Difference between revisions of "Nudging"
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− | ==Introduction== |
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− | The ability of a climate model to realistically reproduce the state of the atmosphere, |
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− | the weather, is useful to study processes with short timescales. Nudging, or Newtonian |
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− | relaxation, is a simple form of data assimilation that adjusts dynamical variables of free |
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− | running Global Climate Models (GCMs) using meteorological re-analysis data to give a |
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− | realistic representation of the atmosphere at a given time. |
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− | The variables in the Unified Model (UM) that can be currently nudged are: horizontal wind |
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− | components (u and v) and potential temperature (θ) by using the ECMWF ERA-40(1) or |
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− | operational(2) data, UM Analyses or the Japanese 25-year Re-Analysis (JRA-25)(3). |
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− | The nudging code in the UM carries out two main operations: bringing the analysis data |
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− | onto the model grid as well as time; and relaxing the dynamical variables to this data. |
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− | The interpolation to model grid and time step is currently done through simple bi-linear |
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− | interpolation, using Ln(Surface pressure) for vertical interpolation of data on hybrid levels. |
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− | The analyses are introduced into the model by the addition of a non-physical term to the |
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− | model equations. The change in a variable (ΔX) is given by |
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− | ΔX = F(X) + G(Xana - Xmod) |
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− | where, |
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− | Xana is the value of the nudged variable in the analyses; |
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− | Xmod is the value of the variable in the model and |
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− | G is a constant, known as the ‘relaxation parameter’. |
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− | The value of G can be fairly arbitrary, though keeping it too small will render the technique |
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− | ineffective, while making it too strong can introduce instabilities. The recommended value |
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− | of G is 1/(time frequency of analysis data), which is usually 6 hours. |
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− | A detailed description of the methodology, UM nudging experiments and sensitivity |
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− | studies can be found in Telford et al, 2008(4). |
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− | The Nudged model was developed originally at the University of Cambridge under the |
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− | Met Office - NCAS (National Centre for Atmospheric Sciences) collaboration for UM6.1 |
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− | but finally lodged in the main UM trunk at version 7.7. It is also available at UM versions |
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− | 7.1 and 7.3 on the MONSooN(5) system. It is represented in the UM by a dedicated code |
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− | section viz Section 39. |
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− | ==Description of the Analysis Files== |
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− | The original analysis data have to be processed before they can be used with the |
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− | UM. For example, in case of ERA-40 the variables are obtained at T159 resolution on |
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− | ECMWF hybrid-p levels at six hourly interval and interpolated to N48 using convsh. The |
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− | pre-processed files are in NetCDF6 with standardised dimension and variable names. |
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− | As described in Section 1, the ECMWF ERA-40 data is expected to be in hybrid-p levels |
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− | and contain Ln(Psurf). No vertical interpolation is done for UM analyses and hence these |
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− | have to be on the exact levels as the proposed simulation. The ECMWF operational data |
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− | and JRA-25 data are expected to be on standard pressure levels. Each NetCDF file must |
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− | contain a single time record. The file naming convention for the data files based on the |
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− | existing available file types is as follows: |
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− | - ECMWF ERA-40 data: ecm-e40_1deg-model-levs_YYYYMMDDHH_all.nc |
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− | - ECMWF Op Analysis : ecm-e40_1deg-model-levs_YYYYMMDDHH_all.nc |
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− | - UM Analysis : HPYYYYMMDDHH00.GLOUM6.nc |
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− | - JRA-25 data : JRAYYYYMMDDHH_all.nc |
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− | Currently pre-processed ERA-40 files are available from Jan 1978 to Aug 2009 |
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− | at 6 hourly interval. These are located at /data/cr/cce/hadnd/era40/ on the IBM. |
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− | The available data for other sources covers a limited period only. Note that the UM has |
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− | not yet been tested extensively with ECMWF pressure level data or UM analyses. |
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− | For obtaining additional ERA40 data, or data for other sources, please contact Paul |
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− | Telford at Cambridge Univ. [pjt50@cam.ac.uk]. |
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− | ==Setting up a UM nudging job== |
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− | The major change required for using nudging with the UM, especially the climate models |
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− | is that the run has to be changed to use Gregorian (365 day) calendar. This is because |
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− | most operational/ analysis datasets are produced according to this calendar. The 360 day |
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− | calendar may be used only for short term runs (less than a month) as the discontinuities |
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− | in the data, such as the exclusion of the 31st of different months are likely to cause |
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− | instabilities in the simulation. |
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− | The following changes are required in UMUI to add nudging to a given job: |
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− | 1. Input/ Output Control and Resources à General Configuration & Control : |
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− | Unselect the Use 360 day calendar option if selected (e.g. climate model runs) |
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− | 2. Input/ Output Control and Resources à Time convention and SCRIPT |
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− | Environment Variables ; |
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− | Change the location of ANCIL_ATMOS and ANCIL_LAND to point to ancillaries |
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− | compatible with 365 day calendar. On the MONSooN, a set of compatible ancillaries |
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− | normally required by HadGEM3-A N96L38 jobs is available at /projects/ukca/mdalvi/ |
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− | ancil_365/ and /projects/ukca/mdalvi/ancil_365/LAND respectively. |
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− | 3. Input/ Output Control and Resources à User Hand_Edit files; |
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− | Add to the hand_edits list: /home/mdalvi/umui_jobs/hand_edits/nudg/nudg_on_Lxx.ed, |
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− | where xx stand for the vertical levels of current job. |
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− | The hand_edit adds the required control parameters, compilation flags and STASH |
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− | items to respective namelists during job processing. The recommended values of the |
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− | nudging control parameters have been set by default in the hand_edits. However, |
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− | these parameters have also been described at the end of this section, in case the user |
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− | wishes to modify them for a particular UM configuration. |
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− | 4. FCM Configuration à FCM Options for Atmosphere and Reconfiguration; |
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− | Include the following two branches: |
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− | fcm:um_br/dev/hadzm/VN7.3_nudging_new/src. This contains the main nudging code |
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− | fcm:um_br/dev/hadzm/VN7.3_GregorianCal_fixes/src. This branch contains minor |
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− | fixes which are required for long term Gregorian calendar simulations. |
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− | 5. Atmosphere à Control à Post processing, Dumping & Meaning à Dumping |
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− | and meaning; |
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− | Select the option Regular frequency dumps for Gregorian-calendar Meaning. This |
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− | should automatically set the frequency of restart dumps to 24 hourly and in the |
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− | Climate meaning section, the profiles to Monthly, Seasonal and Annual means. |
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− | (Multi-annual means are not currently possible for Gregorian Climate Meaning) |
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− | 6. Since the restart dumps will now be created daily, it is recommended that under the |
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− | Post Processing à Main Switch + General Questions pane, Automatic Post- |
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− | processing is selected along with the Delete Superseded restart dumps option. If |
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− | archiving is not desired, select the No Archiving option. |
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− | 7. Description of Nudging Control Parameters in the Hand_Edit |
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− | The user modifiable control parameters are set at the beginning of the hand_edits |
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− | nudg_on_Lxx.ed. |
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− | i) ndg_relax_uvalue; ndg_relax_vvalue, ndg_relax_tvalue; These are the Relaxation |
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− | Parameters which control the amount by which the corresponding model variable |
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− | (U,V,T) is nudged towards analysis data. The recommended value for this parameter is |
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− | 1/(time interval between analyses in sec). |
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− | A too small value may be ineffective, while a too large value may lead to model |
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− | instability. |
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− | ii) ndg_lev_bottom: This is the lowermost model level from which the nudging will be |
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− | applied. The lowermost levels should be excluded due to errors in interpolating from |
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− | the analysis levels to the UM levels. In general, it is advisable not to apply nudging in |
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− | the boundary layer. |
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− | iii) ndg_lev_top; This is the uppermost model level up to which the variables will be |
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− | nudged. This value is chosen to avoid the topmost 'sponge' layers of the analyses. |
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− | iv) ndg_on_lev_bottom, ndg_on_lev_top; These are the number of levels to go from none |
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− | to full-strength nudging. They specify the span of layers, starting from the lowermost |
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− | (and topmost) levels selected above, over which the relaxation parameter is gradually |
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− | applied from zero to full strength. |
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− | v) ndg_datapath; This is the location, on the MONSooN where the pre-processed |
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− | analysis files are kept. All the files required for the simulation have to be present in this |
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− | folder, with one data record in each file. |
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− | vi) ndg_hours_perdata; Time Interval (in hours) between analysis data files. This has to |
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− | be uniform throughout the simulation period. |
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− | vii) ndg_analysis_source; The source and type of analysis (currently 4) that can be used |
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− | for nudging. The codes for the various analysis sources are as follows: |
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− | ECMWF ERA-40 |
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− | (0) |
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− | ECMWF operational analysis (1); |
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− | UM Analyses |
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− | (2); |
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− | Japanese Analysis (JRA-25)- |
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− | - |
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− | (3) |
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− | - |
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− | References |
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− | 1. ECMWF 40-year Re-Analysis (ERA-40). http://www.ecmwf.int/research/era/do/get/era-40 |
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− | 2. ECMWF Operational Pressure level Analysis dataset: |
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− | http://www.ecmwf.int/products/data/archive/descriptions/od/oper/an/pl/index.html |
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− | 3. Japanese 25-year Re-Analysis (JRA-25): http://jra.kishou.go.jp/JRA-25/index_en.html |
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− | 4. Telford, P. J., Braesicke, P., Morgenstern, O., and Pyle, J. A.: Technical Note: Description and |
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− | assessment of a nudged version of the new dynamics Unified Model, Atmos. Chem. Phys., 8, |
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− | 1701-1712, doi:10.5194/acp-8-1701-2008, 2008 |
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− | http://www.atmos-chem-phys.net/8/1701/2008/acp-8-1701-2008.html |
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− | 5. MONSooN: The Met Office – NERC Supercomputing Node for Unified Model collaborations. |
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− | http://collab.metoffice.gov.uk/twiki/bin/view/Main/WebHome |
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− | 6. NetCDF: Network Common Data Format: http://www.unidata.ucar.edu/software/netcdf/ |
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− | Further details and updates: |
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− | http://collab.metoffice.gov.uk/twiki/bin/view/Project/UKCA/UkcaNudging |
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− | (requires password) |
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− | http://www.atm.ch.cam.ac.uk/community/pg/pages/view/449/ |