Observation of spectacular, regular and lasting cloud top formation near the tropopause

We describe what appears to be a field of upside-down mammatus clouds (termed here as mammatocumulus) spanning several hundreds of kilometers at the top of a large frontal system in the North Pacific storm track. CloudSat cloud profiling radar imagery shows large lobes of upwardly protruding cloud that are 10 to 20 km across and 2 to 4 km deep. The lobes are spaced by equally large regions of downward protruding clear air. Observations (radar, lidar, IIR), meteorological data from reanalysis and theoretical arguments, support a hypothesis that these lobes formed as a consequence of very strong thermal radiative cooling at the cloud top. Because the frontal cloud system was broad and optically dense, the concentrated cloud top cooling went into creating a turbulent mixed-layer that deepened downward. The mammatocumulus lobes are simply the visible upward component of the dynamic circulations, spaced in cells of 10 km diameter. Looking at the same system a day later, CloudSat shows evidence of cloud decay while the mammatocumulus clouds linger, leaving a turbulent cirrus sheet hundreds of kilometers in extent near the tropopause. We suggest that such mammatocumulus formation implies a potential for a radiative-dynamic initiation of clouds that ultimately exert a tropospheric radiative warming while facilitating mixing across the tropopause.