Nudging in LMDZ

The concept

Nudging is simply adding a forcing term to the GFD equation governing a variable $$V$$ to drive it towards a known value $$V_{analysis}$$ over a given time scale $$\tau$$: \begin{align} \frac{\partial V}{\partial t} = \frac{\partial V}{\partial t}_{GCM} + \frac{V_{analysis}-V}{\tau} \end{align} Where $$V$$ can be zonal wind u, meridional wind v, relative humidity Q, pressure P or temperature T

Nudging is often used in conjunction with zoomed configurations, where strong nudging towards reanalyses in enforced outside the zoomed region.

Nudging parameters in LMDZ

The master parameter to turn nudging on is ok_guide (most often all nudging parameters are put in a guide.def file included in run.def) which must be set to "y" (or equivalently ".true."):

ok_guide = y

("guide" means "nudge" in French).

Then one must select which fields will be nudged via the guide_* parameters:

guide_u=y
guide_v=y
guide_T=y
guide_P=n
guide_Q=n

In the example above, nudging is done on horizontal winds (u,v) and on temperature (T) only.

In addition, the user must provide two time scales $\tau$, for each of the nudged variables; one for the inner zoom area tau_max_* and another relative to outside the zoomed area tau_min_*. These time scales are stated in unit of days, e.g.

tau_min_u= 0.0208333
tau_max_u= 10.
tau_min_v= 0.0208333
tau_max_v= 10.
tau_min_T= 0.0208333
tau_max_T= 10.

In this example all 3 variables are nudged with a time scale of 10 days, i.e very loosely, within the zoomed region and a time scale of 30 minutes (30.*60/86400.= 0.0208333), i.e. very strongly, outside the zoomed region.

Nudging files

In addition to stating which variable is nudged and the related time scales, one must provide the fields towards which to nudge (these must be on the same grid as the simulation and must be created separately, e.g. using a re-interpolation of ERA-interim files). These files must be in the directory where LMDZ runs and must be called u.nc, v.nc, T.nc, etc.

Nudging contribution output

One can request that the nudging term be stored in an output file, guide_ins.nc by setting (in guide.def):

guide_sav=y

14/12/2021