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Huaqing Cai

Abstract

Comparisons between tornadic and nontornadic mesocyclones using the concept of fractal geometry are presented. Both the maximum vertical vorticity (ζ max) and pseudovorticity (ζ pv) associated with a mesocyclone at low levels are found to be scaling with the horizontal grid spacing (ε) according to a power-law relationship. The linear least square best fitting of ln (ζ max) or ln (ζ pv) versus ln (ε) for different scales can be obtained for each mesocyclone at a certain time, and it is named the vorticity (pseudovorticity) line of a mesocyclone. Different mesocyclones have different vorticity (pseudovorticity) line slopes that are closely related to the fractal dimension of vorticity (pseudovorticity) of a mesocyclone as a possible fractal structure. Various factors that may affect the accurate estimate of the vorticity (pseudovorticity) line of a mesocyclone are also discussed in detail.

Differences between tornadic and nontornadic mesocyclones are found in terms of the slope of vorticity (pseudovorticity) lines based on three tornadic and two nontornadic mesocyclones. A possible reason why previous studies were not able to identify the difference(s) between tornadic and nontornadic mesocyclones is discussed. Self-similarity (scale invariance), which is a basic characteristic of a fractal structure, seems to be valid between tornado and mesocyclone scales based on the analysis of the vorticity (pseudovorticity) line of the tornadic Kellerville, Texas, mesocyclone. It is hypothesized that a steeper slope of the vorticity (pseudovorticity) line may be indicative of a tornadic mesocyclone.

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Roger M. Wakimoto and Huaqing Cai

Abstract

Analysis of a supercell storm that did not produce a tornado near Hays, Kansas, is presented. A well-defined midlevel mesocyclone was apparent throughout most of the storm’s life cycle. Numerous shallow circulations were observed along the rear-flank gust front during the data collection period. Six of these circulations strengthened into intense low-level mesocyclones. Each of these mesocyclones failed to produce a tornado. The strongest low-level mesocyclone, referred to as vortex #4, underwent a life cycle that was consistent with other tornadic mesocyclones documented in the literature. These results illustrate that the presence of a long-lived mesocyclone at low levels is not sufficient for tornadogenesis to occur.

The kinematic structure of the low-level mesocyclone that did not produce a tornado is compared with a tornadic mesocyclone from another storm in order to understand the characteristic differences between these circulations. The results lead to the conclusion that the presence of a low-level mesocyclone, occlusion downdraft, and updraft/downdraft structure that spirals cyclonically around the circulation are not sufficient conditions for tornadogenesis. Retrieved perturbation pressure and buoyancy fields are used to examine the forcing mechanism of the occlusion downdraft. A downward-directed pressure gradient appears to be the primary forcing mechanism of this downdraft. Perturbation temperature retrievals suggest that the occlusion downdraft is accompanied by a warm core.

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Huaqing Cai and Roger M. Wakimoto

Abstract

An analysis of the observed propagation of the Garden City supercell storm based on data collected by an airborne Doppler radar is presented. The environmental wind profile was characterized by a straight hodograph. Wind syntheses were used to retrieve the perturbation pressure fields in order to isolate the important dynamical processes controlling storm movement. The pressure perturbations produced by the linear and nonlinear effects were comparable in magnitude. The rightward bias in storm motion was primarily a result of the forcing produced by the vertical gradients of perturbation pressure. This finding is consistent with the results based on numerical simulations. The vertical gradients of the linear perturbation pressure were important in determining storm motion early in its life cycle; subsequently, the nonlinear effects dominated. A decomposition of the nonlinear forcing into contributions from cyclostrophically balanced flow and nonlinear shear was also presented. This partitioning revealed that both the forcing produced by the mesocyclone and the horizontal shear that was baroclinically produced by the gradient of buoyancy along the flanks of the updraft were important effects to consider.

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Roger M. Wakimoto and Huaqing Cai

Abstract

An analysis of an oceanic front situated near a col defined by the surface pressure field is presented. There have been few observational examples of this type of front presented in the literature. The primary source of information for this study was data recorded by an aircraft equipped with a Doppler radar. The front was approximately two-dimensional and the cross-frontal scale at low levels was 30–40 km. A prefrontal low-level jet was identified in the high-resolution analyses and was shown to be supergeostrophic. Surface pressure measurements and the horizontal temperature gradients were used to calculate the geostrophic wind and the thermal wind imbalance (TWI) in the alongfront direction. Large negative values of TWI (the vertical shear is less than predicted for the given horizontal temperature gradient) were located near a region of frontogenesis. The strong ageostrophic component of the wind parallel to the front suggests that the alongfrontal component of the wind may not have been in geostrophic balance at the time of the observations.

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Huaqing Cai and Robert E. Dumais Jr.

Abstract

Traditional pixel-versus-pixel forecast evaluation scores such as the critical success index (CSI) provide a simple way to compare the performances of different forecasts; however, they offer little information on how to improve a particular forecast. This paper strives to demonstrate what additional information an object-based forecast evaluation tool such as the Method for Object-Based Diagnostic Evaluation (MODE) can provide in terms of assessing numerical weather prediction models’ convective storm forecasts. Forecast storm attributes evaluated by MODE in this paper include storm size, intensity, orientation, aspect ratio, complexity, and number of storms. Three weeks of the High Resolution Rapid Refresh (HRRR) model’s precipitation forecasts during the summer of 2010 over the eastern two-thirds of the contiguous United States were evaluated as an example to demonstrate the methodology. It is found that the HRRR model was able to forecast convective storm characteristics rather well either as a function of time of day or as a function of storm size, although significant bias does exist, especially in terms of storm number and storm size. Another interesting finding is that the model’s ability of forecasting new storm initiation varies substantially by regions, probably as a result of its different skills in forecasting convection driven by different forcing mechanisms (i.e., diurnal heating vs synoptic-scale frontal systems).

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Roger M. Wakimoto, Chinghwang Liu, and Huaqing Cai

Abstract

Analysis of a supercell storm that produced an F1 tornado near Garden City, Kansas, is presented. This event provided one of the first opportunities to synthesize data collected by a new airborne radar platform called ELDORA (Electra Doppler radar) developed by the National Center for Atmospheric Research. The early stages of development of the midlevel mesocyclone and the entire evolution of the low-level mesocyclone are captured over a 70-min period. The low-level mesocyclone began as an incipient shallow circulation along a synoptic-scale trough. The circulation intensified and grew in depth via vortex stretching under the influence of a strong updraft. As this rotation built up from the boundary layer, it initially remained separate and distinct from the midlevel mesocyclone. Subsequently, the two mesocyclones merge to produce a single column of rotation 4–5 km in diameter. An occlusion downdraft develops within the mesocyclone circulation during the last passes by the storm signaling the beginning of the tornadic phase. Perturbation pressure retrievals provide conclusive evidence that this downdraft is driven by a downward-directed pressure gradient force.

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Roger M. Wakimoto, Huaqing Cai, and Hanne V. Murphey

Two remarkable supercell storms developed on 22 June 2003 in eastern Nebraska. One of the thunderstorms, located near the town of Aurora, Nebraska, produced the largest known hailstone on record. Receiving far less attention was an adjacent supercell that was equally impressive and is referred to as the Superior, Nebraska, supercell. The two supercells formed during the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment (BAMEX), operated in the spring and summer of 2003. One of the main platforms used during BAMEX was the airborne Electra Doppler Radar (ELDORA). ELDORA was deployed on the Superior supercell several hours after it initiated. Striking in one of the flybys past the storm was the characteristics of the parent circulation. The Superior supercell was associated with a mesocyclone that was the largest (~9 km in diameter) and the most intense (118ms−1 velocity differential) ever documented. Ground-based observations from a nearby Weather Surveillance Radar-1988 Doppler (WSR-88D) located in Hastings, Nebraska (UEX), could not resolve the Doppler velocities correctly owing to the intensity of the mesocyclone. The environmental conditions, satellite imagery, and Doppler radar observations of this supercell are presented.

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Roger M. Wakimoto, Hanne V. Murphy, and Huaqing Cai

Abstract

Airborne radar analysis combined with cloud photogrammetry of a supercell that formed on 31 May 1995 near San Angelo, Texas, is presented. The superposition of the Doppler wind syntheses with the cloud pictures provides a unique view of the relationship between the low-level mesocyclone and the developing lowered cloud base. The mesocyclone accompanying the storm went through two periods of intensification. The end of the first period of intensification did not lead to the formation of a tornado. An occlusion downdraft formed but was primarily driven by negative buoyancy rather than a downward-directed perturbation pressure gradient. Recent studies have suggested that cold downdrafts inhibit the formation of tornadoes. The present study supports this hypothesis and provides the first quantitative view of the vertical structure of this downdraft. The mesocyclone evolved through a second period of intensification that produced a weak tornado that was visually apparent from the aircraft. No occlusion downdraft was resolved during this latter period. The unusual aspect of this tornado is that it appeared to develop outside of the low-level mesocyclone. This type of tornadogenesis has not been previously shown.

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Huaqing Cai, Wen-Chau Lee, Tammy M. Weckwerth, Cyrille Flamant, and Hanne V. Murphey

Abstract

The detailed analysis of the three-dimensional structure of a dryline observed over the Oklahoma panhandle during the International H2O Project (IHOP_2002) on 11 June 2002 is presented. High-resolution observations obtained from the National Center for Atmospheric Research Electra Doppler Radar (ELDORA), S-band dual-polarization Doppler radar (S-Pol), water vapor differential absorption lidar (DIAL) Lidar pour l'Etude des Interactions Aérosols Nuages Dynamique Rayonnement et du Cycle de l'Eau (LEANDRE II; translated as Lidar for the Study of Aerosol–Cloud–Dynamics–Radiation Interactions and of the Water Cycle) as well as Learjet dropsondes are used to reveal the evolution of the dryline structure during late afternoon hours when the dryline was retreating to the northwest. The dryline reflectivity shows significant variability in the along-line direction. Dry air was observed to overrun the moist air in vertical cross sections similar to a density current. The updrafts associated with the dryline were 2–3 m s−1 and were able to initiate boundary-layer-based clouds along the dryline. The formation of this dryline was caused by high equivalent potential temperature air pushing northwestward toward a stationary front in the warm sector.

Middle-level clouds with radar reflectivity greater than 18 dBZ e near the dryline were detected by ELDORA. A roll boundary, which was associated with larger convergence and moisture content, was evident in the S-Pol data. It is found that the instability parameters most favorable for convection initiation were actually associated with the roll boundary, not the dryline. A storm was initiated near the roll boundary probably as a result of the combination of the favorable instability parameters and stronger upward forcing. It is noted that both the 11 June 2002 dryline and the roll boundary presented in this paper would not be identified if the special datasets from IHOP_2002 were not available.

Although all model runs [fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), Meso Eta, and Rapid Update Cycle (RUC)] suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms. The reasons why the dryline failed to produce any storm inside the IHOP_2002 intensive observation region are discussed. Both synoptic-scale and mesoscale conditions that were detrimental to convection initiation in this case are investigated in great detail.

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Huaqing Cai, Wen-Chau Lee, Michael M. Bell, Cory A. Wolff, Xiaowen Tang, and Frank Roux

Abstract

Uncertainties in aircraft inertial navigation system and radar-pointing angles can have a large impact on the accuracy of airborne dual-Doppler analyses. The Testud et al. (THL) method has been routinely applied to data collected by airborne tail Doppler radars over flat and nonmoving terrain. The navigation correction method proposed in Georgis et al. (GRH) extended the THL method over complex terrain and moving ocean surfaces by using a variational formulation but its capability over ocean has yet to be tested. Recognizing the limitations of the THL method, Bosart et al. (BLW) proposed to derive ground speed, tilt, and drift errors by statistically comparing aircraft in situ wind with dual-Doppler wind at the flight level. When combined with the THL method, the BLW method can retrieve all navigation errors accurately; however, it can be applied only to flat surfaces, and it is rather difficult to automate. This paper presents a generalized navigation correction method (GNCM) based on the GRH method that will serve as a single algorithm for airborne tail Doppler radar navigation correction for all possible surface conditions. The GNCM includes all possible corrections in the cost function and implements a new closure assumption by taking advantage of an accurate aircraft ground speed derived from GPS technology. The GNCM is tested extensively using synthetic airborne Doppler radar data with known navigation errors and published datasets from previous field campaigns. Both tests show the GNCM is able to correct the navigation errors associated with airborne tail Doppler radar data with adequate accuracy.

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