Questions on CAM6 nudging parameters

[omillin]

Hi,

I am looking to produce some nudged simulations in the CESM2 CAM6 subseasonal prediction system. Some of these will be everywhere (constant window function) and some will be in particular boxes using a window function horizontally and vertically (tropical troposphere, extratropical NH stratosphere, etc). I have a couple of questions regarding the parameters to execute the nudging:

1. Within the namelist descriptions on CESM2.2.0 CAM6.3 Namelist Definitions and 9. Physics modifications via the namelist — camdoc documentation I see that with the window functions you specify the central lat and lon of the window you want and then the width, followed by delta values (e.g., Nudge_Hwin_latDelta, Nudge_Hwin_lonDelta). For example, say I wanted to nudge every longitude but between 20S-20N with 10 degree latitude transition each side, would you just set Nudge_Hwin_lat0 = 0, Nudge_Hwin_lon0 = 180., Nudge_Hwin_latWidth = 40.0, Nudge_Hwin_lonWidth = 999.0, Nudge_Hwin_latDelta = 10.0, Nudge_Hwin_lonDelta = 0? The main question really is then would such a setup do the desired nudging between 20S to 20N, and then linearly transition up to free running at 30S and 30N or does the transition start earlier in the nudging window box?
2. Secondly, would the above type of transition therefore also be valid in the vertical windowing but defined in terms of model indices?
3. If you want to nudge from the top of the model to a given level, is the right way to use the Nudge_Vwin_Lindex = 0 in the same way that Nudge_Vwin_Hindex = 33 is used for the surface?
4. Finally, is there a preferred mode to use for the Nudge_TimeScale_Opt and Nudge_Force_Opt parameters or is the default the best?

Thank you!

Ollie