Add rollover of surface fluxes (cdn) at high wind speed

[vlarson]

The current calculation of the neutral drag coefficient at 10 m (cdn) in subroutine shr_flux_atmOcn and subroutine shr_flux_atmOcn_diurnal does not include a moderating effect at wind speeds greater than 30 m/s. This rollover effect can be incorporated using equation (11a,b) of Large and Yeager (2009, Climate Dynamics).

[billsacks]

@vlarson @mvertens now that shr_flux_mod has been moved out of cime: if this is still needed, can you please open an issue in https://github.com/escomp/cesm_share (or https://github.com/escomp/cmeps - where @mvertens intends to move the module based on discussion today).

[vlarson]

@zarzycki, does the "rollover" effect described above need to be added to CESM? I don't know whether it is worth including.

[zarzycki]

Personally, I don't see any downside -- at lower surface wind speeds the LY09 equation collapses to what is currently in CIME. I have never done a gridpoint PDF of U10 at 1deg, but I can't fathom there being more than a handful of grid cells that ever see that wind speed so the solution should be effectively identical.

However, we have found benefits for tropical cyclones pressure-wind relationships at 0.25deg with the drag saturation (i.e., LY09).

FWIW, my patch is below (with some other Cd profiles I was playing around with as well...)

    qsat(Tk)   = 640380.0_R8 / exp(5107.4_R8/Tk)
-   cdn(Umps)  =   0.0027_R8 / Umps + 0.000142_R8 + 0.0000764_R8 * Umps
+!++CMZ
+! Large and Yeager 2009
+   cdn(Umps)  =  0.0027_R8 / min(33.0000_R8,Umps) + 0.000142_R8 + 0.0000764_R8 * min(33.0000_R8,Umps) - 3.14807e-13_r8 * min(33.0000_R8,Umps)**6
+! Capped Large and Pond by wind
+!   cdn(Umps)  =   0.0027_R8 / min(30.0_R8,Umps) + 0.000142_R8 + 0.0000764_R8 * min(30.0_R8,Umps) 
+! Capped Large and Pond by Cd
+!   cdn(Umps) = min(0.0025_R8, (0.0027_R8 / Umps + 0.000142_R8 + 0.0000764_R8 * Umps ))
+! Large and Pond
+!   cdn(Umps)  =   0.0027_R8 / Umps + 0.000142_R8 + 0.0000764_R8 * Umps
+!--CMZ
    psimhu(xd) = log((1.0_R8+xd*(2.0_R8+xd))*(1.0_R8+xd*xd)/8.0_R8) - 2.0_R8*atan(xd) + 1.571_R8
    psixhu(xd) = 2.0_R8 * log((1.0_R8 + xd*xd)/2.0_R8)

[github-actions]

This issue is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 5 days.

[github-actions]

This issue was closed because it has been stalled for 5 days with no activity.

[jedwards4b]

I don't see an issue in cmeps - is this still needed?

[zarzycki]

I do not believe the code has been edited in CMEPS: https://github.com/ESCOMP/CMEPS/blob/98dcf46c8886104b95cddfd5b02588b3dd9f6722/cesm/flux_atmocn/shr_flux_mod.F90#L262C1-L262C72

I still think it would be a worthwhile update myself. It should collapse to the current implementation at wind speeds typically seen in CESM but taper the drag coefficient at high wind speeds occasionally seen in ne120 and finer.

[jedwards4b]

So the question was - have you opened an issue in cmeps?

[zarzycki]

So the question was - have you opened an issue in cmeps?

Done.