Tuesday, May 6, 2015
|Radar showing hook echo, 9:45 pm CDT, April 26, 2015. Retrieved from Dallas-Fort Worth Scanner Twitter Page.|
|Radar showing supercell with extremely well-defined hook echo heading towards Norman area. Credit: a Facebook thread.|
|Radar image at 5:50 pm CDT showing reformed hook echo heading towards Norman/Moore. The red polygon denotes the area under a tornado warning, the yellow diamond shows the tornado location.|
Many of us have heard the term “Doppler Radar,” but most of the images you see on TV have nothing to do with Doppler Radar. They simply give an approximation of how heavy the precipitation is by measuring the how much of the radar signal broadcast is reflected back to the radar site. Usually, the objects that do the reflecting are different types of precipitation, but they can be many different things. Back here, we sometimes see a radar signal at night even under clear skies due to birds migrating. In fact, as a little aside, let me show you a comparison between two radar images taken this past Sunday. The one at 8:45 pm has no signal (the stuff you see is just random noise), while the one at 09:52, just an hour later, is completely lit up. And it’s all because of birds migrating north for the summer.
|08:45 pm PDT, Sun 03 May 2015. Credit: UW Atmospheric Sciences Online Weather Data Archive|
|09:52 pm PDT, Sun 03 May 2015. Credit: UW Atmospheric Sciences Online Weather Data Archive|
But back to Doppler Radar. All of the NWS radars across the countries are equipped with Doppler capabilities, meaning they can make use of the Doppler effect to find out the velocities of particles in the atmosphere and whether they are moving towards or away the radar. The Doppler Effect refers to how an object’s apparent frequency to an observer is higher while approaching and lower while receding than its actual frequency. We’ve all experienced this with trains and ice cream trucks… as soon as they pass by, the pitch lowers. Doppler radars make use of this same effect, and are able to not only decipher whether an object is approaching or receding, but how fast it is moving. They make use of pulse-Doppler techniques, which are quite complicated and will be discussed in another blog.
In any event, the radar below shows a clear hook echo signature. Not only that; the velocities are extremely high. The maximum negative velocity (i.e. the max speed of the particles traveling away from the radar at Norman) is 124 knots, while the maximum positive velocity (the max speed of the particles towards the Norman radar) is 146.2 knots. These are incredibly high values, and are indicative of an extremely strong mesocyclone and tornado.
|Doppler radar image taken around 6:05 pm CDT.|
Other tornado warnings are continuing to pop up around the region, and I have a feeling we’ll continue to see more tornadoes throughout the night. The latest warning from the NWS is for a “large and extremely dangerous tornado” near Lake Chickasha, which is around 30 miles west of Norman. If you have any friends or family in that region, alert them now. The entire region from northern Nebraska southward to central Texas is under a tornado watch until 9 pm and storms are continuing to fire up across the region. Let’s hope that none of them hit any populated areas.