What elevation for spot forecasts?



  • How can I check the elevation being used for a spot forecast, the one that displays below the map? The temperatures and snowfall forecasts in the meteograms absolutely do not correspond with the precip type and temp data displayed on the map, for the corresponding point on the map. Is this due to a wide grid being used? If so, it makes the forecasts very misleading for mountainous areas.



  • Any response to this?



  • @ivo
    @dmn

    https://community.windy.com/topic/4657/reference-altitude

    It would be very usefull if windy could provide us
    the reference altitude (model's orography).
    Hope Ivo will include this in the next upgrade !


  • administrators

    Are you sure model on map corresponds to meteogram? Send screenshots



  • @ivo
    Let's take a look at the meteogram for Strassburg Airport (LFST), France.
    At "about location" section, we see the altitude: 146m.
    I believe this is the real height for the location and not the model's orography.
    Why?
    Because whatever model I use, it stays the same.
    If it was the height of model's grid point then it must be different
    between ECMWF and GFS as these two models have different grid.
    0_1510521908694_0aa37e8f-61f6-4c17-bbf1-1b57c63bf0f1-εικόνα.png

    Now, let's take a look at a ECMWF's meteogram
    (is not for today, I found it in: "User guide to ECMWF forecast products")
    page 57
    https://www.ecmwf.int/files/user-guide-ecmwf-forecast-products
    0_1510522378778_5a1b30d1-7bdb-42b6-914b-912cfc7f800a-εικόνα.png
    The station's height (as displayed in the title) is 141m.
    But if we see the temperature diagram
    0_1510522507232_e57c8ed3-462c-436c-bc17-6e4da712cd70-εικόνα.png
    we realize that the temperature is adjusted by the 6.5°K/km* difference
    between the station height (141 m, as displayed in the title)
    and the model's orography (179 m),
    as displayed in the title of the temperature panel.

    • 6.5°K/km is the lapse rate for the Standard Atmosphere.

    So, if windy would provide us the height of model's orography (in any place)
    we could adjust the temperature to the real height for a location.
    For mountainous regions it is very helpfull.

    P.S. I hope you understand what I tried to say. I'm not a fluent english speaker!



  • This post is deleted!


  • @ivo I have trouble posting a cogent screenshot examples because:

    1. if I change the zoom level on the map, then the spot forecast value changes. This makes it impossible to compare the map Fx with the meteogram Fx.

    2. if I click the spot location on a known 3000m peak, and then I change the elevation level for the forecast using the slider, I get different values for "Surface" and "3000m". Which elevation slider setting should I use for a forecast on terrain which is 3000m asl? I can find no consistent relationship. This makes it impossible to compare the map Fx with the meteogram Fx.


  • administrators

    @meteo-gr

    Yes it is terrain elevation.

    No we do not display model elevation

    No we do not adjust interpolated values to its terrain height (interpolation in between two model levels). @TZ did some coding in this field but not finished it yet.

    We do not have ANY METEOROLOGISTS in Windy team which is shame (the one I would love to have in team is Swiss), therefore we lack a knowledge in this field. If you have suggestions how to interpolate model's parameters to terrain height, please let us know. Send us some formulas to use or some document how to do it please.



  • @ivo
    Thanks for the answer.
    The first (easy) step for you, is to display (for a location) model's elevation
    (if it is available in the Grib file) next to terrain elevation.

    Then, the "amateur forecaster" can correct the forecasted temperature using the Standard Atmosphere lapse rate.

    Example: I am on a ski resort with terrain elevation 2.000 m.
    The model forecasts temperature +2 C and rain (bad news!).
    If I have (from windy) the indication that model's elevation is 1.500 m
    (500 m lower than true elevation)
    I can correct the forecasted temp (and turn rain into snow!).

    How?
    I know that the Standard Atmosphere lapse rate is 6.5 degrees / 1.000 m.
    That equals to 3.2 degrees / 500 m.
    So, I must subtract 3.2 from the forecased value (+2 C).
    The result (+2-3.2=-1.2 C) is a more realistic forecasted temperature
    (and the negative temp. indicates snow!).

    Thats why will snow although the model forecasts rain!

    Of course, in a relatively smooth terrain, if the difference between actual terrain elevation and model elevation is less than 150m, the temperature correction is only 1 degree C and it doesn't worth make such computations.

    P.S. 1 : For better understanding, I copy the following from "User guide to ECMWF forecast products" https://www.ecmwf.int/files/user-guide-ecmwf-forecast-products , page 10
    "2.4.3. Orography
    Because valleys and mountain peaks are smoothed out by the model orography the direct model output of 2 m temperature may represent an altitude significantly different from the real one.
    A more representative height might be found at one of the nearby grid points.
    Any remaining discrepancy can be overcome by a correction using the Standard Atmosphere lapse rate or statistical adaptation (see Appendix B-6)."

    P.S. 2. For Standard Atmpsphere, see:
    https://en.wikipedia.org/wiki/International_Standard_Atmosphere


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