Understanding Doppler Weather Radar

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When you turn on the TV to watch the local weather report, most likely you will see the local on-air meteorologist display and explain radar imagery.  You have probably noticed that on days when there is no precipitation falling nearby, the radar does not show a whole lot.  However when there is precipitation nearby, and especially when severe weather is in the area, suddenly the radar is alive with a whole lot of colors and movement.

The radar is a complex tool, and it is vitally important to National Weather Service operations as it is the tool meteorologists use to inform the public of any hazardous weather that is in their path.  In other words, they use the Doppler radar to issue the severe thunderstorm and tornado warnings you see scroll across the bottom of your screen occasionally.

How Doppler Radar Works

The radar transmits pulses of microwave radiation. Part of the energy of each pulse bounces off raindrops, insects, snowflakes, etc. back to the radar.

The image above (courtesy of the National Weather Service) illustrates how the radar emits a pulse of microwave radiation, and how part of the energy from this pulse bounces off a rain drop (or snow flake), back to the radar.  Meteorologists analyze the strength of the returned pulse and the time it took to travel to the rain drop and back.

Base Reflectivity

The most visible and recognizable component to the Doppler radar images you see on TV is the base reflectivity imagery.  These types of images paint the picture of what is occurring nearby.  No matter what the precipitation type is, radar images rely on a lot of different colors to indicate the intensity of the precipitation.  The colors represent the strength of returned energy to the radar expressed in values of decibels (dBZ). The color scale is located at the lower right of each image. As dBZ values increase so does the intensity of the rainfall.

Base Reflectivity image showing precipitation in Oklahoma. Photo courtesy of the National Weather Service.

 

Despite the vast improvements to Doppler radar over the years, it simply cannot read everything.  The reason for this is relatively straightforward. The radar is projected about 0.5° off the ground.  The radar scan “sees” the lower parts of storms when they’re close to the radar and higher parts of storms when they’re further away from the radar (due to Earth’s curvature).  The radar then tilts upward and does another rotation for a higher elevation scan. This process repeats several times, depending on which scanning mode it’s in.

At increasing distance, the radar is viewing higher and higher in storms and the beam may overshoot the most intense parts.  Photo courtesy of the National Weather Service.

In other words, when meteorologists are monitoring storms quite a distance away, the radar beam will miss all the “important” information contained in the lower levels of the storm, such as rotation which could be indicative of a developing tornado (although not all rotation within storms is indicative of a developing tornado.  I will discuss rotation in storms next.)  These limitations of the radar may make you wonder how meteorologists can accurately warn on a storm located quite a distance away.  There is a work around to this issue.  National Weather Service meteorologists have the ability to select an adjacent radar, typically located around 200 miles away, which may offer a more advantageous position to sample storms.  Also trained storm spotters (members of the public who attend special classes each year to learn how to discern various weather phenomena) often act as the “eyes and the ears” of the National Weather Service and relay reports of such weather phenomena.

Velocity

What separated the Doppler radar from previous generation National Weather Service radars is its ability to detect motion.  Although Base Reflectivity is important to diagnose precipitation intensity, this is only half the picture.  Meteorologists rely on velocity imagery to indicate whether there is rotation within a storm, detect strong winds from storms, detecting the speed of frontal boundaries, and even the general motion of winds near the radar.

Base velocity, like Base Reflectivity, provides a picture of the basic wind field from the lowest (0.5°) elevation scan. But to see the wind, there needs to be radar “returns” before the radar can determine the velocity.  Remember, the radar beam elevation increases with increasing distance from the radar. Therefore, the reported value will be for increasing heights above the earth’s surface.

Also, know WHERE the radar is located in the image. The radial velocity colors only has the proper meaning if you know how it is blowing relative to the location of the radar. Outbound winds (red colors) on one radar might be inbound winds (green colors) at an adjacent radar. If the radar cannot determine (called range folding) inbound or outbound then it paints the wind in purple.

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Velocity image from Charleston, WV.  The location of the radar is in the middle, near where the small white circle is located.  The greens indicate inbound winds and the reds indicate outbound winds.  In other words, winds are out of the east.

 

Putting it together

Now that you have a good basic understanding of the Doppler radar, I want to show you how National Weather Service meteorologists use these two radar depictions to warn on a tornado.  Even to an untrained eye, it is clear that something is going on in the images below.  You see a lot of the reds and purples in the left panel, indicating intense rainfall. I’m sure your eye is drawn to the area of green and reds converging together.  This is what is called an inbound/outbound couplet and this is a very strong indicator of strong rotation within a storm.  National Weather Service meteorologists spend a lot of time learning how to operate and interpret radar images and this is about as clear as it gets of indicating a tornado on the ground.  Sure enough, these images were taken from the Tuscaloosa, AL tornado back in 2011.

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A side-by-side view of the Tuscaloosa-Birmingham tornado that occurred on April 27, 2011.  The left panel is the Base Reflectivity panel, and the right panel indicates the winds associated with the storm.  The tight couplet of inbound and outbound winds indicate the location of the tornado.  Photo courtesy of the National Weather Service.

Concluding remarks

Doppler radar is a complex and multi faceted tool meteorologists at the National Weather Service rely on to do their job, which is to save the life and property of the public.  This post just serves as a brief introduction to this tool.  For another perspective on how the Doppler radar works, you can check out another meteorologist’s blog here.

 

 

 

 

What Do Those Weather Terms Mean?

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Have you ever thought of the differences between the terms “partly cloudy” versus “mostly sunny”?  Does one sound more optimistic than the other?  Well in this second installment of a two-part series, I will offer a perspective on the rhetoric used in general weather forecasts.  The first part of this two-part blog series centered on the rhetoric used in severe weather watches and/or warnings.  While it is certainly important to understand what the various terms mean when dangerous weather warnings are issued, having an understanding of basic weather terminology can facilitate the understanding of basic weather messages, including the local forecast.

Sky Condition

When you hear the forecast for your area, you will hear if it will be cloudy, sunny, etc. This describes the predominant sky condition based on the percentage of opaque (not transparent) cloud cover.  Although there is some subjectively in what someone perceives as the character of the sky, the National Weather Service has some general guidelines in describing sky condition.

Cloudy / Overcast  88-100%

Mostly Cloudy / Considerable Cloudiness  70-87%

Partly Sunny / Mostly Cloudy  51-69%

Mostly Sunny / Partly Cloudy  26-50%

Sunny / Mostly Clear  6-25%

Sunny / Clear  0-5%

So as you can see the terms “partly sunny” and “mostly cloudy” are interchangeable.  As are “mostly sunny” and “partly cloudy”.  The choice of words is really dependent on the mood of the weather forecaster, sort of as a seeing the glass as half full thing.  For more enlightenment on this issue, click here for a brief discussion on this topic from another meteorologist.

Precipitation Chances

According to the National Weather Service, the Probability of Precipitation (POP) is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Measurable precipitation is equal to or greater than 0.01”.   Terms such as occasional, intermittent, or periods of are used to describe a precipitation event that has a high probability of occurrence (80% +) but is expected to be of an “on and off” nature.

When precipitation is in the forecast, often you will hear a percentage assigned to it. These percentages represent the probability assigned for the type of precipitation specified.  The rhetoric used is slightly different when stratiform (non-convective) rain and/or snow is expected versus convective precipitation events.  Below are the percentages typically assigned to a precipitation forecast and the rhetoric used with these percentages.

20 percent = slight chance of rain or snow or isolated thunderstorms

30, 40, and 50 percent = chance of rain or snow or scattered thunderstorms

60 and 70 percent = likely rain or snow or numerous thunderstorms

Temperature

Believe it or not, there is a lot of ambiguity in a forecast temperature.  One meteorologist may think a temperature in the lower 40s can include temperatures up to 45 degrees, and another meteorologist may think the lower 40s only includes 41-43 degrees.  Then there is the issue when the terminology “around” is used.  How about the adjectives used to describe the character of the temperature?  Again this is highly subjective, and is also location dependent.  If you are from Chicago and a high temperature of 42 degrees in January sounds like a nice, mild winter day.  However if you are from a warmer climate, New Orleans for example, a high of 42 sounds pretty chilly.  What it boils down to is the perception of “hot”, “cold”, “chilly”, and etc. are location dependent.  Being from the Chicago area, I associate temperatures 90 degrees and above as “hot” (I also think the mid to upper 80s are hot but 90, in my opinion, is the lower threshold for the “hot” adjective to be used.  Not so in Texas.  When I lived in Amarillo, my colleagues thought I was nuts when I expressed that I considered 90 degrees as hot.  They did have a point as I did live there during the hottest summer I ever experienced; 30 days of high temperatures of at least 100 degrees!  Now that is downright, furnace-like hot!

Winds

The last weather terminology I would like to provide some insight into are winds. Although a bit more straightforward than sky cover, precipitation chances, and temperatures, National Weather Service like to append adjectives to wind forecasts. Below are the commonly used adjectives for winds.

0-5 mph Calm / Light / Light & variable

5-15 mph / 10-20 mph (None used)

15-25 mph Breezy (mild weather) Brisk or Blustery (cold weather)

20-30 mph / 25-35 mph Windy

30-40 mph Very windy

40 mph or greater Strong, dangerous, high, damaging (High Wind Warning criteria)

Concluding remarks

Hopefully this second installment of this series of blog posts have provided you with enough information so you can make better sense of the daily weather forecast.  In addition, you may now have a better understanding of why certain adjectives are used in forecasts.  So if someone asks you if you know the difference between “partly sunny” and “mostly cloudy”, you will be able to explain to them what these weather terms mean.