Dual Pol Radar Shedding Light on "Wintry Mix"

A recent article in the New Yorker tried in vain to dissect and understand the term “wintry mix,” only to grimly report it’s a weather phenomenon vile and disgusting and that forecasters state it to cover their backsides when a variety of winter precipitation is to descend upon man.
Far from vile and disgusting, a wintry mix is just that: a mixture of winter precipitation—snow, sleet, freezing rain—falling from the sky. No more, no less. Its mention will return to forecasts this weekend as a moisture-laden storm in the nation’s midsection plows into Arctic air and treks across the inland South and into the East next week. Rest assured: research and new technology are ready and are allowing forecasters to view wintry mix in amazing detail, better than ever before, improving predictions of the phenomena by leaps and bounds.
Recently published research on dual polarization (dual pol) weather radar in use, in a handful of AMS journals, is shining a spotlight on its capability to determine different types of precipitation falling at the same time, including the once-dreaded wintry mix. Instead of shying away from such forecasts, meteorologists using the nation’s network of Doppler radars, upgraded in recent years to include polarimetric technology, are beginning to get really good at chronicling the wintry mix in their forecasts.
While the New Yorker implied meteorologists disdain for the term, wintry mix actually is looking more beautiful than ever to scientists–so nice we put the words on the cover of the latest BAMS: “Snow Globe: Dual Pol Deciphers Wintry Mix.”
This cover article in BAMS, by Picca et al., looks at New England’s monster blizzard of 9 February 2013, which unloaded more than 3 feet of snow on much of central Connecticut and Long Island. Dual pol radar’s unique modes deciphered the wintry mix inside an intense snowband producing lightning and snowfall rates of 3-6 inches per hour.

A composite of products from the dual pol radar on Long Island, New York (KOKX) shows reflectivity (ZH; top), differential reflectivity (ZDR; middle), and correlation coefficient (CC; bottom) of a heavy band of now and ice in the Northeast blizzard of 9 February 2013 (from Picca et al., BAMS). (Top) Reports of precipitation types around the time of the radar products provide ground-truth to the radar signatures. The speckled areas of reduced CC in southern Connecticut and around KOKX are a result of ground clutter. The black dot indicates the location of KOKX, and the star represents the location of the Stony Brook University surface observations. The dashed and dotted outlines indicate the two areas 1 and 2 of mixed phase precipitation. The underlined “LS” is the location of a “large sleet” report.

A similar article in Weather and Forecasting, by Griffin et al., documents for the first time polarimetric radar signatures of the same intense convective band of snow. The transition zone from freezing to non-freezing air (0°C isotherm) was exceptionally distinct in the radar signatures.
PPI displays of the polarimetric variables at (a)–(c) 2216 UTC 8 Feb and (d)–(f) 0236 UTC 9 Feb 2013 at 0.58 elevation. The 08C RAP model TW at the surface is overlaid (boldface, dashed). At 2216 UTC, pure dry snow was located within colder temperatures north of the 08C isotherm, while wet snow and mixed-phase hydrometeors occurred within warmer temperatures south of the 08C isotherm in (a)–(c). The solid black line indicates the location of the 1448 azimuth RHI. At 0236 UTC, dry snow was predominant, while wet snow and ice pellets were also observed within the max ZH region, within negative surface temperatures, north of the 08C isotherm in (d)–(f).
Displays of the polarimetric (i.e., dual pol) variables at (a)–(c) 2216 UTC 8 Feb and (d)–(f) 0236 UTC 9 Feb 2013 — during the Northeast blizzard (from Griffin et al., WAF). At 2216 UTC, pure dry snow was falling within colder temperatures north of the model-indicated 0°C isotherm (bold black dashed line), while wet snow and mixed-phase hydrometeors occurred within warmer temperatures south of the 0°C isotherm in (a)–(c).  At 0236 UTC, dry snow was predominant, while wet snow and ice pellets were also observed within the max ZH region, within below-freezing surface temperatures north of the 0°C isotherm in (d)–(f).

In the Journal of Applied Meteorology and Climatology (JAMC), the article by Kumjian et al. discusses the use of intensive radar measurements to study the finescale structure of more than a dozen Colorado Front Range snowstorms. And in Monthly Weather Review, Geerts et al. explain in their article how a specifically synthesized dual Doppler radar technique in an airborne platform was able to directly measure hydrometeor vertical motion, improving the accuracy of the radar.
CSU-CHILL RHI along the 181.998 azimuth at 0852 UTC 9 Apr 2013 for (a) ZH and(b) ZDR. Arrows show the locations of generating cells.
Vertical slices through a 9 April 2013 Colorado snowstorm from Colorado State University’s CHILL dual-pol radar show (a) reflectivity (ZH) as well as (b) differential reflectivity (ZDR), which indicates particle shape and size (from Kumjian, JAMC). Arrows show the locations of generating cells.

Conceptual model of a vertical slice through a generating cell with a shroud echo with example particle types present. The shroud of large ZDR and low ZH values (yellow color) indicates the presence of pristine anisotropic crystals with platelike or dendritic habits. The core of the generating cell (bluish color) is characterized by more snow aggragates or rimed crystals, the larger of which are descending (blue dashed lines) The core is also where the strongest updraft speeds (and thus supersaturations with respect to ice) are located, indicated the black vertical arrow).
In Kumjian’s JAMC article, a conceptual model of a vertical slice through a generating snow cell reveals example particle types. The yellow color indicates the presence of pristine anisotropic snow crystals with platelike or dendritic habits. The core of the generating cell (bluish color) is characterized more by snow aggragates or rimed crystals, the larger of which are descending (blue dashed lines) The core is also where the strongest updraft speeds (black arrow) are located.


A Chat with the Iceman

Thorsten Markus, on sea ice in Antarctica.

By Maureen Moses, AMS Education Program
I hope you all had a good Earth Science Week last week! The theme was “Careers in the Earth Sciences,” and the AMS Education Program participated in a twitter chat with NASA Polar Scientist Thorsten Markus, who admits that as a high schooler in Germany science wasn’t his passion, but becoming a musician was. Now head of NASA Goddard’s Cryospheric Science Lab, Dr. Markus makes measurements of ice thickness in Antarctica.
Chat participants included a whole classroom full of eighth graders. Dr. Markus had plenty of advice on how a future polar scientist with an adventuresome streak can make a splash in a cool field! Here are some of the questions he fielded–edited and excerpted from the full chat archived on Twitter:

I’m here with 25 8th grade Earth Science students and one student would like to know what the day to day duties are as a polar scientist.
It’s extremely playful — playing with lots and lots of satellite data and learning something new every day.
Do you get to travel to cool places or are you processing data in an office?
Oh man, yes. I used to go to the Arctic and Antarctic and also flew over them in helicopters and planes.
What was your favorite experience in the field as a scientist?
Seeing the penguins coming out of the water and then standing right next to us. Fantastic!
When you decided becoming a rock star might not happen, why did you choose physics over math for a major?
Physics is pretty much applied Math — you deal with everyday problems… and actually learn how to solve some.
Which class helped you the most to get where you are today?
Maybe Math, but the arts fostered my creativity, for thinking outside the box
What level math did you have to go to? (for the future polar scientists out there). THX for the response!
I have a Ph.D. in physics, which involves a lot of math — but there’s also chemistry, biology and geography.
What is the difference between glacier ice data and sea ice data… Do they tell different stories?
Very different. Glacier ice is fresh water from mountains or ice sheets whereas sea ice is frozen ocean.
Are they affected differently by climate change?
Glaciers are balanced by snowfall and temperature, while with sea ice, also ocean properties play a big role.
So sea ice is inherently more volatile/variable?
I’d like to say sea ice is more complex, but then the ice sheet people might get angry 😉
What is/will be the impact of disappearing ice sheet on the global climate?
Melting of the ice sheets will increase sea level and affect ocean circulation because of the fresh water influx.
When can we expect to see Antartica’s ice retreating because of climate change. If it keeps stable or increasing, what can be made of that?
The Arctic and Antarctica are two different systems and global warming does not mean it warms uniformly everywhere.
What do you say to people who claim there’s a “debate” about climate change?
I don’t think there’s a “debate” about whether there’s climate change. The debate is by how much we’re responsible for it.
How good are the current models in predicting Arctic and Antarctic ice response to the climate warming?
I think the models are remarkable — certainly not perfect, but what prediction is perfect?
What climate data scares you the most (has the greatest implication for scary future events)?
The global ocean circulation, because it shows that things we do to the Chesapeake Bay may affect far away places.
Does any of the research you do tell us anything about other sheets of ice in cosmos?
As a matter of fact, I was involved in research about the Jupiter icy moons. So yes, there are analogies.
Who do you regard as your inspiration?
It was Keith Richards, now it’s the balance of the earth system… isn’t it remarkable how it all works together?

Eurasian Cold Snap a Product of Arctic Warmth

In a paradox that it seems only nature can muster—like quelling a year-long drought in the western two-thirds of Texas with record snows—it turns out that warming going on in the Arctic this winter is the likely culprit behind killer cold and snow that had been plaguing Eastern Europe since late January. And it is now also likely linked to the colder-than-normal winter occurring in Japan and Western Asia.
Weather patterns since the fall have set up in such a way as to leave large expanses of Arctic Ocean, particularly the Barents and Kara Seas north of Scandinavia and Russia, nearly ice-free. In fact, an image from early February posted to the Arctic Sea Ice Blog shows this area north of the Arctic Circle as open water for the first time in what it refers to as the “new Arctic regime (2005-present),” when it should be completely frozen over.
The changing weather patterns resulting from this new era of record sea ice melt seem to have helped build up a huge helping of frigid air over Siberia that came crashing southward as soon as the storm track shifted, which it inevitably does throughout the year. Instead of Western Europe like the last two winters, this time the bitter cold targeted Eastern Europe, delivering snow-laden storms to nations of the Eastern Mediterranean and even North Africa, as well as far Western Asia, Korea, and Japan.
Numerous researchers have been working the last few years to figure out what’s going on. The environmental news and technology site Bits of Science wrote a timely online article explaining the findings of recent published research and then tying the studies to the different teleconnections patterns that influence Eurasia’s winter weather — the Arctic Oscillation and its focused cousin, the North Atlantic Oscillation. Together, these pressure anomaly-driven climate patterns can dictate where winter’s worst cold and snow and best warmth become established.
A new study headed for publication in the Journal of Climate in March further extends the influence of the warming Arctic. The article by the Research Institute for Global Change’s Jun Inoue et al. contends that the storm track over the Barents and Lara Seas takes a more northerly route when sea ice there is reduced. In a positive feedback mechanism, the northward-shifted storm track pulls warm air over the Arctic Ocean. At the same time, cold air builds over Siberia and the Norwegian coast, and becomes poised to spill southward, or to the west and the east.

Barents Sea Storm Tracks
Sea-level pressure anomaly (hPa) and typical cyclone paths (red arrow: light-ice years, blue arrow: heavy-ice years). In the light-ice years, the cyclone path shifts northward and the Siberian High expands up to the Arctic coast.

“Such warm Arctic and cold continental conditions are referred to as a warm-Arctic cold-Siberian (WACS) anomaly, and the WACS anomaly could be a procurer of severe weather in the downstream region,” the authors report.
As with the collective research on the warming Arctic’s influence on mid-latitude winter weather, Inoue et al. conclude that using sea-ice variability from this region of the Arctic would improve the reliability of the seasonal weather predictions in individual years.

Mapping Ice Flow in Antarctica

A recently released map of the speed and direction of ice flows across Antarctica not only reveals some previously undiscovered geographical features, but also suggests a new explanation for how ice moves across the continent. Researchers constructed the map after studying billions of data points taken from a number of polar-orbiting satellites. After accounting for cloud cover, solar glare, and various land features, the scientists were able to determine the shape and speed of glacial formations across Antarctica. They found that some formations moved as much as 800 feet per year, and they also discovered a previously unknown ridge that runs east-to-west across the continent. The NASA animation below shows the ice flow patterns. “This is like seeing a map of all the oceans’ currents for the first time,” says Eric Rignot of the University of California—Irvine, who led the study (subscription required for access to the full article). “It’s a game changer for glaciology.” The observations also showed that the ice moves by slipping and sliding along the land, and not by being crushed and broken down by ice above it, as had previously been theorized by many glaciologists. That difference is critical to forecasting sea level rise in decades to come since a loss of ice at the water’s edge means “we open the tap to massive amounts of ice in the interior,” according to Thomas Wagner of NASA’s cryospheric program.

Charting the Course of Arctic Warmth…and Oceanography

While many parts of the country have recently been experiencing conditions that residents might call “Arctic,” the Arctic region itself has been warming since at least the early 1990s, reaching warmth unprecedented in the last century. The consequences for global climate are potentially critical―particularly if fresh water from melting ice and increased atmospheric precipitation in the Arctic slow the overturning circulation of the North Atlantic. With Arctic sea ice melting dramatically in recent years, scientists  are trying to understand the influence of the warmer water that flows into the Arctic from the North Atlantic.
At the National Oceanography Centre (NOCS) in Southampton, United Kingdom, scientists using high-resolution computer models found that from 1989 to 2009, about 50% of the salty North Atlantic water entering the Arctic Ocean came through Fram Strait, a deep channel between Greenland and the Norwegian island of Spitsbergen that connects the Nordic Seas to the Arctic Ocean. The Barents Sea contributes about as much Atlantic water to the Arctic, but the Fram Strait water carried most of the heat that has been a primary cause of Arctic ice melting.
An example of the modeling in this study, published in the January 2010 issue of Journal of Marine Systems, can be seen in the image below, which shows a computer simulation of ocean temperatures at a depth of 100 meters and sea ice thickness in September 2006. The pathways of warm saline water toward the Arctic have previously been poorly understood, but here the 8-km resolution defines three distinct pathways for this water to move under the more pure Arctic water, thus pumping heat northward between 50 and 170 meters below the surface.
“Computers are now powerful enough to run multidecadal global simulations at high resolution,” said NOCS scientist Yevgeny Aksenov. “This helps to understand how the ocean is changing and to plan observational programs so as to make measurements at sea more efficient.”
Ocean-climate interactions are a primary focus of the ocean science research priorities recommended by the U.S. National Science and Technology Council’s Joint Subcommittee on Ocean Science and Technology (JSOST) in their 2007 report, “Charting the Course for Ocean Science in the United States for the Next Decade: An Ocean Research Priorities Plan and Implementation Strategy.” As our understanding continues to evolve regarding the ocean and its influence on the Earth system, the priorities outlined in this report have also evolved. A town hall meeting on “Refreshing Our Ocean Research Priorities” (Monday, 12:15–1:15 p.m., B212) at the upcoming AMS Annual Meeting will explore some of these developments and give participants a forum to discuss topics of interest with the chairs of JSOST.