The mean day of the first 3 records are the min, max, average day.
st-vsm1.navydsrc.hpc.mil 135> head -n 13 53X_archMN.2011_05.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 31 40298.00 0 0.000 -4.9933121E+01 1.1652845E+01
srfhgt = 31 40329.00 0 0.000 -1.9516382E+01 1.3948812E+01
surflx = 31 40313.50 0 0.000 -5.8567511E+02 3.4555576E+02
st-vsm1.navydsrc.hpc.mil 136> head -n 13 53X_archMN.2011_06.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 61 40298.00 0 0.000 -4.9525333E+01 1.1496494E+01
srfhgt = 61 40359.00 0 0.000 -1.9453716E+01 1.3996861E+01
surflx = 61 40328.50 0 0.000 -5.4944458E+02 4.0910971E+02
So, 53X_archMN.2011_06 is the average of months 5 and 6 instead of the average of month 6 only.
st-vsm1.navydsrc.hpc.mil 141> head -n 13 928_archMN.2017_05.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 9 42490.00 0 0.000 -5.5614990E+01 1.0263707E+01
srfhgt = 9 42499.00 0 0.000 -1.9434381E+01 1.4649388E+01
surflx = 9 42494.50 0 0.000 -6.4700427E+02 4.2935178E+02
st-vsm1.navydsrc.hpc.mil 143> head -n 13 928_archMN.2017_06.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 30 42521.00 0 0.000 -5.4116100E+01 1.1333158E+01
srfhgt = 30 42551.00 0 0.000 -1.8963694E+01 1.4270756E+01
surflx = 30 42536.00 0 0.000 -5.1975604E+02 4.9222437E+02
928_archMN.2017_05 is the average of the 1st 9 days of the month. You might be better off replacing it with the average of 04 and 06. I don't have access to the original daily means, so I can't recreate the 05 mean.
st-vsm1.navydsrc.hpc.mil 146> head -n 13 930_archMN.2018_01.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 26 42736.00 0 0.000 -5.4221161E+01 1.0937086E+01
srfhgt = 26 42766.00 0 0.000 -1.8886330E+01 1.4824276E+01
steric = 26 42751.46 0 0.000 -1.9253153E+01 1.5430040E+01
st-vsm1.navydsrc.hpc.mil 147> head -n 13 930_archMN.2018_02.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 28 42766.00 0 0.000 -5.4764805E+01 1.1061132E+01
srfhgt = 28 42794.00 0 0.000 -1.8907375E+01 1.4859701E+01
steric = 28 42780.00 0 0.000 -1.9161093E+01 1.5634953E+01
For 930_archMN.2018_01 the 26 records span 30 days, so this monthly mean is likely good enough.
The first 4 months of 2021 has a few missing days, but the monthly means are likely OK:
st-vsm1.navydsrc.hpc.mil 157> head -n 13 930_archMN.2020_01.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 30 43465.00 0 0.000 -5.8329739E+01 1.2863902E+01
srfhgt = 30 43496.00 0 0.000 -1.9257404E+01 2.0680132E+01
steric = 30 43480.27 0 0.000 -1.9444895E+01 2.0270014E+01
st-vsm1.navydsrc.hpc.mil 158> head -n 13 930_archMN.2020_02.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 23 43496.00 0 0.000 -5.8832039E+01 1.2166242E+01
srfhgt = 23 43520.00 0 0.000 -1.8969286E+01 1.7897949E+01
steric = 23 43507.93 0 0.000 -1.9555454E+01 1.8675064E+01
st-vsm1.navydsrc.hpc.mil 159> head -n 13 930_archMN.2020_03.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 28 43526.00 0 0.000 -5.8588848E+01 1.2023198E+01
srfhgt = 28 43556.00 0 0.000 -1.9483227E+01 1.7171019E+01
steric = 28 43541.86 0 0.000 -1.9607971E+01 1.8599995E+01
st-vsm1.navydsrc.hpc.mil 160> head -n 13 930_archMN.2020_04.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 29 43557.00 0 0.000 -5.7692612E+01 1.0504010E+01
srfhgt = 29 43586.00 0 0.000 -1.9525816E+01 1.7358118E+01
steric = 29 43571.50 0 0.000 -1.9535723E+01 1.7724287E+01
st-vsm1.navydsrc.hpc.mil 161> head -n 13 930_archMN.2020_05.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 31 43586.00 0 0.000 -5.7624783E+01 1.0317311E+01
srfhgt = 31 43617.00 0 0.000 -1.9878511E+01 1.5056312E+01
steric = 31 43601.50 0 0.000 -1.9584261E+01 1.6352856E+01
Finally, 2021_02 spans 28 days with 25 records which is likely OK:
st-vsm1.navydsrc.hpc.mil 163> head -n 13 930_archMN.2021_02.b | tail -n 4
field no. recs mean day k dens min max
montg1 = 25 43862.00 0 0.000 -5.9888428E+01 1.2668935E+01
srfhgt = 25 43890.00 0 0.000 -1.9812866E+01 1.7272903E+01
steric = 25 43875.82 0 0.000 -1.9481983E+01 1.9681286E+01
Alan.