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Discussion papers
https://doi.org/10.5194/acp-2020-104
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-104
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Mar 2020

Submitted as: research article | 11 Mar 2020

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This preprint is currently under review for the journal ACP.

Turbulent and Boundary Layer Characteristics during VOCALS-REx

Dillon S. Dodson and Jennifer D. Small Griswold Dillon S. Dodson and Jennifer D. Small Griswold
  • Department of Atmospheric Sciences, University of Hawaii, Manoa, Honolulu, HI, USA

Abstract. Stratocumulus clouds have a significant impact on climate due to their large spatial extent, with areas of enhanced coverage termed stratocumulus decks. How turbulence evolves with time and influences the stratocumulus deck properties however, in particular throughout the vertical profile of the boundary layer, is still lacking through model parameterizations of the small-scale flow. Collecting in situ data to better understand the turbulence and physical processes occuring within the stratocumu- lus deck therefore key to better model parameterizations. Boundary layer and turbulent characteristics, along with synoptic scale changes in these properties over time, are examined using data collected from 14 research flights made with the CIR- PAS Twin Otter Aircraft. Data was collected during the VOMOS Ocean-Cloud-Atmosphere-Land Study-Regional Experiment (VOCALS-REx) at Point Alpha in October and November of 2008 off the cost of South America (20°S, 72°W).

Findings show that the influence of a synoptic system on Nov 1st and 2nd brings in a moist layer above the boundary layer, leading to a deepening cloud layer and precipitation during passage, and a large increase in boundary layer height and cloud thinning after passage. The maximum value in turbulent kinetic energy (TKE) was measured on Nov. 1st due to precipitation destabilizing the sub-cloud layer while a minimum occurred on Nov. 2nd after precipitation had ceased due to turbulent mixing overturning the boundary layer and depleting the initial turbulent energy produced from the evaporation of precipitation below cloud base. Turbulent properties averaged over all 14 flights reach a maximum near cloud middle (between normalized in- cloud values of 0.25–0.75), with well mixed boundary layers experiencing two peaks in TKE, one near cloud base due to latent heat release and another near cloud top due to evaporational cooling. Overall, it appears that turbulence measured at Point Alpha is weaker than that measured over the open ocean to the west of Point Alpha, and that measured during other scientific campaigns. Synoptic scale analysis suggests that as the geopotential height decreases, the boundary layer height and entrainment zone thickness increases, accompanied by a decrease of in-cloud and below-cloud turbulence, and vice versa.

Dillon S. Dodson and Jennifer D. Small Griswold

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Dillon S. Dodson and Jennifer D. Small Griswold

Dillon S. Dodson and Jennifer D. Small Griswold

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Latest update: 01 Apr 2020
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Short summary
The results here reinforce findings from previous in situ studies of the marine boundary layer. It is found that turbulence is maximized in the middle of the stratocumulus layer from latent heating effects. Precipitation acts to increase turbulence in the sub cloud layer, while acting to stabilize the entire boundary layer after the evaporation of precipitation in the sub cloud has stopped. Negative correlation is present between the boundary layer height and turbulence.
The results here reinforce findings from previous in situ studies of the marine boundary layer....
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