A numerical modelling study of the geostrophic adjustment process following deep convection
A numerical modelling study of the geostrophic adjustment process following deep convection
复制标题
深对流后地转调整过程的数值模拟研究
DOI：
10.1002/qj.49712051903
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发表时间：
1994
影响因子：
8.9
通讯作者：
M. E. Gray
中科院分区：
文献类型：
--
作者：
G. Shutts;M. E. Gray
基于滞弹性运动方程的高分辨率数值模型用于模拟单个深对流羽流增长后的地转调整过程。人们的兴趣集中在平衡状态的形式以及进入重力波、耗散和平衡流的能量比例。描述了三种模拟：非旋转情况、中间旋转情况 (f= 10−3S−1) 和高旋转情况 (f= 2 × 10−3S-1)。使用这些人为的高旋转速率（在陆地大气的背景下）被视为缩短调整时间的装置，从而使研究目标在计算上可行。模型模拟是二维的，并使用潮湿过程的理想化规范，其中降水是瞬时的并且不允许有云。模型大气的初始状态是平静的和水平分层的，除了表面附近有一个温暖潮湿的气泡。积分进行 50000 秒，此时达到准稳态流动状态。
在旋转情况下，最终的准平衡状态采用中性浮力水平处具有均匀绝对动量的明确定义的透镜形状区域和从表面向上延伸的强烈的垂直剪切线前沿的形式。响应由两个长度尺度来表征：基于对流深度的罗斯贝变形半径，以及与对流质量和旋转速率等因素相关的中尺度云尺度。
结果发现 - 对流中释放的动能的 30% 被平衡流捕获 - 比早期研究中显示的要大得多，并且不依赖于此处使用的高旋转速率。简单的缩放参数用于将平衡能量保留的效率解释为对流质量的函数，并定义旋转环境中可通过单个羽流对流的质量的上限。这暂时被确定为中尺度对流系统的最大可能规模。
提出了平衡状态的分析模型，并显示该模型非常适合模拟流动。该模型的成功强调了混合在数值模型中发挥的次要作用。
A high resolution numerical model based on the anelastic equations of motion is used to simulate the geostrophic adjustment process that follows the growth of a single, deep convective plume. Interest focuses on the form of the balanced state and the proportions of energy which go into gravity waves, dissipation and balanced flow. Three simulations are described: a non-rotating case, an intermediate rotation case (f= 10−3S−1) and a high rotation case (f= 2 × 10−3S-1). The use of these artificially high rotation rates (in the context of the terrestial atmosphere) is regarded as a device for shortening the adjustment time thereby rendering the objective of the study computationally feasible. The model simulations are two-dimensional and use an idealized specification of the moist process in which precipitation is instantaneous and no cloud is permitted. The initial state of the model atmosphere is calm and horizontally-stratified except for a warm, moist bubble near the surface. Integrations are carried out for 50000 s by which time quasi-steady flow states are achieved.
In the rotating cases, the final quasi-balanced state takes the form of a well-defined lens-shaped region of uniform absolute momentum at the neutral buoyancy level and an intense, vertical shear line front extending upwards from the surface. The response is characterized by two length scales: the Rossby radius of deformation based on the depth of the convection, and a mesoscale cloud scale related to the amount of mass which convects and the rotation rate, amongst other factors.
It is found that - 30% of the kinetic energy released in convection is captured in balanced flow- considerably larger than indicated in earlier studies and not dependent on the high rotation rates used here. Simple scaling arguments are used to account for the efficiency of balanced energy retention as a function of the amount of mass that convects and to define an upper bound on the amount of mass that can be convected through a single plume in a rotating environment. This is tentatively identified with a maximum possible scale for mesoscale convective systems.
An analytic model for the balanced state is presented and shown to fit the simulated flow very well. The success of this model underlines the secondary role played by mixing in the numerical model.