There are going to be a lot of media articles in the next week about why Hurricane Isaac's strength and track couldn't be forecast better -- and comparisons between the technology during Katrina and today (such as this one). Rather than concentrate on the negative, I want to show you some of the amazing technology that has helped us forecast this storm.
In 2005, high-resolution computer models like the HRRR did not exist. Today, we can get a snapshot of dozens of weather variables every 15 minutes out about 15 hours via this model. The screenshots presented here show some of these variables and explain what they mean. It's a lesson in meteorology, but also (I think) art. Forecast models are finally high resolution enough to show storms as the human eye sees them -- and even better.
The first screenshot above is of a "simulated radar" of Composite Reflectivity. Here, the model is guessing what the radar would look like at the future forecast hour. Not surprisingly, Isaac looks like a hurricane. I've blogged before about how helpful this can be in planning for storm chasing. This next graphic is not quite as easy to explain:
Ensemble Composite Reflectivity: This map takes the last three model "runs" of that simulated radar and overlays them. The idea (while a little hard to see with a hurricane) is that you can tell whether the model is trending slower or faster with the movement of the storm.
Vertically Integrated Liquid: This next map shows a weather variable which assumes that you took a "column" of air from the ground up to the top of the atmosphere. How much moisture would it contain? You'd think hurricanes would rank high here, but actually hail storms have the strongest VIL signatures -- sometimes over 80 on the scale. Because hurricanes don't contain hail, Isaac only ranks in the middle of the scale -- and that only because of his heavy rain in the inner bands.
500mb Absolute Vorticity & Height: The image above shows the vorticity, or, spin in the atmosphere at about 18,000 feet. Believe it or not, meteorologists use these upper-air forecast maps, not surface "frontal" maps, to forecast the movement of high and low pressure systems. Vorticity is used to find where storms are and (not surprisingly) Isaac is off the charts, but so are low pressure systems and thunderstorms. The circles indicate points of equal "height" of the 500 millibar pressure layer (in meteorology, weather maps are not produced at a certain height, but on a level of equal pressure). Even though it's high up in the atmosphere, Isaac shows a significant "lowering in heights" as low pressure systems usually do.
700mb Vertical Velocity & Height: The map above shows the heights of a higher (closer to the Earth) pressure level, but the colors indicate "vertical velocity." The higher the air can be buoyed, the stronger the storms. In the case of hurricanes, the highest VV is inside the highest thunderstorms, in the bands of the storm (not in the eye).
Max Updraft Velocity: This is a more specific way of looking at the lift in the atmosphere; it is an attempt to measure the updrafts in thunderstorms that would lead to large hail and dangerous storms. Again, this signature is strongest in the bands of the storms where the large thunderstorms are.
200mb Winds: This is not a particularly exciting-looking image, but I put it here to point out that hurricanes have a large effect on the entire atmosphere above them. Here at the 200 millibar level (about 40,000 feet, aka the jet stream level) a significant increase in winds is seen. If you noticed that the wind barbs are showing a clockwise circulation, you probably wondered why. At the surface, low pressure systems rotate counter-clockwise in the Northern Hemisphere, but above them, the air vents clockwise.
Outgoing Longwave Radiation Flux, Top of Atmosphere: What this colorful image attempts to do is predict the amount of radiation coming from the tops of clouds -- simulating an Infrared Satellite Image, so what you're looking at is essentially a simulated IR satellite image. Neat, huh?
Surface 3-Hour Pressure Change: This graphic is one of the few maps that doesn't look like a hurricane, because it's not measuring a weather variable -- but rather a change in pressure, over a three-hour period. Since the storm is moving northwest, atmospheric pressure is falling ahead of the storm, and rising behind it. The change is off the charts because atmospheric pressure doesn't normally change that much.
Precipitable Water: Similar to Vertically Integrated Liquid, but not radar-based and measured in different units, this map shows how much rain would fall at any point if you could precipitate all moisture out of the atmosphere in a vertical column. There's nothing wetter than a hurricane, and Isaac proves this with colors at the far end of the scale.
Aviation Flight Rules: This funny-looking graphic attempts to break down the "flight rules" for aircraft, into either Visual, Marginal Visual, or Instrument, answering the question which is "How can I see out my windshield?" Of course, in a hurricane you wouldn't be able to see anything out your window, so it gets filed under "Instrument." It's much higher-resolution than the Flight Rules graphics we had in the 1990s, which were just one dot for each airport, based on conditions there.
Echo Tops: Another aviation-related, radar-based product, this map shows at what height above the Earth's surface the tops of the storms are. Most of the storm's cloud tops are around 34,000 feet, but the highest thunderstorms in the eyewall spike to above 46,000. In the eye, of course, there are no heights because it is clear.
Cloud Ceiling: This is essentially the opposite of echo tops -- how high is the "ceiling" or cloud bottoms? This is important for airline pilots as well. Most of Hurricane Isaac shows up as very low clouds, as you would see from the ground. In this image, there are some low clouds, even in the eye at this time.
Surface Convective Available Potential Energy: And last but not least, a measure of the instability in the atmosphere which is used to predict thunderstorm outbreaks (the CAPE needs to be >2,000 for storms). The most interesting thing about this graphic is that the instability gets wrapped around the storm with the wind and rain. Because CAPE measures thunderstorm potential, it is not high in Isaac.
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