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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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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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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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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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Enoch Jo
and
Sonia Lasher-Trapp

Abstract

Entrainment is a key process that can modulate the intensity of supercells, and a better understanding of its impact could help improve forecasts of thunderstorms and the precipitation they produce. In Part III of this series, the three distinct mechanisms of entrainment identified during the mature stage of idealized supercell thunderstorms in Part I (overturning “ribbons” of horizontal vorticity, “disorganized turbulent eddies,” and the “storm-relative airstream”) are examined as the absolute humidity of the environment is decreased. The existence of these mechanisms in a more realistic simulated storm environment is also established. Entrainment is calculated as fluxes of air across the storm core surface; passive fluid tracers help determine the resulting dilution of the storm updraft. Model microphysical rates are used to examine the direct impacts of entrainment on hydrometeors within the storm updraft as well as precipitation that falls to the ground. Results show that as mixed-layer humidity decreases, the “ribbons” and turbulent eddy mechanisms decrease in intensity, but their effects on precipitation production change little. With decreasing humidity in the 3–4 km AGL layer, the storm-relative airstream entrains less humid low-level air into the storm core, decreasing the vertical mass flux, and therefore the precipitation produced by the storm. When the humidity in the mid- to upper troposphere (4–20 km AGL) is decreased, precipitation is significantly reduced, but not due to the effects of the entrained air. Rather, enhanced evaporation and sublimation of falling precipitation decreases the overall precipitation efficiency of the storm.

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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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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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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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Holly Mallinson
,
Sonia Lasher-Trapp
,
Jeff Trapp
,
Matthew Woods
, and
Sophie Orendorf

Abstract

Severe convective storms (SCS) and their associated hazards present significant societal risk. Understanding of how these hazards, such as hailfall, may change due to anthropogenic climate change is in its infancy. Previous methods used to investigate possible changes in SCS and their hail used climate model output and were limited by their coarse spatiotemporal resolution and less detailed representations of hail. This study instead uses an event-level pseudo–global warming (PGW) approach to simulate seven different hailstorms in their historical environments, and again in five different end-of-century PGW environments obtained from the worst-case scenario increases in CO2 of five different CMIP5 members. Changes in large-scale environmental parameters were generally found to be consistent with prior studies, showing mostly increases in CAPE, CIN, and precipitable water, with minor changes in vertical wind shear. Nearly all simulated events had moderately stronger updrafts in the PGW environments. Only cold-season events showed an increase in hail sizes both within the storms and at the surface, whereas warm-season events exhibited a decrease in hail sizes at the surface and aloft. Changes in the event-total hailfall area at the ground also showed a seasonal trend, with increases in cold-season events and decreases in warm-season events. Melting depths increased for all PGW environments, and these increases likely contributed to greater rainfall area for warm-season events, where an increase in smaller hail aloft would be more prone to melting. The differences in PGW simulation hail sizes in cold-season and warm-season events found here are likely related to differences in microphysical processes and warrant future study.

Significance Statement

It is uncertain how severe thunderstorm hazards (such as hail, tornadoes, and damaging winds) may change due to human-induced climate change. Given the significant societal risk these hazards pose, this study seeks to better understand how hailstorms may change in the future. Simulated end-of-century storms in winter months showed larger hail sizes and a larger area of event-total hailfall than in the historical simulations, whereas simulated future storms in spring and summer months showed smaller hail sizes and a reduction in the area where hail fell. An analysis of traditional environmental and storm-scale properties did not reveal a clear distinction between cold-season and warm-season hailstorms, suggesting that changes in small-scale precipitation processes may be responsible.

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