Sunday, February 12, 2006

Column 12: Winter storms (originally posted December 1, 2005)

Stefan from North American Met suggested a column about this, and I think it's a great idea: what happens in winter weather systems. Specifically, the suggestion was for me to discuss precipitation regimes and locations.

Well, the main thing to consider when talking about any weather system, be it a supercell thunderstorm, a hurricane, or whatever, is the airflow within the system. A large-scale winter weather system is made up of air and stuff (water, cloud condensation nuclei, all sorts!) moving up and down and all around in a seemingly chaotic yet surprisingly ordered fashion.

Warm moist air is moving into the system from the south, and cold dry air is moving into it from the north. See this link for a discussion about that--it's essentially the Norwegian cyclone model. Click here.

The warm air moves parallel to the ground until it hits more dense air--the interface of these 2 airmasses being a (warm) front. The warm air rides up over the cold air, and at the mixing zone the moisture in the warm air is cooled to or below its dewpoint, causing cloud and precipitation.

At the cold front, the opposite is happening. The cold air is moving southward, and when it meets the warm air at the interface between the two (aka the cold front), it forces the less dense warm air to rise. As the warm air does so, much like at the warm front, it mixes, cools and condenses, causing precipitation.

The airflow is obviously more complex than this, but going into the entire flow regime is almost a University-level course itself. Let's just say that warm air from the south moves north, rises at the cold front, and eventually turns eastward with the jet stream. Cool-ish air north of the warm front moves westward, parallel to the front, rises gradually and is eventually turned eastward by the jet stream. Cold air essentially moves southward, "wedging" the warm air upward.

There are many factors influencing what kind and how heavy the precipitation will be. These things include 1) surface convergence, 2) moisture content of the air, 3) upper support, and 4) instability.

Surface convergence is an obvious choice because it means air coming together. When air comes together, such as at a front, the excess mass has to go somewhere. Now unless there's a giant vacuum hole in the ground, physics says that the air must rise. So logically we can expect precipitation to be enhanced near fronts.

The moisture content of the airmass is of course a no-brainer. The more moisture you have available to condense, the more moisture you can put on the ground in the form of precipitation.

Upper support is a more nebulous thing to gauge, until you've seen it in action a few times. Examples of upper support include increasing vorticity (translating to increasing lift) aloft and my personal favourite, the divergence at the left exit region of a jet stream. (The latter is something I personally call the "sweet spot" when it comes to winter precipitation--I've seen 5 to 7 cm of snow per hour fall from the sweet spot in the maritimes--most notably St. John's. But here in southern Manitoba, it can even be a pretty sweet spot--dumping 2 to 3 cm every hour, a pretty crazy snowfall rate for here.)

And instability can of course increase precipitation rates. If your air has a tendency to rise without prompting, well, it's going to have a better chance of condensing all that moisture it's involving.

Where, then, do we get the heaviest precipitation from a winter storm? Well, it seems the juxtaposition of the 4 factors mentioned above is where it happens. In this example, you can see on the satellite picture where they came together over southern Manitoba: there were fronts in the regoin; copious moisture was being advected from the Gulf of Mexico; vorticity was increasing aloft along with the sweet spot being over us; and there was just enough cold air aloft and heat/moisture at the surface to make the atmosphere unstable.
Here's the satellite example.

So now we've figured out the areas of heaviest precipitation. But what kind will the precipitation be?

When diagnosing precipitation type, we usually employ a top-down approach: that is, we do a thought experiment where we follow a precipitation particle from birth way up in the clouds to its impact at the ground.

To produce precipitation particles in the cold rain process (for the nerds out there, this is known as the Bergeron-Findeisen process) you need temperatures between -10 and -20 C and saturation. And of course, cloud condensation nuclei. These ingredients produce both ice crystals and supercooled water droplets. The supercooled droplets freeze on impact with the ice crystals, making them grow so that they eventually become large enough to fall.
When these ice crystals/now snowflakes are falling, they fall through all sorts of regimes. It is what the snowflakes encounter that ultimately determine the precipitation type.

If the snowflakes fall into a layer of above-freezing air that extends to the surface, the precipitation reaches the ground as rain (or wet snow if the above-freezing layer is sufficiently shallow).
If the snowflakes fall and hit the ground without encountering above-freezing temperatures, then obviously you have snow.
If the snowflakes fall through an above-freezing layer but then fall through a layer of sub-freezing temperatures close to the ground, the precipitation type depends on the thickness of the above-freezing layer and the thickness of the below-freezing layer. You can thus surmise that if the above-freezing layer is relatively thick and the below-freezing layer is relatively thin, freezing rain is most likely. If the opposite is true, ice pellets (known in the USA as sleet) are most likely. In between is one of the great forecast challenges in the winter. And what makes forecasting precipitation type fun.

In winter storms, the most likely place for freezing rain and/or ice pellets is just north of the warm front. You have cold air at the surface, and the warmer air has risen to above the surface: perfect conditions to melt and re-freeze the precipitation particles. In some rare cases, freezing rain and ice pellets can happen behind a cold front, but it's not a common thing.

So ends a long explanation of precipitation in winter storms.


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