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Numerical Weather Prediction (NWP)

Raziel & tecer

Contact: tecer@hacknology.de - tecer@jabber.square-wave.de - DECT 4950


Intro

This course is...

...a very brief introduction to NWP ...an even shorter introduction to GIS ...all about running an NWP model, i.e. ...pre-processing the input data ...running the model ...post-processing the output data


This course is not...

...complete by almost all means ...about fancy weather visualizations ...providing a production-ready setup ...about data assimilation and other special topics ...too much about maths/meteorology/physics/...


Prelude

NWP model

Input data → pre-processor (data assimilation) → model → post-processor → output data

model := simulate processes in the atmosphere / boundary transitions

Evolve the current condition (several key parameters at many levels) to the next time step using physical rules (ODEs/PDEs)


WRF

  • Developed by NCAR @Boulder, CO
  • 2 main components: -- WPS (preprocessor) -- WRF (main integration)
  • For post-processing: UPP (or NCL)
  • Mostly Fortran
  • 300+ pages manual
  • Impossible to use for the not-inaugurated → NCAR offers tutorials twice a year

Part 1

Preparing the software


Strongly recommended:

Use the VirtualBox image provided at https://www.hacknology.de/vortrag/2016/wrf/ as it contains all the required software and data sets.

Alternatively, set up the software manually, preferably in a Ubuntu 16.04 VM (username "wrf"), the software and data package is also provided at https://www.hacknology.de/vortrag/2016/wrf/


Compiling NetCDF

WRF uses NetCDF as the main data format, hence the NetCDF libraries and the Fortran bindings are required. Unfortunately, the distribution packages usually do not work, so NetCDF must be compiled manually before compiling WRF.

> cd /home/wrf/src > tar -xf ../packages/netcdf-4.4.1.1.tar.gz ; cd netcdf-4.4.1.1 > ./configure --prefix=/home/wrf/netcdf --disable-dap \ --disable-netcdf-4 --disable-shared > make && make install


Compiling NetCDF Fortran bindings

Most of WRF is Fortran code, so the NetCDF Fortran bindings are required as well.

> cd /home/wrf/src > tar -xf ../packages/netcdf-fortran-4.4.4.tar.gz ; cd netcdf-fortran-4.4.4 > CPPFLAGS=-I/home/wrf/netcdf/include ./configure --prefix=/home/wrf/netcdf \ --disable-shared > make && make install

Now, NetCDF and the NetCDF Fortran bindings are installed at /home/wrf/netcdf.


Compiling WRF

The compilation of WRF takes some time. > cd /home/wrf/src > tar -xf ../packages/wrf-3.8.1.tar.bz2 ; cd WRFV3 > export NETCDF=/home/wrf/netcdf > export WRFIO_NCD_LARGE_FILE_SUPPORT=1 > ./configure In the following dialogue, select GNU / dmpar (7) (requires OpenMPI, alternatively, you can select GNU / serial, but then the only 1 core will be used for the actual computation)

> ./compile -j1 em_real >& compile.log (for some reason, the OpenMPI version does not like to be compiled in parallel, hence the -j1 option) Sit back and enjoy the interlude...


Interlude 1

Map projections


World model

90%


Cylindrical

90%


Mercator

90%


Polar stereographic

90%


Lambert

90%


Map projection recommendations

  • Lambert: mid latitudes
  • Mercator: low latitudes
  • Polar-stereographic: high latitudes
  • Lon-Lat: global
  • Generally: minimize the distortion!
  • Conformity: Locally, angles are preserved
  • Areas are not preserved

Back to the preparation...


Compiling WPS

WPS is the WRF preprocessing system. > cd /home/wrf/src > tar -xf ../packages/wps-3.8.1.tar.bz2 ; cd WPS > export NETCDF=/home/wrf/netcdf > ./configure In the dialogue, select Linux / gfortran / serial (OpenMPI does not improve anything here). > ./compile >& compile.log


Compiling UPP

UPP is the only actually working and usable post-processing system. > cd /home/wrf/src > tar -xf ../packages/DTC_upp_v3.1.tar.gz ; cd UPPV3.1 > ./configure In the dialogue, select Linux / gfortran / serial (dmpar won't work - and doesn't improve much anyway!) > ./compile >& compile.log


Now, the software is set up. Next...

Part 2

Preprocessing


WPS

WPS is the pre-processing package for WRF.

Main settings:

  • timespan
  • area of interest (map projection + extents)

It prepares

  • static data
  • initial data

by extracting the relevant parts and interpolating the data to the area of interest (horizontally).


WPS configuration

Configuration file: WPS/namelist.wps

Important settings: start_date and end_date: forecast time span interval_seconds: output time step duration e_sn, e_we: number of grid cells south-north/west-east geog_data_res: resolution of static data dx, dy: grid cell dimension in map units (f.e. meters) map_proj: map projection ("lambert", "mercator", "polar", "lat-lon") Projection parameters: ref_lat, ref_lon: grid center true_lat{1,2}, stand_lon: Lambert-parameters geog_data_path: absolute path to the static data (/home/wrf/data/WPS_GEOG)


Pre-processing static data

Static data := Digital elevation model (DEM), land-use, land-sea-mask, (SST),...

> cd /home/wrf/src/WPS > ./geogrid.exe

Result: geo_em file(s) containing static data horizontally interpolated to AOI.


Initial data / BC

Several sources, here: GFS (NOAA/NCEP global model), 4 runs per day. Temporal resolution: hourly forecast up to 120 hours, 3-hourly 120-240 hours, 12-hourly to 384 hours Spatial resolution: 0.5 degrees or 0.25 degrees (lon-lat)

Download (freely available) from http://www.ftp.ncep.noaa.gov/data/nccf/com/gfs/prod/

File format: Grib2, one file per time step, containing several hundred bands (=many parameters at many levels)


What is in there?

90%


Pre-processing initial/BC data

> cd /home/wrf/src/WPS > ./link_grib.csh /home/wrf/data/gfs.2016121700/gfs.*.gb2 > ln -sf ungrib/Variable_Tables/Vtable.GFS Vtable > ./ungrib.exe > ./metgrid.exe

Result: met_em file(s) containing all the static and initial condition/boundary condition data for all time steps, horizontally interpolated to the AOI.


What is in there?

90%


Part 3

Model run (finally!)


WRF

Main settings:

  • timespan
  • area of interest
  • physics options

Initial condition is evolved using physical rules while respecting the boundary conditions.


WRF configuration

Configuration file: WRFV3/test/em_real/namelist.input

Important settings: Start and end dates, time interval and domain definition as in WPS configuration history_interval: output data every N minutes Settings for physics schemes


Vertical interpolation

> cd /home/wrf/src/WRFV3/test/em_real > ln -sf ../../../WPS/met_em.* . > ./real.exe

Result: wrfinput and wrfbdy files.

Strictly speaking: another pre-processing step.


WRF Model run

> ./wrf.exe

if WRF was compiled with OpenMPI support (strongly recommended), it can use multiple cores via > mpirun -n 2 wrf.exe

Sit back and have a drink...


Interlude 2


Equations and how to solve them

Derivatives? Differences? PDEs? ODEs? Why large matrices?


ODE example

$\ddot{x} = 2$, $t \in [0,1]$ $x(0) = x(1) = 3$

Approximate the 2nd derivative by $\ddot{x}(t) \approx \frac{x(t-h)-2x(t)+x(t+h)}{h^2}$

("Finite difference method")


Numerical approach

$\frac{1}{h^2}(x_{i-1}-2x_i+x_{i+1}) = 2$, $i = 1,...,N$ for the $N$ inner grid points $x_i := x(t + ih)$, $h := \frac{1}{N+1}$.


Solving the equation at given points in time

We solve this equation f.e. for $N=3$ time steps, thus $i=1:$ $\frac{1}{h}(2x_1 + x_2) = 2 - \frac{1}{h}x_0$ $i=2:$ $\frac{1}{h}(x_1 -2x_2 + x_3) = 2$ $i=3:$ $\frac{1}{h}(x_2 -2x_3) = 2 - \frac{1}{h}x_4$

Linear equations $\rightarrow$ $Ax = b$ with a $3$x$3$ matrix $A$ and $x=(x_1,x_2,x_3)$.

Of course, we are considering large $N$ (plus more complicated and higher dimensional equations), hence large systems of linear equations / (typically sparse) matrices.


Post-processing

UPP to the rescue!


Running UPP

UPP is the unified Post Processing system, to my knowledge the only feasible way of producing usable output.

> cd /home/wrf/src/WRFV3/test/em_real/postprd > ln -s ../wrfout_d01_* . > cd /home/wrf/src/UPPV3.1/scripts > ./run_unipost_xmas


Transforming the output

The final output files are called WRFPRS_d01.00..WRFPRS_d01.180, they are in the Grib1 format. To see what is inside > gdalinfo WRFPRS_d01.00 | less You'll see there are many parameters in many different levels (>300 bands). To transform the grib files into something more useful > gdal_translate -b 294 WRFPRS_d01.00 snowc.000.tiff (This will take band number 294, which is the snow cover, and convert the file into a GeoTIFF, which is much easier to handle than a grib file.


Finally, reproject the data from the original (Lambert-)projection to a Lon-Lat grid: > gdalwarp -t_srs epsg:4326 -dstnodata "-1" snowc.000.tiff snowc.4326.000.tiff

(you could also use epsg:3857 for webmercator as used in most web map applications, or any other projection).


Visualisation

Start QGis.

Add vector layer: Ctrl+Shift+V, select ~/data/ne/ne_10m_admin_0_countries.shp

Add raster layer: Ctrl+Shift+R, select snowc.4326.tiff


Congratulations!

You've completed a WRF model run, forecasting the weather from December 17, 2016 for 180 hours, and you visualized some of the output data!

Hope you enjoyed the tutorial!


Tasks

Some suggestions:

  • change pre-processing for initial and boundary conditions to 1-hourly
  • change the WRF output to 1-hourly
  • change the spatial extent / resolution
  • fetch new GFS data to change the temporal domain