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Thijs Heus - One of the best experts on this subject based on the ideXlab platform.
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the contributions of shear and turbulence to cloud overlap for Cumulus Clouds
Journal of Geophysical Research, 2020Co-Authors: William J Calabrase, Anthony Sulak, Shaw D Rya, Thijs HeusAbstract:Vertical cloud overlap, the ratio of cloud fraction by area and by volume, for Cumulus Clouds are studied using large-eddy simulations (LES) due to the inefficient, wide-range values of cloud overl...
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direct numerical simulation of evaporative cooling at the lateral boundary of shallow Cumulus Clouds
Journal of the Atmospheric Sciences, 2013Co-Authors: Dick Abma, Thijs Heus, Juan Pedro MelladoAbstract:AbstractThis study investigates the dynamics of the subsiding shell at the lateral boundary of Cumulus Clouds, focusing on the role of evaporative cooling. Since the size of this shell is well below what large-eddy simulations can resolve, the authors have performed direct numerical simulations of an idealized subsiding shell. The system develops a self-similar, Reynolds number–independent flow that allows for the determination of explicit scaling laws relating the characteristic length, time, and velocity scales of the shell. It is found that the shell width grows quadratically in time, linearly with the traveled distance. The magnitude of these growth rates shows that evaporative cooling, in its most idealized form, is capable of producing a fast-growing shell with numbers that are consistent with observations of the subsiding shell around real shallow Cumulus Clouds: for typical thermodynamic conditions in Cumulus Clouds, a velocity on the order of 1 m s−1 and a thickness on the order of 10 m are estab...
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large eddy simulation of organized precipitating trade wind Cumulus Clouds
Atmospheric Chemistry and Physics, 2013Co-Authors: Axel Seife, Thijs HeusAbstract:Abstract. Trade wind Cumulus Clouds often organize in along-wind cloud streets and across-wind mesoscale arcs. We present a benchmark large-eddy simulation which resolves the individual Clouds as well as the mesoscale organization on scales of O(10 km). Different methods to quantify organization of cloud fields are applied and discussed. Using perturbed physics large-eddy simulation experiments, the processes leading to the formation of cloud clusters and the mesoscale arcs are revealed. We find that both cold pools as well as the sub-cloud layer moisture field are crucial to understand the organization of precipitating shallow convection. Further sensitivity studies show that microphysical assumptions can have a pronounced impact on the onset of cloud organization.
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observational validation of the compensating mass flux through the shell around Cumulus Clouds
Quarterly Journal of the Royal Meteorological Society, 2009Co-Authors: Thijs Heus, H J J Jonke, Harry E A Van Den Akke, Freek C J Pols, Donald H LenschowAbstract:The existence of a subsiding shell around Cumulus Clouds has been observed before in several aircraft measurement campaigns. Recent results from large-eddy simulations (LES) showed that the downward mass flux through the shell compensates for a significant fraction of the upward mass flux through the cloud. In this study, aeroplane measurements from the Rain In Cumulus over the Ocean (RICO) field campaign are used to verify the existence of this compensating mass flux. Just as in the LES results, the in-shell downward mass flux is found to be significant. However, a few differences were found in comparison with the LES results; most of them were explained by taking into account the difference between the two-dimensional slabs in LES and the one-dimensional lines from aeroplane observations. Copyright © 2008 Royal Meteorological Society
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mixing in shallow Cumulus Clouds studied by lagrangian particle tracking
Journal of the Atmospheric Sciences, 2008Co-Authors: Thijs Heus, Gertja Van Dijk, H J J Jonke, Harry E A Van Den AkkeAbstract:Mixing between shallow Cumulus Clouds and their environment is studied using large-eddy simulations. The origin of in-cloud air is studied by two distinct methods: 1) by analyzing conserved variable mixing diagrams (Paluch diagrams) and 2) by tracing back cloud-air parcels represented by massless Lagrangian particles that follow the flow. The obtained Paluch diagrams are found to be similar to many results in the literature, but the source of entrained air found by particle tracking deviates from the source inferred from the Paluch analysis. Whereas the classical Paluch analysis seems to provide some evidence for cloud-top mixing, particle tracking shows that virtually all mixing occurs laterally. Particle trajectories averaged over the entire cloud ensemble also clearly indicate the absence of significant cloud-top mixing in shallow Cumulus Clouds.
Ala M Lyth - One of the best experts on this subject based on the ideXlab platform.
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factors controlling secondary ice production in Cumulus Clouds
Quarterly Journal of the Royal Meteorological Society, 2017Co-Authors: Yahui Huang, Ala M Lyth, T W Choularto, Zhiqiang CuiAbstract:Aircraft measurements of two Cumulus Clouds were made during the Ice and Precipitation Initiation in Cumulus campaign over the British Isles. The 18 May 2006 cloud had high concentrations of ice particles and conditions were conducive for the Hallett-Mossop (HM) process of secondary ice production, but the 13 July 2005 cloud had low concentrations. A bin-resolved cloud model was used to investigate several factors that are known to control the HM process using the observations of the two Clouds. For the 2006 cloud, the model results show that the fast production of graupel by directly freezing of supercooled raindrops through collisional collection with ice particles was crucial to the activation of the HM process. Switching-off raindrop freezing led to much delayed and suppressed formation of graupel particles, and hence, a negligible HM process. Sensitivity studies were performed on the concentration of primary ice particles required to kick-start the HM process. It was found that a concentration of the first ice as low as 0.01 L− 1 could be sufficient, as long as there was a large enough concentration of cloud droplets (small and large) available when a significant number of graupel particles developed in the HM temperature zone. For the modelled 2005 cloud, the HM process did not operate effectively mainly because of the low concentration of supercooled raindrops and hence graupel. The HM process was also hindered by the relatively greater number of aerosols, and higher temperatures at cloud base and top.
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the production of warm rain in shallow maritime Cumulus Clouds
Quarterly Journal of the Royal Meteorological Society, 2013Co-Authors: Ala M Lyth, Yahui Huang, Zhiqiang Cui, Jaso H Lowenstei, S Davies, K S CarslawAbstract:The problem of the production of warm rain by collision and coalescence has been studied for over half a century and several processes have been suggested to explain the observed production, which is more rapid than generally possible with models. A straightforward scenario in relatively shallow maritime Cumulus Clouds is one where cloud drops simply grow by condensation and coalescence, with no appeal to enhancement due to entrainment or turbulence or indeed the presence of giant and ultragiant aerosols that may exist in the boundary layer. However, it is difficult in a field experiment to measure the concentrations of aerosols and the time evolution of the droplet size distribution or reflectivity in individual Clouds. The Rain in Cumulus Over the Ocean (RICO) field experiment overcame some of these difficulties due to the abundance of Clouds and the statistical sampling strategy at all significant altitudes. In this article, we present the results of the rate of increase in the radar reflectivity in a couple of cases. Comparisons with a cloud model strongly suggest that the development of warm rain can be explained using the observed aerosol distribution alone. Sensitivity studies suggested that giant and ultragiant aerosols were unimportant for the production of rain in these Clouds. Copyright © 2012 Royal Meteorological Society
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a study of thermals in Cumulus Clouds
Quarterly Journal of the Royal Meteorological Society, 2005Co-Authors: Ala M Lyth, Sonia Lashertrapp, William A CoopeAbstract:Air motions in the thermals contained within shallow Florida Cumulus Clouds were observed to be similar to the circulation observed in laboratory thermals. There was outward flow in the updraughts of individual thermals at most levels and there were usually downdraughts observed at the edges of the updraught or of the cloud. Widespread inward flow towards the centre of the cloud and a narrow, but strong, updraught was occasionally observed, reminiscent of the tail found at the rear of the laboratory thermals. A region of reduced liquid water content was frequently observed in the centre of several thermals where the updraught and horizontal 1D divergence were strongest, and complete holes were observed on two occasions. Although horizontal wind shear was generally weak, it was significant in a few cases, causing the flow pattern in the cloud to be asymmetric. Ascending regions of cloud with high values of liquid water content (cloud cores) were commonly observed at all altitudes, but generally the percentage of Clouds measured with high liquid water content decreased with altitude. The observations of airflow and liquid water content structure in warm Cumulus Clouds described in this paper are consistent with the schematic model of a thermal where a core of high liquid water content survives for several kilometres above cloud base, but erodes as it ascends. Copyright © 2005 Royal Meteorological Society
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the role of giant and ultragiant nuclei in the formation of early radar echoes in warm Cumulus Clouds
Journal of the Atmospheric Sciences, 2003Co-Authors: Ala M Lyth, Sonia Lashertrapp, William A Coope, Charles A Knigh, J LathamAbstract:Abstract Observations of the formation of the first radar echoes in small Cumulus Clouds are compared with results of a stochastic coalescence model run in the framework of a closed parcel. The observations were made with an instrumented aircraft and a high-powered dual-wavelength radar during the Small Cumulus Microphysics Study (SCMS) in Florida. The principal conclusion is that coalescence growth on giant and ultragiant nuclei may be sufficient to explain observations. The concentration of cloud droplets varied from under 300 cm−3 when surface winds were from the ocean, to over 1000 cm−3 when the wind direction was from the mainland. Although there is a slight tendency for the altitude of the first 0-dBZ echo to be lower on average in maritime than in continental Clouds there were several cases where it was higher. The model results suggest that the lack of correlation is consistent with drops forming on giant and ultragiant nuclei. The first 0-dBZ echo was observed to form at higher altitudes in cloud...
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an investigation of ice production mechanisms in small cumuliform Clouds using a 3d model with explicit microphysics part ii case study of new mexico Cumulus Clouds
Journal of the Atmospheric Sciences, 2000Co-Authors: Mikhail Ovtchinnikov, Yefim L Koga, Ala M LythAbstract:Abstract A new 3D model with explicit liquid- and ice-phase microphysics and a detailed treatment of ice nucleation and multiplication processes is applied to study ice formation and evolution in Cumulus Clouds. Simulation results are compared with in situ observations collected by the National Center for Atmospheric Research King Air aircraft in a cloud over the Magdalena Mountains in New Mexico on 9 August 1987. The model reproduces well the observed cloud in terms of cloud geometry, liquid water content, and concentrations of cloud drops and ice particles (IP). Primary ice nucleation is shown to produce IP in concentrations on the order of 103 m−3 (1 L−1) once the cloud top reaches −10° to −12°C. At mature and early dissipating stages of cloud development, ice production is dominated by the rime-splintering (Hallett–Mossop) mechanism, which in some regions generates up to 5 × 104 m−3 (50 L−1) IP in about 10 min. The predicted maximum of IP concentration is in agreement with observations. The sampling t...
William A Coope - One of the best experts on this subject based on the ideXlab platform.
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a study of thermals in Cumulus Clouds
Quarterly Journal of the Royal Meteorological Society, 2005Co-Authors: Ala M Lyth, Sonia Lashertrapp, William A CoopeAbstract:Air motions in the thermals contained within shallow Florida Cumulus Clouds were observed to be similar to the circulation observed in laboratory thermals. There was outward flow in the updraughts of individual thermals at most levels and there were usually downdraughts observed at the edges of the updraught or of the cloud. Widespread inward flow towards the centre of the cloud and a narrow, but strong, updraught was occasionally observed, reminiscent of the tail found at the rear of the laboratory thermals. A region of reduced liquid water content was frequently observed in the centre of several thermals where the updraught and horizontal 1D divergence were strongest, and complete holes were observed on two occasions. Although horizontal wind shear was generally weak, it was significant in a few cases, causing the flow pattern in the cloud to be asymmetric. Ascending regions of cloud with high values of liquid water content (cloud cores) were commonly observed at all altitudes, but generally the percentage of Clouds measured with high liquid water content decreased with altitude. The observations of airflow and liquid water content structure in warm Cumulus Clouds described in this paper are consistent with the schematic model of a thermal where a core of high liquid water content survives for several kilometres above cloud base, but erodes as it ascends. Copyright © 2005 Royal Meteorological Society
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the role of giant and ultragiant nuclei in the formation of early radar echoes in warm Cumulus Clouds
Journal of the Atmospheric Sciences, 2003Co-Authors: Ala M Lyth, Sonia Lashertrapp, William A Coope, Charles A Knigh, J LathamAbstract:Abstract Observations of the formation of the first radar echoes in small Cumulus Clouds are compared with results of a stochastic coalescence model run in the framework of a closed parcel. The observations were made with an instrumented aircraft and a high-powered dual-wavelength radar during the Small Cumulus Microphysics Study (SCMS) in Florida. The principal conclusion is that coalescence growth on giant and ultragiant nuclei may be sufficient to explain observations. The concentration of cloud droplets varied from under 300 cm−3 when surface winds were from the ocean, to over 1000 cm−3 when the wind direction was from the mainland. Although there is a slight tendency for the altitude of the first 0-dBZ echo to be lower on average in maritime than in continental Clouds there were several cases where it was higher. The model results suggest that the lack of correlation is consistent with drops forming on giant and ultragiant nuclei. The first 0-dBZ echo was observed to form at higher altitudes in cloud...
Jeanlouis Enguie - One of the best experts on this subject based on the ideXlab platform.
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the onset of precipitation in warm Cumulus Clouds an observational case study
Quarterly Journal of the Royal Meteorological Society, 2010Co-Authors: Frederic Urne, Jeanlouis EnguieAbstract:Data collected with a ground radar and an instrumented aircraft during the Small Cumulus Microphysics Study (SCMS) are analysed to examine the physical processes that control the onset of precipitation. On 10 August 1995, the Meteo-France Merlin-IV successively sampled the core of the top cell in three convective Clouds, following the cells from their initial stage of young vigorous ascending turrets up to their decaying stage. Simultaneous range–height indicator (RHI) scans were also performed by the National Center for Atmospheric Research CP2 radar. Although the three Clouds showed similar values of the droplet number concentrations and were able to generate precipitation embryos, the first two collapsed after reaching an altitude of 3 km above sea level, without producing any precipitation, while the third one reached a higher level of 4 km and produced significant precipitation. This case-study illustrates how sensitive the onset of precipitation is to cloud dynamics, revealing that in a conditionally unstable atmosphere hardly noticeable changes in cloud thermodynamics and microphysics may lead to major changes in cloud vertical development and precipitation. Copyright © 2010 Royal Meteorological Society
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droplet spectra broadening in Cumulus Clouds part ii microscale droplet concentration heterogeneities
Journal of the Atmospheric Sciences, 2001Co-Authors: Laure Chauma, Jeanlouis EnguieAbstract:Cloud samples of narrow spectra observed in adiabatic cores of Cumulus Clouds are selected and the droplet spatial distribution is examined in order to document microscale heterogeneities of the droplet concentration. Counting statistics and the Fishing test are applied and compared to the properties of a random spatial distribution, that is, to Poisson statistics. These tests suggest that microscale heterogeneities of the concentration are not significant in adiabatic cores. A conceptual model is then developed to estimate the lifetime that would be required for the observed heterogeneities to generate the observed spectra broadening. The model implies that the lifetime of the heterogeneities should reach unrealistic values to significantly contribute to the observed broadening. The statistical properties of direct numerical simulations of the droplet inertial coupling with turbulence are then compared to the properties of the actual samples. It appears that the properties of the actual samples are closer to the Poisson reference than those of the fields generated by the models. It is concluded that the microscale observations of the droplet spatial distribution in adiabatic cores do not support the hypothesis that inertial coupling is a significant source of spectra broadening.
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droplet spectra broadening in Cumulus Clouds part i broadening in adiabatic cores
Journal of the Atmospheric Sciences, 2001Co-Authors: Jeanlouis Enguie, Laure ChaumaAbstract:Measurements of cloud droplet spectra performed with the Fast-Forward Scattering Spectrometer Probe during the Small Cumulus Microphysics Study (1995) are analyzed. Fifty cloud samples with narrow droplet spectra are selected. They are characterized by values of liquid water content slightly below the adiabatic value. Each observed spectrum is then compared to a narrow adiabatic spectrum predicted at the same level with the current theory of condensational growth in an adiabatic cloud cell, initialized with a reference spectrum measured right above the activation level, at cloud base. Broadening is characterized for each observed spectrum by the probability density function of condensational growth expressed as the Lagrangian integral of the ratio of supersaturation to vertical velocity, along the droplet trajectories. In particular it appears that the derived density functions show high probabilities of very low and very large values of condensational growth. The large values are related to a high relative density of big droplets in the measured spectra, higher than predicted by the adiabatic model. The contribution of the instrument to this feature is examined with a model of probe functioning. The simulations suggest that most of those big droplets are instrumental artifacts. The remaining broadening is parameterized by a linear relationship between the mean value and the standard deviation of the density function of condensational growth. This result will be used to examine the respective contributions to spectra broadening of microscale heterogeneities of the droplet concentration, in Part II, and of the mixing processes, in Part III of this series.
Joh H Seinfeld - One of the best experts on this subject based on the ideXlab platform.
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statistical comparison of properties of simulated and observed Cumulus Clouds in the vicinity of houston during the gulf of mexico atmospheric composition and climate study gomaccs
Journal of Geophysical Research, 2008Co-Authors: Hongli Jiang, Graham Feingold, Patrick Y Chuang, Haflidi H Jonsso, Richard C Flaga, Joh H SeinfeldAbstract:We present statistical comparisons of properties of Clouds generated by Large Eddy Simulations (LES) with aircraft observations of nonprecipitating, warm Cumulus Clouds made in the vicinity of Houston, TX during the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS), carried out in the summer of 2006. Aircraft data were sampled with the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter airplane. Five flights (days) that are most suitable for studying aerosol-cloud interactions are selected from the 22 flights. The model simulations are initiated with observed environmental profiles. The simulations are used to generate an ensemble of thousands of Cumulus Clouds for statistically meaningful evaluations. Statistical comparisons focus on the properties of a set of dynamical and thermodynamical variables, sampled either in the cloud or the cloud updraft core. The set of variables includes cloud liquid water content (LWC), number mixing ratio of cloud droplets (Nd), cloud effective radius (re), updraft velocity (w), and the distribution of cloud sizes. In general, good agreement between the simulated and observed Clouds is achieved in the normalized frequency distribution functions, the profiles averaged over the cloudy regions, the cross-cloud averages, and the cloud size distributions, despite big differences in sample size between the model output and the aircraft data. Some unresolved differences in frequency distributions of w and possible differences in cloud fraction are noted. These comparisons suggest that the LES is able to successfully generate the Cumulus cloud populations that were present during GoMACCS. The extent to which this is true will depend on the specific application.
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analysis of humidity halos around trade wind Cumulus Clouds
Journal of the Atmospheric Sciences, 2003Co-Authors: Jia Wang, Andrew Freedma, Haflidi H Jonsso, Richard C Flaga, Robe A Mcclatchey, Joh H SeinfeldAbstract:Regions of enhanced humidity in the vicinity of Cumulus Clouds, so-called cloud halos, reflect features of cloud evolution, exert radiative effects, and may serve as a locus for new particle formation. Reported here are the results of an aircraft sampling campaign carried out near Oahu, Hawaii, from 31 July through 10 August 2001, aimed at characterizing the properties of trade wind Cumulus cloud halos. An Aerodyne Research, Inc., fast spectroscopic water vapor sensor, flown for the first time in this campaign, allowed characterization of humidity properties at 10-m spatial resolution. Statistical properties of 60 traverses through cloud halos over the campaign were in general agreement with measurements reported by Perry and Hobbs. One particularly long-lived cloud is analyzed in detail, through both airborne measurement and numerical simulation, to track evolution of the cloud halos over the cloud’s lifetime. Results of both observation and the simulation show that cloud halos tend to be broad at lower levels and narrow at upper levels, and broader on the downshear side than on the upshear side, broadening with time particularly in the downshear direction. The high correlation of clear-air turbulence distribution with the halo distribution temporally and spatially suggests that the halo forms, in part, due to turbulent mixing at the cloud boundary. Radiative calculations carried out on the simulated cloud and halo field indicate that the halo radiative effect is largest near cloud top during mature and dissipation stages. The halo-enhanced atmospheric shortwave absorption, averaged over this period, is about 1.3% of total solar absorption in the column.
