Running WRF-Chem with ERA-Interim data is basically similar with GFS data. The main difference is the pre-processing steps. While GFS (FNL or forecast) dataset contains everything the user needs for the pre-processing, ECMWF (ERA-Interim, Era5 etc) dataset requires more efforts from user to determine parameters included in the data before downloading and processing it. It may seems bothersome and complicated, but determination of parameters is really important to understand how WRF model works.
The key point here is WRF's Vtable file, which contains all parameter needed for simulation. Once you understand the contents of the file (in accordance with the type of input data), you can basically use WRF with many kinds of input dataset.
To make long story short, here are steps for running WRF-Chem with ERA-Interim dataset:
1. Downloading Data
- Go to ECMWF website, and log-in to it. An account is required for downloading data, thus if you don't have one, just register a new account. It's free.
- Once you logged in, browse into Forecast > Datasets > Archived Datasets > Reanalysis Datasets, and finally, select Download on ERA-Interim section. You will be directed to ERA-Interim, Daily data page.
- Now, the most important thing, WRF needs two kinds of level type for the data: surface and levels. Aside from surface level, there are at least two kinds of level type could be used for WRF: pressure levels and model levels. Which level type you use will depend on Vtable you use for WRF (wrf directory/ungrib/Variable_Tables). For this example, I use Vtable.ERA-interim.ml, which means I will use model levels type dataset. Another important thing, aside from daily data, you will need Invariant data for the data field. This data is static and not time-varied as daily data, hence you only need to download it once.
- For the surface level data, on Type of Level section, select Surface, and put a tick on the month and year section. Regardless the data period of your interest, you have to download the data for at least one whole month. Next, select the time of the data. If you want to use 6-hourly boundary condition data, put check to all time box. For the step (model cycle), choose 0. Finally, you should choose 18 parameters of the surface level data (which are listed in WRF Vtable file): 2m temperature, 10m U wind component, 10m V wind component, mean sea level pressure, sea surface temperature, sea ice area fraction, skin temperature, snow density, snow depth, surface pressure, soil temperature levels (1,2,3 and 4) and finally, volumetric soil water layers (1,2,3 and 4). Click Retrieve GRIB. The website will summarize your request and if you think it's correct, click Retrieve now, and wait until the download link appears. Depending on the size of the data you requested, this process may take up to 1 minute to hours to complete. All those parameters will be downloaded as single file. Give it suitable name e.g. era-int-201711-sfc.grib.
- For the model levels data, on Type of Level section, select Model levels. The steps are almost similar with surface level data download. This time you only need to select the month, year, time and 5 parameters on all levels: specific humidity, temperature, U component of wind, V component of wind and geopotential. Click Retrieve GRIB, and the rest will be same as surface level data download procedure. Anyway, since geopotential data is only available for one level, you could download it separately from the others. For example, the downloaded data will be named era-int-201711-geo.grib (for geopotential) and era-int-201711-mod.grib (for the other parameters). Therefore, there will be two model levels data files.
- Finally, don't forget to download the Invariant data. On ERA Interim Fields section, select Invariant, and put check on Land-Sea Mask, then click Retrieve GRIB and so on. Name it into era-int-invariant.grib.
- In the end, based on this example, there are four files you should have from the website: era-int-201711-sfc.grib (surface levels), era-int-201711-geo.grib (model levels geopotential), era-int-201711-mod.grib (model levels other than geopotential) and era-int-invariant.grib (land-sea mask invariant).
2. WRF Pre-processing
- Make sure you have already set the model domain (by geogrid.exe) and set the dynamic link of Vtable to Vtable.ERA-interim.ml file.
- Link all ERA-Interim data downloaded by link_grib.csh script, and execute ungrib.exe. Wait until the process finished.
- The next step after ungrib is very important. ECMWF model levels data uses sigma levels, while WRF requires pressure levels, thus you need to calculate the pressure level of the corresponding sigma level using utility calc_ecmwf_p.exe (WPS/util/calc_ecmwf_p.exe). You have to do this before executing metgrid.exe. Failing to do so may result on model crashes or unrealistic simulation results (as I previously experienced). Assuming this is the first time you use ERA-Interim data, open namelist.wps and add 'PRES' to the list of prefixes in the fg_name namelist variable.
fg_name = 'FILE', 'PRES',
- Make a dynamic link of the calc_ecmwf_p.exe to the current working WPS directory and make a text file with this following template, then save and name it: 'ecmwf_coeffs':
0 0.000000 0.00000000
1 20.000000 0.00000000
2 38.425343 0.00000000
3 63.647804 0.00000000
4 95.636963 0.00000000
5 134.483307 0.00000000
6 180.584351 0.00000000
7 234.779053 0.00000000
8 298.495789 0.00000000
9 373.971924 0.00000000
10 464.618134 0.00000000
11 575.651001 0.00000000
12 713.218079 0.00000000
13 883.660522 0.00000000
14 1094.834717 0.00000000
15 1356.474609 0.00000000
16 1680.640259 0.00000000
17 2082.273926 0.00000000
18 2579.888672 0.00000000
19 3196.421631 0.00000000
20 3960.291504 0.00000000
21 4906.708496 0.00000000
22 6018.019531 0.00000000
23 7306.631348 0.00000000
24 8765.053711 0.00007582
25 10376.126953 0.00046139
26 12077.446289 0.00181516
27 13775.325195 0.00508112
28 15379.805664 0.01114291
29 16819.474609 0.02067788
30 18045.183594 0.03412116
31 19027.695313 0.05169041
32 19755.109375 0.07353383
33 20222.205078 0.09967469
34 20429.863281 0.13002251
35 20384.480469 0.16438432
36 20097.402344 0.20247594
37 19584.330078 0.24393314
38 18864.750000 0.28832296
39 17961.357422 0.33515489
40 16899.468750 0.38389215
41 15706.447266 0.43396294
42 14411.124023 0.48477158
43 13043.218750 0.53570992
44 11632.758789 0.58616841
45 10209.500977 0.63554746
46 8802.356445 0.68326861
47 7438.803223 0.72878581
48 6144.314941 0.77159661
49 4941.778320 0.81125343
50 3850.913330 0.84737492
51 2887.696533 0.87965691
52 2063.779785 0.90788388
53 1385.912598 0.93194032
54 855.361755 0.95182151
55 467.333588 0.96764523
56 210.393890 0.97966272
57 65.889244 0.98827010
58 7.367743 0.99401945
59 0.000000 0.99763012
60 0.000000 1.00000000
- Execute the utility and wait until it completes calculating the pressure levels. This results new files with prefix 'PRES' in current directory. Once finished, you can execute metgrid.exe. This will complete the pre-processing steps.
3. Running WRF
- Open namelist.input in WRF's run directory, and pay attention to these following variables, since they are closely related to ECMWF data, e.g. number of metgrid pressure levels. Modify the values as follows:
&domains
num_metgrid_levels = 61 [default 32]
&physics
surface_input_sources= 1 [default 3]
- Execute real.exe and finally, wrf.exe to run the model.
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