Cloud processing, cloud evaporation and Angström exponent

Roelofs, G. J.; Kamphuis, V.

doi:10.5194/acpd-8-12721-2008

Cited by 5 publications

(7 citation statements)

References 26 publications

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“…The exchange between cloudy air and environmental air, including the activated and unactivated aerosol particles, must therefore be represented realistically in atmospheric models aimed to study indirect aerosol effects. Further, due to entrainment of aerosol into clouds more particles will be subject to cloud processing (e.g., Roelofs and Kamphuis, 2009) which is relevant for the aerosol direct effect.…”

Section: Discussionmentioning

confidence: 99%

“…Since the initial CDNC is determined directly at cloud base, understanding how meteorological characteristics and aerosol properties influence aerosol activation at the cloud base has received a lot of attention (e.g., Kulmala et al, 1993). In the second place, the distribution of the liquid water with droplet size further influences both scattering and precipitation formation processes (e.g., Roelofs and Jongen, 2004;Roelofs and Kamphuis, 2009). A model study of cloud microphysics therefore requires a realistic representation of both CDNC and the drop size distribution.…”

Section: Introductionmentioning

confidence: 99%

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Influence of entrainment of CCN on microphysical properties of warm cumulus

2009

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Abstract. We use a 1-D cloud model with explicit microphysics and a binned representation of the aerosol size distribution to investigate the influence of entrainment of cloud condensation nuclei (CCN) on the microphysical development of warm cumulus clouds. For a more realistic representation of cloud drop spectral width, the model separates droplets that grow on aerosol that is initially present in the cloud from droplets growing on entrained aerosol. Model results are compared with observations of trade wind cumulus microphysics from the Rain in Cumulus over the Ocean experiment (RICO, 2004(RICO, -2005. The results indicate that CCN are entrained throughout the entire cloud depth, and inside the cloud part of these may be activated. Compared to a simulation where entrainment of ambient CCN is neglected this leads to higher cloud droplet number concentrations (CDNC) and a continuous presence of droplets in the range smaller than ∼5 µm that is consistent with the observations. Cloud dynamics are sensitive to the entrainment parameter as well as to the applied initial vertical velocity, as expressed by the liquid water content and cloud top height. However, simulated cloud drop spectra remain relatively unaffected for the specific conditions during RICO.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Influence of entrainment of CCN on microphysical properties of warm cumulus

2009

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“…Regions of enhanced humidity have the greatest horizontal extent near cloud top when an overlying stable layer exists [Perry and Hobbs, 1996;Lu et al, 2003], which in the tropics occurs around an altitude of 2 km [Rauber et al, 2007]. Also at cloud top, detrained cloud drops evaporate and leave behind larger, but fewer, aerosols compared to the entrained air due to droplet collision-coalescence within the cloud [Roelofs and Kamphuis, 2009]. Collision-coalescence occurs more often in thicker clouds, so its impact on aerosol size and hence, backscatter, is greatest at the tops of thicker clouds.…”

Section: Resultsmentioning

confidence: 99%

“…Hygroscopic growth increases both the median radius R and width s, leaving number concentration N unchanged [Gerber, 1985]. Droplet collision-coalescence decreases N and increases R, though precipitation scavenging efficiently removes the largest cloud droplets, thereby decreasing s [Peter et al, 2006;Roelofs and Kamphuis, 2009] and limiting the in-crease in R. Other scavenging processes (nucleation, diffusion, and impaction scavenging) decrease N and increase s [Flossmann et al, 1985]. We perform backscatter calculations to understand what changes in a typical maritime aerosol size distribution would produce our observations in Table 1.…”

Section: Resultsmentioning

confidence: 99%

Enhanced aerosol backscatter adjacent to tropical trade wind clouds revealed by satellite‐based lidar

Tackett¹,

Girolamo²

2009

Geophysical Research Letters

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[1] There have been many proposed processes by which clouds modify aerosol properties, suggesting that aerosol properties near clouds are different than far from clouds. We provide the first satellite-based lidar observations of backscatter and color ratio, quantities directly related to aerosol properties, as a function of distance to cloud edge for boundary layer clouds over the western tropical Atlantic. We show backscatter and color ratio are enhanced adjacent to cloud edge, particularly near cloud top and cloud base. Specifically, layer integrated median backscatter increases by 31 ± 3% and 42 ± 2%, at wavelengths of 532 nm, and 1,064 nm, respectively, and layer averaged color ratio increases by 15 ± 5%. Backscatter calculations suggest our observations adjacent to cloud are best explained by an aerosol size distribution with reduced number concentration, increased median radius, and decreased width compared to far from cloud.

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“…Such a size is typical for water droplets in clouds 27,29 . Therefore, we attribute these values to water aerosols (fog, orographic clouds) present close to mountain's surface, also typical for the season in that area, as indicated by the meteorological data.…”

Section: Time-averaged Aerosol-backscattering-related ǻNgström Exponentmentioning

confidence: 99%

Dynamical characteristics of atmospheric layers over complex terrain probed by two-wavelength lidar

Peshev¹,

Dreischuh²,

Deleva³

et al. 2010

SPIE Proceedings

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Time-resolved measurements of the distribution and dynamics of aerosols in atmospheric layers over complex terrains differing in their orographic characteristics, performed at two or more wavelengths, offer an opportunity for revealing peculiarities and evolution of different aerosol fractions, in view of their importance for local climate, air circulations, rainfalls, ecology, etc. In this work, an investigation of dynamical characteristics of the atmospheric aerosol over adjacent city-, plain-, and mountain zone is reported. Measurements are carried out at two wavelengths (1064 nm and 532 nm) by using two channels of an aerosol lidar based on a powerful Nd:YAG laser. Representative results of lidar measurements over the zone of investigation are described, obtained during a winter day. Range profiles of the atmospheric backscattering coefficient, range-corrected lidar signal, normalized standard deviation, and backscatteringrelated Ǻngström exponent are presented and analysed, as averaged over the time and/or range of measurements, as well as in their temporal evolution. Taking advantage of the two-wavelength lidar sounding, statistical quasi-quantitative analysis of the spatial density distribution and temporal dynamics of both the fine-mode and coarse-mode aerosol fractions is accomplished in orographic aspect. Domains of most intensive dynamics are determined for the two aerosol fractions. Obtained results show the strong impact of the heterogenic orography on the atmospheric dynamics, as well as the ability of the utilised lidar system for investigating the distribution and dynamics of aerosol fractions over large areas with high spatial and temporal resolution.

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Cloud processing, cloud evaporation and Angström exponent

Cited by 5 publications

References 26 publications

Influence of entrainment of CCN on microphysical properties of warm cumulus

Influence of entrainment of CCN on microphysical properties of warm cumulus

Enhanced aerosol backscatter adjacent to tropical trade wind clouds revealed by satellite‐based lidar

Dynamical characteristics of atmospheric layers over complex terrain probed by two-wavelength lidar

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