Summary and Conclusions

The characterization and analysis of the dynamics and microphysics of a significant, wet downburst was conducted using various observations and analysis methods in this dissertation. The downburst was a macroburst in size with surface winds in excess of 35 m s-1 (>80 mph) and

hailstones in excess of 4 cm diameter. Surface observations from the Oklahoma Mesonet measured a 6.6-hPa pressure rise that was coincident with an intense peak rain rate of 213 mm hr-1 at the center of the downburst. The downburst was unique due to the combination of its size,

longevity, and intense precipitation rate.

For the first time documented, an HCA was applied to PRD to gain further understanding of the microphysical evolution of a downburst (Chapter 5). The HCA analyses were utilized to develop a conceptual model that characterizes the hydrometeor evolution of the parent downburst thunderstorm. Through the analyses, it was seen that graupel aloft made a transition to a nearly all rain and hail mixture. This large area of mixed rain and hail eventually descended to the ground, causing the downburst. Increased ZDR and decreased ρhv at the bottom of this mixed-

phase precipitation core were assumed to be due to the increased presence of melting hailstones. This observation indicated that melting of hailstones contributed to some of the negative buoyancy in the downburst.

The mixed-phase precipitation was coincident with a vertical velocity of ~-20 m s-1,

which was found through dual-Doppler analysis. Using this value and Mesonet observations, it was estimated that the thermodynamic part had a much greater contribution to the surface mesohigh than the dynamic part.

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Rapid-scan radar data from RaXPol captured the quick expansion of the downburst as it moved to the east-southeast toward Norman (Chapter 6). Through analysis of the RaXPol data, the downburst was found to have grown from a microburst at ~2.1 km horizontal scale to a macroburst at ~6.4 km in less than 7 min. As the downburst expanded, its near-surface horizontal winds intensified from 23 m s-1 to 42 m s-1.

The rapid-scan observations also captured the development of features such as a horizontal rotor, vertical vortices, multiple gust front heads, and an elevated nose on the leading edge of the gust front. The horizontal rotor may have split the hail core into two segments as indicated by reduced ρhv separating around the rotor and a localized ZH minimum collocated with

the rotor. Along the leading edge of the gust front, there was a line of small vertical vortices. There was a slight reduction in ρhv and ZDR along with enhanced σv associated with these

vortices. These vertical vortices may have corresponded to small gustnadoes. There were also three distinct gust front heads, each with a localized wind maximum, behind the initial gust front. There were three wind shifts ≥ 15° observed by the Mesonet that may have corresponded to these gust front heads. The elevated nose was at ~75 m ARL, which resulted in a 2 to 3 min lag from the most intense winds being measured at the surface.

RaXPol also captured several descending surges of mixed-phase precipitation cores aloft, indicated by a reduction in ρhv. These descending cores provided a continued stream of

precipitation loading and melting hail, which may have aided in the continued expansion and intensity of the downburst.

The presence of mixed-phase precipitation was substantiated through a dual-frequency comparison between KOUN and RaXPol (Chapter 7) and the use of a variational retrieval (Chapter 8). From the dual-frequency comparison, it was seen that there is increased sensitivity

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to wet hail for ρhv at X-band when compared to S-band. This reduction in ρhv is attributed to

increased variance of δ due to resonance effects. The comparison also demonstrated the benefits of ρhv at X-band when compared to ZH and ZDR due to its immunity to attenuation.

Finally, when applied to the downburst case, the variational retrieval was another method beyond the HCA that utilizes scattering theory to determine the presence of mixed-phase precipitation (Chapter 8). Since the observation operators were derived with the assumption of pure rain, the retrieval failed where there was hail contamination. These results matched the results of the HCA

Using the results from this study, some conclusions can be inferred on the dynamics of the downburst. As noted, the primary driving mechanisms for the downward motion due to buoyancy in downbursts are 1) thermal buoyancy through evaporation and/or melting of hydrometeors and 2) precipitation loading.

From back of the envelope calculations (Chapter 4), it appeared that thermal buoyancy may have played a slightly larger role in the downburst than precipitation loading; however, these terms were not steady-state during the duration of the downburst event. In reality, the magnitudes of these terms likely varied during the downburst event.

Initially, thermal buoyancy may have dominated with a dry subcloud layer and a high LCL height. The potential for thermal buoyancy may have decreased toward the latter part of the downburst event with the decreasing evaporation due to the environment becoming more saturated. From the RaXPol data, it was seen that as the downburst grew in size and intensity that there was an increase in the stream of descending mixed-phase precipitation well-behind the gust front. This implies precipitation loading may have had more of an effect during this part of the downburst event as the environment may have been closer to saturation. However, mixed-phase

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precipitation may have also contributed to negative thermal buoyancy from melting hail since melting could still occur even if the environment was saturated. From the KOUN data it was seen that there was some melting of hail near the surface.

Therefore, it can be speculated that on average, both thermal buoyancy and precipitation loading may have had approximately the same order of magnitude effect on the downburst’s downward acceleration with perhaps varied degrees of influence through the evolution of the downburst. In the developing stage of the downburst, thermal buoyancy through evaporation may have dominated. At the latter stages, descending surges of mixed-phase precipitation provided a continued source of both precipitation loading and melting hail that aided in the continued expansion, intensity, and longevity of the downburst.