oalib
Search Results: 1 - 10 of 100 matches for " "
All listed articles are free for downloading (OA Articles)
Page 1 /100
Display every page Item
An aircraft case study of the spatial transition from closed to open mesoscale cellular convection over the Southeast Pacific  [PDF]
R. Wood,C. S. Bretherton,D. Leon,A. D. Clarke
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2011, DOI: 10.5194/acp-11-2341-2011
Abstract: Aircraft measurements are presented from the 27/28 October 2008 case study of the VOCALS Regional Experiment (REx) over the remote subtropical southeast Pacific (18° S, 80° W). Data from two aircraft that took measurements approximately twelve hours apart but in the same advected airmass are used to document a remarkably sharp spatial transition in marine boundary layer (MBL), cloud, and aerosol structure across the boundary between a well-mixed MBL containing overcast closed mesoscale cellular stratocumulus, and a pocket of open cells (POC) with significantly lower cloud cover. Long (~190–250 km) straight and level flight legs at three levels in the marine boundary layer and one level in the lower free troposphere permit sampling of the closed cells, the POC, and a 20–30 km wide transition zone with distinctly different structure from the two airmasses on either side. The POC region consists of intermittent active and strongly precipitating cumulus clouds rising and detraining into patches of drizzling but quiescent stratiform cloud which is optically thin especially toward its edges. Mean cloud-base precipitation rates inside the POC are several mm d 1, but rates in the closed cell region are not greatly lower than this. This latter finding suggests that precipitation is not a sufficient condition for POC formation from overcast stratocumulus. Despite similar cloud-base precipitation rates in the POC and overcast region, much of the precipitation (>90%) evaporates below cloud in the overcast region, while there is significant surface precipitation inside the POC. In the POC and transition region, although the majority of the condensate is in the form of drizzle, the integrated liquid water path is remarkably close to that expected for a moist adiabatic parcel rising from cloud base to top. The transition zone between the POC and the closed cells often consists of thick "boundary cell" clouds producing mean surface precipitation rates of 10–20 mm d 1, a divergent quasi-permanent cold/moist pool below cloud, a convergent inflow region at mid-levels in the MBL, and a divergent outflow near the top of the MBL. The stratiform clouds in the POC exist within an ultra-clean layer that is some 200–300 m thick. Aerosol concentrations (Na) measured by a PCASP in the diameter range 0.12–3.12 μm in the center of the ultra-clean layer are as low as 0.1–1 cm 3. This suggests that coalescence scavenging and sedimentation is extremely efficient, since Na in the subcloud layer, and droplet concentration Nd in the active cumuli are typically 20–60 cm 3. The droplet conce
An aircraft case study of the spatial transition from closed to open mesoscale cellular convection over the Southeast Pacific
R. Wood, C. S. Bretherton, D. Leon, A. D. Clarke, P. Zuidema, G. Allen,H. Coe
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2011,
Abstract: Aircraft measurements are presented from the 27/28 October 2008 case study of the VOCALS Regional Experiment (REx) over the remote subtropical southeast Pacific (18° S, 80° W). Data from two aircraft that took measurements approximately twelve hours apart but in the same advected airmass are used to document a remarkably sharp spatial transition in marine boundary layer (MBL), cloud, and aerosol structure across the boundary between a well-mixed MBL containing overcast closed mesoscale cellular stratocumulus, and a pocket of open cells (POC) with significantly lower cloud cover. Long (~190–250 km) straight and level flight legs at three levels in the marine boundary layer and one level in the lower free troposphere permit sampling of the closed cells, the POC, and a 20–30 km wide transition zone with distinctly different structure from the two airmasses on either side. The POC region consists of intermittent active and strongly precipitating cumulus clouds rising and detraining into patches of drizzling but quiescent stratiform cloud which is optically thin especially toward its edges. Mean cloud-base precipitation rates inside the POC are several mm d 1, but rates in the closed cell region are not greatly lower than this. This latter finding suggests that precipitation is not a sufficient condition for POC formation from overcast stratocumulus. Despite similar cloud-base precipitation rates in the POC and overcast region, much of the precipitation (>90%) evaporates below cloud in the overcast region, while there is significant surface precipitation inside the POC. In the POC and transition region, although the majority of the condensate is in the form of drizzle, the integrated liquid water path is remarkably close to that expected for a moist adiabatic parcel rising from cloud base to top. The transition zone between the POC and the closed cells often consists of thick "boundary cell" clouds producing mean surface precipitation rates of 10–20 mm d 1, a divergent quasi-permanent cold/moist pool below cloud, a convergent inflow region at mid-levels in the MBL, and a divergent outflow near the top of the MBL. The stratiform clouds in the POC exist within an ultra-clean layer that is some 200–300 m thick. Aerosol concentrations (Na) measured by a PCASP in the diameter range 0.12–3.12 μm in the center of the ultra-clean layer are as low as 0.1–1 cm 3. This suggests that coalescence scavenging and sedimentation is extremely efficient, since Na in the subcloud layer, and droplet concentration Nd in the active cumuli are typically 20–60 cm 3. The droplet concentrations in the quiescent stratiform clouds are extremely low (typically 1–10 cm 3), and most of their liquid water is in the form of drizzle, which mainly evaporates before reaching the surface. The cloud droplet concentration in the overcast region decreases strongly as the transition region is approached, as do subcloud accumulation mode aerosol concentrations, suggesting that coalescence scav
Droplet:A Virtual Brush Model to Simulate Chinese Calligraphy and Painting
Droplet: A Virtual Brush Model to Simulate Chinese Calligraphy and Painting

XiaoFeng Mi,Min Tang,Jin-Xiang Dong,
Xiao-FengMi
,MinTang,Jin-XiangDong

计算机科学技术学报 , 2004,
Abstract: This paper proposes a virtual brush model based on droplet operation to simulate Chinese calligraphy and traditional Chinese painting in real time. Two ways of applying droplet model to virtual calligraphy and painting are discussed in detail. The second droplet model is more elaborated and can produce more vivid results while being slightly more time-consuming. The novel feature of the proposed droplet virtual brush model successfully enables the simulation painting system to overcome the poor expressional ability of virtual brush based on particle system and avoids the complex evaluation of physical brush with solid model. The model, derived from the actual calligraphy and painting experience, due to the simplicity of the droplet operation and its powerful expressive ability considerably improves the performance of the simulation system and maintains painting effect comparable with real brush by supporting special Chinese brush effect such as dry brush, feng and stroke diffusion.
The fine-scale structure of the trade wind cumuli over Barbados – an introduction to the CARRIBA project  [PDF]
H. Siebert,J. Bethke,E. Bierwirth,T. Conrath
Atmospheric Chemistry and Physics Discussions , 2012, DOI: 10.5194/acpd-12-28609-2012
Abstract: The CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados) project with focus on trade wind cumuli over Barbados is introduced. The project is based on two one-month field campaigns in November 2010 (climatic wet season) and April 2011 (climatic dry season). Observations are based on helicopter-borne and ground-based measurements in a square of 100 km2 off the coast of Barbados. CARRIBA is accompanied by long-term observations at the Barbados Cloud Observatory located at the East coast of Barbados since early in 2010 and which provides longer-term context for the CARRIBA measurements. Deployed instrumentation and sampling strategy are presented together with a classification of the meteorological conditions. The two campaigns were influenced by different air masses advected from the Caribbean area, the Atlantic Ocean, as well as the African continent which led to distinct aerosol conditions. Therefore, pristine conditions with low aerosol particle number concentrations of ~100 cm3 were alternating with periods influenced by Saharan dust or aerosol from biomass burning resulting in comparable high number concentrations ~500 cm3. The later was originating from both, the Caribbean area and Africa. The shallow cumulus clouds responded to the different aerosol conditions with a wide range of mean droplet sizes and number concentrations. Effective radii in the range of 7 to 18 μm have been observed. Finally, the four leading topics of CARRIBA – Clouds, Aerosol, Radiation and tuRbulence – are motivated and illustrated by selected findings and measurement examples.
Parametric representation of the cloud droplet spectra for LES warm bulk microphysical schemes
O. Geoffroy, J.-L. Brenguier,F. Burnet
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2010,
Abstract: Parametric functions are currently used to represent droplet spectra in clouds and to develop bulk parameterizations of the microphysical processes and of their interactions with radiation. The most frequently used parametric functions are the Lognormal and the Generalized Gamma which have three and four independent parameters, respectively. In a bulk parameterization, two parameters are constrained by the total droplet number concentration and the liquid water content. In the Generalized Gamma function, one parameter is specified a priori, and the fourth one, like the third parameter of the Lognormal function, shall be tuned, for the parametric function to statistically best fit observed droplet spectra. These parametric functions are evaluated here using droplet spectra collected in non-or slightly precipitating stratocumulus and shallow cumulus. Optimum values of the tuning parameters are derived by minimizing either the absolute or the relative error for successively the first, second, fifth, and sixth moments of the droplet size distribution. A trade-off value is also proposed that minimizes both absolute and relative errors for the four moments concomitantly. Finally, a parameterization is proposed in which the tuning parameter depends on the liquid water content. This approach significantly improves the fit for the smallest and largest values of the moments.
Parametric representation of the cloud droplet spectra for LES warm bulk microphysical schemes  [PDF]
O. Geoffroy,J.-L. Brenguier,F. Burnet
Atmospheric Chemistry and Physics Discussions , 2009,
Abstract: Parametric functions are currently used to represent droplet spectra in clouds and to develop bulk parameterizations of the microphysical processes and of their interactions with radiation. The most frequently used parametric functions are the Lognormal and the Generalized Gamma which have three and four independent parameters, respectively. In a bulk parameterization, two parameters are constrained by the total droplet number concentration and the liquid water content. In the Generalized Gamma function, one parameter is specified a priori, and the fourth one, like the third parameter of the Lognormal function, shall be tuned, for the parametric function to statistically best fit observed droplet spectra. These parametric functions are evaluated here using droplet spectra collected in non-or slightly precipitating stratocumulus and shallow cumulus. Optimum values of the tuning parameters are derived by minimizing either the absolute or the relative error for successively the first, second, fifth, and sixth moments of the droplet size distribution. A trade-off value is also proposed that minimizes both absolute and relative errors for the four moments concomitantly. Finally, a parameterization is proposed in which the tuning parameter depends on the liquid water content. This approach significantly improves the fit for the smallest and largest values of the moments.
Large-eddy simulation of mesoscale dynamics and entrainment around a pocket of open cells observed in VOCALS-REx RF06
A. H. Berner, C. S. Bretherton,R. Wood
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2011,
Abstract: Large-eddy simulations of a pocket of open cells (POC) based on VOCALS Regional Experiment (REx) NSF C-130 Research Flight 06 are analyzed and compared with aircraft observations. A doubly-periodic domain 192 km × 24 km with 125 m horizontal and 5 m vertical grid spacing near the capping inversion is used. The POC is realized in the model as a fixed 96 km wide region of reduced cloud droplet number concentration (Nc) based on observed values; initialization and forcing are otherwise uniform across the domain. The model reproduces aircraft-observed differences in boundary-layer structure and precipitation organization between a well-mixed overcast region and a decoupled POC with open-cell precipitating cumuli, although the simulated cloud cover is too large in the POC. A sensitivity study in which Nc is allowed to advect following the turbulent flow gives nearly identical results over the 16 h length of the simulation (which starts at night and goes into the next afternoon). The simulated entrainment rate is nearly a factor of two smaller in the less turbulent POC than in the more turbulent overcast region. However, the inversion rises at a nearly uniform rate across the domain because powerful buoyancy restoring forces counteract horizontal inversion height gradients. A secondary circulation develops in the model that diverts subsiding free-tropospheric air away from the POC into the surrounding overcast region, counterbalancing the weaker entrainment in the POC with locally weaker subsidence.
Spectral dispersion of cloud droplet size distributions and radar threshold reflectivity for drizzle

Xie Xiao-Ning,Liu Xiao-Dong,

中国物理 B , 2010,
Abstract: From first principles, we find that the radar threshold reflectivity between nonprecipitating clouds and precipitating clouds is strongly related to not only the cloud droplet number concentration but also the spectral dispersion of cloud droplet size distributions. The further investigation indicates that the threshold value is an increasing function of spectral dispersion and cloud droplet number concentration. These results may improve our understanding of the cloud-precipitation interaction and the aerosol indirect effect.
Precipitating Condensation Clouds in Substellar Atmospheres  [PDF]
Andrew S. Ackerman,Mark S. Marley
Physics , 2001, DOI: 10.1086/321540
Abstract: We present a method to calculate vertical profiles of particle size distributions in condensation clouds of giant planets and brown dwarfs. The method assumes a balance between turbulent diffusion and sedimentation in horizontally uniform cloud decks. Calculations for the Jovian ammonia cloud are compared with results from previous methods. An adjustable parameter describing the efficiency of sedimentation allows the new model to span the range of predictions made by previous models. Calculations for the Jovian ammonia cloud are consistent with observations. Example calculations are provided for water, silicate, and iron clouds on brown dwarfs and on a cool extrasolar giant planet. We find that precipitating cloud decks naturally account for the characteristic trends seen in the spectra and colors of L- and T-type ultracool dwarfs.
Vertical transport of pollutants by shallow cumuli from large eddy simulations
G. Chen, H. Xue, G. Feingold,X. Zhou
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2012,
Abstract: This study investigates the vertical transport of a passive tracer in a shallow cumulus boundary layer using large eddy simulations. The tracer source is at the surface in one case, and in the inversion layer in the other case. Results show that shallow cumulus clouds can significantly enhance vertical transport of the tracer in both cases. In the case with surface-borne pollutants, cloudy regions are responsible for the upward transport, due to the intense updrafts in cumulus clouds. In the case where pollutants are aloft, cloud-free regions are responsible for the downward transport, but the downward transport mainly occurs in thin regions around cloud edges. This is consistent with previous aircraft measurements of downdrafts around cumulus clouds and indicates that the downward transport is also cloud-induced. Cumulus convection is therefore able to both vent pollutants upward from the surface and fumigate pollutants in the inversion layer downward into the lower boundary layer.
Page 1 /100
Display every page Item


Home
Copyright © 2008-2017 Open Access Library. All rights reserved.