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Matthew S. Van Den Broeke
,
Jerry M. Straka
, and
Erik N. Rasmussen

Abstract

Preliminary schematics of polarimetric signatures at low levels in southern plains classic supercells are developed for pretornado, tornado, and tornado demise times from a small collection of cases, most of which are cyclic tornado producers. Characteristic signatures and patterns are identified for the reflectivity factor (Z HH), the differential reflectivity (Z DR), the correlation coefficient (ρ hv), and the specific differential phase (K DP). Signatures likely related to an ongoing tornado are also discussed. Major findings in Z HH at tornado times include “wings” of higher values often extending away from the updraft region, a stronger gradient on the west side of the echo appendage, and a local maximum at the storm location favorable for tornadogenesis. Increasing cyclonic curvature of the hook-echo region was noted through the tornado life cycle. The Z DR tended to indicate hail shafts most commonly at tornado times, with the highest storm values typically located along the storm’s forward flank throughout the tornado life cycle. A Z DR minimum often occurred at the tornado-favorable location, while low Z DR occasionally trailed the tornado region. Storm-minimum ρ hv typically occurred at the tornado-favorable location at tornado times and in hail shafts or heavy rain areas at other times. Another region of low correlation was the storm updraft, while the highest storm correlation was typically found in the downwind light-precipitation shield. The K DP typically exhibited a storm-core temporal maximum at tornado times, with the highest storm values in regions of hail and heavy rain and the lowest values in the downwind light-precipitation region. Values in the tornado-favorable region were typically near zero and sometimes strongly negative.

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Jerry M. Straka
,
Dusan S. Zrnić
, and
Alexander V. Ryzhkov

Abstract

A new synthesis of information forming the foundation for rule-based systems to deduce dominant bulk hydrometeor types and amounts using polarimetric radar data is presented. The information is valid for a 10-cm wavelength and consists of relations that are based on an extensive list of previous and recent observational and modeling studies of polarimetric signatures of hydrometeors. The relations are expressed as boundaries and thresholds in a space of polarimetric radar variables. Thus, the foundation is laid out for identification of hydrometeor types (species), estimation of characteristics of hydrometeor species (size, concentrations, etc.), and quantification of bulk hydrometeor contents (amounts). A fuzzy classification algorithm that builds upon this foundation will be discussed in a forthcoming paper.

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Leigh G. Orf
,
John R. Anderson
, and
Jerry M. Straka

Abstract

A parameter study of colliding microburst outflows is performed using a high-resolution three-dimensional model. The colliding microburst pairs me simulated in a domain of 18 km × 16 km × 4.25 km with 50-m resolution. Microburst pairs are examined in varying space and time separations, and the authors find that for certain geometries strong elevated wind fields are generated from the interactions between outflows. For a narrow range of space-time geometries, this elevated wind field is extremely divergent. An examination of the F-factor aircraft hazard parameter reveals that both the divergent wind fields and microburst downdraft cores are regions of danger to jet aircraft. Trajectory analysis reveals that the air composing the elevated jets can be traced back to the shallow outflow formed beneath each microburst core. An analysis of the parcel kinetic energy budget indicates that the pressure domes beneath and between the microbursts are the primary mechanisms for directing energy into the elevated jets.

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Sonia G. Lasher-Trapp
,
Charles A. Knight
, and
Jerry M. Straka

Abstract

The growth of ultragiant aerosol (UGA) in a Lagrangian framework within a simulated three-dimensional cloud is analyzed and compared with radar and aircraft observations of a cumulus congestus collected during the Small Cumulus Microphysics Study (SCMS). UGA are ingested into the simulated cloud and grow by continuous collection; the resulting radar reflectivity factor and raindrop concentrations are evaluated at 1-min intervals. The calculations produce a substantial echo (>30 dBZ) within a short time (18 min), containing few raindrops (0.3 L−1). The calculated radar echo is very sensitive to the amount of UGA ingested into the modeled cloud and its liquid water content. The modeled radar echo and raindrop concentrations are consistent with the observations in that the differences fall within the modeling and measurement limitations and uncertainties.

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Jerry M. Straka
,
Erik N. Rasmussen
, and
Sherman E. Fredrickson

Abstract

A mobile weather observing system (mobile mesonet) was designed to augment existing meteorological networks in the study of severe local storms and other mesoscale weather phenomena in conjunction with the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX). Fifteen mobile mesonet units were built, each consisting of meteorological instruments mounted on standard automobiles. for high temporal and spatial resolution observations. While the most accurate measurements are possible from stationary mobile mesonet vehicles, accurate observations also are possible from moving vehicles. The mobile mesonet instruments measure pressure (600–1100 mb), temperature (−33° to 48°C), relative humidity (0%–100%), and wind direction and speed (0°–360° and 0–60 m s−1). Onboard each vehicle, a Global Positioning System (GPS) receiver and a flux-gate compass obtain universal time, vehicle location (latitude, longitude, altitude), and vehicle heading and speed. A standard laptop computer stores data, computes derived variables, and provides real-time data display. Instrument compatibility with the Oklahoma Mesonet allows for high-quality instrument calibration and maintenance.

The purpose of this paper is to provide a technical overview of the mobile mesonet system. The rationale for choice of instrumentation and justification for method of exposure are discussed. The performance of the mobile mesonet is demonstrated with two examples of data collected during VORTEX-1994 and comparisons with data from an Oklahoma Mesenet site.

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Aaron Kennedy
,
Jerry M. Straka
, and
Erik N. Rasmussen

Abstract

A new three-dimensional reflectivity echo in the rear flank of supercells known as the descending reflectivity core (DRC) has been documented in the literature by Rasmussen et al. The DRC is an enhanced region of reflectivity presumed to occur in the rear-flank downdraft (RFD) of a supercell. In the four cases they studied, this feature descended with time from the rear-echo overhang at 3–6 km in height into the supercell appendage. In addition, the DRC often occurred prior to tornadogenesis. The purpose of this paper is to serve as a more thorough analysis of DRCs using a larger sample of storms. The frequency of DRCs is explored within isolated supercells with persistent rear-flank appendages, and in particular at times preceding reported tornado onset in those supercells. Of the 64 supercells included within this study, 59% produced DRCs, with 30% of these DRCs occurring within 10 min prior to 5 min after tornadogenesis. This study included 89 reported tornadoes and 71 DRCs. Statistical analysis of the dataset reveals that while DRCs are sometimes associated with tornadoes, they presently have limited usefulness for tornado nowcasting. Improvements to Weather Surveillance Radar-1988 Doppler (WSR-88D) resolution and further classification of DRCs may help discriminate between tornadic and nontornadic appendages in the future, however.

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Paul M. Markowski
,
Erik N. Rasmussen
,
Jerry M. Straka
, and
David C. Dowell

Abstract

Low-level cooling beneath the cirrus anvil canopies of supercell thunderstorms is documented in two Verification of the Origins of Rotation in Tornadoes Experiment cases and in the 17 May 1981 Arcadia, Oklahoma, supercell. Surface temperature decreases of 3°C or more occurred beneath the anvils within 45 min of the onset of overcast conditions. Cooling was confined to the lowest few hundred meters of the boundary layer, and believed to be due mainly to a deficit in the energy budget following a reduction of incoming shortwave radiation. In the three cases studied, the vertical wind shear was strong; thus, mixing prevented the formation of an inversion layer.

Strong insolation at the ground outside of the anvil shadows coupled with the cooling beneath the cirrus canopies led to corridors of baroclinity along the shadow edges. It is shown that residence times in these baroclinic zones may be long enough for parcels to acquire considerable horizontal vorticity (e.g., ∼10−2 s−1) en route to a storm updraft. Enhancement of the horizontal vorticity of parcels ingested by an updraft may have implications for the dynamics of storm rotation.

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Paul M. Markowski
,
Jerry M. Straka
,
Erik N. Rasmussen
, and
David O. Blanchard

Abstract

In this paper, storm-relative helicity (SRH) and low-level vertical shear of the horizontal wind fields were investigated on the mesoscale and stormscale in regions where tornadoes occurred for four case studies using data collected during the Verification of the Origin of Rotation in Tornadoes Experiment. A primary finding was that SRH was highly variable in both time and space in all of the cases, suggesting that this parameter might be difficult to use to predict which storms might become tornadic given the available National Weather Service upper-air wind data. Second, it was also found that the shear between the lowest mean 500-m wind and the 6-km wind was fairly uniform over vast regions in all of the four cases studied; thus, this parameter provided little guidance other than that there was possibly enough shear to support supercells. It was contended that forecasters will need to monitor low-level features, such as boundaries or wind accelerations, which might augment streamwise vorticity ingested into storms. Finally, it was suggested that one reason why one storm might produce a tornado while a nearby one does not might be due to the large variations in SRH on very small spatial and temporal scales. In other words, only those storms that move into regions, small or large, with sufficient SRH might produce tornadoes.

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Jelena Andrić
,
Matthew R. Kumjian
,
Dušan S. Zrnić
,
Jerry M. Straka
, and
Valery M. Melnikov

Abstract

Polarimetric radar observations above the melting layer in winter storms reveal enhanced differential reflectivity Z DR and specific differential phase shift K DP, collocated with reduced copolar correlation coefficient ρ hv; these signatures often appear as isolated “pockets.” High-resolution RHIs and vertical profiles of polarimetric variables were analyzed for a winter storm that occurred in Oklahoma on 27 January 2009, observed with the polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D) in Norman. The Z DR maximum and ρ hv minimum are located within the temperature range between −10° and −15°C, whereas the K DP maximum is located just below the Z DR maximum. These signatures are coincident with reflectivity factor ZH that increases toward the ground. A simple kinematical, one-dimensional, two-moment bulk microphysical model is developed and coupled with electromagnetic scattering calculations to explain the nature of the observed polarimetric signature. The microphysics model includes nucleation, deposition, and aggregation and considers only ice-phase hydrometeors. Vertical profiles of the polarimetric radar variables (ZH , Z DR, K DP, and ρ hv) were calculated using the output from the microphysical model. The base model run reproduces the general profile and magnitude of the observed ZH and ρ hv and the correct shape (but not magnitude) of Z DR and K DP. Several sensitivity experiments were conducted to determine if the modeled signatures of all variables can match the observed ones. The model was incapable of matching both the observed magnitude and shape of all polarimetric variables, however. This implies that some processes not included in the model (such as secondary ice generation) are important in producing the signature.

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Jerry M. Straka
,
Matthew S. Gilmore
,
Katharine M. Kanak
, and
Erik N. Rasmussen

Abstract

One- and two-moment parameterizations are integrated over hydrometeor diameters D(0, ∞) for vapor diffusion and the continuous collection growth processes. For the conditions specified, the total number concentration of collector particles should be conserved. To address the problem, the gamma distribution function is used for the spectral density function. Predicted variables can include total mixing ratio q, total number concentration Nt , and characteristic diameter Dn (inverse of the distribution slope λ). In all of the cases, the slope intercept no is diagnosed or specified. The popular one- and two-moment methods that are explored include the one-moment method in which q is predicted, no is specified, and Nt and Dn are diagnosed; the one-moment method in which q is predicted, Dn is specified, and Nt and no are diagnosed; the two-moment method in which q and Dn are predicted and Nt and no are diagnosed; and the two-moment method in which q and Nt are predicted and no and Dn are diagnosed. It is demonstrated for the processes examined that all of the schemes 1) fail to conserve Nt for the collector particles when Nt should be conserved and 2) have other unphysical attributes, except for the two-moment method in which q and Nt are predicted. In recent years there has been a dramatic increase in the use of more-sophisticated microphysical parameterizations in cloud, mesoscale, and climate models, and it is increasingly important for a model user to be cognizant of the strengths and weaknesses of the parameterizations in complex models.

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