Regional Nudging

Warning

  • Again, care must be taken when using the results of this model version (pre-release).

  • Regional Nudging presented below only works in NorESM2.2 (i.e., not in earlier versions).

  • Also, only historical AMIP experiment on Betzy/Sigma2 has been tested.

Implementation

Regional nudging has been implemented in the latest version of the CESM (CESM2.2, CAM6.3) model, thus the details of the implementation can be found here. Below we show an example of such implementation in NorESM2.2, which is based on the CESM2.2.

Once you have created a case folder ~/<noresm-base>/cases/<case-name> (e.g., see here), you can specify nudging (global or regional) in user_nl_cam within that folder. The variables are described here in more detail, but we provide some information here as well (see also example below):

  • Nudge_Path, Nudge_File_Template specify path & file-name structure of target data (e.g. ERA5 - for more details on available nudging target data below).

  • Nudge_Force_Opt specifies whether we nudge to the next target data (=0) or linearly interpolate between consecutive target data points (=1).

  • Nudge_TimeScale_Opt specifies whether nudging timescale is constant (=0) or gets stronger closer to target values (=1).

  • Nudge_Times_Per_Day specifies the frequency of target data per day (=4 for 6-hourly target data)

  • Model_Times_Per_Day specifies the number of model timesteps per day we wish to nudge (for a model with a 30-minute timestep - like NorESM2.2-AMIP - set to 48 if you wish to nudge every timestep).

  • Nudge_Uprof (and equivalents for V, T, Q, PS) states what kind of nudging in, e.g., U we want (=0 no nudging, =1 global nudging, =2 regional nudging):

    • For global nudging of U,V specify Nudge_Uprof = Nudge_Vprof = 1, Nudge_PSprof = Nudge_Tprof = Nudge_Qprof = 0.

    • For regional nudging of U,V specify Nudge_Uprof = Nudge_Vprof = 2, Nudge_PSprof = Nudge_Tprof = Nudge_Qprof = 0.

  • Nudge_Ucoef (and equivalents for V, T, Q, PS) specifies the strength of nudging for the specific variable, e.g. U (value between 0 and 1).

  • Nudge_Beg_Year, Nudge_Beg_Month, Nudge_Beg_Day, Nudge_End_Year, Nudge_End_Month, Nudge_End_Day specify the start (Beg) and end (End) date (Year, Month, Day) of nudging. See example below.

  • Nudge_Hwin_lat0, Nudge_Hwin_latWidth, Nudge_Hwin_latDelta specify the size of the regional nudging window in latitudinal direction:

    • Nudge_Hwin_lat0 is central latitude of nudging window.

    • Nudge_Hwin_latWidth is the width of the nudging window in latitude direction.

    • Nudge_Hwin_latDelta specifies over how many latitudes we taper-off the data (typically applied to +/- latDelta around the edge of the window).

  • Nudge_Hwin_lon0, Nudge_Hwin_lonWidth, Nudge_Hwin_lonDelta specify the size of the regional nudging window in longitudinal direction (equivalent to the latitudinal window above).

  • Nudge_Vwin_Hindex, Nudge_Vwin_Hdelta, Nudge_Vwin_Lindex, Nudge_Vwin_Ldelta specify the size of the “regional” nudging window in vertical direction:

    • Nudge_Vwin_Hindex & Nudge_Vwin_Lindex specify the window in terms of model indices: High Hindex (model surface) and Low Lindex (model top) transition levels. For nudging over all model levels set Lindex=0 and Hindex=NLEV+1 (in NorESM2.2-AMIP, Hindex = 33).

    • Nudge_Vwin_Hdelta & Nudge_Vwin_Ldelta specify over how many levels we taper off the data in terms of number of model level indices. For no tapering-off set the transition lengths to 0.001, otherwise set it to a value larger than 0.

  • Nudge_Hwin_Invert, Nudge_Vwin_Invert is a logical specifying whether we nudge within the nudging window (.false.) or outside the nudging window (.true.) - separately for horizontal (Hwin) and vertical (Vwin) window.

Target Data

At the moment there is the following capability for nudging in NorESM2.2:

  • Anomaly nudging to historical AMIP experiment climatology with ERA5 anomalies over the period 1979-2016. Data is available for PS, U, V variables only. Set the following path & file-name in user_nl_cam:

    Nudge_Path = '/cluster/projects/nn9039k/CAM_Nudging/met_data/era5_PSUV_f09L32_6hrAnomalySingleRecord/'

    Nudge_File_Template= 'era5-ano_PSUV_f09L32_6hr_r0_%y%m%d%s.nc'

  • Full nudging to ERA5 data (i.e., climatology + anomalies) over the period 1979-2016. Data is available for PS, U, V variables only. Set the following path & file-name in user_nl_cam:

    Nudge_Path = '/cluster/projects/nn9039k/CAM_Nudging/met_data/era5_PSUV_f09L32_6hrFullFieldSingleRecord/'

    Nudge_File_Template= 'era5_PSUV_f09L32_6hr_r0_%y%m%d%s.nc'

  • Full nudging to ERA-Interim data (i.e., climatology + anomalies) over the period 2001-2015. Data is available for PS, U, V, T, Q variables. Set the following path & file-name in user_nl_cam:

    Nudge_Path = '/cluster/projects/nn9039k/CAM_Nudging/met_data/ERAI_f09L32_vn2_2/'

    Nudge_File_Template= '%y-%m-%d-%s.nc'

For now, we have only tested U & V nudging, while PS nudging is also available in ERA5 and T,Q nudging is available in ERA-Interim. We expect to increase the number of variables available for nudging with ERA5 (to, e.g., T, Q) at a later time.

If you do not have access to the data specified above please contact us (lina.boljka@uib.no).

Example

Here is an example of the nudging part of the namelist script (user_nl_cam) for regional nudging over the North Pacific.

user_nl_cam

&nudging_nl

Nudge_Model = .true. ! (set to .false. for no nudging)

Nudge_Path = ‘/cluster/projects/nn9039k/CAM_Nudging/met_data/era5_PSUV_f09L32_6hrAnomalySingleRecord/’

Nudge_File_Template= ‘era5-ano_PSUV_f09L32_6hr_r0_%y%m%d%s.nc’

Nudge_Force_Opt = 1 ! (=0 for target at next future time ; =1 for linear interpolation between 2 target datapoints)

Nudge_TimeScale_Opt = 0 ! (=0 for weak nudging [constant] & =1 for strong nudging [near target points])

Nudge_Times_Per_Day= 4 ! (for 6-hourly target data for nudging - e.g. ERA5)

Model_Times_Per_Day= 48 ! (for a model with a 30-minute timestep - like NorESM2.2-AMIP)

Nudge_Uprof = 2 ! (=0 for nudging off ; =1 for global nudging ; =2 for regional nudging)

Nudge_Ucoef =1.00 ! (= anything between 0 and 1)

Nudge_Vprof = 2

Nudge_Vcoef =1.00

Nudge_Tprof =0

Nudge_Tcoef =1.00

Nudge_Qprof =0

Nudge_Qcoef =1.00

Nudge_PSprof =0

Nudge_PScoef =0.00

Nudge_Beg_Year = 1979

Nudge_Beg_Month= 1

Nudge_Beg_Day = 1

Nudge_End_Year = 2013

Nudge_End_Month= 12

Nudge_End_Day = 31

Nudge_Hwin_lat0 = 45.

Nudge_Hwin_latWidth= 45. ! (set to 999. for full longitudinal circle)

Nudge_Hwin_latDelta= 5.0

Nudge_Hwin_lon0 = 180.

Nudge_Hwin_lonWidth= 100. ! (set to 999. for full latitudinal circle)

Nudge_Hwin_lonDelta= 5.

Nudge_Hwin_Invert =.false. ! (set to .true. for inverted nudging window)

Nudge_Vwin_Hindex = 33.

Nudge_Vwin_Hdelta = 0.001 ! (const vertical window ; for non-const. set to a larger value, i.e., number of levels over which it tapers off)

Nudge_Vwin_Lindex = 13. ! (=0. full vertical extent ; =13. troposphere only & taper off in lower stratosphere [lev 13 ~150 hPa; lev 15 ~200 hPa; lev 11 ~100 hPa] ; =32. surface layer only)

Nudge_Vwin_Ldelta = 2. ! (=2. taper-off over +/- 2 levels ; =0.001 const. vertical window)

Nudge_Vwin_Invert =.false. ! (set to .true. for inverted nudging window)

/

Visualisation of the Nudging Window

To visualise the nudging window used (e.g., prior to implementing it in the model) do the following:

  1. Navigate to Cam Tools folder with Nudging Window scripts within NorESM2.2 dierctory <noresm-base>:

cd ~/<noresm-base>/components/cam/tools/nudging/Lookat_NudgeWindow/

  1. Copy the user_nl_cam script from ~/<noresm-base>/cases/<case-name> directory to the Lookat_NudgeWindow folder and read the README file there. Make sure that you are in an environment where ncl is installed before running the following commands:

>> WRAPIT Read_Namelist.f90 Read_Namelist.stub

>> ncl Lookat_NudgeWindow.ncl

This should show figures with a X11 display, but if you set, e.g., wks = gsn_open_wks("png","Wcoef") in Lookat_NudgeWindow.ncl it will save the file as a pdf instead.

The above example of user_nl_cam yields a nudging window in the horizontal and in the vertical as shown below.

_images/Wcoef_new.png _images/Wcoef_new2.png

Topography data

Also, topography data from a reanalysis can be specified in user_nl_cam, although be aware that ERA5 topography may be very different from model topography and thus care must be taken!

user_nl_cam

&cam_initfiles_nl

use_topo_file=.true.

bnd_topo = ‘/cluster/shared/noresm/inputdata/noresm-only/inputForNudging/ERA_f09f09_32L_days/ERA_bnd_topo_noresm2_20191023.nc’

/

At the moment only the ERA-Interim topography data is available (as specified in the example above), i.e., we have not performed any tests with ERA5 topography.