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Enoch Jo and Sonia Lasher-Trapp

Abstract

Supercell thunderstorms can produce heavy precipitation, and an improved understanding of entrainment may help to explain why. In Part I of this series, various mechanisms of entrainment were identified in the rotating stage of a single simulated supercell thunderstorm. The current study examines the strength and effectiveness of these mechanisms as a function of the environmental vertical wind shear in eight different supercell simulations. Entrainment is calculated directly as fluxes of air over the surface of the storm core; tracers are used to assess the resulting dilution of the moistest air ingested by the storm. Model microphysical rates are used to compare the impacts of entrainment on the efficiency of condensation/deposition of water vapor on hydrometeors within the core, and ultimately, upon precipitation production. Results show that the ascending “ribbons” of horizontal vorticity wrapping around the updraft contribute more to entrainment with increasing vertical wind shear, while turbulent eddies on the opposite side of the updraft contribute less. The storm-relative airstream introduces more low-level air into the storm core with increasing vertical wind shear. Thus, the total entrainment increases with increasing vertical wind shear, but the fractional entrainment decreases, yielding an increase in undiluted air within the storm core. As a result, the condensation efficiency within the storm core also increases with increasing vertical wind shear. Due to the increase in hydrometeors detrained aloft and the resulting enhanced evaporation as they fall, the precipitation efficiency evaluated using surface rainfall decreases with increasing vertical wind shear, as found in past studies.

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Jennifer L. Bewley and Sonia Lasher-Trapp

Abstract

A modeling framework representing variations in droplet growth by condensation, resulting from different saturation histories experienced as a result of entrainment and mixing, is used to predict the breadth of droplet size distributions observed at different altitudes within trade wind cumuli observed on 10 December 2004 during the Rain in Cumulus over the Ocean (RICO) field campaign. The predicted droplet size distributions are as broad as those observed, contain similar numbers of droplets, and are generally in better agreement with the observations when some degree of inhomogeneous droplet evaporation is considered, allowing activation of newly entrained cloud condensation nuclei. The variability of the droplet growth histories, resulting primarily from entrainment, appears to explain the magnitude of the observed droplet size distribution widths, without representation of other broadening mechanisms. Additional work is needed, however, as the predicted mean droplet diameter is too large relative to the observations and likely results from the model resolution limiting dilution of the simulated cloud.

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Charles A. Doswell III and Sonia Lasher-Trapp

Abstract

Meteorological observing networks are nearly always irregularly distributed in space. This irregularity generally has an adverse impact on objective analysis and must be accounted for when designing an analysis scheme. Unfortunately, there has been no completely satisfactory measure of the degree of irregularity, which is of particular significance when designing artificial sampling networks for empirical studies of the impact of this spatial distribution irregularity. The authors propose a measure of the irregularity of sampling point distributions based on the gradient of the sums of the weights used in an objective analysis. Two alternatives that have been proposed, the fractal dimension and a “nonuniformity ratio,” are examined as candidate measures, but the new method presented here is considered superior to these because it can be used to create a spatial “map” that illustrates the spatial structure of the irregularities in a sampling network, as well as to assign a single number to the network as a whole. Testing the new measure with uniform and artificial networks shows that this parameter seems to exhibit the desired properties. When tested with the United States surface and upper-air networks, the parameter provides quantitative information showing that the surface network is much more irregular than the rawinsonde network. It is shown that artificial networks can be created that duplicate the characteristics of the surface and rawinsonde networks; in the case of the surface network, however, a declustered version of the observation site distribution is required.

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Daniel H. Moser and Sonia Lasher-Trapp

Abstract

Cumulus clouds are frequently observed as comprising multiple successive thermals, yet numerical simulations of entrainment have not investigated this level of detail. Here, an idealized simulated cumulus congestus consisting of three successive thermals is used to analyze and understand their role in maintaining the high liquid water content in the core of the cloud, which past 1D modeling studies have suggested can ultimately determine its ability to precipitate. Entrainment and detrainment are calculated directly at the edge of the cloud core at frequent time intervals. Entrainment maxima occur at the rear of the toroidal circulation associated with each thermal and thus are transient features in the lifetime of multithermal clouds. The evolution of the least diluted parcels within each thermal shows that the entrainment rates alone cannot predict the erosion of the high liquid water content cores. A novel analysis of samples of entrained and detrained air within each successive thermal illustrates tendencies for even positively buoyant air, containing condensate, to be entrained by later thermals that rise in the wakes of their predecessors, limiting their dilution. The later thermals can achieve greater depths and produce precipitation when a single thermal could not. Future work is yet needed to evaluate the generality of these results using multiple clouds simulated in different environments with less-idealized modeling frameworks. Implications for current cumulus parameterizations are briefly discussed.

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Sonia Lasher-Trapp and Justin P. Stachnik

Abstract

Numerous studies have indicated the potential for giant and ultragiant aerosol particles to expedite the warm-rain process as a result of their extreme sizes. The central question regarding their importance is, Are they present in large enough numbers to influence the microphysics of the clouds significantly? Thus, quantification of these particles and their variability is paramount. New observations collected during the second Alliance Icing Research Study (AIRS II) are presented as evidence of the presence and variability of giant and ultragiant aerosol particles over a continental region—in this case, within the eastern Great Lakes region and parts of the midwestern United States and Canada during one month in winter 2003. Sources and factors contributing to the amount of these particles observed in the lower atmosphere were difficult to identify separately; future studies incorporating high-resolution weather modeling are likely needed.

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Holly M. Mallinson and Sonia G. Lasher-Trapp

Abstract

Downdrafts extending from convective clouds can produce cold pools that propagate outward, sometimes initiating new convection along their leading edges. Models operating at scales requiring convective parameterizations usually lack representation of this detail, and thus fail to predict this convective regeneration and longer episodes of convective activity. Developing such parameterizations requires an improved understanding of the physical drivers of cold pools, and detailed studies of the roles of all the contributing microphysical processes have been lacking. This study utilizes a set of 12 simulations conducted within a single convective environment, but with variability in the microphysical fields produced by varying parameters influencing warm-rain or ice processes. Time-integrated microphysical budgets quantify the contribution of each hydrometeor type to the total latent cooling occurring in the downdrafts that form and sustain the cold pool. The timing of the onset of the cold pool is earlier in cases with a stronger warm rain process, but both graupel and rain were equally as likely to be the dominant hydrometeor in the downdraft first forming the cold pool. Graupel sublimation is the dominant term in sustaining the cold pool in all simulations, but the evaporation of rain has the strongest correlation to the cold pool expansion rate, depth, and intensity. Reconciling the current results with past studies elucidates the importance of considering: graupel sublimation, the latent cooling only in downdrafts contributing to the cold pool, and latent cooling in those downdrafts at altitudes that may be significantly higher than the melting level.

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Daniel H. Moser and Sonia Lasher-Trapp

Abstract

Cumulus clouds modify their immediate surroundings by detraining their warm, humid updrafts. When clouds are closely spaced, this conditioning of the local environment may alter the properties of the air entrained by neighboring clouds and slow their dilution. This effect has not been quantified, nor has its importance been determined for influencing the amount of convective rainfall from a system of neighboring clouds. Here, a series of idealized numerical simulations, which are based on an observed line of precipitating cumulus congestus clouds, is performed using increasingly smaller cloud spacing to investigate how cloud proximity may alter entrainment, cloud development, and convective rainfall. For clouds of radius R, which is approximately 1 km in these simulations, distances between updraft centers from 4R through 9R are tested. Over this range, the initial clouds all exhibit negligible differences in the directly calculated entrainment rates and in the thermodynamic characteristics of the entrained air. Instead, for cloud separation distances of less than 6R, the subcloud inflow is increasingly disturbed, limiting initial cloud depths and slowing updraft speeds and precipitation onset. Ultimately, however, these same cases produce a new generation of clouds that are stronger and produce more rainfall than for all other cases. The smaller cloud separation distance allows precipitation outflows from the initial clouds to meet and force new, stronger cloud updrafts. For this second generation of clouds, their entrained air is slightly more humid, but the stronger updrafts and ingestion of residual ice and precipitation from earlier clouds appear to be most important for enhancing their rainfall.

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Robert J. Trapp, Kimberly A. Hoogewind, and Sonia Lasher-Trapp

Abstract

The effect of anthropogenically enhanced greenhouse gas concentrations on the frequency and intensity of hail depends on a range of physical processes and scales. These include the environmental support of the hail-generating convective storms and the frequency of their initiation, the storm volume over which hail growth is promoted, and the depth of the lower atmosphere conducive to melting. Here, we use high-resolution (convection permitting) dynamical downscaling to simultaneously account for these effects. We find broad geographical areas of increases in the frequency of large hail (35-mm diameter) over the United States, during all four seasons. Increases in very large hail (50-mm diameter) are mostly confined to the central United States, during boreal spring and summer. And, although increases in moderate hail (20-mm diameter) are also found throughout the year, decreases occur over much of the eastern United States in summer. Such decreases result from a projected decrease in convective-storm frequency. Overall, these results suggest that the annual U.S. hail season may begin earlier in the year, be lengthened by more than a week, and exhibit more interannual variability in the future.

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Kathleen Quardokus, Sonia Lasher-Trapp, and Eric M. Riggs

Participating in scientific research as an undergraduate student provides an opportunity to increase understanding of how scientific knowledge is advanced, to learn new research tools, to develop the ability to critically analyze new ideas, and to practice disseminating scientific findings. This experience unfortunately has traditionally been limited to students that can participate in select programs (e.g., summer research experiences, undergraduate positions in a faculty member's research group, special topics courses, independent study, or internships).

A new laboratory course has been developed to provide sophomore- level atmospheric science students with the opportunity to participate in an authentic research project within the structure of an academic semester. The course consists of two modules based upon research topics currently under investigation by faculty (here, specific problems in cloud microphysics and severe weather research). Students participate in learning activities, work as a research team, and formally present research findings. Phenomenological evaluation of the new course through interviews, surveys, and student performance assessments, using constant comparative analysis, suggests these students improve their ability to understand and perform authentic research. The students attribute their success to the “scaffolding” structure of the course, peer collaboration, and their own high level of enthusiasm. Results also imply that students gain some clarification of their career options.

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Sonia G. Lasher-Trapp, Charles A. Knight, and Jerry M. Straka

Abstract

The growth of ultragiant aerosol (UGA) in a Lagrangian framework within a simulated three-dimensional cloud is analyzed and compared with radar and aircraft observations of a cumulus congestus collected during the Small Cumulus Microphysics Study (SCMS). UGA are ingested into the simulated cloud and grow by continuous collection; the resulting radar reflectivity factor and raindrop concentrations are evaluated at 1-min intervals. The calculations produce a substantial echo (>30 dBZ) within a short time (18 min), containing few raindrops (0.3 L−1). The calculated radar echo is very sensitive to the amount of UGA ingested into the modeled cloud and its liquid water content. The modeled radar echo and raindrop concentrations are consistent with the observations in that the differences fall within the modeling and measurement limitations and uncertainties.

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