A dynamical mechanism for tropopause sharpening
A dynamical mechanism for tropopause sharpening
复制标题
DOI：
10.1127/0941-2948/2004/0013-0477
复制
发表时间：
2004-12
影响因子：
1.2
通讯作者：
V. Wirth
中科院分区：
文献类型：
--
作者：
V. Wirth
观察到的复合剖面浮力频率平方N 2在对流层顶区的特征，包括一个明显的峰值上面一个非常尖锐的对流层顶。早期的工作表明，这些功能是，至少部分是由于保守的平衡动力学的影响，在形成的异常从一个大规模的背景大气。这一结果是利用ERTEL位涡（PV）的守恒和可逆性的孪生原理得到的。解释仍然很困难，因为N2主要受垂直风的影响，这在所使用的PV方法中是高度隐含的。目前的工作提供了一个机制的解释对流层顶锐化明确集中在垂直风及其对N2的影响。通过叠加正压形变场，强迫出一个平衡的平板对称参考流。由此产生的Sawyer-Eliassen二次环流进行了数值计算。这种次级环流对气旋来说意味着垂直辐散和涡旋中心N2的实质性减少，而对反气旋来说意味着垂直辐合和N2的实质性增加。随后的演变导致气旋和反气旋之间的不对称，使净效应保持在平均水平。
Observed composite profiles of buoyancy frequency squared N 2 in the extratropical tropopause region show characteristic features including a pronounced peak right above a very sharp tropopause. Earlier work suggested that these features are, at least partly, due to the impact of conservative balanced dynamics during the formation of anomalies from a large scale background atmosphere. This result was obtained using the twin principle of conservation and invertibility of ERTEL potential vorticity (PV). The interpretation remained difficult, since N 2 is mostly affected by the vertical wind, which is highly implicit in the PV based method used. The current work provides a mechanistic explanation of tropopause sharpening by explicitly focussing on the vertical wind and its impact on N 2 . A balanced slab symmetric reference flow is forced by superimposing a barotropic deformation field. The resulting Sawyer-Eliassen secondary circulation is computed numerically. This secondary circulation implies vertical divergence and a material decrease of N 2 in the vortex center for cyclones, while it implies vertical convergence and a material increase of N 2 for anticyclones. The ensueing evolution leads to an asymmetry between cyclones and anticyclones, allowing a net effect to remain on average.