Radiative and Evaporative Cooling in the Entrainment Zone of Stratocumulus – the Role of Longwave Radiative Cooling Above Cloud Top

vanZanten, Margreet C.

doi:10.1023/a:1013129713315

Cited by 17 publications

(13 citation statements)

References 21 publications

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“…Figure shows four sets of such patterns for TO10 which are given the name “pods” (for a detailed description of pods for POST flights as well as analyses of their EIL, see Hill []). Similar vertical patterns have been used in Sc by VanZanten and Duynkerke [], for a brief time during DYCOMS II, and by Katzwinkel et al . [].…”

Section: Observationsmentioning

confidence: 92%

“…Mixture fraction analysis relates buoyancy ( b ) of mixtures consisting of cloudy air at cloud top and of air above the EIL to mixing fractions given by

χ = m_{2} / (m_{1} + m_{2})

where m 1 is the mass of air from above the EIL in the mixture and m 2 is the mass of cloudy air in the mixture. This analysis has been used frequently [ Siems et al ., ; Wang and Albrecht , ; Shao et al ., ; Stevens , ; VanZanten and Duynkerke , ; de Roode and Wang , ; Mellado et al ., ; Kurowski et al ., ; Yamaguchi and Randall , ; Hill , ; S. A. Hill et al, The entrainment interface layer of stratocumulus‐toped boundary layers during POST, submitted to Atmosphere Research , 2013]. The analysis predicts in part the amount of negative buoyancy caused by LWC evaporation in mixtures of corresponding χ values.…”

Section: Microphysicsmentioning

confidence: 99%

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Entrainment rates and microphysics in POST stratocumulus

et al. 2013

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[1] An aircraft field study (POST; Physics of Stratocumulus Top) was conducted off the central California coast in July and August 2008 to deal with the known difficulty of measuring entrainment rates in the radiatively important stratocumulus (Sc) prevalent in that area. The Center for Interdisciplinary Remotely-Piloted Aircraft Studies Twin Otter research aircraft flew 15 quasi-Lagrangian flights in unbroken Sc and carried a full complement of probes including three high-data-rate probes: ultrafast temperature probe, particulate volume monitor probe, and gust probe. The probes' colocation near the nose of the Twin Otter permitted estimation of entrainment fluxes and rates with an in-cloud resolution of 1 m. Results include the following: Application of the conditional sampling variation of classical mixed layer theory for calculating the entrainment rate into cloud top for POST flights is shown to be inadequate for most of the Sc. Estimated rates resemble previous results after theory is modified to take into account both entrainment and evaporation at cloud top given the strong wind shear and mixing at cloud top. Entrainment rates show a tendency to decrease for large shear values, and the largest rates are for the smallest temperature jumps across the inversion. Measurements indirectly suggest that entrained parcels are primarily cooled by infrared flux divergence rather than cooling from droplet evaporation, while detrainment at cloud top causes droplet evaporation and cooling in the entrainment interface layer above cloud top.

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Section: Observationsmentioning

confidence: 92%

“…Mixture fraction analysis relates buoyancy ( b ) of mixtures consisting of cloudy air at cloud top and of air above the EIL to mixing fractions given by

χ = m_{2} / (m_{1} + m_{2})

Section: Microphysicsmentioning

confidence: 99%

Entrainment rates and microphysics in POST stratocumulus

et al. 2013

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“…The last term in this expression represents the fraction of radiative flux divergence that reduces the enthalpy but not the buoyancy. The reason for this distinction is that radiative cooling decreases q e s (h, q t ), the saturation vapor humidity, which leads to condensation [(∂ t q ) pha > 0] and thus condensational warming, which compensates part of the radiative cooling (e.g., Shao et al 1997, van Zanten & Duynkerke 2002. This effect explains that the buoyancy in Figure 4 varies slower with ψ in cloudy-air regions than in clear-air regions.…”

Section: Interaction Between Evaporative and Radiative Coolingmentioning

confidence: 98%

“…The variable ψ quantifies diabatic effects (Moeng et al 1995, Shao et al 1997, van Zanten & Duynkerke 2002, Yamaguchi & Randall 2012, de Lozar & Mellado 2015b. Hence, cloud-top entrainment is often discussed in terms of pure mixing (which includes latent-heat effects) and deviations thereof by radiative transfer and microphysical effects.…”

Section: Evaporative Coolingmentioning

confidence: 99%

Cloud-Top Entrainment in Stratocumulus Clouds

Mellado

2017

Annu. Rev. Fluid Mech.

162

159

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It has been considered that the unevenness of cloud top and cloud base has a great influence on solar and atmospheric radiation. Thus, to understand the cloud base structure of a widespread stratocumulus cloud, observations of stereoscopic photographs were made in which the optical axis of two wide-lens cameras was pointed upwards in a vertical direction. The observations were carried out in the summer and fall of 1986 and 1987. A set of stereophotographs was processed by an image analyzer and the edge lines of unevenness of the cloud base were obtained. Using the edge lines of unevenness of a pair of stereophotographs, 100 to 200 corresponding points were selected on the edge lines and the heights of points were obtained. As a result, it was recognized that the stratocumulus clouds analyzed have an unevenness of 300 m or more at the cloud base. Furthermore, a correlation between the height of cloud base and brightness of cloud base using a set of stereophotographs was obtained. Using the correlation, the cloud base structure, that is, the variation in cloud base height, was analyzed. As a result, it was demonstrated that the relationship between the size and depth of unevenness of the stratocumulus clouds resembled a hemisphere when the wind velocity was weak and wind shear was not strong up to cloud base height. It was recognized, however, that when the wind velocity and wind shear were strong up to the cloud base height, the unevenness at the cloud base changed from a hemispherical shape to a flattened shape.

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“…The consequent turbulent motion and entrainment leads to mixing between the cloudy air and the relatively warmer and drier air above, which produces evaporation of cloud droplets. Often this evaporative cooling contributes to a negative buoyancy (Vanzanten and Duynkerke, 2002) leading to a further instability processcalled buoyancy reversal -related to the reduction of the buoyancy of the entrained air parcel to below that of the unmixed cloudy air, so that the parcel sinks through the cloud producing a positive feedback between downdraft, turbulent mixing and evaporating cooling (Lock and MacVean, 1999). In this case, even if in the CTBL the longwave radiative cooling at the cloud top constitutes a major source of generating and maintaining turbulence (Moeng, 2000), the entrainment of free-atmospheric air can also generate turbulent kinetic energy, differently from the cloud-free boundary-layer case (Lock and Mac Vean, 1999).…”

Section: Introductionmentioning

confidence: 99%

Sodar Detected Top-Down Convection in a Nocturnal Cloud-Topped Boundary Layer: A Case Study

Martano

Cava

Mastrantonio

et al. 2005

Boundary-Layer Meteorol

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Nocturnal convection, originating in a well-mixed marine cloud-topped boundary layer, advected onshore, was observed using a Doppler sodar on the Tyrrhenian coast in Italy. The horizontal and vertical dimensions of the downdrafts were evaluated. The oscillation frequency triggered by the downdrafts at the inversion layer, derived from the harmonic analysis of the sodar measured vertical velocity (w), is compared with the Brunt-Vaisala frequency, obtained from the rawinsonde temperature profile. A similarity function for the r 2 w vertical profile was used to fit the sodar experimental data and to retrieve the depth of the mixing layer and the sensible heat flux at the top of the cloud layer. The results are in agreement with the convection layer depth observed in the sodar echoes facsimile record, and with the energy budget evaluated at the top of the cloud layer using the rawinsonde profiles.

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Radiative and Evaporative Cooling in the Entrainment Zone of Stratocumulus – the Role of Longwave Radiative Cooling Above Cloud Top

Cited by 17 publications

References 21 publications

Entrainment rates and microphysics in POST stratocumulus

Entrainment rates and microphysics in POST stratocumulus

Cloud-Top Entrainment in Stratocumulus Clouds

Sodar Detected Top-Down Convection in a Nocturnal Cloud-Topped Boundary Layer: A Case Study

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