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  • 1
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    The Importance of the Shape of Cloud Droplet Size Distributions in Shallow Cumulus Clouds. Part II: Bulk Microphysics Simulations (2017)
    American Meteorological Society
    Details
    Publikationsdatum: 2017-01-01
    Beschreibung: In this two-part study, relationships between the cloud gamma size distribution shape parameter, microphysical processes, and cloud characteristics of nonprecipitating shallow cumulus clouds are investigated using large-eddy simulations. In Part I, the dependence of the shape parameter (which is closely related to the distribution width) on cloud properties and processes was investigated. However, the distribution width also impacts cloud process rates and in turn cloud properties, and it is this aspect of the relationship that is explored in Part II and is discussed in the context of aerosol–cloud interactions. In simulations with a bulk microphysics scheme, it is found that the evaporation rates are much more sensitive to the value of the shape parameter than to the condensation rates. This is due to changes in both the rate of removal of mass and the rate of removal of fully evaporated droplets. As a result, cloud properties such as droplet number concentration, mean droplet diameter, and cloud fraction are strongly impacted by the value of the shape parameter, particularly in the subsaturated regions of the clouds. These changes can be on the same order of magnitude as changes due to increasing or decreasing the aerosol concentration by a factor of 16. Particular attention is paid to the impact of the shape parameter on cloud albedo. The cloud albedo increases as the shape parameter is increased as a result of the changes in evaporation. The magnitude of the increase is about 4 times larger than previous estimates. However, this increase in cloud albedo is largely offset by a decrease in the cloud fraction, which results in only small increases to the domain-average albedo. Implications for the aerosol relative dispersion effect are discussed.
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
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    Cold Pool-Land Surface Interactions in a Dry Continental Environment (2018)
    American Geophysical Union
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    Publikationsdatum: 2018-06-02
    Digitale ISSN: 1942-2466
    Thema: Geographie , Geologie und Paläontologie
    Publiziert von American Geophysical Union
    Standort Signatur Erwartet Verfügbarkeit
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  • 3
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    Meteorological and Land Surface Properties Impacting Sea Breeze Extent and Aerosol Distribution in a Dry Environment (2018)
    American Geophysical Union
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    Publikationsdatum: 2018-01-04
    Beschreibung: The properties of sea breeze circulations are influenced by a variety of meteorological and geophysical factors that interact with one another. These circulations can redistribute aerosol particles and pollution and therefore can play an important role in local air quality, as well as impact remote sensing. In this study, we select 11 factors that have the potential to impact either the sea breeze circulation properties and/or the spatial distribution of aerosols. Simulations are run to identify which of the 11 factors have the largest influence on the sea breeze properties and aerosol concentrations and to subsequently understand the mean response of these variables to the selected factors. All simulations are designed to be representative of conditions in coastal sub tropical environments and are thus relatively dry, as such they do not support deep convection associated with the sea breeze front. For this dry sea breeze regime, we find that the background wind speed was the most influential factor for the sea breeze propagation, with the soil saturation fraction also being important. For the spatial aerosol distribution, the most important factors were the soil moisture, sea-air temperature difference, and the initial boundary layer height. The importance of these factors seems to be strongly tied to the development of the surface-based mixed layer both ahead of and behind the sea breeze front. This study highlights potential avenues for further research regarding sea breeze dynamics and the impact of sea breeze circulations on pollution dispersion and remote sensing algorithms. ©2017. American Geophysical Union. All Rights Reserved.
    Print ISSN: 2169-897X
    Digitale ISSN: 2169-8996
    Thema: Geologie und Paläontologie , Physik
    Publiziert von American Geophysical Union
    Standort Signatur Erwartet Verfügbarkeit
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  • 4
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    Reply to “Comments on ‘A CloudSat–CALIPSO View of Cloud and Precipitation Properties across Cold Fronts over the Global Oceans’” (2018)
    American Meteorological Society
    Details
    Publikationsdatum: 2018-04-01
    Print ISSN: 0894-8755
    Digitale ISSN: 1520-0442
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 5
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    The Role of Cold Pools in Tropical Oceanic Convective Systems (2018)
    American Meteorological Society
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    Publikationsdatum: 2018-07-20
    Beschreibung: The processes governing organized tropical convective systems are not completely understood despite their important influences on the tropical atmosphere and global circulation. In particular, cold pools are known to influence the structure and maintenance of midlatitude systems via Rotunno–Klemp–Weisman (RKW) theory, but cold pools may interact differently with tropical convection because of differences in cold pool strength and environmental shear. In this study, the role of cold pools in organized oceanic tropical convective systems is investigated, including their influence on system intensity, mesoscale structure, and propagation. To accomplish this goal, high-resolution idealized simulations are performed for two different systems that are embedded within a weakly sheared cloud population approaching radiative–convective equilibrium. The cold pools are altered by changing evaporation rates below cloud base in a series of sensitivity tests. The simulations demonstrate surprising findings: when cold pools are weakened, the convective systems become more intense. However, their propagation speeds and mesoscale structure are largely unaffected by the cold pool changes. Passive tracers introduced into the cold pools indicate that the convection intensifies when cold pools are weakened because cold pool air is entrained into updrafts, thereby reducing updraft intensity via the cold pools’ initial negative buoyancy. Gravity waves, rather than cold pools, appear to be the important modulators of system propagation and mesoscale structure. These results reconfirm that RKW theory does not fully explain the behavior of tropical oceanic convective systems, even those that otherwise appear consistent with RKW thinking.
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 6
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    Toward an Algorithm for Estimating Latent Heat Release in Warm Rain Systems (2016)
    American Meteorological Society
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    Publikationsdatum: 2016-06-01
    Beschreibung: This paper outlines an approach for estimating latent heating, surface rainfall rate, and liquid water path in warm rain from downward-viewing W-band radar observations using a Bayesian Monte Carlo algorithm. The algorithm utilizes observed vertical and path-integrated characteristics of precipitating liquid clouds to identify the most appropriate hydrometeor and latent heating structures in a large database of profiles generated using a cloud-resolving model. These characteristics are selected by applying multiple performance metrics to synthetic retrievals. Analysis of the retrievals suggests that a combination of cloud-top, rain-top, and maximum reflectivity heights; vertically integrated reflectivity and attenuation; and a measure of near-surface intensity is sufficient to constrain bulk properties and the vertical structure of warm rain systems. When applied to observations at CloudSat resolution, biases in retrieved liquid water path and surface rainfall rate are small (less than 10%). The algorithm also captures the vertical structure of latent heating, although the magnitudes of integrated heating and cooling exhibit nearly compensating low biases. Random errors are larger owing to the limitations of single-frequency radar observations in constraining drop size distributions. Uncertainties in the altitudes of peak heating and cooling at the pixel scale are typically less than one vertical level, while uncertainties in vertically resolved estimates of heating and cooling rates are on the order of a factor of 2. The utility of the technique is illustrated through application to case studies from airborne radar data from the VAMOS Ocean–Cloud–Atmosphere–Land Study field campaign and satellite observations from CloudSat.
    Print ISSN: 0739-0572
    Digitale ISSN: 1520-0426
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 7
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    The Importance of the Shape of Cloud Droplet Size Distributions in Shallow Cumulus Clouds. Part I: Bin Microphysics Simulations (2017)
    American Meteorological Society
    Details
    Publikationsdatum: 2017-01-01
    Beschreibung: In this two-part study, the relationships between the width of the cloud droplet size distribution and the microphysical processes and cloud characteristics of nonprecipitating shallow cumulus clouds are investigated using large-eddy simulations. In Part I, simulations are run with a bin microphysics scheme and the relative widths (standard deviation divided by mean diameter) of the simulated cloud droplet size distributions are calculated. They reveal that the value of the relative width is higher and less variable in the subsaturated regions of the cloud than in the supersaturated regions owing to both the evaporation process itself and enhanced mixing and entrainment of environmental air. Unlike in some previous studies, the relative width is not found to depend strongly on the initial aerosol concentration or mean droplet concentration. Nonetheless, local values of the relative width are found to positively correlate with local values of the droplet concentrations, particularly in the supersaturated regions of clouds. In general, the distributions become narrower as the local droplet concentration increases, which is consistent with the difference in relative width between the supersaturated and subsaturated cloud regions and with physically based expectations. Traditional parameterizations for the relative width (or shape parameter, a related quantity) of cloud droplet size distributions in bulk microphysics schemes are based on cloud mean values, but the bin simulation results shown here demonstrate that more appropriate parameterizations should be based on the relationship between the local values of the relative width and the cloud droplet concentration.
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 8
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    Wind–Flux Feedbacks and Convective Organization during the November 2011 MJO Event in a High-Resolution Model (2018)
    American Meteorological Society
    Details
    Publikationsdatum: 2018-01-01
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 9
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    The Microphysical Roles of Lower-Tropospheric versus Midtropospheric Aerosol Particles in Mature-Stage MCS Precipitation (2017)
    American Meteorological Society
    Details
    Publikationsdatum: 2017-11-01
    Beschreibung: Simulations of two leading-line, trailing-stratiform mesoscale convective system (MCS) events that occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E) have been used to understand the relative microphysical impacts of lower- versus midtropospheric aerosol particles (APs) on MCS precipitation. For each MCS event, four simulations were conducted in which the initial vertical location and concentrations of cloud droplet nucleating APs were varied. These simulations were used to determine the precipitation response to AP vertical location. Importantly, the total integrated number and mass of the initial aerosol profiles used in the sensitivity simulations remained constant, such that differences in the simulations could be directly attributable to changes in the vertical location of cloud droplet nucleating APs. These simulations demonstrate that lower-tropospheric APs largely influenced the precipitation response directly rearward of the leading cold pool boundary. However, farther rearward in the MCS, the relative impact of lower- versus midtropospheric APs largely depended on the MCS structure, which varied between the two events because of differences in line-normal wind shear. Midtropospheric APs were able to activate new cloud droplets in the midtropospheric levels of convective updrafts and to enhance mixed-phase precipitation through increased cloud riming, and this microphysical pathway had a more significant impact on mixed-phase precipitation in weaker line-normal wind shear conditions. This result exposes the importance of properly representing midtropospheric APs when assessing aerosol effects on clouds. This study also demonstrates the utility of assessing aerosol effects within the different regions of MCSs.
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 10
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    Updraft Vertical Velocity Observations and Uncertainties in High Plains Supercells Using Radiosondes and Radars (2020)
    American Meteorological Society
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    Publikationsdatum: 2020-10-20
    Beschreibung: Observations of the air vertical velocities (wair) in supercell updrafts are presented, including uncertainty estimates, from radiosonde GPS measurements in two supercells. These in situ observations were collected during the Colorado State University Convective Cloud Outflows and Updrafts Experiment (C3LOUD-Ex) in moderately unstable environments in Colorado and Wyoming. Based on the radiosonde accelerations, instances when the radiosonde balloon likely bursts within the updraft are determined, and adjustments are made to account for the subsequent reduction in radiosonde buoyancy. Before and after these adjustments, the maximum estimated wair values are 36.2 and 49.9 m s−1, respectively. Radar data are used to contextualize the in situ observations and suggest that most of the radiosonde observations were located several kilometers away from the most intense vertical motions. Therefore, the radiosonde-based wair values presented likely underestimate the maximum values within these storms due to these sampling biases, as well as the impacts from hydrometeors, which are not accounted for. When possible, radiosonde-based wair values were compared to estimates from dual-Doppler methods and from parcel theory. When the radiosondes observed their highest wair values, dual-Doppler methods generally produced 15–20 m s−1 lower wair for the same location, which could be related to the differences in the observing systems’ resolutions. In situ observations within supercell updrafts, which have been limited in recent decades, can be used to improve our understanding and modeling of storm dynamics. This study provides new in situ observations, as well as methods and lessons that could be applied to future field campaigns.
    Print ISSN: 0027-0644
    Digitale ISSN: 1520-0493
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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