Mammatus-Like Structure along the Cloud Base in the Outflow Layer Observed in Several Typhoons

Abstract:

The clouds in the typhoon outflow layer are present above 0°C altitude and are mainly composed of ice particles. It has recently been reported that the outflow-layer clouds affect the path of typhoons (tropical cyclones) through radiation. In this study, microstructure of the outflow-layer clouds was examined. From a C-band precipitation radar observation, protuberances of radar echoes were found along the cloud base of the outflow layer in Typhoon Francisco (2013). The protuberances of echoes are referred to as mammatus-like echoes in this study. Subsequent Ka-band cloud radar observations showed details of the mammatus-like echoes in Typhoons Chaba (2016), Cimaron (2018), and Neoguri (2019). The horizontal and vertical scales were 1-3 km, and 0.5-1 km, respectively. Radar echoes obtained at high elevation scans showed no structural changes with azimuth. This indicates that the structure was composed of numerous downward projecting echoes, rather than having a two-dimensional structure like the wave-like clouds. For Typhoon Cimaron (2018), vertical observations showed downward Doppler velocities in hanging echo regions, and upward Doppler velocities in between the hanging echoes. Considering the difference between positive and negative peaks of the Doppler velocities, upward and downward air velocities were 1.5-2 m s^-1 around the mammatus-like echoes. This vertical air motion was significantly larger than that in the upper part of the outflow-layer clouds. For Typhoon Chaba (2016), five radiosonde observations showed that an almost constant layer of potential temperature was formed near the cloud base. Below the cloud base, there was a dry layer with relative humidity with respect to ice of < 20%. These profiles can indicate sublimation of ice particles falling from the outflow-layer clouds. This study showed that the mammatus-like structure frequently occurred at the boundary between the outflow-layer clouds and the underlying dry layer, with upward and downward air motions of several meters per second. This mammatus-like structure may facilitate disappearance of the typhoon outflow layer clouds.