Hurricane Sandy and her merger with a strong autumn storm system are making history along the U.S. eastern seaboard. But for a time earlier in her life, Sandy provided a bit of mystery to forecasters – showing why what you see in a satellite picture is not always what you get at the ground.
Shown below are three infrared images of Sandy as she was approaching Cuba from October 24-25.
In the absence of observations, meteorologists perform the Dvorak technique to determine the maximum wind strength, or intensity; Cyclone Center uses a modified version of this technique to analyze historical tropical cyclones. The expert who put these images together said that he would assign a minimum intensity of 115 kt. for all three of these times. That would have made Sandy a Category-4 hurricane on the Saffir-Simpson scale, capable of catastrophic damage. An automated Dvorak technique produced a similar intensity, and official intensity estimates from the U.S. National Hurricane Center and the U.S. Satellite Analysis Branch were also over 100 kt.
That may have been the end of the story if it were not for one key piece of additional information – data from “Hurricane Hunter” aircraft that were sampling the storm at the same times these images were taken. They determined that the surface winds were about 75-80 kt, at least 20 kt. lower than the Dvorak estimates. So what’s going on here?
This instance illustrates some of the challenges that forecasters and analysts have when trying to determine the strongest winds in a tropical cyclone. In cases where a tropical cyclone intensifies rapidly, as here, the cloud pattern typically leads the surface wind increase. So an analyst using the Dvorak technique may get an instantaneous wind value that may be much higher than the actual surface wind speed (which hasn’t had time to increase yet). Because of this, the Dvorak technique takes into account the storm’s recent intensity and does not allow storm to “jump” too high from one time to another.
Even when we have aircraft data, it is impossible for 1 or 2 planes to sample the entire storm. So it is quite likely that the point in the eyewall with the maximum winds does not get observed, especially in cases when the wind field is changing rapidly.
In Sandy’s case – a dangerous tropical cyclone close to populated areas – observations from inside the storm have provided forecasters with a pretty good idea about the wind speeds. But imagine a storm like Sandy swirling out in the middle of the Pacific, thousands of kilometers from civilization, with only satellite pictures and data available for estimating her strength. It is pretty easy to see how we don’t always get the intensity right. In fact, we don’t even know what the “right” intensity is! But from a scientific perspective, these storms are just as important as the ones that ravage our coastlines. By having an accurate account of their strength, we may, for instance, be able to determine how tropical cyclones worldwide have been reacting to our changing climate.
And that is the whole point of Cyclone Center – to have all of you provide us with your analysis of storms, so that we can determine not only the “best” intensity, but also get an idea about how certain we can be about it.