13 November 2018

The Cloud That Appears on Top of The Most Dangerous Storms

Posted by Dan Satterfield

We meteorologists knew that the new GOES satellites would be revolutionary, and a new paper presented at an AMS conference on severe storms is a good example of just that. It’s about a cloud signature visible on high- resolution imagery that can lead to more lead time on severe weather warnings and fewer false alarms. It’s called an AACP: Above Anvil Cirrus Cloud, and when a forecaster sees one on top of a supercell thunderstorm, that’s a strong indication that this storm will produce high winds, hail and tornadoes.

The paper (see above) is really worth reading, but I want to give a brief summary for those who do not have access to the journal. You may be able to hear the talk and see the slides here as well. 

This is an AACP or an Above Anvil Cirrus Plume. They are strong indicators of a supercell that is going to produce dangerous and damaging weather.

First Some Background

 When a supercell storm develops, the updraft of warm air will rise very rapidly into the troposphere but when the bubble of warm air passes into the stratosphere things suddenly change. That bubble of buoyant air now finds itself in an environment where the air starts getting warmer as you go up. This is the opposite of the troposphere, and it’s why you don’t see clouds last long in the stratosphere. It’s a harsh environment and clouds that find themselves there are living on borrowed time!

This view from the International Space Station shows a classic supercell with an anvil and an overshooting top. The anvil marks the tropopause- the boundary between the cloud-friendly troposphere and the cloud desert of the stratosphere.

When a fast-rising updraft of air reaches the stratosphere it will often punch through into the stratosphere producing what we meteorologists call an overshooting top. (see the image). This alone is a very good indicator of a severe storm since you need a very strong updraft to do this. Most storms reach the tropopause and flatten out into the familiar anvil. If you ever wondered why you see these anvils, now you know. The atmosphere above is the stratosphere and the air is warming, so updraft and clouds fall back down as they find themselves colder than the air around them.

AACP Clouds
What has been known for several decades though is that often when an overshooting top develops a cirrus cloud will form downstream of it IN THE STRATOSPHERE and it will linger for a while. Dr Fujita of the Fujita scale fame studied them in the 1980’s, but now with GOES 16 and 17 we can see them in real time and the paper by Bedka et.al shows how important they are to recognize. Interestingly since they are in the warmer stratosphere, these cirrus show up as being warmer than the anvil on IR imagery!

Slide from the presentation at the AMS Conf.on Severe Local Storms. Note the blue AACP indicating it is warmer than the anvil. Normally this would indicate it is lower but it is obviously above it and it’s warmer because it’s in the stratosphere where temperatures begin to rise.

Another thing you may not realize is that many many storms both in the U.S. and around the world happen in areas where radar coverage is very poor, so having a signature that indicates a storm is severe from the satellite, can mean the difference between a warning and no alert at all. Below is the summary slide from the presentation at the AMS Conference on Severe Local Storms.

It’s been my impression for quite a long while that many meteorologists who work in severe weather regions are not using satellite meteorology to its full advantage, and this is particularly true of broadcast meteorologists who rely far too much on the radar. We get a new image from the Doppler radars about every 4-5 minutes, but with GOES 16 we can get an image real time at 30 second to one-minute intervals. Not taking advantage of that seems wrong-headed IMHO.

AACP’s and lighting jumps are two important severe weather signatures that cut false alarm rates and increase lead time. Neither was possible on a hemispheric-wide basis before the new generation of GOES satellites. A big hat tip to the authors of the paper for doing some good science that will lead to better warnings. This post is my effort to get the word out!