On the Abrupt Change in the Structure of the Baiu Front Over the China Continent in Late May of 1979

摘要

The evolution of the Baiu frontal zone over the China Continent during the early summer of 1979 is examined based on the observation data. The Baiu frontal zone is a quasi-stationary cloud zone with a large horizontal gradient of specific humidity. In late May when the Baiu frontal zone still stays in the South China Plain the air temperature increases rapidly in the North China Plain and the horizontal temperature gradient in the lower layer in the Baiu frontal zone vanishes.A comparison is made between the characteristics of the Baiu frontal zone over the China Continent for the two periods (I) (1-20 May) and (II) (10-29 June). In the period (I) the stratification is stable and the frontal surface with a relatively large temperature gradient tilts northward with height in the Baiu frontal zone. The widely spread stratiform clouds as well as convective clouds cover the frontal zone. On the other hand, the stratification is convectively unstable and the Baiu frontal zone consists of active deep cumulus clouds in the period (II). It is suggested that the vanishment of the horizontal temperature gradient in late May greatly affects the difference between the stabilities for moist convection in the two periods.The ground surface temperature attains the nearly maximum value in late May and the amount of sensible heat supplied from the ground to the atmosphere is large in the North China Plain. The mixed layer with a thickness of 2000-3000m develops there through dry convection. The sensible heat supplied from the ground cancels out the cold air advection due to the transient eddies and contributes to the maintenance of the high air temperature there. It is also suggested that the air temperature in the lower layer increases rapidly in a few days after the passage of a synoptic-scale disturbance by the sensible heat from the ground there. Thus it is concluded that the heating from the ground in the North China Plain contributes greatly to the change in the structure of the Baiu frontal zone over the China Continent.