地形上空边界层流中低层锋面结构的理论研究I:冷锋、均匀地转流

A THEORTICAL STUDY OF LOW-LEVEL FRONTAL STRUC- TURE IN THE BOUNDARY LAYER OVER OROGRAPHY PART Ⅰ: COLD FRONT AND UNIFORM GEOSTROPHIC FLOW

文中利用一个包含地形、边界层摩擦作用的二层锋面简化模型 ,讨论了地形、边界层对冷锋锋面结构、环流的动力学影响作用问题。冷锋的倾斜主要与冷锋暖区的地转流分布、锋面移动速度、锋面与地形的相对位置有关。当冷锋位于迎风坡时 ,其坡度减小 ,位于背风坡时 ,其坡度增大。在静止冷锋存在两类不同的环流系 ,即位于大气低层、地面锋附近的反时针环流系和远离地面锋的顺时针环流系。静止冷锋位于背风坡 ,其冷域中的这支闭合环流增强 ,范围增大 ,而位于冷锋界面上的环流转向点沿锋面上移 ,暖域中沿锋面的暖空气运动范围变大。当静止冷锋位于迎风坡时 ,结果相反。冷锋移动较慢时 ,其冷域远离地面锋的这支顺时针环流主体可被地形完全阻塞在迎风坡。当静止冷锋移离地形时 ,由于地形作用可在锋面暖域、地形下游形成一个背风槽。地形对锋区的垂直运动影响主要通过地形与边界层共同作用 ,改变锋区流场辐合辐散的分布及地形强迫抬升两条途径实现。由于边界层摩擦的辐合作用 ,在地面冷锋的前缘可形成一支范围较窄、强度较大的上升运动带 ,当冷锋位于迎风坡时 ,其强度增强 ,当冷锋位于背风坡时 ,其强度减弱。当冷锋位于背风坡时 ,在暖区沿锋面上升的暖空气运动范围增大 。

In this study a simplified two layer frontal model is incorporated to investigate the effect of orography and boundary layer friction on the structure and circulation of surface cold front (SCF). The inclination of SCF mainly depends on the geostrophic wind in the warm sector, frontal moving speed and the position of SCF relative to orography. Frontal slope is increased in the upward side,but decreased in the leeward side. There are two different circulation regimes in the cold sector for the steady cold front,i.e. a closed anti clockwise one in the low level near the SCF and a clockwise one away from the SCF. In the upward side the intensity and scope of the closed circulation in cold sector are enlarged and the turning point of frontal circulation at the frontal interface is lifted which leads to an enhancing upward motion of warm air ahead of the SCF. There is a contrary result when the cold front moves to the leeward side. The clockwise circulation in the cold air can be obstructed wholly in the upward side by orography for the moving slowly steady cold front. A lee trough over the leeward side is induced by the interaction of orography with the cold front. The vertical motion in frontal zone would be induced by the orography dynamic lifting and the distribution of convergence which related to the interaction between the boundary layer friction, frontal structure and orography. An intense upward motion ahead of the SCF may produced by the boundary layer friction that can be inflated in the upward side and reduced in the leeward side. There are multiple vertical upwarding zones in the warm side when the cold front passages in the leeward side.