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R. B. SMITH, C. E.

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R. J. Hung and R. E. Smith

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Gravity waves with wave periods of 13 to 15 min and horizontal phase velocities of 90 to 220 m s−1 were present in ground-based observations of the upper atmosphere during time periods when tornadoes were occurring and gravity waves with wave periods of 20 to 25 min and horizontal phase velocities of 100 to 200 m s−1 were detected when a hurricane was present. Combinations of available neutral atmosphere data and model parameter values were used with a group ray tracing technique in an attempt to locate the sources of these waves. Computed sources of the waves with periods of 13 to 15 min were located within 50 km of the locations where tornadoes touched down from 2 to 4 h later. In the case of the waves with periods of 20 to 25 min it was found that the computed location of the source was roughly where the hurricane would be located 3 h after the waves were excited. The applicability of the present study to a tornado and hurricane warning system is noted.

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R. E. Smith and R. J. Hung

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A three-dimensional, nine-element, high-frequency cw Doppler sounder array has been used to detect ionospheric disturbances during periods of severe weather, particularly during periods with severe thunderstorms and tornadoes. One typical disturbance recorded during a period of severe thunderstorm activity and one during a period of tornado activity have been chosen for analysis in this note. The observations indicate that wave-like disturbances possibly generated by the severe weather have wave periods in the range 2–8 min which place them in the infrasonic wave category.

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E. M. Frisby and J. R. Smith

Data for fifty high-altitude balloon flights made by Raven Industries, Inc. between January, 1957, and October, 1960, have been released for analysis. Raven flight altitudes and trajectories supplement those used by A. D. Belmont. They serve to confirm and in a few instances to modify his “meanwhile” conclusions, based on the data currently available.

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John E. Zimmerman, Phillip J. Smith, and David R. Smith

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A study of the sensitivity of a weak winter extratropical cyclone to latent heat release (LHR) is presented using 48-h simulations of the cyclone's evolution derived from three versions of the LFM model: a MOIST simulation in which full model physics was employed, a DRY simulation in which all latent heating was removed, and a DOUBLE MOIST simulation in which the effect of latent heating on the temperature field was doubled. Results indicate that a deepening cyclone occurs in the DOUBLE MOIST simulation, a near steady-state cyclone in the MOIST simulation, and a filling cyclone in the DRY simulation. Thus, for this case the presence and intensity of LHR is of critical importance to this cyclone's intensification. In addition, using height tendency diagnoses, it is concluded that for this case in the lower troposphere the dominant LHR influence is direct, through the explicit diabatic heating forcing in the height tendency equation. In contrast, in the middle and upper troposphere this direct LHR role is no longer dominant, but rather shares its importance with the indirect effect, represented by the influence of LHR on the dynamical forcing mechanisms.

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R. J. Hung, T. Phan, and R. E. Smith

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Penetrative convection, thunderstorms, squall lines, etc., all generate atmospheric gravity waves which can be observed by a ground-based ionospheric Doppler sounder array. Sources of these waves can be determined from reverse ray tracing computations. Case studies of gravity waves associated with isolated tornadic storms on 13 January 1976 were summarized to establish the minimum data sampling time required for correct spectral analysis and ray tracing computations. It was concluded that the data sampling time can be reduced to two to three times the wave period while still obtaining a reasonably good power spectral density. It was also demonstrated that the data sampling time can be reduced to two to three times the time delay of the wave arrival between two station pairs while still obtaining a justifiably good cross-spectral analysis. Computed source locations of the observed gravity waves are compared with conventional and satellite meteorological data.

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R. J. Hung, T. Phan, D. C. Lin, R. E. Smith, R. R. Jayroe, and S. West

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Enhanced convection-initiated gravity waves associated with an isolated tornado in the absence of a squall line are investigated. Ray-tracing computations based on data observed on 29 May 1977 indicated that the wave sources were located in north-central Oklahoma. Comparison with a radar echo map during the time period when the waves were excited showed that the waves were generated by an isolated cloud with enhanced convection. GOES infrared digital data during the time period from wave excitation to tornado touchdown were analyzed. Results showed that the cloud where the gravity waves were excited was characterized by both a very low temperature at the cloud top and a very high expansion rate of the cold cloud-top area. The lead time between the excitation of the gravity waves and the tornado touchdown is discussed in conjunction with the growth rate of the clouds associated with the tornado.

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David R. Smith, Michelle E. Pumphry, and John T. Snow

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An error analysis of the Purdue Regional Objective Analysis of the Mesoscale (PROAM) scheme is described. PROAM is an interactive, multiple iteration objective analysis package for surface meteorological data. It utilizes a Gaussian weighting function similar to that of Barnes. A problem with applying interpolation routines to surface data is that the surface station network has a highly nonuniform spatial distribution. Smith and Leslie have shown that this nonuniformity can impact the accuracy of the analysis. The objective of this investigation is to determine quantitatively the contribution that this nonuniformity of station locations has on the analysis of data for two different distributions of stations.

Experiments were done by varying the values of important analysis parameters in order to generate error curves for each parameter. Analytic functional representations for pressure and temperature were used to generate input data free of observational error, as well as to produce exact analyses. With exact fields available, root-mean-square errors and average grid point errors were calculated for analyses using uniform, actual and edited station distributions.

Results show that the PROAM scheme performs a better interpolation of the observations when the stations are more uniformly distributed. Root-mean-square and average grid point errors for analyzed fields decrease as the uniformity of the station distribution increases. Furthermore, elimination of some data stations (when several are clustered within a small area) can result in an analysis with smaller errors than a field with more stations but a less uniform distribution. The error analysis performed in this study suggests methods which can minimize errors in the analyzed fields.

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R. M. Brown, L. A. Cohen, and M. E. Smith

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Recent studies of particulate and gaseous materials in the atmosphere have raised important questions about diffusion at distances of 10–100 km. A photometric densitometer, initially developed for a quantitative study of oil-fog concentrations at ground level, has been adapted for use in an aircraft. Real-time measurements of ground-level and airborne particle concentrations are presented to distances of 120 km, and the implications of these data in terms of large-scale dispersion are discussed.

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Jessica R. Smith, Henry E. Fuelberg, and Andrew I. Watson

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Cloud-to-ground lightning data from the National Lightning Detection Network are used to create a warm season (May–September) lightning climatology for the northern Gulf of Mexico coast for the 14-yr period 1989–2002. Each day is placed into one of five flow regimes based on the orientation of the low-level flow with respect to the coastline. This determination is made using the vector mean 1000–700-hPa wind data at Lake Charles and Slidell, Louisiana. Flash densities are calculated for daily, hourly, and nocturnal periods.

Spatial patterns of composite 24-h and nocturnal flash density indicate that lightning decreases in an east-to-west direction over the region. Flash densities for the 24-h period are greatest over land near the coast, with relative maxima located near Houston, Texas; Lake Charles, Baton Rouge, and New Orleans, Louisiana; Biloxi, Mississippi; and Mobile, Alabama. Flash densities during the nocturnal period are greatest over the coastal waters.

Lightning across the northern Gulf coast is closely related to the prevailing low-level synoptic flow, which controls the sea breeze, the dominant forcing mechanism during the warm season. Southwest flow, the most unstable and humid of the five regimes, exhibits the most flashes. In this case, sea-breeze-induced convection is located slightly inland from the coast. Northeast flow, the driest and most stable of the regimes, exhibits the least amount of lightning. The large-scale flow restricts the sea breeze to near the coastline.

Geographic features and local mesoscale circulations are found to affect lightning across the region. Geographic features include lakes, bays, marshes, swamps, and coastline orientations. Thermal circulations associated with these features interact with the main sea breeze to produce complex lightning patterns over the area.

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