The Dangers of Downbursts from Pulse Severe Thunderstorms

On Saturday, August 16, 2014, the Storm Prediction Center put all of North Texas into it's lowest probability category for severe weather. 

Our area didn't even make it into a SEE TEXT region, yet with our moderately unstable air and weak wind shear environment, conditions were favorable for pulse thunderstorms which were capable of producing a phenomenon called DOWNBURSTS.

A pulse thunderstorm is a single-cell thunderstorm forms and dissipates relatively quickly and typically doesn't yield any severe weather.  However, some pulse storms grow fast enough and drop enough rain to produce a downburst.  A downburst occurs when rain-cooled air falling out of a thunderstorm hits the ground and begins to spread out in all directions.  This expansion of winds (called outflow winds) can sometimes reach 100 MPH and cause quite a bit of damage!  The easiest way to visualize this is to imagine pouring water out of a glass and onto the ground.  As the water slams into the ground, it immediately fans out in all directions, pushing air in front of it.  Is this forward motion that can drive winds as far as 50 miles away from the original storm.  Here is a good visual representation, shown in cross-section. (h/t Wikipedia)

There are actually 2 varieties of downbursts - wet and dry.  A wet downburst happens when rain falling out of the thunderstorm reaches the ground.  A dry downburst happens when rain falling from a storm evaporates before hitting the ground.  In either case, it's the outward-spreading winds from this cooler, sinking air that is responsible for causing damage.  Downbursts smaller than 2.5 miles in diameter are called micro downbursts or microbursts.  Downbursts greater than 2.5 miles in diameter are called macro downbursts or macrobursts.

Let's examine a real-world example of downbursts.  This example begins northwest of Fort Worth, TX.  At 3:02 PM, the National Weather Service issued a severe thunderstorm warning for northwest Tarrant and northeast Parker county.

 

Although the radar returns look unimpressive, there were some interesting things that were about to happen.  One of the dangers with downbursts and the storms associated with them is the speed in which they form, drop their rain-cooled air, and eventually collapse.  The following 5 snapshots of radar reflectivity and radar velocity start at 3:12 PM (about 10 minutes after the initial warning was issued) and ends at 3:31 PM - a span of only 19 minutes!! 

 

TIME 3:12 PM

The first thunderstorm core is in the process of growing. 

 

 

 

TIME 3:17 PM

The first thunderstorm core is still in the process of growing, while a new core is forming to the northwest.

 

TIME 3:22 PM

The first thunderstorm core has matured fully and has begun to produce a downburst (visible on the right side of the image).  The velocity view clearly shows that winds directly underneath the thunderstorm are hitting the ground and are spreading out.  The red colors indicate winds that are blowing away from the radar, while the green colors show winds blowing toward the radar.  The second thunderstorm core continues to grow.

 

 

 

TIME 3:27 PM

The first thunderstorm core is now in the dissipating (or dying) stage.  Reflectivity values (intensity of rain) has dramatically dropped off.  The outflow winds from the original downburst have spread further and further apart.  The second thunderstorm core continues to build up.

 

 

 

TIME 3:31 PM

The first thunderstorm core is now simply an area of heavy rain, while the second thunderstorm core has finally reached its mature stage.  It too has begin to form a downburst beneath itself as indicated by the diverging velocity signatures.

 

 

Let me reiterate that these 2 thunderstorm cores formed, matured, created downbursts, and began to dissipate in about 30 minutes total.  When damage can occur from these storms in such a small amount of time, it's crucial to be aware of the weather in your area.  Boaters and others enjoying open waters are likelier to get caught off guard as the there is little in the way to shield outflow winds that blow across these bodies of water.  Luckily, the downbursts in northwest Tarrant county did not produce any damage to my knowledge.  However, numerous power outages along with downed power lines and trees were reported just north of the downtown Dallas area due so similar conditions.

 

-Andrew

Doppler Radar and Non-Meteorological Targets

One of the cool side benefits of the WSR-88D radars is the ability to detect non-meteorological targets.  Smoke from wild fires, flocks of birds, swarms on insects and even debris from tornadoes can all be seen on radar given the right circumstances.  On Saturday, August 2, 2014, the kids had gotten up extra early, presumably from the excitement of leaving for vacation the next day.  When I sat down at the kitchen table, I noticed that my radar program was still on from the night before.  Gulping down my morning coffee in an attempt to wake up, I noticed something pretty cool.  Here is a time-lapse sequence of what I saw....



See those expanding rings in northeast Dallas and down by Waco?  These are known as Roost Rings.  These rings are actually thousands of birds leaving their nests in their morning hunt for insects.

One way that we can tell that these echoes are biological in nature and not some kind of strange precipitation is by using something called Correlation Coefficient (CC).  This radar product looks at how diverse the radar returns are.  If the values are homogeneous, the higher the CC value will be.  The more values are different, the lower the CC value will be. 

As an example, look at this:

This is a great example of normal precipitation.  The radar beam sees the rain as similar sized drops (more or less) and as a result, the CC values show up with warm colors (reds and oranges). 

Now, look at the roost rings:

The roost rings have very low CC values - which tells us that the objects the radar is seeing are not uniform in size and shape and, more than likely, biological in nature.

Although these roost rings are not uncommon, you usually only see them during the morning hours.  Why is that?  Well, atmospheric conditions play a large role.  During the mornings, there is usually something called a temperature inversion present.  This means that the temperature near the ground is cooler than the air just above it.  As a result, the radar beam gets bent downward (an effect known as superrefraction), making objects near the surface more visible..