急流加速产生的高空锋生和低空锋生

本文用数值模拟方法证实了曾庆存提出的一种锋生机制,即高空急流加速或高空动量输入通过地转适应过程造成高空锋生.我们采用二维非弹性数值模式模拟了五种急流加速分布产生的锋生过程:三个低空锋生,分别由中尺度地面、中尺度低空和次天气尺度低空急流加速产生;二个高空锋生,分别由高空次天气尺度和天气尺度急流加速产生.模拟的结果表明:(1)通过地转适应产生的位温梯度的大小依赖于急流的空间结构以及Rossby变形半径和急流扰动尺度的比值,比值越大,适应锋生越强.(2)在相同的尺度条件下急流加速产生的高空锋生比低空锋生更强.(3)地转适应过程将加速的高空急流动量向下层传输,使锋区变陡并向对流层中、下层延伸,这种对流层高低层的非地转耦合可能是低空急流形成的另一种机制.(4)在模拟的五个过程中急流振荡的主频率约是惯性频率的1.7倍.

Upper-level frontogenesis and lower-level frontogenesis forced by jet acceleration

The dynamic effects of jet accelerations are analysed numerically in this study . Results from this study are as floows. (1) The magnitude of the potential temperature gradients established by geostrophic adj ustment depends on the particular structure of the jet, the ageostrophic jet acceleration, and the ratio between the scale of the disturbance and the Rossby radius of deformation. (2) Upper-level frontogenesis was observed to be more intense than lower level frontogenesis on similar scales. (3) The geostrophic adjustment process which leads to upper-level frontogenesis tends to extend the frontal zone downwards into the middle and lower troposphere. This is accomplished through the downward transport of jet momentum which in turn acts to steepen the frontal zone . The ageostrophic coupling between the upper and lower troposphere may be a possible mechanism for the establishment of lower level jets . (4) The period of jet oscillations generated within the five simulations generally ranged from that of the inertial oscillation (approximently 17 hours) to about 5.5 hours.