Thermal Adaptation, Overshooting, Dispersion, and Subtropical Anticyclone Part I: Thermal Adaptation and Overshooting
Wu Guoxiong and Liu Yimin (State Key Laboratory of Numerical Modeling Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029)
Based upon potential vorticity theory, this paper investigates the feature of atmospheric motion as a result of diabetic heating, and stresses the principle of atmospheric thermal adaptation to external thermal forcing. It is proved that, during thermal adaptation the baroclinic component of potential vorticity along the boundary of the heating region becomes negative and larger than the barotropic component, where the atmosphere becomes symmetrically unstable. It is shown that over the top layer of a heating region, although there is no diabatic heating, but still exist ascent, divergence, anticyclonic circulation, and cold center. This is proved to be a result of the so-called “over shooting” accompanied with the thermal adaptation. Budget analysis of total potential vorticity reveals that, through thermal adaptation, the vorticity generation due to surface friction forms an upper-layer source of negative vorticity, and affects atmospheric motions. These are further verified by a series of numerical experiment.