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[ferret_users] Plotting differences is not giving correct answer, time axis problem?



Dear Ferret fans,

I've been scratching my head over some results Ferret is giving me. I am trying to plot the difference between two fields of NO2. The data from the fields is shown below. In the first column I have a monthly mean NO2 field (variable no2_mm in the netcdf file). In the second column I have a time average computed from 6hrly output, ie. no2[t=@ave] where the data in the file runs from 1st March to 31st March.

If I try to take the difference as shown in the 3rd column I get wrong answers. When I do the difference I get a warning from Ferret that the time axes are not compatible (*** NOTE: Ambiguous coordinates on T axis: NO2_MM[D=1]-NO2[D=2,T=@AVE]) ; presumably because my monthly mean field has a date of 31/3 whereas ferret uses a date in the middle of the month?

But I can't figure out what Ferret is doing. It's obviously not subtracting the numbers in the way I want to. My guess is it's shifted the time axis somehow? The differences around 87W for example are clearly not calculated right.

Apart from the fact this is rather scary, how do I fix it?  Thanks.

    Glenn

--

             LATITUDE: 40.5N
             Z (sigma_level): 31
Column 1: NO2_MM[D=ptom_t042_199703mm,T=31-MAR-1997 00:00] is Nitrogen Dioxide monthly mean (ppbv) Column 2: NO2[D=ptom_t042_1997030106,T=01-MAR-1997 03:00:31-MAR-1997 21:00@AV] is Nitrogen Dioxide (ppbv) Column 3: EX#3 is NO2_MM[D=ptom_t042_199703mm]-NO2[D=ptom_t042_1997030106,T=@AVE]
             NO2_MM    NO2   EX#3
180E   /  65:   0.02   0.02 -0.003
177.2W /  66:   0.02   0.02 -0.002
174.4W /  67:   0.02   0.02  0.001
171.6W /  68:   0.02   0.02  0.003
168.8W /  69:   0.02   0.02  0.007
165.9W /  70:   0.02   0.02  0.006
163.1W /  71:   0.02   0.02  0.003
160.3W /  72:   0.02   0.02  0.000
157.5W /  73:   0.02   0.02 -0.005
154.7W /  74:   0.02   0.02 -0.008
151.9W /  75:   0.02   0.02 -0.007
149.1W /  76:   0.02   0.02 -0.005
146.2W /  77:   0.02   0.02 -0.005
143.4W /  78:   0.02   0.02 -0.006
140.6W /  79:   0.02   0.02 -0.009
137.8W /  80:   0.04   0.04  0.003
135W   /  81:   0.07   0.06  0.027
132.2W /  82:   0.07   0.07  0.038
129.4W /  83:   0.04   0.04  0.017
126.6W /  84:   0.04   0.04  0.019
123.8W /  85:   0.32   0.31  0.144
120.9W /  86:   0.67   0.67 -0.216
118.1W /  87:   0.40   0.40 -0.206
115.3W /  88:   0.26   0.25 -0.220
112.5W /  89:   1.88   1.87 -0.096
109.7W /  90:   1.55   1.50  0.221
106.9W /  91:   1.65   1.62  0.250
104.1W /  92:   1.96   1.94  0.094
101.2W /  93:   1.25   1.26  0.091
98.4W  /  94:   1.58   1.64  0.330
95.6W  /  95:   3.38   3.48  1.099
92.8W  /  96:   2.39   2.41  0.960
90W    /  97:   4.17   4.23  1.620
87.2W  /  98:   6.94   7.22  2.057
84.4W  /  99:   5.84   6.09  2.405
81.6W  / 100:   7.34   7.85  0.619
78.8W  / 101:   5.93   6.23  0.376
75.9W  / 102:  12.74  13.41 -1.871
73.1W  / 103:   6.06   6.32 -1.817
70.3W  / 104:   1.52   1.56 -0.706
67.5W  / 105:   0.80   0.81 -0.422
64.7W  / 106:   0.36   0.37  0.109
61.9W  / 107:   0.24   0.24  0.079
59.1W  / 108:   0.17   0.17  0.094
56.2W  / 109:   0.14   0.14  0.088
53.4W  / 110:   0.10   0.10  0.073
50.6W  / 111:   0.08   0.08  0.054
47.8W  / 112:   0.08   0.08  0.036
45W    / 113:   0.08   0.08  0.006
42.2W  / 114:   0.07   0.07 -0.010
39.4W  / 115:   0.07   0.07 -0.009
36.6W  / 116:   0.10   0.10  0.006
33.8W  / 117:   0.22   0.22  0.023
30.9W  / 118:   0.16   0.16 -0.039
28.1W  / 119:   0.06   0.05  0.012
25.3W  / 120:   0.09   0.09  0.009
22.5W  / 121:   0.22   0.21 -0.023
19.7W  / 122:   0.25   0.24 -0.038
16.9W  / 123:   0.26   0.26 -0.093
14.1W  / 124:   0.40   0.40  0.026
11.2W  / 125:   1.44   1.43 -0.135
8.4W   / 126:   0.98   0.99  0.077
5.6W   / 127:   1.46   1.54  0.214
2.8W   / 128:   1.57   1.63  0.168
0E     / 129:   0.93   0.93  0.103


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