Difference between revisions of "Nudging"

Line 1:

−

==Introduction==

−

The ability of a climate model to realistically reproduce the state of the atmosphere,

−

the weather, is useful to study processes with short timescales. Nudging, or Newtonian

−

relaxation, is a simple form of data assimilation that adjusts dynamical variables of free

−

running Global Climate Models (GCMs) using meteorological re-analysis data to give a

−

realistic representation of the atmosphere at a given time.

−

The variables in the Unified Model (UM) that can be currently nudged are: horizontal wind

−

components (u and v) and potential temperature (θ) by using the ECMWF ERA-40(1) or

−

operational(2) data, UM Analyses or the Japanese 25-year Re-Analysis (JRA-25)(3).

−

The nudging code in the UM carries out two main operations: bringing the analysis data

−

onto the model grid as well as time; and relaxing the dynamical variables to this data.

−

The interpolation to model grid and time step is currently done through simple bi-linear

−

interpolation, using Ln(Surface pressure) for vertical interpolation of data on hybrid levels.

−

The analyses are introduced into the model by the addition of a non-physical term to the

−

model equations. The change in a variable (ΔX) is given by

−

ΔX = F(X) + G(Xana - Xmod)

−

where,

−

Xana is the value of the nudged variable in the analyses;

−

Xmod is the value of the variable in the model and

−

G is a constant, known as the ‘relaxation parameter’.

−

The value of G can be fairly arbitrary, though keeping it too small will render the technique

−

ineffective, while making it too strong can introduce instabilities. The recommended value

−

of G is 1/(time frequency of analysis data), which is usually 6 hours.

−

A detailed description of the methodology, UM nudging experiments and sensitivity

−

studies can be found in Telford et al, 2008(4).

−

The Nudged model was developed originally at the University of Cambridge under the

−

Met Office - NCAS (National Centre for Atmospheric Sciences) collaboration for UM6.1

−

but finally lodged in the main UM trunk at version 7.7. It is also available at UM versions

−

7.1 and 7.3 on the MONSooN(5) system. It is represented in the UM by a dedicated code

−

section viz Section 39.

−

==Description of the Analysis Files==

−

The original analysis data have to be processed before they can be used with the

−

UM. For example, in case of ERA-40 the variables are obtained at T159 resolution on

−

ECMWF hybrid-p levels at six hourly interval and interpolated to N48 using convsh. The

−

pre-processed files are in NetCDF6 with standardised dimension and variable names.

−

As described in Section 1, the ECMWF ERA-40 data is expected to be in hybrid-p levels

−

and contain Ln(Psurf). No vertical interpolation is done for UM analyses and hence these

−

have to be on the exact levels as the proposed simulation. The ECMWF operational data

−

and JRA-25 data are expected to be on standard pressure levels. Each NetCDF file must

−

contain a single time record. The file naming convention for the data files based on the

−

existing available file types is as follows:

−

- ECMWF ERA-40 data: ecm-e40_1deg-model-levs_YYYYMMDDHH_all.nc

−

- ECMWF Op Analysis : ecm-e40_1deg-model-levs_YYYYMMDDHH_all.nc

−

- UM Analysis : HPYYYYMMDDHH00.GLOUM6.nc

−

- JRA-25 data : JRAYYYYMMDDHH_all.nc

−

Currently pre-processed ERA-40 files are available from Jan 1978 to Aug 2009

−

at 6 hourly interval. These are located at /data/cr/cce/hadnd/era40/ on the IBM.

−

The available data for other sources covers a limited period only. Note that the UM has

−

not yet been tested extensively with ECMWF pressure level data or UM analyses.

−

For obtaining additional ERA40 data, or data for other sources, please contact Paul

−

Telford at Cambridge Univ. [pjt50@cam.ac.uk].

−

==Setting up a UM nudging job==

−

The major change required for using nudging with the UM, especially the climate models

−

is that the run has to be changed to use Gregorian (365 day) calendar. This is because

−

most operational/ analysis datasets are produced according to this calendar. The 360 day

−

calendar may be used only for short term runs (less than a month) as the discontinuities

−

in the data, such as the exclusion of the 31st of different months are likely to cause

−

instabilities in the simulation.

−

The following changes are required in UMUI to add nudging to a given job:

−

1. Input/ Output Control and Resources à General Configuration & Control :

−

Unselect the Use 360 day calendar option if selected (e.g. climate model runs)

−

2. Input/ Output Control and Resources à Time convention and SCRIPT

−

Environment Variables ;

−

Change the location of ANCIL_ATMOS and ANCIL_LAND to point to ancillaries

−

compatible with 365 day calendar. On the MONSooN, a set of compatible ancillaries

−

normally required by HadGEM3-A N96L38 jobs is available at /projects/ukca/mdalvi/

−

ancil_365/ and /projects/ukca/mdalvi/ancil_365/LAND respectively.

−

3. Input/ Output Control and Resources à User Hand_Edit files;

−

Add to the hand_edits list: /home/mdalvi/umui_jobs/hand_edits/nudg/nudg_on_Lxx.ed,

−

where xx stand for the vertical levels of current job.

−

The hand_edit adds the required control parameters, compilation flags and STASH

−

items to respective namelists during job processing. The recommended values of the

−

nudging control parameters have been set by default in the hand_edits. However,

−

these parameters have also been described at the end of this section, in case the user

−

wishes to modify them for a particular UM configuration.

−

4. FCM Configuration à FCM Options for Atmosphere and Reconfiguration;

−

Include the following two branches:

−

fcm:um_br/dev/hadzm/VN7.3_nudging_new/src. This contains the main nudging code

−

fcm:um_br/dev/hadzm/VN7.3_GregorianCal_fixes/src. This branch contains minor

−

fixes which are required for long term Gregorian calendar simulations.

−

5. Atmosphere à Control à Post processing, Dumping & Meaning à Dumping

−

and meaning;

−

Select the option Regular frequency dumps for Gregorian-calendar Meaning. This

−

should automatically set the frequency of restart dumps to 24 hourly and in the

−

Climate meaning section, the profiles to Monthly, Seasonal and Annual means.

−

(Multi-annual means are not currently possible for Gregorian Climate Meaning)

−

6. Since the restart dumps will now be created daily, it is recommended that under the

−

Post Processing à Main Switch + General Questions pane, Automatic Post-

−

processing is selected along with the Delete Superseded restart dumps option. If

−

archiving is not desired, select the No Archiving option.

−

7. Description of Nudging Control Parameters in the Hand_Edit

−

The user modifiable control parameters are set at the beginning of the hand_edits

−

nudg_on_Lxx.ed.

−

i) ndg_relax_uvalue; ndg_relax_vvalue, ndg_relax_tvalue; These are the Relaxation

−

Parameters which control the amount by which the corresponding model variable

−

(U,V,T) is nudged towards analysis data. The recommended value for this parameter is

−

1/(time interval between analyses in sec).

−

A too small value may be ineffective, while a too large value may lead to model

−

instability.

−

ii) ndg_lev_bottom: This is the lowermost model level from which the nudging will be

−

applied. The lowermost levels should be excluded due to errors in interpolating from

−

the analysis levels to the UM levels. In general, it is advisable not to apply nudging in

−

the boundary layer.

−

iii) ndg_lev_top; This is the uppermost model level up to which the variables will be

−

nudged. This value is chosen to avoid the topmost 'sponge' layers of the analyses.

−

iv) ndg_on_lev_bottom, ndg_on_lev_top; These are the number of levels to go from none

−

to full-strength nudging. They specify the span of layers, starting from the lowermost

−

(and topmost) levels selected above, over which the relaxation parameter is gradually

−

applied from zero to full strength.

−

v) ndg_datapath; This is the location, on the MONSooN where the pre-processed

−

analysis files are kept. All the files required for the simulation have to be present in this

−

folder, with one data record in each file.

−

vi) ndg_hours_perdata; Time Interval (in hours) between analysis data files. This has to

−

be uniform throughout the simulation period.

−

vii) ndg_analysis_source; The source and type of analysis (currently 4) that can be used

−

for nudging. The codes for the various analysis sources are as follows:

−

ECMWF ERA-40

−

(0)

−

ECMWF operational analysis (1);

−

UM Analyses

−

(2);

−

Japanese Analysis (JRA-25)-

−

-

−

(3)

−

-

−

References

−

1. ECMWF 40-year Re-Analysis (ERA-40). http://www.ecmwf.int/research/era/do/get/era-40

−

2. ECMWF Operational Pressure level Analysis dataset:

−

http://www.ecmwf.int/products/data/archive/descriptions/od/oper/an/pl/index.html

−

3. Japanese 25-year Re-Analysis (JRA-25): http://jra.kishou.go.jp/JRA-25/index_en.html

−

4. Telford, P. J., Braesicke, P., Morgenstern, O., and Pyle, J. A.: Technical Note: Description and

−

assessment of a nudged version of the new dynamics Unified Model, Atmos. Chem. Phys., 8,

−

1701-1712, doi:10.5194/acp-8-1701-2008, 2008

−

http://www.atmos-chem-phys.net/8/1701/2008/acp-8-1701-2008.html

−

5. MONSooN: The Met Office – NERC Supercomputing Node for Unified Model collaborations.

−

http://collab.metoffice.gov.uk/twiki/bin/view/Main/WebHome

−

6. NetCDF: Network Common Data Format: http://www.unidata.ucar.edu/software/netcdf/

−

Further details and updates:

−

http://collab.metoffice.gov.uk/twiki/bin/view/Project/UKCA/UkcaNudging

−

(requires password)

−

http://www.atm.ch.cam.ac.uk/community/pg/pages/view/449/

@@ Line 1: / Line 1: @@
-==Introduction==
-The ability of a climate model to realistically reproduce the state of the atmosphere,
-the weather, is useful to study processes with short timescales. Nudging, or Newtonian
-relaxation, is a simple form of data assimilation that adjusts dynamical variables of free
-running Global Climate Models (GCMs) using meteorological re-analysis data to give a
-realistic representation of the atmosphere at a given time.
-The variables in the Unified Model (UM) that can be currently nudged are: horizontal wind
-components (u and v) and potential temperature (θ) by using the ECMWF ERA-40(1) or
-operational(2) data, UM Analyses or the Japanese 25-year Re-Analysis (JRA-25)(3).
-The nudging code in the UM carries out two main operations: bringing the analysis data
-onto the model grid as well as time; and relaxing the dynamical variables to this data.
-The interpolation to model grid and time step is currently done through simple bi-linear
-interpolation, using Ln(Surface pressure) for vertical interpolation of data on hybrid levels.
-The analyses are introduced into the model by the addition of a non-physical term to the
-model equations. The change in a variable (ΔX) is given by
-ΔX = F(X) + G(Xana - Xmod)
-where,
-Xana is the value of the nudged variable in the analyses;
-Xmod is the value of the variable in the model and
-G is a constant, known as the ‘relaxation parameter’.
-The value of G can be fairly arbitrary, though keeping it too small will render the technique
-ineffective, while making it too strong can introduce instabilities. The recommended value
-of G is 1/(time frequency of analysis data), which is usually 6 hours.
-A detailed description of the methodology, UM nudging experiments and sensitivity
-studies can be found in Telford et al, 2008(4).
-The Nudged model was developed originally at the University of Cambridge under the
-Met Office - NCAS (National Centre for Atmospheric Sciences) collaboration for UM6.1
-but finally lodged in the main UM trunk at version 7.7. It is also available at UM versions
-.1 and 7.3 on the MONSooN(5) system. It is represented in the UM by a dedicated code
-section viz Section 39.
-==Description of the Analysis Files==
-The original analysis data have to be processed before they can be used with the
-UM. For example, in case of ERA-40 the variables are obtained at T159 resolution on
-ECMWF hybrid-p levels at six hourly interval and interpolated to N48 using convsh. The
-pre-processed files are in NetCDF6 with standardised dimension and variable names.
-As described in Section 1, the ECMWF ERA-40 data is expected to be in hybrid-p levels
-and contain Ln(Psurf). No vertical interpolation is done for UM analyses and hence these
-have to be on the exact levels as the proposed simulation. The ECMWF operational data
-and JRA-25 data are expected to be on standard pressure levels. Each NetCDF file must
-contain a single time record. The file naming convention for the data files based on the
-existing available file types is as follows:
-- ECMWF ERA-40 data: ecm-e40_1deg-model-levs_YYYYMMDDHH_all.nc
-- ECMWF Op Analysis : ecm-e40_1deg-model-levs_YYYYMMDDHH_all.nc
-- UM Analysis : HPYYYYMMDDHH00.GLOUM6.nc
-- JRA-25 data : JRAYYYYMMDDHH_all.nc
-Currently pre-processed ERA-40 files are available from Jan 1978 to Aug 2009
-at 6 hourly interval. These are located at /data/cr/cce/hadnd/era40/ on the IBM.
-The available data for other sources covers a limited period only. Note that the UM has
-not yet been tested extensively with ECMWF pressure level data or UM analyses.
-For obtaining additional ERA40 data, or data for other sources, please contact Paul
-Telford at Cambridge Univ. [pjt50@cam.ac.uk].
-==Setting up a UM nudging job==
-The major change required for using nudging with the UM, especially the climate models
-is that the run has to be changed to use Gregorian (365 day) calendar. This is because
-most operational/ analysis datasets are produced according to this calendar. The 360 day
-calendar may be used only for short term runs (less than a month) as the discontinuities
-in the data, such as the exclusion of the 31st of different months are likely to cause
-instabilities in the simulation.
-The following changes are required in UMUI to add nudging to a given job:
-. Input/ Output Control and Resources à General Configuration & Control :
-Unselect the Use 360 day calendar option if selected (e.g. climate model runs)
-. Input/ Output Control and Resources à Time convention and SCRIPT
-Environment Variables ;
-Change the location of ANCIL_ATMOS and ANCIL_LAND to point to ancillaries
-compatible with 365 day calendar. On the MONSooN, a set of compatible ancillaries
-normally required by HadGEM3-A N96L38 jobs is available at /projects/ukca/mdalvi/
-ancil_365/ and /projects/ukca/mdalvi/ancil_365/LAND respectively.
-. Input/ Output Control and Resources à User Hand_Edit files;
-Add to the hand_edits list: /home/mdalvi/umui_jobs/hand_edits/nudg/nudg_on_Lxx.ed,
-where xx stand for the vertical levels of current job.
-The hand_edit adds the required control parameters, compilation flags and STASH
-items to respective namelists during job processing. The recommended values of the
-nudging control parameters have been set by default in the hand_edits. However,
-these parameters have also been described at the end of this section, in case the user
-wishes to modify them for a particular UM configuration.
-. FCM Configuration à FCM Options for Atmosphere and Reconfiguration;
-Include the following two branches:
-fcm:um_br/dev/hadzm/VN7.3_nudging_new/src. This contains the main nudging code
-fcm:um_br/dev/hadzm/VN7.3_GregorianCal_fixes/src. This branch contains minor
-fixes which are required for long term Gregorian calendar simulations.
-. Atmosphere à Control à Post processing, Dumping & Meaning à Dumping
-and meaning;
-Select the option Regular frequency dumps for Gregorian-calendar Meaning. This
-should automatically set the frequency of restart dumps to 24 hourly and in the
-Climate meaning section, the profiles to Monthly, Seasonal and Annual means.
-(Multi-annual means are not currently possible for Gregorian Climate Meaning)
-. Since the restart dumps will now be created daily, it is recommended that under the
-Post Processing à Main Switch + General Questions pane, Automatic Post-
-processing is selected along with the Delete Superseded restart dumps option. If
-archiving is not desired, select the No Archiving option.
-. Description of Nudging Control Parameters in the Hand_Edit
-The user modifiable control parameters are set at the beginning of the hand_edits
-nudg_on_Lxx.ed.
-i) ndg_relax_uvalue; ndg_relax_vvalue, ndg_relax_tvalue; These are the Relaxation
-Parameters which control the amount by which the corresponding model variable
-(U,V,T) is nudged towards analysis data. The recommended value for this parameter is
-/(time interval between analyses in sec).
-A too small value may be ineffective, while a too large value may lead to model
-instability.
-ii) ndg_lev_bottom: This is the lowermost model level from which the nudging will be
-applied. The lowermost levels should be excluded due to errors in interpolating from
-the analysis levels to the UM levels. In general, it is advisable not to apply nudging in
-the boundary layer.
-iii) ndg_lev_top; This is the uppermost model level up to which the variables will be
-nudged. This value is chosen to avoid the topmost 'sponge' layers of the analyses.
-iv) ndg_on_lev_bottom, ndg_on_lev_top; These are the number of levels to go from none
-to full-strength nudging. They specify the span of layers, starting from the lowermost
-(and topmost) levels selected above, over which the relaxation parameter is gradually
-applied from zero to full strength.
-v) ndg_datapath; This is the location, on the MONSooN where the pre-processed
-analysis files are kept. All the files required for the simulation have to be present in this
-folder, with one data record in each file.
-vi) ndg_hours_perdata; Time Interval (in hours) between analysis data files. This has to
-be uniform throughout the simulation period.
-vii) ndg_analysis_source; The source and type of analysis (currently 4) that can be used
-for nudging. The codes for the various analysis sources are as follows:
-ECMWF ERA-40
-(0)
-ECMWF operational analysis (1);
-UM Analyses
-(2);
-Japanese Analysis (JRA-25)-
--
-(3)
--
-References
-. ECMWF 40-year Re-Analysis (ERA-40). http://www.ecmwf.int/research/era/do/get/era-40
-. ECMWF Operational Pressure level Analysis dataset:
-http://www.ecmwf.int/products/data/archive/descriptions/od/oper/an/pl/index.html
-. Japanese 25-year Re-Analysis (JRA-25): http://jra.kishou.go.jp/JRA-25/index_en.html
-. Telford, P. J., Braesicke, P., Morgenstern, O., and Pyle, J. A.: Technical Note: Description and
-assessment of a nudged version of the new dynamics Unified Model, Atmos. Chem. Phys., 8,
--1712, doi:10.5194/acp-8-1701-2008, 2008
-http://www.atmos-chem-phys.net/8/1701/2008/acp-8-1701-2008.html
-. MONSooN: The Met Office – NERC Supercomputing Node for Unified Model collaborations.
-http://collab.metoffice.gov.uk/twiki/bin/view/Main/WebHome
-. NetCDF: Network Common Data Format: http://www.unidata.ucar.edu/software/netcdf/
-Further details and updates:
-http://collab.metoffice.gov.uk/twiki/bin/view/Project/UKCA/UkcaNudging
-(requires password)
-http://www.atm.ch.cam.ac.uk/community/pg/pages/view/449/

Difference between revisions of "Nudging"

Revision as of 10:33, 10 July 2010