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ERA5 Data - How is a wind gust defined?

I'm working with wind data, that I buy from a company, which delivers basically reformatted ERA5 weather data from ECMWF (https://www.ecmwf.int/). Their historic wind data includes "Wind gusts (sfc)" (m/s) and I cannot find out how this "Wind gust" is defined.

My understanding is that a wind gust is basically just the maximum value of wind velocity of a number of averages within an observation period. Say I measure wind velocity as average over 1 hour, then look at the 10-minute averages of that same 1 hour and the largest of those is my "Wind gust" (V_600_3600 - see here for nomenclature:http://ijsetr.com/uploads/621345IJSETR1044-354.pdf).

But the ERA data are a re-analysis so of course the gust is basically just a factor on top of the average 1hour wind. What I cannot figure out, is, which gust definition this uses. Is it 10-min (V_600_3600) or 1-min (V_60_3600) or even 3-seconds (V_3_3600).

The support of my data-company insists that "Wind gust" is "the maximum wind velocity within an hour" (they deliver hourly data over long time periods - years) but they insist that this "Wind gust"-value cannot be defined in terms of a particular averaging-time period like 10-minute gust or similar.

我单词查找树d to find the wind gust definition for ERA5 - the only thing I found is this definition here, that defines the "Instantaneous 10m wind gust" as being equal to the 3s wind gust (V_3_3600):https://eqc-cms.predictia.es/pub/qar/reanalysis-1412/pdf

Is this the correct definition? I'm a bit confused, since this contradicts the company support guys (which are specialist for this thing). They insist, there is not time-averaging-period attached to the definition of "Wind gust" which is, in my understanding, not sensible. Theremustbe some time information attached to any definition of "wind gust" as far as I understand? But then again I'm not a meteorologist and might miss something basic.

Obliged for any help to clarify this respectively lets me understand this better.

Answer

The [WMO definition of wind gust](https://space.oscar.wmo.int/variables/view/wind_gust) for observing stations is, > Maximum value, over the observing cycle, of the 3-second running average wind speed. which is the definition you've found. The ECMWF doc you've linked to says, > This duration is shorter than a model time step, and so the ECMWF Integrated Forecasting System (IFS) deduces the magnitude of a gust within each time step from the time-step-averaged surface stress, surface friction, wind shear and stability. The details of this calculation can be found in the [IFS documentation](https://www.ecmwf.int/en/elibrary/20198-ifs-documentation-cy47r3-part-iv-physical-processes) (S3.9.4) and there's a more readable summary in an old ECMWF newsletter by [Bechtold and Bidlot (2009)](https://www.ecmwf.int/en/elibrary/17487-parametrization-convective-gusts). My interpretation is that this is a parameterization of V_3_X that's calculated every model time step, so X is the atmospheric time step length (e.g., about 90 to 1200 s, depending on model resolution) for the instantaneous output field. The aggregated period maximum output fields that are also available will be simply the maximum V_3_X over that period.

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$\begingroup$ Great information much obliged! The IFS documentation in particular is a treasure trove of information. I've been not looking for gusts, but also roughness definitions, zero-level height adjustments etc. and its all in there! Thanks a bunch! $\endgroup$
–squarespiral
Oct 22, 2022 at 16:57

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