Mechanism for the Formation of Arc-Shaped Cloud Lines over the Tropical Oceans

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