Air Mass Transformation along Trajectories of Airflow and Its Relation to Vertical Structures of the Maritime Atmosphere and Clouds in Yamase Events

Abstract

The Yamase is a cool easterly wind that is observed in summer along the eastern coast of the northern part of the main island (Honshu) of Japan. It usually accompanies a boundary layer cloud, the “Yamase cloud.” The origin of the Yamase is the cool polar maritime air mass that develops over the North Pacific, including the Bering Sea and the Sea of Okhotsk. The characteristics of the Yamase are controlled by air mass transformation over the western North Pacific. Campaign observations of the Yamase were performed using the marine vessel Koufu-maru of the Japan Meteorological Agency over the sea east of northern Honshu in summer, from 2001 to 2007. We studied two Yamase events and examined the heat flux along back trajectories as well as the heat and moisture budget. The vertical structure of the Yamase was strongly dependent on the history of the air over the ocean, despite the many factors that influence boundary layer clouds. In the June 2003 event, the stepwise upward development of Yamase clouds observed at the Koufu-maru site was related to the influence of Hokkaido Island and an oceanic front on Yamase flow trajectories. For the July 2006 event, the temperature profiles observed from the Koufu-maru changed from stable layer type to mixed layer type. Changes in the visibility and oceanic heating along Yamase air trajectories were also observed. Ocean heating increased when the trajectories changed from westward to southward across an oceanic front located to the east of 144°E in the Kuroshio-Oyashio extension. The meridional SST gradient was smaller over water off northern Honshu, where the Oyashio current prevailed. A heat and moisture budget analysis using aerological data observed by the Koufu-maru and three weather stations in northern Japan showed weak sensible heating and weak moisture sink when the Yamase wind prevailed. We ascribed the weak sensible heating to the small air-sea temperature difference, which was caused by the weak meridional SST gradient and offset by radiative cooling at the cloud top.