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David O. Blanchard
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David O. Blanchard

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Damage surveys in the aftermath of tornadoes occurring in the forested regions of the Mogollon Rim in northern Arizona have been assessed using the enhanced Fujita scale (EF scale) damage indicator (DI) and degree of damage (DOD) tables. These surveys often revealed different DODs within close proximity as well as different spatial patterns and areal extent of tree damage exhibiting the same DOD, making the determination of wind speed and EF-scale ratings challenging. A localized tornado outbreak occurred across northern Arizona on 6 October 2010, producing at least 11 tornadoes and substantial areas of forest damage. Remarkably, one of these tornadoes passed over a three-dimensional sonic anemometer. Wind data from this sensor are compared with tree damage in the adjacent forest to assess the performance of the EF-scale metrics for damage to trees.

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David O. Blanchard

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The case presented here is submitted as an example of a previously undocumented type of interaction between a supercell thunderstorm and a frontal boundary. During the afternoon of 8 June 1995, a supercell thunderstorm formed near a quasi-stationary frontal boundary and then moved northeast across Beaver County in the Oklahoma Panhandle. Its motion took it away from the boundary and deeper into the cool air. As the storm matured and strengthened, a portion of the boundary to the south of the supercell moved northward and briefly became entrained in the low-level circulation of the storm. This northward advance of the boundary was subsequently followed by a southward motion back to near its original location. High-density spatial and temporal observations from the Oklahoma Mesonet and the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) Mobile Mesonetwork are presented to document the northward advance of the boundary into the supercell circulation.

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David O. Blanchard

Mesoscale convective systems observed in the southern High Plains during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-central (PRE-STORM) field program were analyzed using radar and rawinsonde data. Although radar data indicate that no two systems are identical, basic recurring mesoscale structures are evident. Based on these recurrent features, the systems have been classified into three types of mesoscale convective patterns: linear mesoscale systems, occluding mesoscale systems, and chaotic mesoscale systems. Examples of all three types are discussed. High-density rawinsonde data collected in the regions ahead of the mesoscale systems have been averaged to produce composite soundings; the composites exhibit differences in both thermodynamic and wind structure between types.

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David O. Blanchard

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An unusual severe weather event with supercell thunderstorms developed across portions of northern Arizona in the midst of the warm-season North American monsoon—a regime characteristically dominated by a subtropical upper-level high over the southwestern United States. The approach of a midlatitude, cold-core, upper-level low brought an environment of enhanced shear and increased instability supportive of supercells. This atypical system is described and how a correct interpretation of the winds and hodograph would allow a forecaster to maintain situational awareness is discussed.

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David O. Blanchard

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Comparisons of convective available potential energy (CAPE) with standard instability indices for evaluating the convective potential of the atmosphere such as the lifted index (LI) reveal only moderate correlations. This is because the LI is a measure of single-level buoyancy while CAPE is a measure of both integration depth and the buoyancy. Normalizing the CAPE values by the depth over which the integration takes place provides an index (NCAPE) that is independent of the depth and is a convenient measure of the mean parcel buoyancy. This normalization effectively distinguishes between environments with similar CAPE but exhibiting different buoyancy and integration depth. Also, because the vertical distribution of CAPE can have an important effect on convective updraft strength, it is advantageous to vertically partition CAPE and NCAPE into multiple layers. NCAPE may provide a more useful indicator of buoyancy in environments in which the depth of free convection is shallow and total CAPE is small. It is suggested that NCAPE computations be used in combination with CAPE for evaluation of convective potential.

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Frederick Sanders and David O. Blanchard

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This study of the Oklahoma–Kansas area on 10 May 1985 undertakes to explain why severe convection developed in only a small portion of northwestern Kansas despite large potential instability for surface air over the entire region and despite the approach of a mobile upper-level trough from the southwest. Special soundings from the O–K PRF-STORM program showed that a persistent thermodynamic lid above the warm moist surface boundary layer separated this layer from the middle and upper troposphere in which the instability could be realized and was almost completely effective in suppressing deep convection.

Only one of the soundings with these characteristics showed temporary removal of this lid, and the only convective storm developed near the place and time of this removal. This coincidence points to removal as the likely, although not certain, cause. Isentropic trajectories showed that adiabatic lifting was the cause, and that this lift was part of a series of mesoscale waves with wavelengths of about 200 km, vertical extent from 1 to 5 km above the ground, and crests approximately parallel to the wind shear in this layer. The shear was highly ageostrophic, representing a strong transverse circulation in the exit region of a jet streak. Thus, the jet dynamics were responsible only indirectly for the convective outbreak by providing a favorable environmental shear for the directly responsible mesoscale disturbance.

A series of prominent mesoscale oscillations of surface dewpoint along the northwestern boundary of the moist surface layer began coincidentally with the convective development and is considered to have been caused by it.

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Andrew I. Watson and David O. Blanchard

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Total area divergence is related to area rainfall using data collected during the Florida Area Cumulus Experiment (FACE) 1975 field experiment over a network that covered 1440 km2. A convergence event is defined as a monotonic decrease in total area divergence of more than 25×10−6 s−1 for more than ten minutes. This change in total area divergence is related to the total amount of area rainfall considered to be associated with the convergence event. For 121 convergence events during July and August 1975, a correlation coefficient of −0.59 is found between change in convergence and rainfall amount. When the ensemble is subdivided, it is found that for slow moving convective systems, or when low-level winds are weak, there is twice the amount of rainfall per convergence event. When middle-level (850–500 mb) relative humidity is in the range 50–65%, the correlation coefficient between convergence and rainfall is −0.81. Data are also partitioned according to stability and buoyancy. Convective outflow and its reflection in total area divergence are examined, and relationships are developed for determining the amount of precipitation for each divergence event. For the 75 rain events during FACE 1975, a correlation coefficient of 0.75 is found between the change in divergence and the rainfall amount.

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David O. Blanchard and Raül E. López

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Although they are a fairly consistent feature, the sea-breeze and lake-breeze convergence lines and the associated convection over south Florida during the summer may vary considerably from one day to the next. Daily radar maps indicate a few basic recurring patterns. Analyses of radiosonde data show significant differences corresponding to the different patterns in the local thermodynamic parameters, most notably the mixing ratio. Changes in the synoptic-scale wind field correspond closely to changes in the observed radar patterns and the local thermodynamic conditions. Explanation of the formation and development of the different patterns of convection is given in terms of the complex interaction between the regional-, synoptic-, peninsular- and local-scale circulations.

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David O. Blanchard and Kenneth W. Howard

A brief overview of the 13 June 1984 Denver hailstorm is presented. This storm produced very large hail in a few locations and copious amounts of small hail over a large area. Documentation of the storm includes data from a surface mesonetwork, cooperative observers and storm spotters, dual Doppler radar, profiler winds, radiosonde, lightning detectors, and photographs of smoke tracers. Integration of these data sets provides an interesting and informative look at this destructive storm.

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