Earlier this week I had the opportunity to talk with a local meteorologist, Katie Dupree, from northern Michigan’s 9&10 News. Through Twitter, I asked her a few questions about how to understand radar. Honestly, I really only knew that green indicated rain, orange indicated heavy rain, and red meant severe weather. She kindly replied with a few awesome tips on how to read radar data. Then it hit me. This information could really help the storm chasing photographers out there. I sent a message to Katie asking if she could write up an article detailing how to get a better understanding of radar. She accepted my request and completely went above and beyond my expectations. I really cannot thank her enough for writing this.
*Note that Katie refers to the RadarScope weather application in this article, however, all radar information in this post still applies to all radar other maps.
So without further ado, Understanding Radar by Katie Dupree:
“Once you’re done reading this, you’ll learn all you need to know about radar (which took me a whole semester in college to learn!). You’ll be a pro, and not only be able to better interpret what you’re looking at, but to know where you might go to take awesome storm pics!
First of all, a general overview of radar values.
NEXRAD (Next Generation Radar) radars work by bouncing radio waves off of particles in the air. These particles can be what we normally expect to show up on radar—rain, snow, sleet, hail, etc. to even bugs and dust (Yes, bugs and dust!). Flocks of birds can be picked up too. The amount of energy that returns back to the radar site after bouncing off this stuff is called “reflectivity”, the variable “Z”. Since reflectivity values can widely vary, it uses a logarithmic scale that is measured in decibels, or dB. So, the “dBZ” values that you see on the radar are essentially a measure of the decibels of reflectivity.
The dBZ values increase as the strength of the signal return of the radar increases. The scale of the dBZ values is related to intensity of rainfall. However, it is important to remember that radar technically only shows areas of returned energy, and not necessarily precipitation. So, the presence of a very weak return below 20dBZ, doesn’t always mean it’s raining. This could be what we call radar “clutter” (dust/bugs), or even virga (rain that is being picked up on radar and evaporating before it makes it to the ground).
The colors correspond to precipitation types and intensities, but again remember—it’s a measure of returned energy—so it can’t technically distinguish different types of precipitation with absolute certainty. However, we can use this as a general guide for different precip types:
10 dBZ (green) – Very light rain or light snow
20 dBZ (green) – Light rain or moderate to heavy snow
30 dBZ (yellow) – Moderate rain or sleet showers
40 dBZ (orange) – Moderate to heavy rain or sleet showers
50 dBZ (red) – Heavy thunderstorms
60 dBZ (pink) – Intense to severe thunderstorms with hail
*If you see values less than 5 dBZ, the radar is operating in clear air mode, which means that it’s likely not raining. This is a different scale used in fair weather to look at things in the atmosphere that are really small (bugs, dust, etc.), that pick up much weaker returns. Clear air mode MAY be turned on in instances of very light rain or snow events, that might not necessarily show up on the regular reflectivity scale. It also gives meteorologists an idea of things like cold fronts and subtle airmass boundaries. I am not afraid to admit that I don’t know how to detect these on radar, and not many meteorologists that I know (on TV anyway) do either. It’s generally studied by those that work in the National Weather Service. So, clear air mode is something that for the means of what you’re trying to do, is not entirely important. Another way to tell that the radar is in clear air mode is to see that the scale has changed from 5 to 75 dBZ for regular reflectivity, to -28 to 28 dBZ. If the radar is in clear air mode, you may see what looks like rain in a circular pattern about 25mi radius from the radar, but it’s usually just ground clutter. Here’s a picture of clear air mode that shows ground clutter, and not rain:
*One thing to remember with all of these products: reflectivity, velocity, and rainfall rates are indicating what has already happened, not what is happening (since it takes a while for the volume scan), or what is going to happen.
So now you may be wondering—ok, so I think I get radar, but what’s this tilt thing? Well. Tilts work at different levels. The radar site operates in somewhat of a cone shape. The lower the tilt angle, the farther distance the radar sweep reaches. The higher the sweep, the shorter the distance that the radar can detect, and the higher up into the atmosphere the sweep reaches. The first tilt is 0.5 degrees from the horizon and sweeps in a circle. The second tilt is 1.5 degrees, the third tilt 2.4 degrees, and the last tilt is 3.4 degrees. There are a lot more, but the first 4 are the most important, and RadarScope doesn’t go past 4, since the sweep moves in a cone shaped circular pattern, once the radar reaches the end of the beam, it is quite far away, and much higher up.
**It is important to remember—since the radar sweep moves in a circular motion, it can not (no matter the tilt angle) see directly above the radar. So, a storm that is directly overhead will look significantly less outstanding on radar.
Here is a picture that I found online that shows you better idea of how radar tilts work: http://blogs.agu.org/wildwildscience/files/2011/03/vcp11.gif
Looking at different tilts is a good indicator of determining vertical storm structure. Looking at the highest tilt will show you where the very highest part of the storm is located. If it is located say, to the west of where the highest value on the lowest tilt is, you can determine that the storm likely has a back sheared anvil. Back sheared anvils are created when the general anvil of a cumulonimbus cloud is spread upwind. When this happens, it generally indicates a VERY strong updraft, since it is formed against the flow aloft (something you may want to take a picture of!).
Now that you know about reflectivity, we can talk about the different radar products.
By reading the above section, you know what base reflectivity is. It merely takes one of the tilts and measures reflectivity. This is the normal radar that you see online, on RadarScope, and on TV. The first tilt is the most common, and the most important because it looks closest to the ground and gives us the best idea of what’s actually hitting the ground.
Composite reflectivity is the same as base reflectivity, except that it is a composite. It combines the highest values of reflectivity from each tilt level, all on one image. This can be good and bad. Composite reflectivity, since it combines the highest values of all tilts, sometimes overestimate dBZ values because different sweeps may overlap what is being seen—AND it usually shows a lot of virga because high tilt scans indicate rain. Because composite reflectivity combines information from all tilts, it is the last product to be produced during a volume scan. Storm chasers go with base reflectivity, because they only have to wait for the radar scan of 1 particular level, instead of 4. Waiting for 4 can take several minutes, and with severe weather, you usually don’t have a matter of minutes.
Base Reflectivity 248 nmi
The 248 nautical mile base reflectivity product offers 2x the range of the standard radar image, but at only half the pixel resolution. In my opinion, since you can change between radars on RadarScope, just use regular base reflectivity and if you need, switch between radars, that way you get the best resolution possible.
Base velocity looks a WHOLE lot different than radar, and may seem a little overwhelming. But..fear not! Most of the stuff on there is not all that important.
Base velocity indicates storm motion toward or away from the radar measured in knots, so it is KEY to know exactly where the radar is located. One knot is one nautical mile per hour, which translates to about 1.15mph. The velocity products determine how fast the particles are moving relative to the radar itself (which you may have guessed is 0…because the radar doesn’t move! XD). Negative values are green, and they indicate motion TOWARD the radar. I don’t know the idiot that came up with (-) is TOWARD, but, there ya go. As you may have guessed, positive values are red, and indicate motion AWAY from the radar. Now…why is this important? Well, for many reasons. But the main reason we look at base velocity is to give us a clue of where rotation is occurring, and that may give us a hint as to where a tornado is (or could likely form).
Since we’re looking at values toward and away from the radar, very bright reds next to very bright greens usually mean rotation.
*One thing to note is that the radar can only detect the component of the velocity vector along the radar beam, so it’s only giving us a slice of the wind field…not the whole thing.
*Just like the regular reflectivity tilts, tilts are the same here.
*****AND just as we can’t see directly above the radar, we can’t see the ground either. In fact, the lowest we can see is at the site of the radar, which could be a hundred feet off the ground. This is why detecting tornadoes is so hard…we don’t have a clear picture of what’s actually on the ground!
Since we can’t see rotation at the ground, using the tilts is quite helpful because if you see bright reds and bright greens right next to each other on all of the tilts, that suggests that the rotation is quite deep through the atmosphere, and therefore a good indication that there’s a tornado on the ground, or very well could be soon.
Here’s a picture of some good rotation using base velocity:
In the real RadarScope app on your iphone, you’ll be able to run your finger over the color table to see the actual speed in knots that the inbound and outbound winds are going.
Storm Relative Velocity
This one is a bit harder to understand. Heck, I even struggle with this one sometimes. Storm relative velocity is simply (or not) base velocity with the average storm motion subtracted out. When storms are moving quickly, it’s easier to spot green/red velocity couplets that are indicative of rotation, which may be masked out by the motion of the storm. Just like base velocity, green is toward, red is away. Below is a SRV image from the same time as above.
You can see that factoring out the overall motion of the storm makes the area of rotation stand out more clearly. Here, they look pretty much the same because the rotation is strong, but in weaker instances, you may not be able to see evident rotation on BV.
This is a pretty nice product to see where the heaviest rain has fallen. It takes the dBZ values, and uses a relationship between the reflectivity and rainfall rate to estimate rainfall. It is usually a pretty good idea of rain fallen, but shouldn’t be used for exact reports as it’s not completely 100% accurate. Also, since snow can be of different densities, it is NEVER an accurate indicator of snow. The general rule of thumb is that 12 inches of snow melts to 1 inch of rain, liquid equivalent. Some very fluffy snows could be 20 inches of snow melts to 1 inch of rain, while some really heavy crusty snow could be like 8 inches snow (or less) to 1 inch rain.
Range folding is something that can happen on both velocity and reflectivity products, and it’s nice to be aware of it because it can messy our data a bit. The radar sends out a pulse, and waits for the return echo. If the pulse goes farther than the span of a general radar (~143 miles), hits something and bounces back (though weakly), it will return much later than it should, and in some instances right after the next pulse is sent out. This confuses the radar and causes it to think that whatever it’s detecting far away is really close to the radar. This is the way it would work. Suppose the maximum range is 140 miles. If a storm is located at 160 miles from the radar, the radar will detect the storm as being 20 miles from the radar instead. Any energy returned to radar beyond 140 miles would be range folded. This occurs because the radar energy bounced off the distant storm is returning to the radar after the radar has already sent out another pulse and is waiting for the return pulse. To some extent, radar software can detect when this happens and exclude the data, but in others, it does not. In this case, the pixels show up purple. So now you know what purple is!!—range folded (bad) data.
Pictures, Pictures, Pictures!:
Thunderheads/cumulonimbus will almost always be your most photogenic weather pictures (besides an actual tornado) unless you encounter a supercell, which is like the holy grail! A supercell is a very large thunderstorm, classified as a supercell because it has a rotating updraft (the most severe!). Supercells are generally the storms that spawn tornadoes (even though they often don’t). To read more about supercells: http://en.wikipedia.org/wiki/Supercell. The best supercell to photograph is an LP supercell (low precipitation), versus HP (high precipitation). As you may have guessed, high precipitation supercells have a lot of rain associated with them, which makes them hard to see (not very picture worthy). Another good reason to stay away from HP supercells is that since you can’t see…it is SO hard to see if there’s a tornado. Even experienced storm chasers fear HP supercells because tornadoes can sneak up on them. So, for general purposes, stay with the LP supercells for better picture quality and your safety!
Now…the bad news… living in Michigan puts a serious damper on the amount of supercells that we get. While they can happen in all 50 states, they are not all that common. So, you may have to only encounter general storms—which can be just as severe!
**It may be in your interest to check out the SPC if you’re really set on trying to get pictures of storms. They put out convective outlooks that will help you decide if it’s possible for severe storms to pop up: http://www.spc.noaa.gov/products/outlook/day1otlk.html. FYI- I tend to think that the terminology that the SPC uses underestimates the risk. I have seen MANY occasions where we’re in the “slight” risk for severe weather, and we’ve just gotten slammed, so to see anything moderate or above is generally a good indicator that severe storms are likely.
As for the safety aspect: Common sense is a MUST. Don’t let your interest in getting a good shot put you in danger. Even professional storm chasers that technically know what they’re doing have gotten burned. Obviously, the general rule of thumb: chase the storm…don’t let it chase you! It’s ALWAYS a good idea to have mobile radar (like RadarScope on your iphone), and a compass. That way you can infer the direction that the storm is traveling, and use your compass to make sure you stay on its good side. The best website I’ve found that will help to explain staying safe while storm chasing is here : http://www.stormtrack.org/library/faq/. The “Safety” section is about half way down the page.
Now that you know all about radar and how to stay safe, go take those pics!!!
Sources for this blog:
http://www.basevelocity.com/RadarScope/support/products.php <<Pics and a lot of this information from this site”
Katie Dupree joined the Doppler 9&10 Weather Team in January 2011. Before she joined 9&10 News, Katie interned at both WDIV in Detroit, and with the Doppler 9&10 Weather team. Katie graduated with a Bachelor of Science in Meteorology from Central Michigan University. She is also a member of the American Meteorological Society and the National Weather Association.
Katie is originally from the Detroit area, and is excited to be working in her home state at Northern Michigan’s News Leader. Her interest in weather came from her fascination with storms, and faithfully watching Detroit’s meteorologists Chuck Gaidica and Rich Luterman while growing up. (From 9&10 News Personnel Bios)