Probing Finescale Dynamics and Microphysics of Clouds with Helicopter-Borne Measurements

Siebert, Holger; Franke, Harald; Lehmann, K.; Maser, R.; Saw, Ewe-Wei; Schell, D.; Shaw, Raymond A.; Wendisch, Manfred

doi:10.1175/bams-87-12-1727

Cited by 105 publications

(109 citation statements)

References 21 publications

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“…ACTOS is attached to the helicopter by means of a 140 m long rope to avoid any influence from the rotor downwash and its typical true airspeed is about 15 m s −1 enabling high-resolution measurements in the cloudy atmosphere. A detailed description of the ACTOS payload and its general performance is given in Siebert et al (2006a). The analysis of this work is mainly based on measurements of the three-dimensional wind vector measured with an ultra-sonic anemometer (hereafter called "sonic"), Solent HS (Gill Instruments, Lymington, UK), the liquid water content (LW C) measured with a particle volume monitor (PVM-100A; Gerber et al 1994), the temperature (T ) measured with a special cloud thermometer (UltraFast Thermometer, UFT, Haman et al 1997), the absolute humidity (a) measured with an open-pass infrared absorption hygrometer (Licor, LI-7500) and the number concentration of interstitial aerosol (N ) measured with a condensation particle counter (CPC, model 3762A, TSI Inc., St. Paul, Minnesota, USA).…”

Section: Methodsmentioning

confidence: 99%

Observation of a Self-Limiting, Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus

Katzwinkel

Siebert

Shaw

2011

Boundary-Layer Meteorol

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High-resolution measurements of thermodynamic, microphysical, and turbulence properties inside a turbulent inversion layer above a marine stratocumulus cloud layer are presented. The measurements are performed with the helicopter-towed measurement payload Airborne Cloud Turbulence Observation System (ACTOS), which allows for sampling with low true air speeds and steep profiles through cloud top. Vertical profiles show that the turbulent inversion layer consists of clear air above the cloud top, with nearly linear profiles of potential temperature, horizontal wind speed, absolute humidity, and concentration of interstitial aerosol. The layer is turbulent, with an energy dissipation rate nearly the same as that in the lower cloud, suggesting that the two are actively coupled, but with significant anisotropic turbulence at the large scales within the turbulent inversion layer. The turbulent inversion layer is traversed six times and the layer thickness is observed to vary between 37 and 85 m, whereas the potential temperature and horizontal wind speed differences at the top and bottom of the layer remain essentially constant. The Richardson number therefore increases with increasing layer thickness, from approximately 0.2 to 0.7, suggesting that the layer develops to the point where shear production of turbulence is sufficiently weak to be balanced by buoyancy suppression. This picture is consistent with prior numerical simulations of the evolution of turbulence in localized stratified shear layers. It is observed that the large eddy scale is suppressed by buoyancy and is on the order of the Ozmidov scale, much less than the thickness of the turbulent inversion layer, such that direct mixing between the cloud top and the free troposphere is inhibited, and the entrainment velocity tends to decrease with increasing turbulent inversion-layer thickness. Qualitatively, the turbulent inversion layer likely grows through nibbling rather than engulfment.

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Section: Methodsmentioning

confidence: 99%

Observation of a Self-Limiting, Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus

Katzwinkel

Siebert

Shaw

2011

Boundary-Layer Meteorol

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“…The Taylor-scale Reynolds number, R λ , varies correspondingly between stratocumulus and cumulus: R λ ∼ 5000 in stratocumulus (Siebert et al, 2010a) and R λ ∼ 30 000-40 000 in cumulus (Siebert et al, 2006a). These values of ε can be misleading in that they give no indication of the extremely large fluctuations in the local dissipation rate that are possible: for example, in small cumulus clouds Siebert et al (2006b) found that the dissipation rate could vary by a factor of 50 over scales of order 15 m (see Figure 5 therein); Meischner et al (2001) also found that the dissipation rate could vary by several orders of magnitude on scales of order 100 m within thunderstorm anvils.…”

Section: Cloud-scale Turbulencementioning

confidence: 99%

“…In addition, one might anticipate that the turbulence intensity should be highest near the cloud edge where turbulent entrainment and mixing between the cloudy air and subsaturated (and typically non-turbulent) air take place (e.g. Siebert et al, 2006aSiebert et al, , 2006b. Numerical simulations by Seifert et al (2010) show that ε reaches a maximum near the cloud top at its windward edge (see Figure 6 therein).…”

Section: Sources Of Cloud Turbulencementioning

confidence: 99%

“…However, there is some evidence (e.g. Siebert et al, 2006aSiebert et al, , 2006b) that ε is higher around the cloud edges than in the cloud core, perhaps as a result of the initial entrainment, which would suggest that l c is larger near the cloud edge (leading to a greater prevalence of homogeneous mixing). In general, though, one would expect that over a sufficiently long time period ε would decrease as a result of mixing.…”

Section: Homogeneous and Inhomogeneous Mixingmentioning

confidence: 99%

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Droplet growth in warm turbulent clouds

Devenish

Bartello

Brenguier

et al. 2012

Quart J Royal Meteoro Soc

243

281

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In this survey we consider the impact of turbulence on cloud formation from the cloud scale to the droplet scale. We assess progress in understanding the effect of turbulence on the condensational and collisional growth of droplets and the effect of entrainment and mixing on the droplet spectrum. The increasing power of computers and better experimental and observational techniques allow for a much more detailed study of these processes than was hitherto possible. However, much of the research necessarily remains idealized and we argue that it is those studies which include such fundamental characteristics of clouds as droplet sedimentation and latent heating that are most relevant to clouds. Nevertheless, the large body of research over the last decade is beginning to allow tentative conclusions to be made. For example, it is unlikely that small-scale turbulent eddies (i.e. not the energy-containing eddies) alone are responsible for broadening the droplet size spectrum during the initial stage of droplet growth due to condensation. It is likely, though, that small-scale turbulence plays a significant role in the growth of droplets through collisions and coalescence. Moreover, it has been possible through detailed numerical simulations to assess the relative importance of different processes to the turbulent collision kernel and how this varies in the parameter space that is important to clouds. The focus of research on the role of turbulence in condensational and collisional growth has tended to ignore the effect of entrainment and mixing and it is arguable that they play at least as important a role in the evolution of the droplet spectrum. We consider the role of turbulence in the mixing of dry and cloudy air, methods of quantifying this mixing and the effect that it has on the droplet spectrum. Copyright

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“…The easternmost island in the Caribbean, Barbados, was used as a base to examine the interplay between undisturbed marine aerosol and warm clouds. Ground-based and airborne measurements were performed, where the measurement platform ACTOS (Airborne Cloud Turbulence Observation System; Siebert et al, 2006) was deployed for the latter. An overview of the measurement activities, which took place during the two periods of CAR-RIBA in November 2010 and April 2011, is given in Siebert et al (2013).…”

mentioning

confidence: 99%

Aerosol arriving on the Caribbean island of Barbados: physical properties and origin

et al. 2016

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Abstract. The marine aerosol arriving at Barbados (Ragged Point) was characterized during two 3-week long measurement periods in November 2010 and April 2011, in the context of the measurement campaign CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados). Through a comparison between groundbased and airborne measurements it was shown that the former are representative of the marine boundary layer at least up to cloud base. In general, total particle number concentrations (N total ) ranged from as low as 100 up to 800 cm −3 , while number concentrations for cloud condensation nuclei (N CCN ) at a supersaturation of 0.26 % ranged from some 10 to 600 cm −3 . N total and N CCN depended on the air mass origin. Three distinct types of air masses were found. One type showed elevated values for both N total and N CCN and could be attributed to long-range transport from Africa, by which biomass burning particles from the Sahel region and/or mineral dust particles from the Sahara were advected. The second and third type both had values for N CCN below 200 cm −3 and a clear minimum in the particle number size distribution (NSD) around 70 to 80 nm (Hoppel minimum). While for one of these two types the accumulation mode was dominating (albeit less so than for air masses advected from Africa), the Aitken mode dominated the other and contributed more than 50 % of all particles. These Aitken mode particles likely were formed by new particle formation no more than 3 days prior to the measurements. Hygroscopicity of particles in the CCN size range was determined from CCN measurements to be κ = 0.66 on average, which suggests that these particles contain mainly sulfate and do not show a strong influence from organic material, which might generally be the case for the months during which measurements were made. The average κ could be used to derive N CCN from measured number size distributions, showing that this is a valid approach to obtain N CCN . Although the total particulate mass sampled on filters was found to be dominated by Na + and Cl − , this was found to be contributed by a small number of large particles (> 500 nm, mostly even in the super-micron size range). Based on a three-modal fit, a sea spray mode observed in the NSDs was found to contribute 90 % to the total particulate mass but only 4 to 10 % to N total and up to 15 % to N CCN . This is in accordance with finding no correlation between N total and wind speed.

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Probing Finescale Dynamics and Microphysics of Clouds with Helicopter-Borne Measurements

Abstract: A new helicopter payload designed to overcome rotor downwash allows better than 10 cm resolution for turbulence and microphysical observations in boundary layer clouds.

Cited by 105 publications

References 21 publications

Observation of a Self-Limiting, Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus

Observation of a Self-Limiting, Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus

Droplet growth in warm turbulent clouds

Aerosol arriving on the Caribbean island of Barbados: physical properties and origin

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