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David O. Blanchard

Abstract

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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Frederick Sanders
and
David O. Blanchard

Abstract

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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Erik N. Rasmussen
and
David O. Blanchard

Abstract

All of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convective available potential energy (CAPE) are examined. Soundings are classified as being associated with nonsupercell thunderstorms, supercells without significant tornadoes, and supercells with significant tornadoes. This classification is made by attempting to pair, based on the low-level sounding winds, an upstream sounding with each occurrence of a significant tornado, large hail, and/or 10 or more cloud-to-ground lightning flashes. Severe weather wind parameters (mean shear, 0–6-km shear, storm-relative helicity, and storm-relative anvil-level flow) and CAPE parameters (total CAPE and CAPE in the lowest 3000 m with buoyancy) are shown to discriminate weakly between the environments of the three classified types of storms. Combined parameters (energy–helicity index and vorticity generation parameter) discriminate strongly between the environments. The height of the lifting condensation level also appears to be generally lower for supercells with significant tornadoes than those without. The causes for the very large false alarm rates in the tornadic/nontornadic supercell forecast, even with the best discriminators, are discussed.

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Andrew I. Watson
and
David O. Blanchard

Abstract

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

Abstract

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
,
William R. Cotton
, and
John M. Brown

Abstract

The hypothesis that inertial instability plays a role in the upscale development of mesoscale convective systems (MCSs) is explored by sampling environments that supported the growth of MCSs in the Preliminary Regional Experiment for STORM (Stormscale Operational and Research Meteorology) (PRE-STORM) network with high quality special soundings. Secondary circulations that occurred in the presence of inertial instabilities were analyzed and documented using rawinsonde data with high spatial and temporal resolution from the PRE-STORM field program. Additional examples of MCS environments were examined using data from the Mesoscale Analysis and Prediction System. Results show strong divergence and cross-stream accelerations occurred at upper-tropospheric levels where inertial instabilities were present. These accelerations were not uniform over the domain but were focused in the regions of instability. Also, the analyses of these data showed that regions of inertial instability may be more commonplace than is typically assumed.

The Regional Atmospheric Modeling System was used to increase the understanding of the basic processes and secondary circulations that enhance MCS growth in inertially unstable environments. Model results indicate that the strength of the divergent outflow was strongly linked to the degree of inertial stability in the local environment. The results also showed a strong dependence on the magnitude of the Coriolis parameter. Finally, experiments using varying degrees of vertical stability indicated that there was also significant sensitivity to this parameter.

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Raul E. Lopez
,
David O. Blanchard
,
Ronald L. Holle
,
Jack L. Thomas
,
David Atlas
, and
Daniel Rosenfeld

Abstract

This work extends the Area Time Integral (ATI) method of Doneaud et al., developed for the lifetime rainfall from an individual storm, and the instantaneous areawide rainfall method of Atlas et al., to the measurement of the cumulative areawide rainfall for periods up to 12 h. The database is the radar and rainfall network data for the three summers of the Florida Area Cumulus Experiment (FACE) II. For 12-h accumulations, V, over the area of 3.6 × 104 km2, we find correlations of 0.92 between radar deduced rainfall and ATI where the latter is computed at intervals from 5 min up to 1 h. The slope of the regression line V/(ATI) is 3.4 mm h−1. Using a gage network with density of 1/11 km2 over an area 1.5 × 104 km2 the correlation coefficient drops to 0.84, still sufficiently high to confirm the validity of the ATI approach. Also, with the gages the V/(ATI) slope decreases to 2.6 mm h−1. The decrease in the correlation is due largely to anomalous propagation which falsely increases storm areas, and partly to the poorer sampling by the gages. The decrease in the rain volume from radar to gage-determined values is probably due to: 1) underestimation of the rain cores by the spaced gages; 2) the use of the wide beam WSR-57 and low threshold for echo area measurements, which detects weak anvil and other precipitation debris to increase the effective echo area without a proportional increase in surface rainfall; and 3) an inappropriate ZR relation. A comparison of the V/(ATI) ratios using either radar or gage rainfall to the value expected theoretically on the basis of the probability distribution of rain rate at Miami shows that one should expect about twice the volume per unit echo area as those observed. This too is believed to be due to the wide beam and the low threshold which tends to enlarge the echo areas excessively. Improved correlations and better agreement with theory are expected at higher radar/rain rate thresholds and with narrower beams.

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Raúl E. López
,
Patrick T. Gannon Sr.
,
David O. Blanchard
, and
Christopher C. Balch

Abstract

Radar and synoptic data obtained during the Florida Area Cumulus Experiment have been used in an exploratory study to investigate the effects of synoptic and regional circulations on the development of convective activity in south Florida. The radar data were used to stratify the days into four groups according to their degree of shower coverage. Mean soundings and typical synoptic maps were constructed for each group. These products were then analyzed to identify and quantify the principal factors that are associated with the production of convective showers. In general, the overall differences in activity among the four groups are a result of the influence of large-scale regional and synoptic flow patterns on the underlying local mesoscale and sea-breeze circulations. This influence is reflected in the properties of the air mass in which the convection is developing, as well as in the additional large-scale dynamic effects that are provided.

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Raúl E. López
,
David O. Blanchard
,
Daniel Rosenfeld
,
William L. Hiscox
, and
Marjorie J. Casey

Abstract

Radar data from the Florida Area Cumulus Experiment were used to study the ensemble characteristics of echo populations and also the structure of echo systems and their phenomenological growth and development process. The diurnal development of the convective field was explored as well. The overall distribution of echoes turned out to be of the truncated lognormal type, which is indicative of growth process that favor the larger, more vigorous clouds. Three principal scales of convective activity were apparent: single cell echoes conglomerates of several individual cells, and large areas of convection that normally form by the joining of existing cloud conglomerates by growth of new cells in the intervening space. The distributions of the radar characteristics of the individual cells were found to be very skewed, which indicates that the large majority of the cells are small and weak, and that only sporadically do a few large and strong cells appear. A considerable number. of the cells originated below the -10°C level indicating that warm rain formation is common in south Florida. Growth curves for the cells indicate fast formation and dissipation stages. A study of the diurnal development of the field of echoes revealed a tendency with time toward more complex echo structures starting from isolated showers to large merged systems. It was also seen that the large multicelled systems are preferred area for the formation of new cells, even at the expense of the population of smaller echoes.

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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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