Stratocumulus cloud-top height estimates and their climatic implications

Paquita Zuidema, David Painemal, Simon De Szoeke, Chris Fairall

Research output: Contribution to journalArticle

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Abstract

A depth-dependent boundary layer lapse rate was empirically deduced from 156 radiosondes released during six month-long research cruises to the southeast Pacific sampling a variety of stratocumulus conditions. The lapse-rate dependence on boundary layer height is weak, decreasing from a best fit of 7.6 to 7.2 K km-1 as the boundary layer deepens from 800 m to 2 km. Ship-based cloud-base heights up to 800 m correspond well to lifting condensation levels, indicating well-mixed conditions, with cloud bases >800 m often 200-600 m higher than the lifting condensation levels. The lapse rates were combined with Moderate Resolution Imaging Spectrometer 11-μm-derived cloud-top temperatures and satellite microwave-derived sea surface temperatures to estimate stratocumulus cloud-top heights. The October-mean cloud-top height structure of the southeast Pacific was then spatially and diurnally characterized. Coastal shoaling is apparent, but so is a significant along-coast cloud-top height gradient, with a pronounced elevation of the cloud-top heights above the Arica Bight at ∼20°S. Diurnal cloud-top height variations (inferred from irregular 4-times-daily sampling) can locally reach 250 m in amplitude, and they can help to visualize offshore propagation of free-tropospheric vertical motions. A shallow boundary layer associated with the Chilean coastal jet expands to its north and west in the afternoon. Cloud-top heights above the Arica Bight region are depressed in the afternoon, which may mean that increased subsidence from sensible heating of the Andes dominates an afternoon increase in convergence/upward motion at the exit of the Chilean coastal jet. In the southeast Atlantic during October, the stratocumulus cloud-top heights are typically lower than those in the southeast Pacific. A coastal jet region can also be identified through its low cloud-top heights. Coastal shoaling of the South Atlantic stratocumulus region is mostly uniform with latitude, in keeping with the more linear Namibian/Angolan coastline. The southeast Atlantic shallow cloudy boundary layer extends farther offshore than in the southeast Pacific, particularly at 15°S.

Original languageEnglish (US)
Pages (from-to)4652-4666
Number of pages15
JournalJournal of Climate
Volume22
Issue number17
DOIs
StatePublished - 2009

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stratocumulus
boundary layer
condensation
coast
sampling
radiosonde
MODIS
subsidence
sea surface temperature
heating

ASJC Scopus subject areas

  • Atmospheric Science

Cite this

Stratocumulus cloud-top height estimates and their climatic implications. / Zuidema, Paquita; Painemal, David; De Szoeke, Simon; Fairall, Chris.

In: Journal of Climate, Vol. 22, No. 17, 2009, p. 4652-4666.

Research output: Contribution to journalArticle

Zuidema, Paquita ; Painemal, David ; De Szoeke, Simon ; Fairall, Chris. / Stratocumulus cloud-top height estimates and their climatic implications. In: Journal of Climate. 2009 ; Vol. 22, No. 17. pp. 4652-4666.
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