Towards a more “scale-aware” orographic gravity wave drag parametrization: Description and initial testing,Quarterly Journal of the Royal Meteorological Society

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Towards a more “scale-aware” orographic gravity wave drag parametrization: Description and initial testing
Quarterly Journal of the Royal Meteorological Society ( IF 3.0 ) Pub Date : 2021-07-15 , DOI: 10.1002/qj.4126
A. Niekerk ₁ , S.B. Vosper ₁

Affiliation

Many current orographic gravity wave drag parametrizations employed in numerical weather prediction and climate models assume idealised elliptical subgrid orography. As a result, they do not fully capture the spectrum of subgrid orographic scales and do not exhibit the desired behaviour when the model grid length is varied. This article describes the motivation, formulation, and testing of a new orographic gravity wave drag parametrization in the Met Office Unified Model that is more “scale-aware”. The scheme circumvents the need to make assumptions about the shape and distribution of the subgrid orography, moving away from elliptical mountains, through the use of Fourier transforms. The reduction in gravity wave surface stress when orographic flow blocking is present is also accounted for. The result is a new scheme that represents subgrid mountain scales more faithfully and consistently, rather than assuming a single mountain scale for each grid box. Tests using both regional and global simulations, ranging in grid spacing from 2–130 km, demonstrate that the scale-aware scheme provides a more constant total, resolved plus parametrized, orographic gravity wave momentum flux, when the grid length is varied over most regions of the globe. Global five-day forecasts also show a significant reduction in the too-strong zonal wind bias within the Northern and Southern Hemisphere upper troposphere to lower stratosphere at grid spacings of 130 and 40 km when the scale-aware scheme is employed. Some discussion on future developments for the scheme is also presented, noting that improvements to the orographic flow-blocking drag component are required before the scheme can be implemented operationally.

中文翻译：

走向更“尺度感知”的地形重力波阻力参数化：描述和初步测试

许多当前用于数值天气预报和气候模型的地形重力波阻力参数化假设理想化的椭圆亚网格地形。因此，当模型网格长度变化时，它们不能完全捕获子网格地形尺度的频谱，也不会表现出所需的行为。本文描述了气象局统一模型中新的地形重力波阻力参数化的动机、制定和测试，该模型更具“尺度意识”。该方案通过使用傅立叶变换避免了对子网格地形的形状和分布进行假设的需要，远离椭圆山。当存在地形流动阻塞时，重力波表面应力的减少也被考虑在内。结果是一个新的方案，它更忠实和一致地表示子网格山脉尺度，而不是为每个网格框假设一个单一的山脉尺度。使用区域和全球模拟进行的测试，网格间距范围为 2-130 公里，表明当网格长度在大多数区域变化时，尺度感知方案提供了更恒定的总量、解析加参数化、地形重力波动量通量的地球。全球五天预报还显示，当采用尺度感知方案时，北半球和南半球对流层上层到平流层下层的太强纬向风偏差显着减少，网格间距为 130 和 40 公里。还提出了有关该计划未来发展的一些讨论，

更新日期：2021-09-06

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