WRF设置模式垂直层
翻译自WRF论坛https://forum.mmm.ucar.edu/threads/stretched-vertical-levels-information.14975/#post-37729
设置模式垂直层
Eta 层由 real 自动计算，基于namelist选项“e_vert.”所设置的层数。然而，完整的eta 层次可以由namelist选项“eta_levels.”来显式的指定。层次的数目必须和eta 面的数目(e_vert)相一致.使用的方法是由namelist选项“auto_levels_opt.”设计的。通常，auto_levels_opt=2, 使用底层和顶层缩放系数(dzstretch_s and dzstretch_u) 来对层次根据log p进行缩放，从厚度“dzbot.”开始，直到最大厚度(max_dz)。当厚度达到max_dz/2时，缩放的变换从“dzstretch_s”到“dzstretch_u”.
另外一个选项时设置auto_levels_opt=1, 这将假设已知前几个层，然后生成等高度间距的层次，直到模式的顶层。
Given are two distributions for 28 and 35 levels.
- dzstretch设置成1.3，则在5km（第13层）以下的厚度将达到1km-*通常不推荐
- dzstretch设置成1.2，则在7km（第19层）左右的厚度将达到1km
- dzstretch设置成1.1，则在5km（第36层）以下13厚度将达到1km
- dzstretch设置成1.1，在最底下的1km有12层，最底下10km有34层
- dzstretch设置成1.2，在最底下的1km有9层，最底下10km有22层
- dzstretch设置成1.3，在最底下的1km有8层，最底下10km有18层
当使用了dzstretch_s和dzstrech_u时，最少的层次数
To avoid max thickness in the upper troposphere, stretching levels must extend above the tropopause before going to constant d (logp). This can be done by using low “dzstretch_u” values (but larger than ~1.02) to reach the tropopause, while also stretching fast enough to compensate the lapse rate.
为了避免对流层上层的最大厚度，拉伸的层次必须在达到常数 d (logp) 之前延伸到对流层顶以上。这可以通过使用较低的“dzstretch_u”值（但大于~1.02）来达到对流层顶，同时拉伸得足够快以补偿递减率。
For additional flexibility, use namelist options
- dzbot: the thickness of the first model layer between full levels (default value is 50 m)
- max_dz: the maximum layer thickness allowed with the default value of 1000 m.
https://www2.mmm.ucar.edu/wrf/users/wrf_users_guide/build/html/initialization.html#initialization-for-real-data-cases
Setting Model Vertical Levels
Note: All namelist options mentioned in this section are set in the &domains record in namelist.input, and this section is specific to real.exe only.
Model vertical levels are determined by one of two ways.
Eta levels are automatically computed by the real program, based on the number of levels set by the namelist value for “e_vert.”
User-specification of each eta level: Users may opt to explicitly define each full eta level by utilizing the namelist option “eta_levels” (model eta levels from 1 to 0), with the total number of eta_levels equaling the number of eta surfaces allocated (namelist value for e_vert).
Option 1 is the most commonly used practice, as the real program ensures a reasonable set of levels. When doing so, the method used to determine the specific levels is based on the setting for namelist option “auto_levels_opt.”
auto_levels_opt = 1 : An older method that assumes a known first several layers, then generates equi-height spaced levels up to the model top.
auto_levels_opt = 2 : The default method, which uses a surface stretching factor (dzstretch_s) and an upper stretching factor (dzstretch_u) to stretch eta levels according to logP, starting from thickness dzbot, and up to the max thickness (max_dz). The stretching transitions from “dzstretch_s” to “dzstretch_u” by the time the thickness reaches max_dz/2.
- The following definitions and default information may also be useful:
dzbot: the thickness of the first model layer between full levels (default value is 50 m)
max_dz: the maximum layer thickness allowed with the default value of 1000 m.
Tips for Choosing Stretching Factors
To avoid max thickness in the upper troposphere, stretching levels must extend above the tropopause before going to constant d (logP). This can be done by using low “dzstretch_u” values (but larger than ~1.02) to reach the tropopause, while also stretching fast enough to compensate the lapse rate.
Typically a thin surface layer should be used.
The best practice is to first choose dzbot, then max_dz (which typically should remain at 1 km), then choose the stretching factors. It is ideal to find the minimum number of levels needed to work with the other values chosen. For e.g., if desiring more levels near the surface, use a smaller dzstretch_s.
Use trial and error, only changing one or two factors at a time, while keeping others constant.
Look at real.exe output to see printouts of values used.
Additional resource: WRF - More Runtime Options WRF Tutorial presentation
See the tables below for reasonable e_vert settings for dzbot=50 and dzbot=30.
