Let's start with some basics. The basic structure of a thunderstorm consists of two parts--warm, moist air rising in an updraft, and cool, rain-laden air sinking in a downdraft.
The kinds of thunderstorms that most often produce long-lived or strong tornadoes are called "supercells". At the heart of a supercell storm is a rotating column of air called the mesocyclone. Just because this rotating column of air is present doesn't mean that the storm is producing or will produce a tornado--all supercells have a rotating mesocyclone, but only some produce a tornado. The presence of rotation in the middle of a storm will somewhat twist and turn how the updraft and downdraft actually look. Let's switch to an idealized block diagram looking down from the top of a storm. The warmer, moister air is usually to the south or southeast of the storm, so that's where the storm starts drawing in its moisture. Because there is a rotating core in supercells, it can draw in warm moist air somewhat laterally instead of just vertically. This air that is drawn in is called the "inflow".
However, because the center of the storm is rotating, some of the downdraft air can get wrapped around the back side of the storm as well. Thus, in most supercells, we see an intense, though narrow band of rain wrapped around on the back side of the mesocyclone. This part of the downdraft is usually called the "rear flank downdraft". Here's a block diagram of the two paths I just metioned:
So what would our resulting radar reflectivity image look like for this kind of situation. We expect to see lots of returns out ahead of the storm because that's where most of the rain-filled downdraft air is going. There would also probably be a weaker area of returns (or no returns) where the inflow region happened to be, since that warm, buoyant and rising and rain is probably not falling through that area. Then we also expect a narrow band of radar reflectivity returns behind the mesocyclone corresponding to that portion of the downdraft that gets wrapped around by the rotating air. The light green outline here shows generally what we'd expect to see on radar:
So what does this look like on an actual radar reflectivity image? Here's a picture of a radar reflectivity image from a supercell thunderstorm from this afternoon in North Carolina that was producing a tornado at the time:
|Radar 0.5 degree Base Reflectivity image from KRAX, 2040Z, April 16, 2011.|
So that's all and good for reflectivity images--we can recognize these kinds of structures that show us what rotation is doing to the environment. But what about seeing rotation itself--looking at the wind velocities? Our Doppler radars can do this, and that's a big reason the Doppler capability was added to the radars. They do have some limitations, though. But first...let's get back to concepts.
We have an area of strong rotation in the mesocyclone. Winds there are probably much faster than they are elsewhere, and they also change speed and direction over a very small spatial area. Our Doppler radars can measure wind speeds, but only in two directions--air moving toward the radar and air moving away from the radar. The radar measures wind speed by looking at phase differences caused by moving targets along the radar beam. However, things moving across the radar beam perpendicular to it do not cause phase shifts along the beam. So, we can actually only measure wind in two directions--towards and away. Turns out that's all we really need.
Let's go back to the mesocyclone diagram and insert a theoretical radar location. Since the radar measures winds moving toward and away from it, the radar's location is going to dictate what the return looks like. In the image below I've randomly added a radar located to the southeast of the storm.
As the radar looks toward the storm, its beam follows a path similar to the dotted line. You can see that to the right of the dotted line, winds in the mesocyclone (which is rotating counter-clockwise here as most mesocyclones do...but that's a topic for another blog...) are generally moving away from the radar. But, to the left of the dotted line, winds are generally moving toward the radar. We can assume that if there's a compact space where winds suddenly shift from moving away from the radar to moving toward the radar, there's probably something rotating there. Unfortunately, we just can't tell what's going on when winds are blowing perpendicular to the radar beam (unless you use another radar located at another location, for instance). But, in general, anything rotating should have one side strongly blowing away from the radar and the other side strongly blowing toward the radar.
What will this look like on a radar velocity image? Usually, warm colors like reds and oranges are used for winds blowing away from the radar. Cool colors like greens and blues are used for winds blowing toward the radar. If we assume that the rotation is very concentrated in the mesocyclone, we'd then expect the radar velocity return to look something like this:
What does this look like in real life? Here's the base velocity image corresponding to the radar reflectivity image of the storm in North Carolina that I showed above.
|Radar 0.5 degree Base Velocity image from KRAX, 2040Z, April 16, 2011.|
So those are the basics. Now let's look at a couple more examples, just for practice. Here's a reflectivity image of the powerful supercell and tornado that did lots of damage right as it moved over downtown Raleigh, North Carolina.
|Radar 0.5 degree Base Reflectivity image from KRAX, 1940Z, April 16, 2011.|
|Radar 0.5 degree Base Velocity image from KRAX, 1940Z, April 16, 2011.|
Of course, you'll notice on these velocity images that there are a lot of places where green and red velocity colors are right next to each other--and obviously they don't all mean rotation. This is why I also discussed the reflectivity images--it's best to use the reflectivity images to know where to look for rotation before looking at the velocity images to verify your suspicions. If you just use the velocity images, you can sometimes get rather lost.
But, then again, sometimes the reflectivity images don't tell you everything and don't show the clear structures we saw in today's North Carolina storms. For example, here's another tornadic supercell in Oklahoma two days ago in reflectivity:
|Radar 0.5 degree Base Reflectivity image from KTLX, 202Z, April 15, 2011.|
|Radar 0.5 degree Base Velocity image from KTLX, 202Z, April 15, 2011..|
As I said before, often it can be somewhat difficult to really tell what is rotation and what's not by just looking at the velocity image alone Often, as we saw above, the reflectivity image may not be very much help either. In these cases, you just have to investigate and be persistent--check higher tilts of the radar to see if there's clearer signs of rotation aloft, look at a loop of images to see any suspicious signs in the recent past, and so on. However, the velocity couplets for strong storms will usually stand out. Just as an exercise, here's a wider view of the reflectivity images of the storms in eastern Oklahoma on Thursday evening:
|Radar 0.5 degree Base Reflectivity image from KSRX, 231Z, April 15, 2011.|
Here's the raw velocity image for that same time:
|Radar 0.5 degree Base Velocity image from KSRX, 231Z, April 15, 2011.|