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Thomas P. Charlock and William L. Smith Jr.
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Walter P. Smith and Robert L. Gall

Abstract

Squall lines possessing nearly all the characteristics of tropical squall lines occasionally develop during the summer monsoon over southern Arizona and northwestern Mexico. Initial thunderstorm formation is over the Continental Divide in the late afternoon and the systems become organized within a few hours. Satellite imagery, cloud-to-ground lightning strike data, and surface observations indicate the squall lines move from east to west or northeast to southwest by discrete propagation faster than the environmental flow at all levels below 20 kPa so that most of the anvil clouds lag behind.

The synoptic-scale circulation is anomalous with a strong ridge located over the western United States and a deep trough located over the eastern United States. West to northwest winds are found in the boundary layer over southern Arizona and northwest Mexico while a deep layer of east winds are observed above. As a result most of the environmental wind shear is confined to the lowest 2.5 km above the ground with very little shear at higher altitudes. The low-level wind shear seems to be required for the westward propagation of thunderstorms and the formation of the squall lines. Extremely dry midtropospheric air develops in the easterly flow through some combination of advection and subsidence and also appears to be an important factor in the development of the squall lines and in the creation of severe thunderstorms.

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F. I. Shimabukuro, P. L. Smith, and W. J. Wilson

Abstract

The daytime and nighttirne distribution of the ozone density in the atmosphere has been determined from ground-based measurements of the emission spectra of the strong 40,4 = 41,3 rotational line of ozone at 101.737 GHz (λ = 2.9 mm), using a least-squares parameter estimation technique. The inversion procedure is described, and a linearized model is used to obtain approximate error bounds on the ozone parameter estimates.

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P. KRISHNA RAO, W. L. SMITH, and R. KOFFLER

Abstract

A composite histogram method is used to objectively derive sea-surface temperature distribution from satellite radiation measurements for the Northern and Southern Hemispheres. Comparisons with conventional observations yield root-mean-square differences of 2°–3°K. Some of the differences can be accounted for by factors such as the coherent noise introduced by the onboard tape recorder, insufficient atmospheric attenuation corrections, and basic differences between the two types of temperature measurements.

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Judah L. Cleveland, Jeffrey A. Smith, and James P. Collins

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Numerical simulations allow users to adjust factor settings in experimental runs to understand how changes in those factors affect the output. However, it is not straightforward to analyze these outputs when multiple input factors are changed, especially simultaneously. For the atmospheric sciences, Stein and Alpert introduced a method they termed “factor separation” in order to separate the “pure contribution” of a factor from “pure interactions” of combinations of factors. Although factor separation appears to be used exclusively within the atmospheric sciences, other communities achieve a similar result by computing “main effects” via design of experiments methods. While both methods yield different estimates for the factor effects or contributions, we show that factor separation effects are identical to “simple effects” in the design of experiments literature. We demonstrate how both factor separation effects and design of experiments main effects correspond to multiple linear regression coefficients with different coding methods; thus, effect estimates produced by each method are equivalent through a variable transformation. We illustrate the application of both methods using a shallow-water simulation. This connection between factor separation and the design of experiments discipline extends factor separation to more applications by making available design of experiments methods for decreasing the computational cost and calculating effects for factors with more than two settings, both of which are limitations of factor separation.

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Guoqi Han, C. L. Tang, and P. C. Smith

Abstract

TOPEX/Poseidon (T/P) altimeter data over the period 1992–98 have been analyzed to examine annual variability of sea surface elevation and currents over the Scotian Shelf and Slope. A modified orthogonal response analysis is used to derive the annual cycle while simultaneously removing the residual tides and other dynamical processes at the appropriate T/P alias periods. An evaluation of the M 2 and K 1 alias variations is carried out, suggesting notable tidal correction errors off Cape Cod and over Georges Bank. The along-track sea surface slopes, which represent surface geostrophic current components normal to the track, are estimated on selected T/P ascending and descending ground tracks. The annual altimetric sea level harmonic is compared with steric height anomalies and wind-driven setup. The comparison indicates that the altimetric sea surface elevation variability is dominated by the baroclinic (and associated barotropic) component and supplemented by the wind-driven and remotely forced components. Altimetric elevations agree favorably with tide-gauge data at Halifax, Nova Scotia, and well with those at St. John's, Newfoundland. Wintertime intensification of the shelf-break flows is indicated in the altimetric surface currents, consistent with the solutions of regional diagnostic model forced by baroclinicity and boundary flows. Altimetric results clearly demonstrate seasonal variability of northeastward slope current stronger in fall and winter and weaker in spring and summer, which is less well resolved in the model. Assimilation of altimetric data into regional circulation models could help improve their prognostic ability to hindcast and nowcast seasonal variability of shelf-edge and slope water circulation. This study also implies a demand for better shelf tidal models to detide altimetric data for extraction of semiannual and shorter-period processes.

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P. L. Smith Jr., C. G. Myers, and H. D. Orville

Abstract

This paper describes and compares various methods for calculating radar reflectivity factors in numerical cloud models that use bulk methods to characterize the precipitation processes. Equations sensitive to changes in the parameters of the particle size distributions are favored because they allow simulation of phenomena causing such changes. Marshall-Palmer-type functions are established to represent hailstone size distributions because the previously available distributions lead to implausibly large reflectivity factors. Simplified equations are developed for calculating reflectivity factors for both dry and wet hail. Some examples are given of the use of the various equations in numerical cloud models.

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W. P. Menzel, W. L. Smith, and T. R. Stewart

Abstract

A CO2 cloud tracking technique to determine simultaneous heights and velocities of cloud motion winds is presented. Using animated CO2 channel imagery from VAS, multi-level cloud situations are separated into high, middle and low level cloud motion wind vectors by the CO2 slicing method. The VAS CO2 channel radiometric values are used in the CO2 absorption method to assign quantitative heights to the cloud vectors; cloud top pressures are determined from the ratio of the deviations in cloud produced radiances and the corresponding clear air values for three CO2 channels in a radiative transfer equation formulation. Two case studies are presented that show CO2 cloud-motion wind vectors to be in good agreement with radiosonde wind observations and CO2 cloud heights to be within a 50 mb rms deviation of radiosonde, bispectral and stereo height determinations.

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A. S. Dennis, P. L. Smith Jr., G. A. P. Peterson, and R. D. McNeil

Abstract

An electronic hailstone momentum sensor has been developed which records hailstone impacts on magnetic tape. The instrument and the programs for analyzing the recorded data are described. Sensors were operated during six hailstorms in 1969 and recorded 624 hailstone impacts. Derived frequency distributions of hailstone size show the median diameter for all stones recorded to be near 0.9 cm. Significant variations exist among storms. Values of the equivalent radar reflectivity factor Z e have been computed for the hailshafts sampled, for X-hand and S-band radar, and for both wet and dry hailstones. The values range up to 70 dBz (107.0 mm6 m−3) and show good agreement with radar-measured values. The results suggest that dual-wavelength radar systems are not much better than S-band sets alone for estimating hailstone size.

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D. J. Musil, E. L. May, P. L. Smith Jr., and W. R. Sand

Abstract

Precipitation particle sizes were measured using a continuous hydrometeor sampler (foil impactor) during penetrations of hailstorms with an armored T-28 aircraft. Data have been analyzed from three penetrations of a storm near Raymer, Colorado, on 9 July 1973 at altitudes between 5.5 and 7.2 km MSL, which correspond to temperatures between about −2°C and −12°C. Other results pertinent to the Raymer storm are discussed in Parts I,II,III and elsewhere in this issue.

Most of the particles were identified as ice particles or ones containing both ice and water; however, significant amounts of liquid particles were found in the updrafts of developing cells at temperatures as cold as −12°C. Particles larger than 5 mm in diameter were typically found along the edges of the updrafts, with the precipitation concentrations being strongly dependent on these larger particles. The downdrafts were composed of ice particles.

Several particle size distributions from one of the penetrations were examined. The distributions are roughly exponential, or bi-exponential when large particles are present.

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