Improved cloud tracking method tolerant to streaky features: application to Venusian cloud images

Abstract An improved cloud tracking method applicable to cloud images dominated by streaky features was developed and applied to Venusian ultraviolet images. By accounting for streaky features in 365-nm images obtained by the Ultraviolet Imager (UVI) on the Akatsuki spacecraft, velocity vectors were obtained in a broad latitudinal range from the equator to high latitudes, where streak patterns have prevented reliable cloud tracking in previous studies. A spectral analysis of the obtained velocities revealed a prominent periodicity of 4.9 days, which is longer than the recurrence period of the background zonal wind. The velocity field of the 4.9-day wave exhibits a hemispherically symmetric vortex pattern extending to latitudes of ~ 60° and is consistent with a hemispherically symmetric gravest Rossby wave. The meridional momentum transport by the 4.9-day wave was found to have a hemispherically symmetric, complicated latitudinal structure, although an analysis of an extended period is needed to reveal an averaged picture. Graphical Abstract