Liu Yi-Chin, Fan Jiwen, Xu Kuan-Man, Zhang Guang J
Pacific Northwest National Laboratory, Richland, Washington, USA.
Air Resources Board, California Environmental Protection Agency, California, USA.
J Atmos Sci. 2018 Jul;75(7):2445-2472. doi: 10.1175/JAS-D-18-0019.1. Epub 2018 Jul 1.
We use 3-D cloud-resolving model (CRM) simulations of two mesoscale convective systems at mid-latitudes and a simple statistical ensemble method to diagnose the scale dependency of convective momentum transport (CMT) and CMT-related properties, and evaluate a parameterization scheme for convection-induced pressure gradient (CIPG) developed by Gregory et al. (GKI97). GKI97 relates CIPG to a constant coefficient multiplied by mass flux and vertical mean wind shear. CRM results show that mass fluxes and CMT exhibit strong scale dependency in temporal evolution and vertical structure. The prevalent understandings of CMT characteristics in terms of upgradient/downgradient transport are applicable to updrafts but not downdrafts across a wide range of grid spacings (4-512 km). For the small-to-median grid spacings (464 km), GKI97 reproduces some aspects of CIPG scale dependency except for underestimating the variations of CIPG as grid spacing decreases. However, for large grid spacings (128512 km), GKI97 might even less adequately parameterize CIPG because it omits the contribution from either the nonlinear shear or buoyancy forcings. Further diagnosis of CRM results suggests that inclusion of nonlinear shear forcing in GKI97 is needed for the large grid spacings, and use of the three-updraft and one downdraft approach proposed in an earlier study may help a modified GKI97 capture more variations of CIPG as grid spacing decreases for the small-to-median grid spacings. Further, the optimal coefficients used in GKI97 seems insensitive to grid spacings, but they might be different for updrafts and downdrafts, for different MCS types, and for zonal and meridional components.
我们使用中纬度地区两个中尺度对流系统的三维云分辨模型（CRM）模拟以及一种简单的统计系综方法，来诊断对流动量输送（CMT）和与CMT相关特性的尺度依赖性，并评估由Gregory等人（GKI97）开发的对流诱导压力梯度（CIPG）的参数化方案。GKI97将CIPG与一个常数系数乘以质量通量和垂直平均风切变相关联。CRM结果表明，质量通量和CMT在时间演变和垂直结构上表现出很强的尺度依赖性。关于CMT特征的普遍理解，即上梯度/下梯度输送，适用于上升气流，但在广泛的网格间距（4 - 512千米）范围内不适用于下沉气流。对于小到中等网格间距（464千米），GKI97再现了CIPG尺度依赖性的一些方面，但低估了随着网格间距减小CIPG的变化。然而，对于大网格间距（128512千米），GKI97对CIPG的参数化可能更不充分，因为它忽略了非线性切变或浮力强迫的贡献。对CRM结果的进一步诊断表明，对于大网格间距，GKI97需要包含非线性切变强迫，并且使用早期研究中提出的三上升气流和一下沉气流方法，可能有助于改进后的GKI97在小到中等网格间距下随着网格间距减小捕获更多CIPG的变化。此外，GKI97中使用的最佳系数似乎对网格间距不敏感，但对于上升气流和下沉气流、不同的中尺度对流系统类型以及纬向和经向分量可能不同。
