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Parameterization of Turbulent Orographic Form Drag and Implementation in GRAPES
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摘要: 分辨率的限制使得不能被模式识别的地形称为次网格尺度地形，次网格尺度地形在热力和动力方面对实际大气有着不可忽略的作用，其效应只能通过参数化的形式回馈给模式。分辨率的提高使得与较小尺度地形相联系的地形湍流拖曳力凸显其重要性。数值模式中地形湍流拖曳力的参数化对完善模式物理过程和改善模式近地层预报效果具有积极意义，其方法包括有效粗糙度法和直接参数化法，而GRAPES模式中并未以任何方法考虑次网格尺度地形的影响。该文通过单柱模式比较了有效粗糙度法和直接参数化法的优劣, 发现后者在有些方面优于前者。最后，将应用于实际的一个直接参数化方案接入GRAPES中尺度模式中，进行个例模拟，并与NCEP再分析资料进行对比，结果表明：考虑地形湍流拖曳力方案对模式预报具有改进作用，尤其对局地低层风场具有积极影响。
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关键词:
- 次网格尺度地形湍流拖曳力;
- 参数化;
- GRAPES模式
Abstract: Due to the limited model resolution, scales of topographies below the model resolution are called sub-grid scale orography. Sub-grid scale orography plays important roles on the atmosphere from the thermal and dynamic aspects, and these effects are fed back to the model atmosphere through parameterization.Generally, the dynamic effects of sub-grid scale orography includes: Sub-grid orographic turbulent form drag caused by turbulent flow over hills; sub grid-scale form drag of orographic flow blocking, separating and lee vortex; buoyancy waves drag caused by breakdown of gravity-waves that excited by flow over mountains and transmitting upward in the upper atmosphere. With resolution raised, turbulent form drag becomes more important which is associated with small-scale orography. Given these facts, the characteristics and effects of the sub-grid orographic turbulent form drag and its parameterization are studied.Usually, effects of the turbulent form drag due to sub-grid scale orography in numerical models are simply considered as enhancing surface roughness, known as the effective roughness method. In recent years, according to asymptotic theory and numerical simulations, a new scheme of sub-grid orographic turbulent form drag is developed. As an independent physical process, the new scheme is of three-dimension features in the model, while the effective roughness method is only considered being on the surface level.It is significant to consider turbulent form drag in NWP model for completing physical processes of model and improving the model prediction of the surface layer. A single-column model is used to simulate the characteristics of turbulent form drag, comparing the advantages and disadvantages between the effective roughness method and direct parameterizing method. Two direct parameterization schemes are also compared, one is developed and applied in NWP model recently, and the other is based on ideal terrain. Finally, the form drag scheme that developed and applied in NWP model is implemented into the GRAPES regional model. Case studies are conducted to investigate the possible influences of form drag on the forecast of near surface wind fields and other variables. -
图 1 有效粗糙度法和直接参数化法 (应用WBH01方案) 对低层气流的作用
(a) 有效粗糙度随坡度和地面粗糙度的变化，(b) 地表总应力 (被摩擦速度的平方标准化) 随坡度的变化，(c) 模式第1层 (5.5 m) 纬向风随坡度变化，(d) 模式第1层 (5.5 m) 经向风随坡度变化
Fig. 1 The influence of effective roughness method and directed method (using WBH01 scheme) on the lower layer
(a) the effective roughness changes with local roughness and slope, (b) the total surface stress (normalized by friction velocity square), (c) the zonal velocity changes of the 1st model level (5.5 m) with slope, (d) the meridional velocity changes of the 1st model level (5.5 m) with slope
图 3 地形湍流拖曳力直接参数化法对纬向风速 (a)、经向风速 (b) 和纬向应力 (c)、经向应力 (d) 的影响
Fig. 3 The influence of form drag on zonal velocity (a), meridional velocity (b) and zonal flux (c), meridional flux (d)
图 4 WBH01方案 (a) 和BBW04方案 (b) 动能倾向诊断
Fig. 4 The diagnosis of kinetic energy tendency for WBH01 scheme (a) and BBW04 scheme (b)
图 5 无量纲化地表地形湍流拖曳力随坡度变化
Fig. 5 The change of the normalized surface form drag with slope
图 6 试验I2B6地形湍流拖曳力造成的区域平均速度倾向随时间的变化 (每个时次间隔3 h)
Fig. 6 The change of area average velocity tendency caused by form drag with time in case I2B6 (the time interval is 3 hours)
图 7 I2B6试验24 h预报场地形湍流拖曳力造成的速度倾向沿30°N垂直剖面图 (阴影)，模式面第1层全风速大小 (绿线) 和地形标准偏差 (红线)
Fig. 7 30°N zonal section of velocity tendency (shaded area) caused by 24-hour prediction form drag, the 1st level wind speed (green line) and orographic standard deviation (red line) in case I2B6
图 9 10 m高度纬向风和经向风速考虑与不考虑BBW04方案的均方根误差和偏差
Fig. 9 RMS error and bias of 10-meter zonal and meridional winds between cases with and without BBW04 scheme
表 1 个例模拟简明表
Table 1 Cases concise table
试验简称 描述 区域 起报时间 O8B15 外围考虑BBW04方案，0.15°分辨率 15°~65°N，70°~145°E 2008-12-08T00：00 O8N15 外围不考虑拖曳力方案，0.15°分辨率 15°~65°N，70°~145°E 2008-12-08T00：00 O2B15 外围考虑BBW04方案，0.15°分辨率 15°~65°N，70°~145°E 2009-08-02T00：00 O2N15 外围不考虑拖曳力方案，0.15°分辨率 15°~65°N，70°~145°E 2009-08-02T00：00 I8B6 嵌套，考虑BBW04方案，0.06°分辨率 20°~38°N，92°~113°E 2008-12-08T00：00 I8N6 嵌套，不考虑拖曳力方案，0.06°分辨率 20°~38°N，92°~113°E 2008-12-08T00：00 I2B6 嵌套，考虑BBW04方案，0.06°分辨率 20°~38°N，92°~113°E 2009-08-02T00：00 I2N6 嵌套，不考虑拖曳力方案，0.06°分辨率 20°~38°N，92°~113°E 2009-08-02T00：00 
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