Convective Patterns within a Field of Stratocumulus

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  • 1 Meteorological Institute, University of Hamburg, Hamburg, Germany
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Abstract

A case study of convective patterns observed within a field of stratocumulus is presented. Measurements were made by two aircraft over the North Sea in August 1984 during the KONTROL experiment. The slightly tilted boundary layer averaged about 900 m deep with a cloud layer of 300 m maximum thickness and cloud coverage between 3 and 6 octas. The sea surface was about 1 K warmer than the air. The boundary layer was capped by a temperature inversion of about 2 K, a sharp moisture decrease of about 3 g kg−1 and an insignificant wind shear.

A spectral analysis of the cloud and moisture field shows a mesoscale cloud structure of about 8 km diameter, accompanied by a secondary flow traceable within the entire boundary layer. This flow is driven by buoyancy near the sea surface and near cloud top. Beside the mesoscale structure, three smaller structures are present which are roughly separated from each other in wavenumber by a factor of about two. These patterns are not traceable within the subcloud layer. The shortest structure of about 1 km is probably the basic convective Rayleigh mode developing in a cloud layer subject to cooling at the top.

The basic state of the boundary layer is tested for cloud top entrainment instability (CEI) and mesoscale entrainment instability (MEI). It is found that the boundary layer is unstable with respect to small-scale cloud-top entrainment processes but stable to mesoscale entrainment disturbances.

Abstract

A case study of convective patterns observed within a field of stratocumulus is presented. Measurements were made by two aircraft over the North Sea in August 1984 during the KONTROL experiment. The slightly tilted boundary layer averaged about 900 m deep with a cloud layer of 300 m maximum thickness and cloud coverage between 3 and 6 octas. The sea surface was about 1 K warmer than the air. The boundary layer was capped by a temperature inversion of about 2 K, a sharp moisture decrease of about 3 g kg−1 and an insignificant wind shear.

A spectral analysis of the cloud and moisture field shows a mesoscale cloud structure of about 8 km diameter, accompanied by a secondary flow traceable within the entire boundary layer. This flow is driven by buoyancy near the sea surface and near cloud top. Beside the mesoscale structure, three smaller structures are present which are roughly separated from each other in wavenumber by a factor of about two. These patterns are not traceable within the subcloud layer. The shortest structure of about 1 km is probably the basic convective Rayleigh mode developing in a cloud layer subject to cooling at the top.

The basic state of the boundary layer is tested for cloud top entrainment instability (CEI) and mesoscale entrainment instability (MEI). It is found that the boundary layer is unstable with respect to small-scale cloud-top entrainment processes but stable to mesoscale entrainment disturbances.

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