Gravity Waves Wednesday Morning

On Wednesday morning, after thunderstorms had moved through the area overnight, an area of freezing drizzle moved across southern Wisconsin. This drizzle was triggered by a gravity wave moving through. You can think of a gravity wave as being similar to the waves that result from tossing a rock into a pond, but these waves are in the atmosphere. 
The most common trigger of gravity waves is air rising over a mountain, but they can happen in locations away from mountains, and in these cases are most often caused by thunderstorms. As a thunderstorm builds upward, it may break into a layer of air that is “stable”. This means that warmer air is located above cooler air. When this happens, some cooler air will be lifted into the warm layer…and since warm air is less dense than cool air, the cool air that was lifted will want to move back down to where it belongs. Because this “parcel” of air now has some momentum, it continues to make a wave pattern. 

This graphic depicts what happens. Think of like water disturbed in a pond or lake.

The satellite image below shows a gravity wave that moved over the lake earlier in the morning. The rippled clouds are a clear indication of a gravity wave over far southern Lake Michigan. There was also some evidence along the Michigan shore near Muskegon, but it is hard to see with the observations on the map.


The next satellite image shows the back edge of the gravity wave that caused the drizzle over southern Wisconsin. The wave extends north-south in this image, through the center of the lake.

The radar loop below shows the drizzle traveling across the area, with the white line on the first frame indicating the sharp edge of the front of the gravity wave. It also shows an interesting result of the gravity wave to the north…as the wave interacted with some left over snow showers, it clearly enhances the reflectivity on the radar. The extra lift caused the snow showers to intensify as the wave intersected them. There were even several reports of thunder and lightning at this time.

More information about gravity waves is available if you search on the web.


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