources."
Impacts vary across the U.S.
The study found that the number of summertime storms producing extreme precipitation is expected to increase across the entire country, though the amount varies by region. The Midwest, for example, sees an increase of zero to about 100 percent across swaths of Nebraska, the Dakotas, Minnesota, and Iowa. But the Gulf Coast, Alabama, Louisiana, Texas, New Mexico, Arizona, and Mexico all see increases ranging from 200 percent to more than 400 percent.
The study also found that the intensity of extreme rainfall events in the summer could increase across nearly the entire country, with some regions, including the Northeast and parts of the Southwest, seeing particularly large increases, in some cases of more than 70 percent.
A surprising result of the study is that extreme downpours will also increase in areas that are getting drier on average, especially in the Midwest. This is because moderate rainfall events that are the major source of moisture in this region during the summertime are expected to decrease significantly while extreme events increase in frequency and intensity. This shift from moderate to intense rainfall increases the potential for flash floods and mudslides, and can have negative impacts on agriculture.
The study also investigated how the environmental conditions that produce the most severe downpours might change in the future. In today's climate, the storms with the highest hourly rainfall intensities form when the daily average temperature is somewhere between 20 and 25 degrees C (68 to 77 degrees F) and with high atmospheric moisture. When the temperature gets too hot, rainstorms become weaker or don't occur at all because the increase in atmospheric moisture cannot keep pace with the increase in temperature. This relative drying of the air robs the atmosphere of one of the essential ingredients needed to form a storm.
In the new study, the NCAR scientists found that storms may continue to intensify up to temperatures of 30 degrees C because of a more humid atmosphere. The result would be much more intense storms.
"Understanding how climate change may affect the environments that produce the most intense storms is essential because of the significant impacts that these kinds of storms have on society," Prein said.

Ask the Experts: Is El Niño to Blame for So Much Weird Weather?


El Niño has become a hot topic. Most people in the general public now know the term, and they have a vague idea that it is some kind of pattern in the Pacific Ocean that means the U.S. will have a warm winter…or snowy winter…or hot summer—or something. Almost every day, somewhere in the country, a meteorologist is blaming El Niño for unusual weather. The perceived wisdoms, and misunderstandings, are widespread. Atmospheric scientists are the first to acknowledge that only certain effects can be linked to a strong El Niño, and that they are unsure about others.
The current 2015–16 El Niño is one of the three strongest ever recorded. The other two occurred in 1982–83 and 1997–98. In between these events El Niño may have been weak or absent, and its cousin, La Niña, may have been strong in some seasons during that period. In the Northern Hemisphere El Niño’s effects peak during the winter, and are typically sorted out and summed up by the following April.

5 Weather Events Happening Right Now That Shouldn't Be


There's nothing unusual at all with a January storm track that runs well inland from the Northeast seaboard, pumping above-freezing air into the I-95 corridor, keeping them mainly rain, as is the case, here.
However, this system was expected to spread rain as far north as the Champlain Valley of Vermont and Upstate New York, places you might expect cold air to hold in place to allow more snow, sleet or freezing rain, rather than rain in January.
This also added to the general sluggish, snowless season start in these areas.
However, a pattern change is shaping up to finally bring weather more reminiscent of January to a good swath of the East during the second half of January.
This may make folks in the Northeast pine for the warm, weird early winter of 2015-2016.