Boundary layer mechanisms in extratropical cyclones
Boundary layer mechanisms in extratropical cyclones
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DOI：
10.1002/qj.30
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发表时间：
2007-01-01
影响因子：
8.9
通讯作者：
Beare, Robert J.
中科院分区：
文献类型：
--
作者：
Beare, Robert J.
本文重新讨论了边界层与旋风分离器相互作用的机理。两种诊断方法进行了比较：埃克曼泵和位涡。Ekman抽吸是由边界层应力引起的辐合和上升。边界层位涡是涡度和层结的统一量。通过对气象局统一模式（Met Office Unified Model）模拟的一个理想化的热带气旋生命周期的比较，发现边界层应力的一个重要分量以及Ekman抽吸作用是由不稳定边界层中的冷走廊带射流所强迫的。相反，边界层深度平均位涡的大部分包含在稳定的暖区。暖区PV的反演表明，当地有一个约2.5百帕的小加深。此外，在不稳定冷区关闭边界层混合比在稳定暖区关闭边界层混合的影响更大。然后研究了气旋及其边界层对基本状态急流强度的敏感性。最大摩擦速度与初始最大射流强度密切相关。这表明了大尺度流动在组织边界层结构中的重要作用。通过改变边界层参数化产生的最小压力的变化密切对应于旋风器平均表面应力的变化。对边界层方案的不同操作变化在三天内对气旋最低压力产生小的补偿变化。(c)版权所有2007年。
This paper revisits the mechanism for the interaction of the boundary layer with extratropical cyclones. Two diagnostic approaches are compared: Ekman pumping and potential vorticity. Ekman pumping derives from the boundary layer stress which induces convergence and ascent. boundary layer potential vorticity contains in a single quantity both the vorticity and stratification. These quantities are compared for an idealized extratropical cyclone life cycle simulated with the Met Office Unified Model.A significant component of the boundary layer stress and thus Ekman pumping at occlusion is forced by the cold conveyor-belt jet in the unstable boundary layer. In contrast, much of the boundary layer depth-averaged potential vorticity is contained within the stable warm-sector region. Inversion of the warm-sector PV indicates a small local deepening of about 2.5 hPa. Moreover, switching off the boundary layer mixing in the unstable cold sector has much more impact than in the stable warm sector.The sensitivity of the cyclone and its boundary layer to basic-state jet strength is then investigated. The maximum friction velocity scales closely with the initial maximum jet strength. This demonstrates the important role of the large-scale flow in organizing the boundary layer structure. Changes in the minimum pressure produced by altering the boundary layer parametrization correspond closely to changes in the surface stress averaged over the cyclone. Different operational changes to the boundary layer scheme produce small and compensating changes to the cyclone minimum pressure over three days. (c) Crown Copyright 2007.