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 Post subject: Interview: Michelle Margraf, NOAA Meteorologist
PostPosted: Mon Oct 02, 2006 7:46 pm 
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Joined: Sat Oct 26, 2002 10:06 pm
Posts: 5274
Location: Here, Now
Name: Michelle Margraf

Title/job description: Meteorologist – Observing Program Leader

What do you do on any given day?
Monitor weather data coming into the National Weather Service for quality. Correct/preventing data errors by training weather observers or fixing/conducting routine maintenance on automated equipment. Serve as liaison to those who share weather data with the National Weather Service. Recruit weather observers in area where data is needed.

In a nutshell, what is the mission of NOAA?
To understand and predict changes in the Earth’s environment and conserve and manage coastal and marine resources to meet our nation’s economic, social and environmental needs.


Let’s start off with an overview of what causes wind in Minnesota? Only Frontal wind?
Wind is natures way of balancing out air pressure. In areas where the pressure is high, winds blow the air away from the high pressure area toward an area where the pressure is low.

If you look at the weather map the winds don’t blow directly from the center of highs and lows because the earth is spinning. That makes winds go counterclockwise around lows, and clockwise around highs (the spin is opposite in the southern hemisphere, interestingly enough). The winds are stronger when the pressure difference between a low and high is greater. Therefore, the stronger a low or high pressure is, the stronger the winds will be.

A front is a place where air from two different places on the earth collide. On one side of a front the air may be cooler than the other side, depending on where the air came from. The air might also have a different amount of humidity. The winds will also be blowing in different directions on either side of a front as well, and most likely are blowing at different speeds. In general, where the air is cooler, the winds are stronger, since cold air is more dense. There is also a component of wind that is trying to balance out the differences in temperatures along a front, so right as the front goes through the wind may be gusty as well. However, the biggest drive for the wind is the high and low pressure areas.


How does the wind change with the time of year?
In general winds are stronger in the winter than in the summer because there is a wider range in temperatures on the planet. Near the equator it’s warm all year round, but in the winter the mid and upper latitudes cool down significantly, which causes stronger low and high pressure areas to form. The lows and highs are natures way of stirring up the atmosphere to help balance out the temperatures. As an example, when it gets extremely cold in northern Canada a high pressure area is present. After a while the high pressure area will push southward to take that cold air away and let that area warm up a bit. As the high pressure with the very cold temperatures moves southward it gradually warms up. It may be -30F here in Minnesota when the “Canadian High Pressure” moves through, but by the time it gets to Texas or the Gulf Coast, it’s closer to 32 degrees. Generally, the colder the air is, the stronger the high pressure is. That’s why the highs that come by in the winter usually bring more wind.

The one thing to note about high and low pressure areas is, when you’re directly under the center of the high or low, the winds are nearly calm. The strongest winds are usually found a bit away from the center where they are circling the high or low. A good way to visualize this is to think of a hurricane with its eye and eye wall.

What causes gusty wind? We are all very much aware of turbulence from structures and land masses, but what other conditions makes wind more gusty?
The rays of the sun heat surfaces on the earth at different rates. Water heats up slower than land, and grassy areas heat up slower than concrete parking lots. Where the air is hot, it rises. This causes the wind that is blowing by the ground to be disrupted as it runs into rising pockets of air. As a result you may feel a gust of wind, then lighter winds for a bit as a piece of air that was disrupted by rising pockets of air moves by you.

What are some indicators of micro bursts? Are they prevalent in the Midwest?
Microbursts and downbursts are rain cooled air that falls toward the ground out of a thunderstorm. The air is cooled when the water vapor it contains is pulled out to make rain droplets. Imagine coming out of the shower with water droplets on you. As you dry off (the water is absorbed by the air), you cool down. Cooler air is more dense, so it falls out of the cloud toward the ground. When this happens, you feel a rush of air. It is usually followed by rain as the water droplets grow and gravity eventually pulls them down toward the ground when they get heavy enough.

The strongest thunderstorms have the strongest microbursts and downbursts. The difference between the two are that microbursts affect a smaller area (a few city blocks). Downbursts can affect an area as large as several miles. Strong thunderstorms are most prevalent in the central and southern U.S. so we are in an area which sees more than our share of microbursts and downbursts. Doppler radar can pick up the speed of the falling air so warnings can go out when the wind will be strong enough to cause damage.

What cloud patterns should we keep an eye peeled for? ie. which cloud patterns might signify a dramatic shift in the wind?
Any shower or thunderstorm has the capability to produce changes in wind direction. When the rain cooled air hits the ground it spreads out in all directions (imagine a droplet of water hitting a pond and the waves go out away from the point of impact). It depends on what side of the storm you are on which direction the wind will turn to. One thing to note is that after half an hour or so, the prevailing wind direction will return.

The other phenomenon that can cause rapid wind shifts is fronts. Most fronts have showers and thunderstorms along them, but some (when the air is too dry to make clouds) may only be noticed as the flag changes directions. It is hard to determine whether the shower or thunderstorm that passes is associated with a front without looking at a weather map. You would also want to check the weather map to see if a “dry” front is passing through.

The best rule of thumb is that if you see a shower or thunderstorm approaching, it is best to prepare for a sudden wind change. To know if the wind change will be temporary or not, you will want to check out a weather map to see if a front is also coming through at the same time.

Check our website to see an hour by hour forecast of wind speed and direction at http://weather.gov/twincities. Put the city and state you want a forecast for in the top left hand box, then click on hourly weather graph at the bottom of the forecast page that comes up to see an hourly wind forecast. The weather map including fronts can also be found our website by clicking on Observations in the left hand side menu, then Daily National Weather Map.

How does water temp to air temp effect the wind on the water and 100ft above the water?
Wind also balances out differences in temperature (in addition to pressure). Since the land heats up faster than the water, on days when the land and water temperatures are large, a wind develops to blow the cool air from the water onto the land to balance the temperature out. This affect is greatest near the ground, but can also be felt at 100 feet. There is less friction the higher you are above the ground, so winds are generally stronger at 100 feet than at the surface.

When you have isobars stacking up between a low and a high and looking uniform in spread, does the character of the wind change as you get closer to either of them?
The closer the isobars are together, the stronger the winds. When isobars are close together it means the pressure is changing rapidly. An isobar is a line of equal pressure. The more lines of pressure there are on a weather map, the greater difference in pressure there are. The wind responds to this imbalance by increasing in speed.

Some additional questions from other members on our forum:

How does a large body of water like Lake Superior or semi-large like Mille Lacs affect the wind? It seems like a west wind gets compressed and gusty on the Duluth shore, while an East wind stays steady off of the lake.

There are two factors that make the winds different on a body of water than on the land. The first is on water there is no friction so when the wind is being caused by a high or low pressure area the winds would be generally be stronger on the lakes. The other factor that causes wind differences on the lake is the temperature of the lake. When the lake is much cooler than the land, it causes the wind to blow off the lake onto the land to equalize the temperature (lake breeze). When the lake is warmer than the land, the winds blow off the land onto the lake (land breeze).

Without knowing what the temperatures of the land or water were like (and if there was a strong low or high pressure in the area) it is hard to tell which factor played a bigger role in the examples listed above. It sounds like the examples above are describing a lake breeze (the east wind) and a land breeze (west wind). Both examples would give the shoreline gusts as the wind moves by to equalize the temperature differences.


Why does the wind blow on Superior more in the fall???
In the fall the waters of Lake Superior are still relatively warm because it takes a long time for water to lose (or gain) heat. The air (and land) cool down much quicker in the fall as cold air arrives from Canada. When cool air blows over a warmer Lake Superior, the difference in temperature causes the winds to increase to try and balance out the temperatures.

How can it be blowing 30mph everywhere around Mill Lacs but on the lake it's blowin 10mph???
I assume that in this case the wind was warmer than the temperature of the lake. The air directly above the water is generally as cold as the water, and when the warmer air hits the colder air, it is forced to rise. Imagine a hot air balloon. The balloon rises because it is warmer than the air around it. The cold air just above the lake’s surface acts like a bubble of cool air over the lake. The gusty winds would pass over this bubble. This phenomenon is called an inversion. We see it a lot at night when our winds decrease at the ground. The overall wind speed hasn’t changed, but the air cools at the ground (especially on cloud free nights when the heat escapes to space). The cool air at the ground creates a bubble which forces the winds to rise above ground level and makes the air light at the ground.

Why do we get that updrafty wind some days on Mille lacs??? which I think happens when we have cold water and hot air.
If the air that moves over the water is warmer than the water, it will rise like a hot air balloon. This creates updrafts of air that you can feel on the lake.

Some days on top of the hill in Duluth it will be blowing something like South wind... then on Park point it will be blowing NE. Why is this??
The wind at the harbor is coming off the water because the land is warmer than the lake. The lake breeze is trying to cool the land off. Cold air cannot rise so it doesn’t make it up the hill. At the top of the hill the wind blows without the effect of the lake. The main winds were from the south that day (which makes sense on a warm day when the land would be warmer than the lake) so that is what you’d see at the top of the hill.

Does the wind accelerate when passing over terrain as over an airplane wing? Does atmospheric pressure have the effect of layering the wind into different strata of wind speed, direction , etc?
The wind would be faster in an area free of friction such as an airplane wing or water (if temperatures were not a factor). Terrain generally slows down the wind because it the air looses energy when it brushes by an object.

What forecast models give the most accurate forecasts of wind speeds? Each computer model has its strengths. Some are better than others with different kinds of weather situations (ie. Winter weather vs. summer, storms coming from the south vs. the north, etc.). The two main models that are used with small scale wind forecasting are the GFS and the NAM. You can see forecasts from these two models on our website at http://www.crh.noaa.gov/mpx/model.php

Thanks so much for taking the time to answer these questions. Your wisdom will help hundreds of us get more out of our passion.

_________________
Tighe


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