API for plotting skew-T log-P thermodynamic diagrams with data from the DWDs ICON-EU weather forecast model.
An example of the output produced:
Using the API is very easy and straightforward. An HTTP GET will start plotting the diagram, resulting in a simple JSON with the model time, valid time and links to a full and lower level (detail) skew-T diagram.
https://icon-skewt-plot.vektorraum.com/<latitude_north>/<longitude_east>/<valid_at_YearMonthDayHour>
The following parameters need to be provided:
| Parameter | Format | Example |
|---|---|---|
| latitude_north | decimal latitude, north positive | 48.12 |
| longitude_east | decimal longitude, east positive (as in ICON) | 16.37 |
| valid_at | YearMonthDayHour with no separators and leading zeros | 2020030512 |
Example Output of the API:
{
"model_time": "2020-03-04T18:00:00.000000000",
"valid_time": "2020-03-05T12:00:00.000000000",
"plot_full": "https://storage.googleapis.com/icon-skewt-plot/plot_48.21N_16.37E_2020-03-04T18_00_2020-03-05T12_00_full.png",
"plot_detail": "https://storage.googleapis.com/icon-skewt-plot/plot_48.21N_16.37E_2020-03-04T18_00_2020-03-05T12_00_detail.png"
}All times are in UTC.
Plotting currently takes about 30-50 seconds. Therefore timeouts need to be set high enough!
While some tools and websites are available for thermodynamic diagrams based upon the GFS model this software provides the tooling for plotting skew-T log-P diagrams based upon the much higher resolution ICON model.
This model currently provides a resolution of about 7km and 60 levels within Europe. The forecasts are available for up to 120 hours, while forecasts up to 78 hours are available in hourly intervals.
DWD provides the ICON EU model output as open data on the DWD open data server.
A second backend, nwp-sounding, which is scaled up dynamically to over 100 instances, is used for speeding up the plotting process. This backend uses the ECMWFs ecCodes library via xarrays integration into ECMWFs cfgrib engine.
For convenience the newest model run, which contains the valid time is looked up automatically based upon the content log, published on the DWD open data server.
Once this run has been determined the NWP sounding backend is called in a highly parallelized fashion. Concurrent calls are made for each parameter, as well as for each chunk of levels, as configured.
This backend downloads the compressed grib files from the open data server. Uses xarray for reading the files and for interpolating to the coordinates provided.
Each backend instance returns the horizontally interpolated grid values for the chunk of levels for which it has been called.
Chunks are recombined into a numpy array of all values, for all 60 levels.
The following parameters of the ICON model are used for plotting the diagram:
- P - pressure
- Used as the main y-axis reference for plotting.
- T - temperature
- QV - specific humidity
- Used for calculating the dewpoint
- U - first component of the wind
- V - second component of the wind
- HHL - geometric height of model half levels above MSL
- Used for calculating the full level height of each pressure level
SkewT plot from the MetPy package, together with matplotlib, are used for plotting the actual diagram.
First temperature (solid red) is plotted, with pressure on the y-axis. Then the dewpoint (solid blue) is added.
A parcel lifted dry adiabatically from the lowest pressure level to the LCL and moist adiabatically is added to the plot (solid yellow).
Finally the plot is completed by adding the thermodynamic lines to the plot:
- Isotherms - slanted solid grey
- Lines of equal temperature
- The 0° Celsius isotherm - solid cyan
- Isobars - horizontal solid grey
- Lines of equal pressure
- Dry adiabats - dashed red
- Moist adiabat - dashed red
- Isohumes - dashed green
- Lines of equal humidity, equal mixing ratio in this plot
On the edge of the diagram wind barbs (in knots) are plotted, based upon the models U and V wind component outputs.
An additional axis is added, showing the actual geometric altitude at different pressure levels. This uses the actual pressure distribution and can be used to convert from pressure to geometric altitude.
For the output two diagrams are produced, one from 1000 hPa to 100 hPa. As well as a second one, showing the lower levels in finer detail from 1000 hPa to 500 hPa. The isotherms in the detail plot are skewed to only 15 degrees to facilitate easier interpretation and reading of the plot.
Many resources are available for understanding these thermodynamic diagrams.
Roland Stull provides a great explanation in his book "Practical Meteorology", which is available for free online, under a creative commons licence. The explanation is given in the chapter on atmospheric stability (page 122).
A second backend application is used for high concurrency. Both applications are deployed using a Docker container, built on Google Cloud Build. The container is then deployed to Google Cloud Run.
Resulting plots are stored in a Google Cloud Storage bucket, which has public access enabled, for serving the plots as a CDN.
