Day 146: Thermal Couplet on Infrared Satellite

Posted by pmarsh on 26 May 2010 | 2 comments

Today was day 26 of VORTEX II and the armada operated in the suburbs of Denver in northeast Colorado. Before the armada could even leave the hotel, a very strong thunderstorm developed in the vicinity of Denver International Airport and produced a brief tornado. The armada scrambled southeast toward the storm as quickly as they could and were able to get an extended deployment on this very slow moving thunderstorm. Unfortunately for VORTEX II (yet fortunately for the residents of northeast Colorado) the thunderstorm appears to have been unable to produce a second tornado. In any event, this will once again be an interesting dataset to analyze!

As previously mentioned, the targeted thunderstorm developed very near the Denver International Airport. The image above is a infrared satellite image of the thunderstorm taken very near the time of the tornado. Without going into all the details, an infrared satellite image is essentially displaying the temperature of the cloud top. In the image above the dark reds indicate the coldest temperatures and the grays indicate the warmer temperatures. A colder temperature means the cloud is higher in the atmosphere than a warmer temperature.

Overlaid on the satellite image are surface reports from various official weather stations in northeast Colorado. The observation that is important is the one overlaid on the thunderstorm as it is the Denver International Airport observation. The symbols to the immediate left of the “KDEN” text are the observed “weather” ocuring at the time the observation was made. One of the two weather symbols is what appears to be a funny looking “R”. This is the symbol for thunderstorm. The observation on the far left – the one that looks like “)(” is pretty unique – it is the weather symbol for “funnel cloud”. So in other words, at the time of this report, the weather observer was able to see a funnel cloud with the thunderstorm near the airport. Amazing!

That would make for an interesting blog post in and of itself, but there is something else unique in this image as well. A “thermal couplet” is displayed. In the image below, I’ve removed the observation to give you a clear look at the infrared image. Can you identify anything that might be considered a “thermal couplet”?

In case you were unable to identify the thermal couplet above, I’ve circled it below. In words, the thermal couplet in infrared satellite imagery is identified as an area of colder cloud tops located next to warmer cloud tops. In this particular set of images, the dark reds located to the southwest of the oranges.

So what is going on here, and why is it important? In simplest form, the answer is “warm air rises and cold air sinks”. The longer answer is a bit more complex. To understand, we need to briefly discuss the physics of thunderstorms.

The thermal couplet is the direct result of one of the two key components of a thunderstorm – the updraft. In the updraft warm, moist air from the surface is rapidly transported upward. As the air in the updraft rises, it expands and cools slightly, however, as long as the air in the updraft is warmer than the air immediately surrounding the updraft, the updraft will continue to rise. At some point the air surrounding the updraft will become warmer than the updraft itself causing the air in the updraft to fall back toward the ground. However, so much momentum will have built up in the updraft that it cannot stop immediately. Part of the updraft will continue to rise into the colder surrounding air before falling back into the main portion of the thunderstorm. This is known as an “overshooting top”. As the air falls back toward the main portion of the thunderstorm it warms slightly.

A good way to visualize this process is to put a water hose under the surface of a pool. If you point the hose toward the surface of the pool and then turn it on you’ll notice that bubbling effect of the surface of the pool. Namely, a hill in the water tends to develop on the surface in the direction of where the hose is pointed. This hill develops because the water streaming out of the hose can’t stop when it hits the surface of the pool. It rises out of the pool and then falls back into the water. This is essentially what is happening in an updraft.

So how does this tie into the image satellite image and create a thermal couplet?

The dark colored area circled above is the cold air associated with the overshooting top. When these dark spots begin appearing in the infrared satellite imagery, a meteorologists knows that an extremely strong updraft is present. This is a necessary ingredient for extremely strong thunderstorms.

The lighter colored area circled below is the relatively warmer air caused by the air falling back to the main part of the thunderstorm. It isn’t as concentrated as the updraft because as the air falls back toward the thunderstorm it is blown downstream by the strong winds located in the upper troposphere and lower stratosphere.

A thermal couplet is an indicator that a thunderstorm is intensifying or already extremely intense. The report of a funnel cloud by the weather observer at the Denver International Airport at the same time as this image just acts to confirm what we were able determine from looking only at satellite imagery!