DEPARTMENT OF GEOSCIENCES                   NAME ________________________

San Francisco State University                                                                 Spring 2011

 

Metr 415/715

Quiz 3

Open Book/Notes

150 pts. (60 minutes)

 

At the rear of the quiz are provided the following KBMX (Birmingham, Alabama) Level

II-derived products for ~2129 UTC  4/27/2011, including the following Level III- equivalent plots: reflectivity plots N0R,  N3R and NCR;  storm  relative velocity plot N0S;  VAD profile NVW and the Storm Attribute Table.  The radar side is located due east of the radar echoes and slightly east of the right margin of the plots.

 

1.    Identification (you donÕt need complete sentences here) (5 pts each for a total of 50 points):

a.     Which Volume Control Pattern was in use at the times shown for Fig. 1 and Fig. 2? 
VCP 212

b.    How often was the radar information updated for Fig. 1 and Fig. 2?
4.8 minutes

c.     What was the elevation angle depicted on Fig. 1?
0.5 degrees

d.    What was the elevation angle depicted on Fig. 2?
3.1 degrees

e.     What was the maximum echo strength value on Fig. 1?
67 dBZ

f.      What is the red circle labeled Q0 on Figs. 2 and 4?
Tornado Vortex Signature

g.     On Fig. 4, the light green pixels (or gates or bins) next to the light red pixels (or gates or bins) indicate a general area of rotation called what?
Mesocyclone

h.    The label Q0 indicates what?
Cell ID Designation or Number

i.      Figure 5 (NVW) is called what?

Vertical Azimuth Display

j.      ND is indicated for many of the levels shown in in Fig. 5.  What does ND mean?

No data are available for that level.

 


2.    Short Essay (Answer two of the three) (30 points each for a total of 60 points)

a.    Figs. 1, 2 and 3 together indicate the presence of a Bounded Weak Echo Region.  Briefly discuss.

The Bounded Weak Echo region refers to the apparent geometry visualized when a strongly developed low and mid level hook echo is surmounted by the divergent signature of precipitation spreading over the updraft area at the top of the storm.  Figure 1 shows the hook in the lower part of the echo. Figure 2, which is a tilt at 3.1 degrees, shows that the radar was intersecting the region near the top of the vault.  The composite reflectivity shown in Fig. 3 indicates that at higher levels the whole top of the storm was overspread with precipitation.  Together these three images suggest a vault like geometry to the echoes, known as the Bounded Weak Echo Region.



b.    Fig. 4 indicates that the thunderstormÕs updraft was rotating.   Briefly discuss.

A mesocyclone, the small rotating circulation with its center beneath the updraft of a supercell thunderstorm, is detectable as a velocity couplet in the velocity or storm-relative velocity data. The couplet is oriented so that a concentrated area of radial winds moving away from the radar appears on one side of the beam axis, while a concentrated area of radial winds moving toward the radar appears on the opposite side of the beam axis. This image shows the couplet in the storm relative velocity field collocated with the hook echo in the reflectivity field. Negative values (blue-green) denote movement toward the radar and positive values (yellow-red) represent movement away from the radar.

 

c.     The Volume Control Pattern in use by the radar is used as a special strategy during times of intense convection.  This strategy is designed to eliminate accuracy issues with the reflectivity and velocity data. Briefly comnment.

Once a pulse of energy is emitted by the radar, it "waits" for the length of time it would take for the return pulse to return (all at about the speed of light) before sending out another pulse. The trouble is, the first pulse will have traveled out beyond the maximum unambiguous range and could have encountered another scatterer (although poorly). That scatterer will return a pulse of energy to the radar that will arrive slightly after the radar has sent out a second pulse. The radar will interpret this return energy as an echo from a source at a very nearby range. This is a false echo and the process by which false echoes appear in this manner is called "range folding". To some extent, range folded data can be "unfolded" by comparing returns the radar receives using different pulse repetition frequencies (artificially changing the maximum unambigous range).  Certain Volume Control Patterns, such as VCP212, are designed to perform this unfolding and eliminate the number of false echoes.







 

 

3.    Short Answer (Answer Both).   (20 points each for a total of 40 points)

a.    The information on the storm relative velocity plot shows that there was 64 knots of inbound velocity and 50 knots of outbound velocity.  Based upon the information in the storm attributes table and mesocyclone recognition guidelines, what was the strength of the mesocyclone? (Please show details of any calculation you did).

(By chance, the calculation is EXACTLY the same as the example we discussed in class)

Mesocyclone Strength is assessed from the rotational velocity.


Rotational Velocity = max inbound + max outbound/2

For this case, maximum inbound was 64 knots and maximum outbound 50 knots.

Rotational Velocity = 64 + 50/2= 57 knots

According to the storm attributes table (Fig. 6) the storm was located 47 nautical miles from the radar.  Reading the table provided on the class website, the mesocyclone strength would be classified as very strong.

b.    The radar makes judgments about future positions of the radar signatures based upon current motion and characteristics of the storm and echoes. What was the direction and speed of the storm and how large was the hail that was associated with it?

The storm was moving at 240 at 49 knots (from the southwest at around 55 mph).  The maximum hail size detected by the radar had a diameter of 2.5 inches.



 

Figure 1.  KBMX Level III Reflectivity Plot, N0R, 2120 UTC, 4/27/2011

 

Figure 2.  KBMX Level III Reflectivity Plot, N03, 2120 UTC, 4/27/2011

 

Figure 3.  KBMX Level III Reflectivity Plot, NCR, 2120 UTC, 4/27/2011

 

 

Figure 4.  KBMX Level III Storm Relative Velocity Plot, N0S, 2129 UTC, 4/27/2011

 

 

Figure 5.  KBMX Level III Plot, NVW, 2101 UTC, 4/27/2011

 

Figure 6.  KBMX Storm Attributes Table, 2129 UTC, 4/27/2011