Analytical solution to a thermodynamic model for the sensitivity of afternoon deep convective initiation to the surface Bowen ratio
Analytical solution to a thermodynamic model for the sensitivity of afternoon deep convective initiation to the surface Bowen ratio
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DOI：
10.1002/qj.3340
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发表时间：
2018-10
影响因子：
8.9
通讯作者：
M. Bhowmick;D. Parker
中科院分区：
文献类型：
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作者：
M. Bhowmick;D. Parker
对流降雨在湿润或干燥的地表上发生的趋势是气候系统中的一个关键反馈过程，在各种空间尺度上影响水文循环，特别是在世界上水资源有限的地区。一个简单的代数解来自基本的物理方程，预测的迹象，这种对流降雨反馈与地面。对流发生的趋势通过对流边界层顶部接近自由对流水平的速率来评估。众所周知的积分模型预测边界层顶部的上升速率，在干燥的表面上往往更快。边界层中等效位温的相关变化决定了自由对流水平下降的速率，通常在湿表面上下降得更快，作为环境剖面、热力学强迫和表面鲍文比的函数。所得到的系统由三个参数控制。两个无量纲参数决定了是否存在湿或干“优势”：边界层顶部的鲍文比和“对流不稳定性参数”，定义为自由对流水平的饱和等效位温垂直梯度与边界层上方的廓线稳定性之比。一个量纲函数，取决于表面通量、边界层深度和剖面稳定性，提供了响应的大小。与以前的工作相比，解决方案是严格的物理原理推导和封装在一个简单的代数形式。理论框架的第一次评估是使用印度上空允许对流的数值模型模拟的数据进行的，这表明方程成功地确定了在干燥表面上触发对流的条件。
The tendency of convective rainfall to initiate over a wetter or drier land surface is a critical feedback process in the climate system, influencing the hydrological cycle on a variety of spatial scales, especially in parts of the world where water is limited. A simple algebraic solution is derived from fundamental physical equations, to predict the sign of this convective rainfall feedback with the surface. The tendency for convection to occur is evaluated by the rate at which the convective boundary‐layer top approaches the level of free convection. Well‐known integral models predict the rate of ascent of the boundary‐layer top, which tends to be faster over a dry surface. The associated changes in equivalent potential temperature in the boundary layer determine the rate at which the level of free convection descends, typically faster over a wet surface, as a function of the ambient profile, the thermodynamic forcing and the surface Bowen ratio. The resulting system is controlled by three parameters. Two nondimensional parameters determine whether there is wet or dry “advantage”; the Bowen ratio at the boundary‐layer top and a “convective instability parameter,” defined as the ratio of the vertical gradient of saturated equivalent potential temperature at the level of free convection to the profile stability just above the boundary layer. A dimensional function, dependent on the surface fluxes, the boundary‐layer depth, and the profile stability, provides the magnitude of the response. In comparison with previous work, the solution is both rigorously derived from physical principles and encapsulated in a simple algebraic form. A first evaluation of the theoretical framework has been made using data from a convection‐permitting numerical model simulation over India, and this indicates that the equations successfully determine the conditions under which convection is triggered over dry surfaces.