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• zero might be a valid value for many meteorological parameters, so it can't be used as a mask
• if we want to compute the mean over the field, the sea points will still be used and will simply result in a lower mean because they are all zero

So we will convert the zeros to missing values using the bitmap() and nobitmap() functions, whose documentation is reproduced here:

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• modify the lsm variable to have missing values where there are currently zeros (visualise to verify)
• read t2m.grib into a new variable t2m
• copy the missing value mask from lsm to t2m (plot t2m to verify)
• compute and print the mean value of t2m - use the function integrate()

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