Nudging

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-401 or operational2 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, 20084. 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 MONSooN5 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/