The Role of Diffusive Effects On Potential Vorticity In Fronts
The Role of Diffusive Effects On Potential Vorticity In Fronts
复制标题
扩散效应对锋面位涡度的作用
DOI：
10.1002/qj.49711850603
复制
发表时间：
1992
影响因子：
8.9
通讯作者：
C. Bishop
中科院分区：
文献类型：
--
作者：
I. Cooper;A. Thorpe;C. Bishop
二维锋的理论研究表明，在阻塞和/或非绝热效应时的摩擦过程意味着低层的正位涡异常。它示出在这里，大量的PV可以产生在一个不断发展的斜压波（被认为是二维半地转Eady波）纯粹由于存在一个无滑移，绝缘的下边界。一个简单的解析表达式给出的时间演化的质量加权体积平均PV，（PV），锋面崩溃的点。对于最不稳定的线性正常模式，它被发现，在锋面不连续的形成时，PV增加了约三分之一的基本状态值。
散装制定的表面通量的热量和动量是用来说明修改PV局部边界层过程中的一般作用。湍流通量线性减少，从最大值在表面上的行星边界层（PBL）的顶部为零，用于恢复一个简单的分析形式的PV发电的趋势。通过垂直积分边界层上的趋势方程，突出显示了中纬度气旋和相关锋面典型气流产生的PV异常幅度。结果表明，边界层中PV的产生取决于边界层深度上的速度和位温的垂直差异，以及边界层顶部的垂直涡度和水平位温梯度。例如，如果在地面风矢量右侧的边界层顶部有较冷的空气，即如果热成风具有与风速计水平的风相反的分量，则存在PV的局部源。使用该公式，由表面过程产生的PV异常与遮挡过程无关。
Pestrian水平混合的潜在温度异常附近的热量进行检查，以预测本地PV异常的大小，与内部的“涡”粘性经常使用的数值模型相一致。
二维Eady波的数值积分与详细的表面层的描述和稳定性相关的涡动粘性系数也表明，在PV的显着增长发生之前，温暖的部门被解除了表面。有一个直接的对应关系，这种增长和水平平均的表面势温度的降低，从无滑移，绝缘边界理论预测。此外，本地PV异常预测的散装PBL表示被证明是在密切的协议与数值模拟中产生的，它使用了一个更全面的计划。
Theoretical studies of two-dimensional fronts have shown that frictional processes at the time of occlusion and/or diabatic effects imply low-level, positive potential vorticity (PV) anomalies. It is shown here that significant amounts of PV can be produced in an evolving baroclinic wave (taken to be the two-dimensional semigeostrophic Eady wave) purely owing to the presence of a no-slip, insulating lower boundary. A simple analytic expression is given for the time evolution of the mass-weighted volume-average PV, (PV), up to the point of frontal collapse. For the most unstable linear normal mode it is found, at the time of the formation of the frontal discontinuity, that the PV has increased by approximately a third of its basic-state value.
The bulk formulation for surface fluxes of heat and momentum is used to illustrate the general role of boundary-layer processes in modifying PV locally. Turbulent fluxes which decrease linearly from maximum values at the surface to zero at the top of the planetary boundary layer (PBL) are used to recover a simple analytical form for the PV generation tendency. the magnitudes of the PV anomalies produced by flows typical of mid-latitude cyclones and associated fronts are highlighted by vertically integrating the tendency equation over the PBL. It is shown that the generation of PV in the PBL depends on the vertical differences of velocity and potential temperature over the depth of the PBL; and on the vertical vorticity and horizontal potential-temperature gradient at the top of the PBL. For example, there is a local source of PV if there is cooler air at the top of the boundary layer to the right of the surface wind vector, i.e. if the thermal wind has a component opposing the wind at anemometer level. the production of PV anomalies by surface processes, using this formulation, is independent of the occlusion process.
Purely horizontal mixing of heat in the vicinity of a potential-temperature anomaly is examined to predict the magnitude of local PV anomalies, consistent with the internal ‘eddy’ viscosity often used in numerical models.
Numerical integrations of a two-dimensional Eady wave with a detailed surface-layer description and stability-dependent eddy viscosity coefficients also illustrate that significant growth in the PV occurs before the warm sector is lifted off the surface. There is a direct correspondence between this growth and the decrease of the horizontally-averaged surface potential temperature, as predicted from the no-slip, insulating boundary theory. Also, the local PV anomalies predicted by the bulk PBL representation are shown to be in close agreement with those produced in the numerical simulation, which uses a more comprehensive scheme.