Mechanism for the formation of arc-shaped cloud lines over the tropical oceans

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  • 1 Max Planck Institute for Meteorology, Bundesstrasse 53, 20146 Hamburg, Germany
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Abstract

Satellite images frequently show mesoscale arc-shaped cloud lines with a spacing of several tens of kilometers. These clouds form in a shallow mixed boundary layer in locations where the near-surface horizontal wind speed exceeds ~7 ms−1. Unlike other mesoscale cloud line phenomena, such as horizontal convective rolls, these cloud lines do not align with the wind direction but form at large oblique angles to the near-surface wind. A particularly distinct event of this pattern developed on Jan 31, 2020 over the western tropical Atlantic. Radiosonde soundings are available for this time and location, allowing a detailed analysis. By comparing observations to theoretical predictions based on Jeffreys’ drag-instability mechanism, it is shown that draginstability waves may contribute to the formation of this cloud pattern. The theory is formulated in only two dimensions and predicts that wave-like horizontal wind perturbations of this wavelength can grow, because they modulate the surface friction in a way that reinforces the perturbations. The theoretical horizontal wavelengths of 40–80 km agree with the observations. Streamlines from the ERA5 reanalysis show that the directional change of the near-surface wind is likely to contribute to the arc-shape, but that a radial propagation of an initial instability is also required to explain the strong curvature. Moreover, ERA5 winds suggest that other known explanations for the formation of cloud lines are unlikely to apply in the case studied here.

Corresponding author: Claudia Christine Stephan, claudia.stephan@mpimet.mpg.de

Abstract

Satellite images frequently show mesoscale arc-shaped cloud lines with a spacing of several tens of kilometers. These clouds form in a shallow mixed boundary layer in locations where the near-surface horizontal wind speed exceeds ~7 ms−1. Unlike other mesoscale cloud line phenomena, such as horizontal convective rolls, these cloud lines do not align with the wind direction but form at large oblique angles to the near-surface wind. A particularly distinct event of this pattern developed on Jan 31, 2020 over the western tropical Atlantic. Radiosonde soundings are available for this time and location, allowing a detailed analysis. By comparing observations to theoretical predictions based on Jeffreys’ drag-instability mechanism, it is shown that draginstability waves may contribute to the formation of this cloud pattern. The theory is formulated in only two dimensions and predicts that wave-like horizontal wind perturbations of this wavelength can grow, because they modulate the surface friction in a way that reinforces the perturbations. The theoretical horizontal wavelengths of 40–80 km agree with the observations. Streamlines from the ERA5 reanalysis show that the directional change of the near-surface wind is likely to contribute to the arc-shape, but that a radial propagation of an initial instability is also required to explain the strong curvature. Moreover, ERA5 winds suggest that other known explanations for the formation of cloud lines are unlikely to apply in the case studied here.

Corresponding author: Claudia Christine Stephan, claudia.stephan@mpimet.mpg.de
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