A Study of Stratocumulus Responses to the Kuroshio Extension Front: In-situ Observations and Mechanism

The effect of sea surface temperature (SST) front on the vertical structures of the marine atmospheric boundary layer (MABL) and marine stratiform clouds is not well understood especially on a daily time scale due to the lack of in-situ observations over the ocean. A research vessel, Dongfanghong 2 of Ocean University of China, captured a rapid development of stratocumulus near the Kuroshio Extension (KE) front on 12 April 2014. The cloud top and cloud base increased and the cloud coverage expanded significantly on the warm flank of the front. The present study used a suite of on board observations to analyze the mechanism for the cloud development. The results are as follows. A positive heat flux center formed on the warm flank (downwind) of the KE front due to the slow atmospheric response to the forcing of the KE front. A weak pressure trough that was associated with the heat flux center intensified, leading to vertical motions in the MABL. The turbulent mixing in the MABL was favorable for the downward transport of westerly momentum from aloft to sea surface, which in turn further intensified the surface heat flux. This positive feedback process provided favorable conditions for the increase in the thickness of the mixed layer in MABL and the height of the cloud base/top downwind the KE front. On the other hand, a warm air advection from Honshu, Japan arrived at the heat flux center. The heat flux was combined with the warm advection, producing a pronounced warm center near the surface at 0900-1200 UTC 12 April. This warm center further led to the deepening of the mixing layer, the rising of the cloud base/top and the rapid development of the clouds. This study helps to understand the mechanism of the response of the MABL and the marine stratiform clouds to SST fronts under synoptic disturbances.