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Winter 1997

Lake-ICE and SNOWBAND: Let it snow, let it snow, let it snow


If you ever had any doubts that science is a tough business, David Kristovich, Robert Rauber, and Peter Sousounis have the proof. They are part of a large team of principal investigators in two experiments that are currently braving a blustery and snowy winter on the U.S. and Canadian sides of the Great Lakes. The researchers are looking for bad weather, and it seems likely that they'll find it.

Location of experimental equipment for Lake-ICE and SNOWBAND. The experiment headquarters is in Ann Arbor, and the aircraft are also based there.

The two experiments, the Lake-Induced Convection Experiment (Lake-ICE) and Snow Band Dynamics (SNOWBAND), will study a group of weather phenomena that occur around the Great Lakes. These range from the well-known lake-effect storms, which heap snow on communities just south and east of the Great Lakes, to the snow bands northwest of low-pressure systems, which bury the Midwest in snow, to the recently discovered mesoscale aggregate vortices, which influence weather as far afield as the East Coast. The scientists are studying the storms using NCAR and the University of Wyoming's aircraft; NCAR, Pennsylvania State University, and the University of Wisconsin's remote sensing equipment; NCAR and the National Severe Storms Laboratory's sounding systems; and the National Weather Service's WSR-88D (Nexrad) radars (see sidebar).

The lakes and their surrounding land, as it happens, have another unique advantage as an experiment site: they are a microcosm of the earth. "We're trying to understand how the atmosphere over water bodies responds to intense heat and moisture input from the surface," says Kristovich (Illinois State Water Survey). This is important in understanding the atmospheric boundary layer, the kilometer of air closest to the earth. "The Great Lakes allow us to study how marine boundary layers evolve in these circumstances without having to sample over the ocean," Kristovich explains. A better understanding of these processes could improve weather and climate forecasting.

The bane of Buffalo: The lake effect

Cities on the south and east sides of the Great Lakes, such as Cleveland and Buffalo, owe much of their snow each year to lake-effect storms. Snowfalls at lakeside may be twice as deep as they are only 100 kilometers inland. These heavy snowstorms occur when a low-pressure system, or cyclone, moves north and east of the lakes. Several processes induce the snows. First, frigid though the lake water may be, it is warmer than the air above it. As cold air passes over the relatively warm lake in fall and winter, the air is warmed, moistened, and destabilized. The resulting clouds produce snow over the lake and along the downwind shores. Since the prevailing winds are from the west, the east ends of the lakes get the heaviest snow. The second contributing factor for the lake effect is the difference in friction between the smoother water and bumpier ground. Air traveling over a lake first accelerates. When it reaches the land it slows and converges with other air masses, intensifiying the snowfall. Finally, hills downwind of the lakes further lift the air flowing off the lake, which deepens the snowfall even more.

Lake-ICE will study lake-effect snowfall in a concentrated effort around Lake Michigan. Scientists plan to use the data from the experiment to address a number of outstanding scientific questions. For example, how does the rapid exchange of moisture and heat affect the growth of the boundary layer? What controls the organization, intensity, and location of heavy lake-effect snowfall? How do lake-effect snow bands interact with small-scale turbulence to transport heat, moisture, and momentum vertically? How do clouds and precipitation affect the boundary layer? "Lake-effect events are convenient laboratories for studying the marine boundary layer," Kristovich emphasizes.

The blizzard generator: Snow bands

Winter cyclones in the Midwest may also create a different kind of extremely heavy snowstorm. In the northwestern part of a cyclone, there can be bands of heavy snow that are many hundred of kilometers long but only about 200-300 km wide. Accompanied by strong winds and sharp changes in surface pressure, the bands often produce blizzards. "Although scientists know that these snow bands occur because several air masses of different temperature and moisture content converge, they don't understand the instabilities within air moving through the snow bands and how they relate to the development of heavy snowfall," says Rauber (University of Illinois). They also don't understand why a larger snow band sometimes evolves into a group of narrower, more intense bands.

(NCAR file photo by Charles Semmer.)

One problem with studying these snow bands is that they are narrow enough to fall through the existing network of regular meterological observations. SNOWBAND is the first study designed to take observations of snow bands on the proper scale.

SNOWBAND will focus on snowstorms that occur on the western side of Lake Michigan. When slow-moving cyclones track northeastward along a path extending about 300 km south of the lake, surface winds north of the cyclone cross Lake Michigan from east to west, producing lake-effect conditions on its western shore--the opposite side from the more familiar lake effect described above. These lake-enhanced snows occur simultaneously with the larger-scale snow bands, contributing to very heavy snows on the west side of the lake.

A group effort: Mesoscale aggregate vortices

About nine years ago, Peter Sousounis (University of Michigan) became curious about how the Great Lakes might affect weather on a larger scale. Henry J. Cox first suggested such a possibility in 1917. "He kind of casually noticed that the lakes seemed to attract and deepen lows in the winter and attract and intensify highs in the summertime," says Sousounis. However, no study had ever been done to understand these effects.

Sousounis picked a single, well-observed cold spell, called a cold-air outbreak, of two days' length in November 1982. Working with Michael Fritsch of Pennsylvania State University, he modeled the cold weather in two ways, with and without the Great Lakes, using the Penn State/NCAR MM4 model. The results were so surprising, he says, that "I was somewhat sceptical that the model was behaving properly." The simulations suggested that places as far away as Philadelphia were being warmed by as much as 2 degrees Celsius. "I grew up in Philadelphia," says Sousounis, "and if you had told me it was warmed by the Great Lakes, I would have laughed."

This year, Sousounis identified a cyclonic circulation that develops within the warm pool of air that is generated by the lakes. He named this circulation a mesoscale aggregate vortex (MAV), because it develops from heating and moistening from all of the Great Lakes. Sousounis's simulations suggest that an MAV has a warm core (something like a tropical system), winds on the order of 15-20 knots, and a pressure disturbance at the surface of 6-7 millibars. The simulations indicate that it is as wide as the upper Great Lakes (Superior and Huron) and about 4 km deep. The modeled MAVs tend to develop most strongly just northeast of Lake Huron.

Although observational data give some confirmation of MAVs, upper-air soundings from around the Great Lakes are few and far between, especially on the sparsely populated Canadian side. Sousounis will gather data with a network of upper-air stations across southern Ontario during Lake-ICE and will do additional modeling to understand how MAVs develop and where they go. "It's not impossible that they can affect weather over the North Atlantic," he says.

What about El Niño?

Some weather forecasters have predicted that this year's El Niño will bring a warmer and drier winter to the Great Lakes region. With equipment and personnel dotted across hundreds of kilometers of the United States and Canada (see map), the scientists certainly have plenty of worries. But they are not worried about El Niño.

"During the last two major El Niños," says Rauber, "a split flow pattern in the jet stream set up over North America during December and January. In those years, storms tended to form east of the Rockies in two places, north of the U.S./Canadian border (the Alberta Clippers) and around Texas and New Mexico. The clippers moved across the Great Lakes into the Northeast U.S., bringing short bursts of cold air and lake-effect snows. Storms along the southern branch of the jet stream moved northeastward into Illinois or Wisconsin, producing snow bands. Although long-lasting cold-air outbreaks were not common, the conditions required for the success of the projects were present." However, a split flow pattern this year is by no means certain. Rauber points out, "The atmosphere usually ignores our best predictions and does the unpredictable."


Wintering over at Aux Berges des 11 Rapides

A CLASS station, with a weather-balloon launch in progress.

"This is going to be a rough one because of the cold," says Larry Murphy (NCAR Atmospheric Technology Division). Murphy is in charge of the cross-chain Loran atmospheric sounding system (CLASS) stations in Ontario and Quebec, which make surface meteorological observations and allow weather-balloon launches. He has had experience with tough experiments; for example, he spent last January weathering high seas on the North Atlantic in the Fronts and Atlantic Storms Experiment. But he is distinctly unenthusiastic about the conditions he'll encounter in Lake-ICE.

Forecasters are predicting temperatures as low as -45 degrees C. Murphy isn't sure how the CLASS computers will handle that kind of cold. "The trailers [housing the computers and other equipment] are not well insulated. We're just going to keep them as warm as we can." Murphy has the responsibility for any on-site repairs needed. Although he's hoping to avoid Murphy's Law, that anything that can go wrong will, "I think the cold is probably going to add to the typical problems. And driving around in that type of weather, it tends to get a little treacherous."

During the operating periods, students from regional universities and seven or eight local people will be hired to do the balloon launches, but "we try to have some NCAR personnel at the more remote locations," Murphy explains. That's why he'll be in Beattyville, Quebec, during the January leg of the experiment. You can easily find it on a map, but finding it on the road is another story. "The first time I went up there, I drove all the way to the next town. I didn't see how I could have missed it; there was nothing there to miss." When he called his contact, he was instructed to turn off the highway at a certain mile marker. There was still nothing to see, but "I went over a hill, and there was this hotel on a lake." That was Beattyville.

The owner of Aux berges des 11 rapides, Murphy discovered, had bought the land for one dollar and built the inn as a stop along the trans-Canadian snowmobile trail. Though the site was isolated, the owner was thrilled to be hosting a CLASS station, even arranging local television coverage for Murphy when he set up the station in October. So although January temperatures are cold in Quebec, at least the hsopitality is warm.


The experiments in a nutshell

Who:
Participants from Colorado State University, Illinois State Water Survey, McGill University, NCAR, Pennsylvania State University, Purdue University, South Dakota School of Mines and Technology, State University of New York at Brockport and at Oswego, University of Illinois, University of Michigan, University of Washington, University of Wisconsin, and Western Michigan University. The Lake-ICE Scientific Steering Committee consists of Kristovich, Donald Lenschow (NCAR), Pierre Mourad (University of Washington), Sousounis, Hans Verlinde (Penn State), and George Young (Penn State). The SNOWBAND principal investigators are Rauber, Mohan Ramamurthy, and Brian Jewett (all of the University of Illinois).

With What:
NCAR Electra and University of Wyoming King Air; University of Wisconsin Volume Imaging Lidar (VIL); five fixed and one mobile cross-chain Loran atmospheric sounding system (CLASS) stations; and three integrated sounding system (ISS) stations.

When:
Two three-week periods, 2-22 December 1997 and 5-24 January 1998.

Where:
Data collected in Michigan, Wisconsin, Illinois, Indiana, Ontario, and Quebec. Operations center at the University of Michigan, Ann Arbor. Forecast modeling support from the National Weather Service, the University of Wisconsin, and the University of Michigan.

Why:
Lake-ICE aims to gain a better understanding of how the Great Lakes influence weather both nearby and farther away, and more generally of how the atmosphere reacts to exchanges of heat and moisture with large bodies of water. SNOWBAND intends to determine the dynamic mechanisms responsible for heavy snowfall in the northwest quadarant of cyclones and lake enhancements of snowfall in this quadrant.


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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 14:43:58 MDT 2000