Warning: "TOO COARSE LONGITUDE RESOLUTION"

[fcheng]

Dear FLEXPART community,

I recently used "flex_extract" to prepare ERA5 data for the FLEXPART model. Everything went fine, except I noticed a warning message saying "TOO COARSE LONGITUDE RESOLUTION" while the calc_etadot program was running.

I was downloading 1˚x1˚ hourly ERA5 data. Maybe the spatial resolution is too coarse for hourly resolution? But I don't know exactly why. Can anyone explain this, please? Also, is there any recommended setting of the spatiotemporal resolution of the ERA5 data for the FLEXPART model?

Below is my data-downloading job. Many thanks for your attention. Hope you have a nice day.

Best,
Fandy

START_DATE 20190103
END_DATE 20190104
DTIME 1
TYPE AN AN AN AN AN AN AN AN AN AN AN AN AN AN AN AN AN AN AN AN AN AN AN AN
TIME 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
STEP 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ACCTYPE FC
ACCTIME 06/18
ACCMAXSTEP 12
CLASS EA
STREAM OPER
DATASET ERA5
GRID 1.
LEFT -179.
LOWER -90.
UPPER 90.
RIGHT 180.
LEVELIST 1/to/137
RESOL 799
SMOOTH 0
ETA 1
GAUSS 0
ETADIFF 0
DPDETA 1
OMEGA 0
OMEGADIFF 0
CWC 1
PREFIX EA
ECTRANS 1

[mqzhang]

The source resolution of ERA5 would be 0.25degree.
GRID 1. Should be GRID 0.25

[pesei]

User mqzhang is right.

The message is trigged by the condition that fewer longitudes are specified than Fourier coefficients available. This way of extracting data would lead to aliasing effects. For the correspondence between grid and spectral resolution, see Tipka et al., Flex_extract v7.1.2 – a software package to retrieve and prepare ECMWF data for use in FLEXPART, Geosci. Model Dev., 13, 5277–5310, ​​https://doi.org/10.5194/gmd-13-5277-2020. Table 1 to 4, and also ​https://www.flexpart.eu/flex_extract/Documentation/vertco.html.

[fcheng]

Hi mqzhang and pesei,

Thanks a lot for your prompt reply. Yes, I know that the native resolution of ERA5 is ~0.25, but it's not always feasible to download and run the model with such high-resolution data (especially for a climatological study).

I appreciate pesei's informative explanation of the warning message. The paper and the online webpage about the choice of GAUSS, RESOL and SMOOTH for a given grid resolution really help address my questions. I will think about which resolution setting I should choose.

Thank you, and hope you have a nice day.

Cheers,
Fandy