Reading a grib2 dataset without loss of metata with R - r

Question

My question follows this previous question, that has been partially answered : Reading a grib2 dataset with 4 dimensions and 2 variables with R
I am trying to read a GRIB2 file with R. This file is a probabilistic meteorological forecast : 10 variables, 1 lead time, 17 longitudes, 23 latitudes, and 51 members.
I can extract that thanks to the terra package and this script :
require(terra)
require(dplyr)
require(data.table)
require(stats)
destfile <- "C:/Users/XXX/Documents/example_grib_file_3"
##Downloading file
grib_data <- terra::rast(destfile)
print(grib_data)
## Convert to data frame
df <- as.data.frame(grib_data, xy=TRUE)
## Colnames is a combination of members (50) X time (57) X variables (2)
colNames <- paste(names(grib_data), as.character(time(grib_data)), sep="_")
colnames(df) <- c("lon", "lat", colNames)
df2 <- data.table::melt(as.data.table(df), c("lon", "lat"))
## Split variable and time
df2$time_UTC <- sub(".*_", "", df2$variable)
df2$variable <- sub("_.*", "", df2$variable)
## Add members
df2 <- df2 %>% group_by(lon, lat, variable, time_UTC) %>% mutate(member=(1:length(value)))
##Convert to array
df_array <- stats::xtabs(value~lon+lat+variable+member+time_UTC, df2, drop=F)
The BIG problem is that I can't retrieve the metadata concerning the perturbation number (member). For now, variables and members are mixed, giving 500 columns, with the variable name repeated 50 times. The member is not always at the same position along the other dimension (variable). For example, one particular member is in position 6 for temperature data, and position 50 for precipitation data.
So the line "Add member" in my code is totally irrelevant and needs the "perturbation number" to arrange the array. If I use eccodes, there is a field called "perturbationNumber", how can I retrieve it from terra and R ?
The example file : https://drive.google.com/file/d/1kUfTdtAdNJpugMpPhdQ6tY4QZaZlUQcA/view?usp=sharing
The example file in grib2 : https://drive.google.com/file/d/1Zsf8uajm5AOXTiCus6PyANZAsJ5cKsc1/view?usp=sharing
These are the different parameters retrieved using eccodes with Python :
grib_ls -l 48.5,3.5,1 -p paramId,name,typeOfLevel,level,dataDate,stepRange,dataType,shortName,step,perturbationNumber PATH_TO_FILE
paramId name typeOfLevel level dataDate stepRange dataType shortName step perturbationNumber value
167 2 metre temperature surface 0 20230124 0 pf 2t 0 35 273.517
167 2 metre temperature surface 0 20230124 0 pf 2t 0 3 273.589
167 2 metre temperature surface 0 20230124 0 pf 2t 0 50 273.229
etc...
Thanks for any help

Answer 1

With your example file
f <- "example_grib_file_3"
library(terra)
#terra 1.7.5
x <- rast(f)
This is the metadata that is available
names(x)[1]
#[1] "SFC (Ground or water surface); 2 metre temperature [C]"
time(x)[1]
#[1] "2023-01-24 UTC"
units(x)[1]
#[1] "C"
And with version 1.7-5 (currently the development version) you can also get raw metadata
metadata(x)[[1]]
# [,1] [,2]
#[1,] "GRIB_COMMENT" "2 metre temperature [C]"
#[2,] "GRIB_ELEMENT" "2T"
#[3,] "GRIB_FORECAST_SECONDS" "0"
#[4,] "GRIB_REF_TIME" "1674518400"
#[5,] "GRIB_SHORT_NAME" "0-SFC"
#[6,] "GRIB_UNIT" "[C]"
#[7,] "GRIB_VALID_TIME" "1674518400"
I do not know if there are ways to get other metdata from this file but in principle that is possible: see your previous question.