time variable in arl files

time variable in arl files

Postby Martina » December 17th, 2013, 10:08 am

Dear all,

I am struggling a bit with the timing of my meteorological data. I have to add that this is my first contact with grib, arl and forecast/reanalysis data so maybe I'm asking questions which are obvious for everybody else.

I am working with CFSR forecast (6hr) and analysis data and calculating trajectories. I'm dealing with both forecast and analysis, since not all fields (for example the 10m winds) are available in the analysis.

I don't understand how the timing is set up in the grib and arl files, since depending how I look at the data, I get different answers:
For example in the arl-header under "Greenwich date/time for which data valid" is this the time for which the forecast is valid or is it the time where the forecast was initiated? I.e. which time is used later in the trajectory calculations, the time indicated here or this time + "Forecast"?
Same thing when looking at the grib data with wgrib or grib_ls, which time is shown?
So basically do I need to shift the Forecast data manually by 6hr to compare Forecast and Analysis at the same moment or is that already taken care off automatically?

I am using MeteoInfo to convert from grib2 to arl and as it looks the midnight grib2 data in some cases is identical to the 6am arl data, so I think something is going wrong somewhere and I need to better understand how this timevariable is dealt with.

Thanks for your input on that question,

Martina
Martina
 
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Re: time variable in arl files

Postby barbara.stunder » December 18th, 2013, 12:41 pm

The arl header in HYSPLIT meteorology files begins with values giving YYMMDDHHFF

YY=year
MM=month
DD=date
HH=hour
FF= forecast hour

YYMMDDHH is the "Greenwich date/time for which data valid". This applies whether it's a forecast or analysis.
FF is the associated forecast hour of that data

Example with 0-h forecast:
13121812 0 - data valid 12UTC 18 Dec 2013, forecast initialization is the same since the forecast hour is 0

Example with 12-h forecast:
1312181212 - data valid 12UTC 18 Dec 2013, forecast initialization 12 hours earlier, or 00UTC 18 Dec 2013

I thought wgrib inventories are labeled as to whether it is the valid time or forecast initialization time plus X forecast hours.

I'm not familiar with MeteoInfo.
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Re: time variable in arl files

Postby yaqiang » December 18th, 2013, 9:08 pm

"I am using MeteoInfo to convert from grib2 to arl and as it looks the midnight grib2 data in some cases is identical to the 6am arl data, so I think something is going wrong somewhere and I need to better understand how this timevariable is dealt with."

Can you provide the grib2 and converted arl sample data files for wrong cases?
yaqiang
 
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Joined: November 22nd, 2012, 4:02 am

Re: time variable in arl files

Postby Martina » December 19th, 2013, 11:11 am

Thanks Barbara, for detailing the ARL convention. That helps me a bit to find my error!
Martina
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Joined: September 27th, 2013, 10:26 am


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