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E. C. Nickerson

Abstract

An examination of hourly rainfall within areas designated small outside (SO) and large outside (LO) reveals that prior to the introduction of a new seeding device (NEI) in 1975, average peninsular rainfall on seed (S) days outside the Florida Area Cumulus Experiment (FACE) target area was slightly less than rainfall on no-seed (NS) days. However, during the NEI period, average station rainfall within the SO area yields S/NS ratios as high as 3 for the 6 h period following the release of the first flare. Moreover, during the 3 h period prior to the time of the first seed, average SO station rainfall on seed days exceeded the rainfall on no-seed days by approximately 60%.

The large S/NS ratios that occurred outside the FACE target area during the NEI period were not associated with anomalously high rainfall on seed days, but rather with anomalously low rainfall on no-seed days. For the SO area, differences between seed and no-seed samples are highly significant, with one-tailed P values as low as 0.001 as determined from a Wilcoxon (Mann-Whitney) test. Natural variability therefore appears to account for statistically significant differences between area-wide rainfall on seed and no-seed days.

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E. C. Nickerson

Abstract

No abstract available.

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Everett C. Nickerson and Virginia E. Smiley

Abstract

A transcendental equation is presented for the Monin-Obukhov length L based upon (i) the Businger-Dyer surface layer formulations, (ii) parameterizations of the moisture flux, ground storage, and radiation terms in the surface energy budget; and (iii) the wind and temperature at 10 m above a surface characterized by a roughness length z o. The surface temperature, friction velocity and sensible heat flux are obtained from the computed value of L.

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N. Huret, N. Chaumerliac, H. Isaka, and E. C. Nickerson

Abstract

Three microphysical formulations are closely compared to evaluate their impact upon gas scavenging and wet deposition processes. They range from a classical bulk approach to a fully spectral representation, including an intermediate semispectral parameterization. Detailed comparisons among the microphysical rates provided by these three parameterizations are performed with special emphasis on evaporation rate calculations. This comparative study is carried out in the context of a mountain wave simulation. Major differences are essentially found in the contrasted spreading of the microphysical fields on the downwind side of the mountain. A detailed chemical module including the dissolution of the species and their transfer between phases (air, cloud, and rain) is coupled with the three microphysical parameterizations in the framework of the dynamical mesoscale model. An assessment of the accuracy of each scheme is then proposed by comparing their ability to represent the drop size dependency of chemical wet processes. The impact of evaporation (partial versus total) upon the partition of species between gas and aqueous phases is also studied in detail.

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J. F. Mahfouf, E. Richard, P. Mascart, E. C. Nickerson, and R. Rosset

Abstract

Various parameterizations of the planetary boundary layer (PBL) currently used in three-dimensional (3D) mesoscale models are compared with a more complex scheme including a turbulent kinetic energy (TKE) equation. In the first set of simulations made with a ID model against the classical Wangara data, the mean wind, temperature and moisture calculated in the PBL are nearly insensitive to the choice of the parameterization. In the second set of simulations, the TKE parameterization is used in a 3D mesoscale model to simulate sea breeze flows over south Florida. A comparison is presented with previous simulations of Pielke, and Pielke and Mahrer, for the mean flow, and with the third-order turbulence closure model of Brière for the turbulent variables, including a discussion of the turbulent energy budget, The analysis of the results obtained with the TKE scheme shows that the predicted turbulent fields are qualitatively realistic and interact significantly with the sea breeze circulation. Finally, a comparison is made between the TKE scheme and the simpler parameterization of Pielke and Mahrer. It shows only slight differences as far as the mesoscale structure of the mean variables is concerned.

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E. M. Agee, J. T. Snow, F. S. Nickerson, P. R. Clare, C. R. Church, and L. A. Schaal

Abstract

An observational study of the West Lafayette, Ind., tornado of 20 March 1976 has been made that presents photographic evidence of tornado formation accompanied by suction “debris” vortices in the surface boundary layer that produced a cycloidal debris pattern. Furthermore, four case studies of cycloidal debris analysis form this pattern using aerial photography support the multiple-suction vortex model proposed by Agee et al. (1975). Average values of the tangential speed of a suction vortex according to loop shift estimates ranged from 26.4 to 44 m s−1 and maximum values according to loop width estimates ranged from 44.0 to 61.6 m s−1 as the tornado grew to its most intense stage. The aerial and ground surveys of the damage track also showed that the cycloidal debris pattern corresponded to a region of F3 and F4 wind-speed damage.

Photographic evidence also documented a tail-cloud formation similar to the Fargo tornado (Fujita, 1959) that rotated cyclonically through the northeast to northwest sector of the tornado cyclone with a tangential speed of 16 m s−1 at a distance of 3 km from the tornado funnel.

Analysis of cloud photography further showed the features of the tornado system at various stages of intensity. Notable was the formation of a broad descending bowl-shaped cloud accompanied by multiple “condensation” funnels that corresponded to the region of strongest tornado intensity with F4 structural damage. This study also illustrates that multiple-suction debris vortices in the surface boundary layer can occur with only one condensation funnel present. This event is recognized as a similar but slightly different and smaller multiple-vortex phenomenon than that required for multiple-condensation funnels that fully extend through the planetary boundary layer.

Features unique to cyclodial debris patterns and the role of suction vortices in producing certain types of damage are also brought out in this study. Also a concentrated region of damage along a pathline of the southerly flow into the multiple-vortex region may be associated with a strong vorticity feeder band similar to that reported by Golden and Purcell (1975). Several such feeder bands may concentrate the vorticity into the core of the tornado cyclone. The tail-cloud phenomenon appears to be related to this flow feature in the tornado cyclone wind field.

Radar data from Marseilles, Ill., and Grissom Air Force Base, Ind., were analyzed for a portion of the lifetime of the parent thunderstorm system to determine the relationship between the path of the tornado cyclone and the Sadorus, Ill., and West Lafayette tornado tracks. Finally, a pressure drop of 44.6 mb was recorded in the path of the tornado cyclone at the time the West Lafayette tornado funnel was developing.

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