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David Sharon

Abstract

Spatial correlation functions of daily rainfall were derived separately for various parts of the study area and for unseeded and seeded days. The structure of respective rainfall fields was studied by means of the relationship between the geometry of iso-correlation contours of daily rainfall and the average geometry and/or kinematics of cloud systems that produce the rainfall. Results were obtained on 1) the prevalent direction of storm movement over the study area and 2) the change in size of rainfall areas under varying conditions.

Evidence is presented of an average increase of about 10 km in the dimensions of rainfall areas on seeded days. It is suggested that expanded rainfall areas have a twofold effect on the augmentation of the total yield of individual storms: 1) an increase of point rainfall resulting from prolonged exposure to moving cloud systems, and 2) an increase in the area thus affected. Consequently, the increase in water yield of a storm over the entire area covered by it, may not be fully represented by point measurements.

In the regional context, a contraction of storm areas was found in the transition to the arid zone. It is suggested that the size of rainfall areas is one of the fundamental factors controlling regional variations of rainfall as well as variations resulting from cloud seeding.

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David Sharon

Abstract

A frequent problem in the statistical analysis of data in environmental sciences is the synthesis of results obtained independently from various sets of data such as from different measuring points or from replicated experiments. Unlike their analysis separately, set' ultimate combined evaluation has often been given in descriptive terms. It is that critical final MW of data synthesis where meta-analysis comes in. Some principles of the method are presented and their usefulness is discussed. The approach is illustrated using previously published results on the hydrological effect of rainfall enhancement in Israel.

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David Sharon

Abstract

The spatial organization of localized showers in the summer rainfall region of South Africa has been studied by means of spatial correlation functions. A predominant linear arrangement of parallel shower belts has been identified, with an orientation between W–E and WNW–ESE. No differences were found in this respect between the driest years in the 1960's and the wettest ones in the 1950's and 1970's, but a linear trend was absent, or at least not prevalent at any fixed orientation, during the dry spell extending from the late 1940's to the early 1950's. The above linear trend has been found to persist also throughout the year, though, in general, rainfall appeared as spatially less organized during summer months than during spring and autumn.

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David Sharon

Abstract

A correlation analysis of daily rainfall in the Jordan Valley has been undertaken, both within the Valley itself and in relation to the surrounding hilly areas in Israel and Jordan. Results show the climatic singularity of the Jordan Valley, concerning not only the well-known rain deficiency, but also a high degree of independence of rainfall in the Valley relative to the areas surrounding it. This independence is explained in terms of the timing of rain events and their localness. On the other hand, the abrupt and massive topography of the Valley does not affect even locally the orientation of cell movement.

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David Sharon

Abstract

A statistical model based on a Monte Carlo simulation technique has been proposed by Gringorten for making estimates of the duration and frequency of weather events. The model was evaluated and its predictive results tested. Two major problems have been found—a high input requirement and systematic errors in the estimates. The errors were analyzed to determine required modifications of the model. It is the contention of this paper that both problems can be resolved and that the further development of the otherwise promising model should be continued.

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Sharon L. Vadas and David C. Fritts
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Sharon L. Vadas and David C. Fritts

Abstract

The authors determine the spectral linear solutions that arise in response to local 3D body forces and heatings in an idealized environment that turn on and off smoothly but not necessarily slowly over a finite interval in time. The solutions include impulsive through slowly varying body forcings. The forcings result in both a mean response, which is typically significantly broadened spatially in one direction, and a gravity wave response, which allows the fluid to reach this state. The gravity wave field depends on both the spatial attributes of the source and the forcing duration. The frequency of the wave response is the “characteristic” source frequency (formed from the source dimensions) if the forcing frequency is greater than the characteristic frequency and is the forcing frequency otherwise. The radiated gravity waves from zonal forcings have vertical wavelengths, which are approximately twice the vertical extent of the forcing, and horizontal wavelengths, which are at least twice the horizontal extent of the forcing. Wave excitation is increasingly inefficient when the forcing frequency is smaller than the characteristic source frequency. In addition, the mean responses are not confined to the source region; in general, significant spatial broadening of the mean responses occurs. If the source's frequency is high and low, the responses are broadened horizontally and vertically, respectively, with the amount depending on the characteristic scales of the source. If the body forcing is in the eastward direction, then much or all of the ensuing zonal mean wind is eastward. However, for many realistic forcing scenarios, a large percentage of the ensuing zonal wind flows westward. These countersigned jets are displaced meridionally about the source. Thus, spatially confined body forcings create both gravity wave and mean responses if the forcings are fast enough; very slowly varying forcings create only mean responses.

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Sharon L. Sessions, Stipo Sentić, and David J. Raymond

Abstract

Tropical convection that occurs on large-enough space and time scales may evolve in response to large-scale balanced circulations. In this scenario, large-scale midtropospheric vorticity anomalies modify the atmospheric stability by virtue of thermal wind gradient balance. The convective vertical mass flux and the moisture profile adjust to changes in atmospheric stability that affect moisture and entropy transport. We hypothesize that the convection observed during the 2011 DYNAMO field campaign evolves in response to balanced dynamics. Strong relationships between midtropospheric vorticity and atmospheric stability confirm the relationship between the dynamic and the thermodynamic environments, while robust relationships between the atmospheric stability, the vertical mass flux, and the saturation fraction provide evidence of moisture adjustment. These results are important because the part of convection that occurs as a response to balanced dynamics is potentially predictable. Furthermore, the diagnostics used in this work provide a simple framework for model evaluation, and suggest that one way to improve simulations of large-scale organized deep tropical convection in global models is to adequately capture the relationship between the dynamic and thermodynamic environments in convective parameterizations.

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Sharon A. Lewis, Margaret A. LeMone, and David P. Jorgensen

Abstract

Airborne Doppler and flight-level data are used to document the structure and evolution of portions of a late-stage horseshoe-shaped squall line system and its effect on vertical momentum and mass transports. This system, which occurred on 20 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment, was similar to many previously studied, but had some unique features. First, a slow-moving transverse band, which formed the southern leg of the horseshoe, drew most of its low-level updraft air from the squall-line stratiform region on its north side rather than the “environment” to the south. Second, a long-lived cell with many properties similar to a midlatitude supercell, formed 150 km to the rear of the squall line. This cell was tracked for 4 h, as it propagated into and then through the cold pool, and finally dissipated as it encountered the convection forming the northern edge of the horseshoe. Finally, as the squall line was dissipating, a new convective band formed well to its rear.

The transverse band and the long-lived cell are discussed in this paper. Quadruple-Doppler radar data, made possible by tightly coordinated flights by the two NOAA P3s, are used to document the flow with unprecedented accuracy. At lower levels, the transverse band flow structure is that of a two-dimensional convective band feeding on its north side, with vertical fluxes of mass and horizontal momentum a good match to the predictions of the Moncrieff archetype model. At upper levels, the transverse band flow is strongly influenced by the squall line, whose westward-tilting updraft leads to much larger vertical velocities than predicted by the model. The long-lived cell, though weak, has supercell-like properties in addition to its longevity, including an updraft rotating in the sense expected from the environmental hodograph and an origin in an environment whose Richardson number falls within the Weisman–Klemp “supercell” regime.

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David G. Strimaitis, Gary E. Moore, and Sharon G. Douglas

Abstract

This paper contains an analysis of data obtained from measurements of the concentration of tracer gases released during the four intensive measurement periods of the 1985 South-Central Coast Cooperative Air Monitoring Program (SCCCAMP). These tracer experiments were designed to document mesoscale circulation patterns in the coastal region of Santa Barbara and Ventura counties in southern California. Analyses of these concentration data are aimed at evaluating the design of the experiments, describing the movement of the tracers and comparing transport patterns to measured winds, and evaluating the ability of trajectory calculations, which are based on winds from a diagnostic wind model to reproduce those patterns of transport.

The study concludes that patterns of recirculation over the 2-day period of the tests are successfully documented, revealing transport in the Santa Barbara Channel that is consistent with the circulation about the midchannel eddy, and diurnal patterns of onshore–offshore flow. Trajectories based on an interpolation of observed winds disagreed with the observed transport patterns of tracer material, especially for releases that initially traveled into the channel during the early morning hours. Apparently, the circulation in the channel and the complex flow near the coast was not sufficiently resolved by the measurement network.

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