21 July 2005

A Matter of Scale

Posted by Dan Satterfield

Meteorologists like to break the atmosphere down into scales. We have the microscale and the meso scale and synoptic scales and global or planetary scales. Most forecasters concern themselves with the synoptic scale.

If you have a house that is 10 km long it is a mesocale house. Ten Meters long and it is a local scale house. 10,000 km long and your house is a planetary scale house!

Synoptic scale processes deal with storm systems like cold fronts and high pressure scales whose size is on the order of around 1000 km. The numerical weather prediction models we run every day are for the most part synoptic scale models.

Here in lies the problem with forecasting around here in the Summer. The models are synoptic scale but the processes that are effecting our weather each day right now are mesocale and even microscale processes.
Our weather models do a lousy job of predicting such things.

What I mean is this. The thunderstorms we have been having are not from a cold front. They are basically air mass storms and they form on mini cool fronts called out flow boundaries. We had an out flow boundary develop just south of some heavy storms in Tennessee Tuesday afternoon and that boundary came rapidly through Huntsville. When it did, we had a 12 degree temp. drop and the winds gusted out of the North up to 16 mph at the station. That boundary causes the air to lift and formed even more storms.

Many times these boundaries will form and persist for a day or two, setting off storms each day and getting modified by the cool air that forms as a result of the evaporational cooling.

They are very hard to detect on SYNOPTIC scale weather maps. May times we do see them light up on satellite images an hour or so before the storms develop. Not much lead time though.

As computers get faster and weather observations become more dense, we will see the development of mesocale models. A lot of research on this is going on right now. I may not live to see it but in a few decades, we will see a huge increase in the reliability of warm season forecasts in this part of the world.

These models are also thought to be a key to increased lead time for severe weather events.

For now though I will rely on the advice of one of my meteorology professors at OU. When I told him that I was going over tot he dark side and was planning on working in television, he said
“Tell them what you know and DON’T tell them what you don’t know!”

Best advice I ever received.