6. Dry Microburst Independent Data Cases

Radar and upper-air sounding data shown in these cases were obtained from the Salt Lake City, UT WSR-88D Doppler radar (KMTX) and upper-air site (KSLC -- WMO 72572) as noted in Fig. 10. The KMTX radar site (41:26N 112:45W) is located on the southern end of a narrow mountain range and peninsula which are situated in the northeast portion of Great Salt Lake. The antenna feed horn is positioned 6573 ft (2004 m) above sea level or approximately 2300 ft above the mean surrounding terrain elevation. The KSLC upper-air site is located 4,225 ft ASL at 40:47N 111:58W.

a. 09 July 1997 South Jordan, UT and Michael AAF (mesonet site S01), UT microburst events

On 09 July 1997, a wind gust to 56 mph (49 kt) was recorded by an anemometer at 2310 UTC at South Jordan, UT (location is approx. 13 nm due south of KSLC in Fig. 10) as reported by Vasiloff and Hogan (1997). Upper-air sounding data from KSLC at 1200 UTC, 09 July 1997 and 0000 UTC, 10 July 1997 (Figs. 11 and Fig. 12) along with surrounding mesonet temperatures indicated the following lapse rates --

1200 UTC: 820 mb (top of surface inversion) to 570 mb (freezing level) = -8.40° C km^-1; 700 to 500 mb = -9.24° C km^-1
0000 UTC: 700 to 577 mb  (freezing level) = -9.72° C km^-1;  surface to 550 mb = -9.80°   to -10.06° C km^-1

On the 0000 UTC sounding, the sub-cloud lapse rate was near or greater than the dry adiabatic (-9.8° C km^-1) lapse rate. Figure 13, Figure 14, Figure 15, Figure 16, and Figure 17 are reflectivity vertical cross-sections (RCS) from the KMTX WSR-88D radar depicting the development and descent of the reflectivity core responsible for producing the near severe wind gust. The reflectivity core was located approximately 60 to 65 nm due south of KMTX and is indicated between the 20 and 25 tick marks on each image. The distance between each tick mark is 5 nm and the radar site is located to the left (north) of each image.

At 2226 UTC (Fig. 13), a 29 dBZ core had developed at approximately 15,500 ft ARL (above radar level) or about 6,000 ft above the freezing level. By 2232 UTC (Fig. 14), the height of the reflectivity core had remained unchanged at 15,500 ft ARL while the peak reflectivity value had increased to 34 dBZ. Using the potential gust table (Table 1), and interpolating between 30 and 35 dBZ for the 34 dBZ core, yields peak predicted gusts of 49 and 51 kt for lapse rates of -9.8° to -10.0° C Km^-1, respectively, which is very close to the observed gust of 49 kt. Notice also that by using peak reflectivity at or above the freezing/melting level, a lead time of at least 30 to 35 minutes was achieved.

By 2238 UTC (Fig. 15), most of the reflectivity core had descended to near or below the melting level (approx. 9,000 ft ARL) with peak reflectivity increasing to 40 dBZ. The higher reflectivity value is the result of "bright banding" (note: the term "bright banding" is generally associated with stratiform precipitation; however, in this module, the term is used to indicate melting and wetting of ice particles) caused by ice particles melting and coalescing into larger conglomerates of water-coated ice which return more power than the smaller water and ice particles located above the freezing/melting level. If 40 dBZ is used in Table 1, then predicted gusts ranging from 53 to 56 kt would be obtained. Even though those predicted gust values would only be overestimates of 8 to 14 percent and could be considered "operationally acceptable," this case is the exception rather the rule since this storm was relatively "dry." Reflectivity values for cores descending below the melting level typically exceed 50 dBZ which would result in grossly overestimated predicted gusts in excess of 64 kt for this case.

By 2244 UTC (Fig. 16), the reflectivity core had descended to below 7,000 ft ARL with the areal size of the 40 dBZ echo increasing both horizontally and vertically. At 2249 UTC, (Fig. 17), the peak reflectivity core had almost completely passed below the bottom of the lowest elevation slice (0.5°) and likely would have already been impacting the ground. Figure 17 is nearly coincident with the time that a semicircular ring of dust could be seen moving east toward South Jordan, UT (Vasiloff and Hogan, 1997).

It is also important to note that the areal coverage of 30+ dBZ was too small for the WSR-88D cell tracking algorithm (SCIT) to identify and track this DMB cell, or provide alphanumeric information on the storm structure (e.g., max dBZ and height of max dBZ value). As a result, forecasters must stay aware of all cells developing in their warning area and not rely totally on algorithm output or alarm functions to alert them to the possibility of a severe microburst.

The Doppler velocity patterns (Fig. 18, Fig. 19, Fig. 20, and Fig. 21) associated with this microburst depicted the typical cloud base convergence signature. However, the convergence values were weak at best and provided little if any quantitative warning guidance. In fact, the strongest convergence values were only 18-22 kt over a 3 nm distance and occurred at 2251 UTC (Fig. 21).

It is possible to miss peak convergence values when performing single Doppler analyses if the strongest winds are perpendicular to the radar beam. In this case, that appears unlikely given that most of the storm motion (northward) was toward the radar site which would have maximized the inflow velocities along the radial chosen in Fig. 18, Fig. 19, Fig. 20, and Fig. 21.

Also on 09 July 1997, an earlier microburst produced a severe wind gust of 55 kt at 2100 UTC at Michael AAF (mesonet site S01). A map of the DUGWAY mesonetwork containing site S01 is shown in Figure 22. The evolution of this event is captured in the reflectivity vertical cross-sections (Fig. 25, Fig. 26, Fig. 27, Fig. 28, and Fig. 29) between 2030 and 2053 UTC. No velocity cross-sections were available due to severe range obscuration (i.e., RF-- range folding) of the velocity data. A time series plot of temperature, wind direction, sustained wind speed, and peak gusts for the mesonet site is shown in Fig. 23.

Figure 24 contains mesonet plots for (a) 2015 UTC, (b) 2030 UTC, (c) 2100 UTC, and (d) 2110 UTC. A peak reflectivity value of 40 dBZ was indicated at 2036 UTC (Fig. 26) at 14,500 ft ARL or about 5,000 ft above the freezing level. Using the Salt Lake City potential gust table  (Table 1)  and 40 dBZ yields peak predicted gusts of 53 and 56 kt for lapse  rates of -9.8 to -10.0° C km^-1, respectively. This is essentially identical to the 55 kt gust that was recorded at 2100 UTC.

Even though both recorded gust events on this day were reasonably close to the predicted values obtained from Table 1, forecasters should use various combinations of dBZ and sub-cloud lapse rates to develop potential gust ranges and interpret the output as an indication of the possibility of strong (> 40 kt) and/or severe (> 50 kt) wind gusts occurring.

b. 27 July 1996 Horizontal Grid (mesonet site S08), UT microburst event

At 2315 UTC, 27 July 1996, a dry microburst storm tracked northward just west of mesonet site S08 which recorded a peak wind gust of 41 kt. A time series plot of temperature, wind direction, sustained wind speed, and peak gusts for the mesonet site is shown in Fig. 30. Upper-air sounding data from KSLC at 1300 UTC, 27 July 1996 and 0000 UTC, 28 July 1996 (Fig. 31 and Fig. 32) along with mesonet temperatures indicated the following lapse rates--

1300 UTC: 820 mb (top of surface inversion) - 570 mb (freezing level) = -8.75° C km^-1
0000 UTC: 820 - 540 mb (freezing level) = -8.40° C km^-1; surface - 540 mb (freezing level) = -8.65° C km^-1

On each sounding, the sub-cloud lapse rate was subadiabatic ( < -9.8° C km^-1). In fact, the atmosphere had stabilized between 1300 and 0000 UTC due to cold air advection in the low-levels and warm air advection in the mid-levels. This is noteworthy given that, in Section 2, it was mentioned that the majority of severe wind gusts occur when the sub-cloud lapse rate is dry adiabatic or superadiabatic.

Figure 33 contains mesonet plots for (a) 2245 UTC, (b) 2300 UTC, (c) 2315 UTC, and (d) 2330 UTC on 27 July 1996. Two separate microbursts emanated from a small cluster of storms situated over the southwest portion of the of the DUGWAY mesonetwork (Fig. 22). The stagnation point of the 2nd microburst ("C") tracked north-northeastward across the mesonetwork and passed just west and north of mesonet site S08. Unfortunately, mesonet data for site S08 was missing at 2300 UTC (Fig. 33b) when the center of the 2nd microburst was located 2-3 nm to the southwest. This would have been the time when the peak wind gust would most likely have occurred since the maximum outflow would have been directed along the direction of propagation. The peak gust of 41 kt was recorded at 2315 UTC from the northwest which would have been on the receding or weaker side of the microburst.

The evolution of this double microburst event can be seen in Fig. 33 and in the reflectivity vertical cross-sections (Fig. 34, Fig. 35, Fig. 36, Fig. 37, Fig. 38) between 2151 and 2215 UTC. A 29 dBZ reflectivity core that produced the first microburst had developed above 14,000 ft ARL (along tick mark 25) at 2151 UTC (Fig. 34). Peak reflectivity values associated with the 1st core increased to 40 dBZ by 2157 UTC (Fig. 35) and to 45 dBZ by 2202 UTC (Fig. 36). The 1st microburst downdraft was already underway by 2202 UTC as indicated by the 40 dBZ echo already extending down to 7,500 ft ARL. By 2209 UTC (Fig. 37), a "bright band" 50 dBZ echo (midway between tick marks 20 and 25) associated with the 1st microburst downdraft had developed at 7,500 ft ARL. The outflow from the 1st microburst may have helped to initiate or enhance the updraft associated with the 2nd microburst. This is indicated by the development of a 50 dBZ core at 14,500 ft ARL (approximately 5,000 above the freezing level) located to the left (north) of the 7,500 ft ARL 50 dBZ core. By 2215 UTC (Fig. 38), the "bright band" 50 dBZ core associated with the 2nd microburst had increased in areal extent and extended down to at least 7,000 ft ARL.

Using the potential gust table (Table 1) and 50 dBZ yields peak predicted gusts of 44 and 48 kt for lapse rates of -8.5° to -8.75° C km^-1, respectively. This is a prediction of near-severe winds, similar to what was reported from the mesonetwork.

c. 09 June 1996 Magna (mesonet site MAG), UT microburst event.

At 0210 UTC, a peak gust of 54 mph (47 kt) was recorded at Magna, UT. The 0000 UTC, 09 June 1997, KSLC sounding, combined with data from surrounding mesonet sites (surface temps 32-34° C), indicated that sub-cloud lapse rates were slightly superadiabatic and likely had only decreased to dry adiabatic by 0200 UTC. The mid-levels had also dried out significantly when compared to the 1200 UTC, 08 June 1997 classic "inverted-V" sounding. The sharp decrease in available moisture may explain why peak reflectivity values (above the freezing level) observed in this storm and in other nearby storms rarely exceeded 30 dBZ. At 0133 UTC, the Magna storm contained a peak reflectivity core value of 30 dBZ at an altitude of 15,700 ft AGL or about 5,000 ft above the freezing level.

Using Table 1 and  30 dBZ yields peak predicted  gusts of 46 and 47 kt for lapse rates of -9.8° and -10.0° C km^-1, respectively, which is essentially identical to the anemometer reported gust of 47 kt. Using the 30 dBZ echo core at 0133 UTC would have also resulted in a "warning" lead time in excess of 30 minutes. For a complete discussion of this particular microburst event, see Vasiloff (1997).