Introduction

This Technical Attachment discusses the use of a WSR-88D Operational Support Facility (OSF) nomogram, created for mesocyclone recognition, used in conjunction with WSR-88D storm relative velocity (SRV) products. In short, a nomogram is a trace of three coplanar curves which are usually straight parallel lines, each graduated for a different variable. In this case, radar-defined variables are used and include weak shear, minimal mesocyclone, moderate mesocyclone, and strong mesocyclone. The nomogramatic method is used as an operational tool to help decipher mesocyclone strength and predict which ones may be tornadic.

Four (three mini-supercell and one supercell) severe weather events from the California Central Valley are used to test the nomogram.

1. 13 May 1995, an F0 tornado event located 14nm west southwest of the Fresno Yosemite International Airport. 2. 12 March 1996, an F0 tornado event located 2nm northwest of the RDA in Hanford. 3. 22 November 1996, an F1 tornado event within Lemoore NAS located 16nm from the RDA in Hanford. 4. 20 January 1997, a severe thunderstorm, which produced multiple funnel clouds, located 25nm to 30nm southeast of the Hanford RDA site. One funnel cloud was observed near the town of Strathmore and another near Exeter.

Background

Several years ago, the OSF devised a nomogram which determines mesocyclone strength. As seen in Fig. 1, four categories of mesocyclone shear strength are depicted. They are defined as weak shear, minimal mesocyclone, moderate mesocyclone, and strong mesocyclone. To use the nomogram, the Vr/shear algorithm is run. From the algorithm results several variables are determined including the diameter of mesocyclone rotation (nm), mesocyclone distance from the RDA sight or range (nm), and mesocyclonic rotational velocity (kts). Using these variables, the mesocyclonic rotational velocities are plotted on the nomogram to determine potential mesocyclone strength.

From OSF results of Midwest supercell events, conclusions state:

1. A severe thunderstorm warning is recommended if a mesocyclone is recognized by the radar. 2. A tornado warning is recommended if a strong mesocyclone is recognized by the radar. 3. Only 30% of all radar defined mesocyclones produce tornadoes. 4. 90% of all radar-defined mesocyclones produce severe weather.

Recently, more and more mesocyclone studies have been submitted to the OSF for research, including studies from east and west coast mini-supercell events. From these events, new nomograms have been developed. The original Midwest nomograms were based on 3.5nm to 5.0nm mesocyclone core diameters, while the newer mini-supercell nomograms assume 1.0nm to 2.0nm core diameters.

Case-Analysis

Mesocyclone/circulation parameters for the four events are shown in Table 1.

TABLE 1
 
Date        Event          Maximum           Range @ maximum     Greatest depth of          Apparent gate 
                           VR/shear          VR/shear            Circulation (AGL)          to-gate shear 
 
05/13/95    F0 tornado     16 kts            29nm                5,627f t -11,613ft         21:05 - 21:23UTC 
                           2.4 degree scan                       1.5 - 3.4 degree scan 
 
03/12/96    F0 tornado     16 kts            06nm                700ft - 1973ft             22:03 - 22:09UTC 
                           2.4 degree scan                       0.5 - 2.4 degree scan 
 
11/22/96    F1 tornado     40 kts            14nm                No Data                    No Data 
               
01/20/97    Funnel cloud   16 k              26nm                Only detected by 1.5       00:14UTC 
                           1.5 degree scan                       degree scan @ 4856ft 
 
            Funnel cloud   12 kt             29nm                Only detected by 1.5       23:50UTC 
                                                                 1.5 degree scan            degree scan @ 5782ft      
 

Using the updated nomograms (Fig. 2), the 22 November 1996 event closely resembles Midwest guidance, where a tornado warning is recommended if a strong mesocyclone is recognized by radar. The data point (40kts at 14nm) is in the higher values of the moderate category, approaching strong mesocyclone values. The other events, which are more common to the California Central Valley, all fall within the weak shear range. Of most importance, two events, 13 May 1995 and 12 March 1996, were tornadic, but had rotational velocity values of only 16kts. These values, at their respective ranges, fall well below the strong mesocyclone category and OSF guidance for tornado warning issuance. The last event, 20 January 1997, produced multiple funnel clouds. No mesocyclones were defined by the mesocyclone algorithm, but rotational velocity values analyzed by hand were 12kts and 16kts.

Since three events, two tornadic and the other non-tornadic, had the same Vr/shear values at about the same range, a comparison was made of the storm relative velocity product. A subtle difference was observed between tornadic and non-tornadic events. In the tornadic events, (13 May 1995 and 12 March 1996), the maximum azimuthal shear resided within multiple radial pixels adjacent to each other, i.e., gate-to-gate (Figs. 3 and 4), while the shear of the non-tornadic events of (20 January 1997) did not (Figs. 5 and 6). These results follow the reasoning that, within sampling constraints, small tornadoes are more likely to be associated with well-structured and developed gate-to-gate shear.

Conclusions

California Central Valley mini-supercell tornadoes are usually associated with very weak rotational velocities. It was found that these mini-supercells, with approximately 1nm mesocyclone cores, can be tornadic with Vr/shear values of only 16kts. This value, at a given distance, falls well below OSF recommendations of tornado warning issuance. Also, the greatest Vr/shear values and organized storm structure were located by using either the 1.5 or 2.4 elevation slice, depending on distance. Results also indicate that gate-to-gate shear is important in discriminating between tornadic and non-tornadic circulations as seen on radar. In addition, these types of tornadoes are anticipated to be well-handled by the upcoming Build 10 Tornado Detection Algorithm. However, it is expected that optimal adaptable parameters will need to be determined (see Vasiloff 1996 for a TDA parameter study in Utah).

Acknowledgments

The author is grateful to Steve Vasiloff, who reviewed this Technical Attachment and provided consultation. The nomograms used in this study are from the Operational Training Branch of the Operational Support Facility in Norman, Oklahoma.

References

Vasiloff, S. V., 1996: WSR-88D TVS parameter study. WR TALITE96-11, NWS WR Homepage.