Page describing on-going effort to clarify the calculation of roughness length in ORCHIDEE and investigate/solve the biais in bare soil evaporation

Testing roughness length for heat in LMDZOR

01-Sep-2015 by Fuxing WANG, Frederique CHERUY.

Background:

The roughness length for heat (z0h) is generally not identical to roughness length for momentum (z0m).
In LMDZOR, z0h = z0m. A short description of related code is (only in coupled mode): ​https://forge.ipsl.jussieu.fr/orchidee/attachment/wiki/Meetings/CMIP6/Physic/condveg_z0cdrag.pdf

Objective:

To test the sensitively of LMDZOR to z0h.

Simulations:

Two simulations (1-year) were done with LMDZOR after 20 years spin-up.
(1) CTL: z0h = z0m; ORCHIDEE-rev 2664, LMDZ-rev 2287; new soil vertical discretization and soil thermodynamics
(2) EXP: The same as CTL, but z0h = 0.1 * z0m.

Results:

Results are compared over different seasons (annual mean, JJA and DJF): ​https://forge.ipsl.jussieu.fr/orchidee/attachment/wiki/Meetings/CMIP6/Physic/z0mh_annual_jja_djf.pdf.
The left side in the figures is "CTL: z0h = z0m", the right side is the 'EXP-CTL' (except the last row which is the " EXP/CTL").
The Ts (temp_sol) increases over most regions. The maximum increase is over 0N-90N in JJA (1-2K), while it is over 30N-60S in DJF (0.5-1.5K).(Fig b)
The pattern of fluxsens is similar for different seasons. It increases over most regions (3 W/m2 for most regions).
The variation of fluxlat (+/- 6 W/m2 over most regions) and precipitation (+/-0.5mm/d over most regions) is less systematic than Ts and fluxsens.