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W. S. Wilson
and
J. P. Dugan

Abstract

Two multi-ship XBT surveys have acquired data on the mesoscale temperature structure in the region of the Kuroshio Extension. Thermal sections and maps illustrate cold features in the thermocline near 31°N and warm ones near 37°N. Isotherm displacement spectra exhibit a plateau of order 103 m2 per cycle per kilometer for scales longer than 500 km, and they roll off between the second and third inverse power of the wavenumber for shorter scales. There is a significant decrease in spectral amplitude south of about 31°N.

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Louis J. Battan
and
David S. Wilson Jr.

Abstract

Visual observations on several mountain peaks in the Santa Catalina Mountains in southeastern Arizona reveal that solid ice particles commonly fall from cumulonimbus clouds in the summer. Hailstones having diameters >2 cm occurred on only one day out of a total of 76 days over a four-summer period. On the other hand, on the top of a 2800-m peak, ice or snow pellets or hail fell on 18 out of 30 observational days during a two-summer period. It seems reasonable to speculate that virtually all thunderstorms in this area contain such particles at some stage in their development.

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R. Lalbeharry
,
J. Mailhot
,
S. Desjardins
, and
L. Wilson

Abstract

A coupled atmospheric and ocean wave system has been developed to study the impact of changes of surface roughness length induced by ocean waves. A two-way coupling between a mesoscale atmospheric model, MC2, and an oceanic wave model, a regional version of WAM Cycle-4, was designed to ensure consistency in the treatment of the atmospheric boundary layer parameterizations between the two models. Two different approaches, based on the wave age of Smith et al. and the wave-induced stress of Janssen, are used to compute a coupling parameter, called the Charnock parameter, expressed as the nondimensional surface roughness length. The coupling between the two models is accomplished by the use of this parameter, which is a function of sea state, instead of the constant value obtained from empirical studies using the well-known Charnock relation.

The impacts on the atmospheric forecasts are discussed in Part I. In Part II, the ocean wave forecasts resulting from this two-way coupling are discussed for four different real cases. The two approaches are evaluated by comparing ocean wave model outputs obtained from the coupled and uncoupled systems against buoy observations. The coupling has some beneficial impact, especially in areas of extreme sea states. The significant wave heights are reduced in the coupled runs and generally show better agreement with the buoy observations. The impact of the coupling also exhibits some dependence on the intensity of the cyclone development, with larger changes occurring in the case of rapidly deepening storms.

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S. H. S. Wilson
,
N. C. Atkinson
, and
J. A. Smith

Abstract

The United Kingdom Meteorological Office (UKMO) has developed an airborne interferometer to act as a simulator for future satellite-based infrared meteorological sounders. The Airborne Research Interferometer Evaluation System (ARIES) consists of a modified commercial interferometer mounted on the UKMO C-130 aircraft. The instrument is sensitive to the wavelength range 3.3–18 μm and has a maximum optical path difference of ±1.037 cm. This paper describes the design and performance of ARIES, discusses instrument calibration, and presents some preliminary results. An important problem associated with the use of the new generation of high-spectral resolution infrared meteorological sounders is that improvements need to be made to knowledge of atmospheric spectroscopy and radiative transfer. These improvements are necessary to extract the promised vertical and absolute resolution in temperature and humidity retrievals from these new high-spectral resolution sounders. By virtue of the extensive instrumentation that is available on the C-130 aircraft for observing and measuring the basic meteorological and atmospheric parameters (e.g., in situ temperature, humidity, and ozone), it is hoped that ARIES will be an important tool for use in studying this issue.

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K. J. Wilson
,
S. L. Barrell
, and
R. Del Beato

Abstract

An approach to verifying position forecasts of line phenomena was developed. The technique, which tests for occurrence/nonoccurrence and the time of arrival of line phenomena, was applied to frontal position prognoses over southeastern Australia. It was found that operational manual prognoses exhibited no skill in cases of frontogenesis or total frontolysis. In other cases, accuracy varied seasonally and geographically, with summer apparently being the most difficult period for frontal prognosis. Frontal position forecasts were successful for 70 percent of frontal crossings in the 27-month trial period, but two of every five forecast events were false alarms. The Hanssen and Kuipers skill discriminant for a 24-hour Yes/No forecast of a frontal crossing was new 0.6.

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E. F. Greneker
,
C. S. Wilson
, and
J. I. Metcalf

Abstract

Joint observations by radar and high-frequency sferics detectors at Georgia Institute of Technology provided unique data on the Atlanta tornado of 24 March 1975. The classic hook echo was detected by radar at a range of about 26 km, 15 min before the tornado touched down. While the tornado was on the ground the sferics burst rate was very low, despite very high values recorded immediately before and after this interval. This observation, together with visual reports of a strong cloud-to-ground discharge at the time of tornado touchdown, suggests an interaction of the tornado with the electric field of the storm.

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B. N. Hanstrum
,
K. J. Wilson
, and
S. L. Barrell

Abstract

A climatology of the prefrontal westerly trough of southern Australia is presented, based on data for the 10-yr period 1976–85. Trough formation was confined to the longitudes of the Australian continent in the warmer months of the year. An average of approximately 15 troughs/yr were observed, evenly distributed each month from September to April, with appreciable interannual variability in incidence. Troughs formed in advance of a major Southern Ocean cold front in a region of differential thermal advection. Genesis occurred primarily over the west of the continent in spring and autumn, but over southeastern Australia in summer.

Variability in the location of formation at different times of year was related to the seasonal shift of the subtropical ridge over the continent, the shape of the southern continental coastline, and periods of low zonal-index flow. A class of troughs prone to rapid intensification after formation was also identified.

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B. N. Hanstrum
,
K. J. Wilson
, and
S. L. Barrell

Abstract

A case study of frontogenesis within a surface prefrontal trough over southern Australia is presented. The trough developed ahead of a surface cold front and, over a period of approximately 24 h, intensified into a mature summertime frontal system while the original front underwent total frontolysis. Two-hourly rawinsonde ascents at three locations were used to examine the structure of the trough both before and after frontogenesis. Diagnosis of the frontogenetic tendencies showed that horizontal deformation of the potential temperature field was the main forcing mechanism.

The process studied occurs frequently in the warmer months of the year, and a conceptual model is proposed for the process of frontogenesis within the trough and frontolysis of the preexisting midlatitude front. Important components include differential thermal advection, the action of Coriolis turning in association with ageostrophic vertical circulations, and horizontal deformation of the thermal field.

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C. Mueller
,
T. Saxen
,
R. Roberts
,
J. Wilson
,
T. Betancourt
,
S. Dettling
,
N. Oien
, and
J. Yee

Abstract

The Auto-Nowcast System (ANC), a software system that produces time- and space-specific, routine (every 5 min) short-term (0–1 h) nowcasts of storm location, is presented. A primary component of ANC is its ability to identify and characterize boundary layer convergence lines. Boundary layer information is used along with storm and cloud characteristics to augment extrapolation with nowcasts of storm initiation, growth, and dissipation. A fuzzy logic routine is used to combine predictor fields that are based on observations (radar, satellite, sounding, mesonet, and profiler), a numerical boundary layer model and its adjoint, forecaster input, and feature detection algorithms. The ANC methodology is illustrated using nowcasts of storm initiation, growth, and dissipation. Statistical verification shows that ANC is able to routinely improve over extrapolation and persistence.

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Matthew J. Bunkers
,
Matthew B. Wilson
,
Matthew S. Van Den Broeke
, and
Devon J. Healey

Abstract

In this exploratory study, storm-motion deviations are examined for concurrent tornadic and nontornadic supercells using 171 cases. This deviation, or “delta,” is defined as the shear-orthogonal distance between the observed supercell motion and a baseline supercell-motion prediction. Larger deltas—representing supercells moving farther right (in a shear-relative sense) compared to the baseline prediction—are hypothesized as more likely to be associated with tornadoes than nearby supercells with smaller deltas, consistent with recent research. Automated radar tracking is used to calculate supercell motion every scan, which then is compared to a model-derived hourly supercell-motion prediction to calculate the deltas. Tornadic supercells have larger average deltas (by 1.9–2.0 m s−1) than nearby nontornadic supercells when using 20- and 30-min storm-motion calculations, and the deltas are larger for the tornadic versus nontornadic supercells ∼80% of the time. Average delta trends also are positive 62%–70% of the time prior to tornadogenesis. The supercell-motion deltas show a modest positive correlation with EF-scale damage rating, indicating a possible relationship between tornado rating and storm deviation. The relative delta differences between tornadic and nontornadic supercells appear more meaningful than the absolute delta magnitudes (i.e., about 70% of tornadic cases with negative average deltas had deltas that were less negative compared to concurrent nontornadic supercells). This concept shows promise as a potential tool to assist operational forecasters in tornado warning decisions.

Significance Statement

Supercells are rotating thunderstorms, and these storms produce the most destructive tornadoes. However, it has been challenging to forecast which supercells will produce tornadoes. In this exploratory study to help better forecast supercell tornadoes, we looked at how the observed supercell motion compared to the predicted motion, based on a commonly used method. We found tornadic supercells tend to move somewhat differently from the predicted motion—compared to nearby nontornadic supercells. This unusual movement often starts prior to tornadogenesis, potentially providing lead time to tornado formation. Pending further validation, development, and testing of real-time analysis tools, this storm-motion behavior could be used by operational forecasters as a factor to help determine when (or when not) to issue a tornado warning for a supercell thunderstorm, thus providing better information to the public.

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