I illustrated NCEP Model Performance with verification statistics for the
18Z GFS forecast cycle because 18Z corresponds to noon CST--near the peak daily temperatures over the Continental Unites States (CONUS). However, that may have been misleading or only told a partial truth.
Let's look at the statistics for 18Z again.
![](https://1.bp.blogspot.com/-yATKyZUjIS8/Vti8dZjD0TI/AAAAAAAAAVw/CM0UWRuK9iQ/s320/biasmp_T_gglb.png) |
18Z analysis cycle GFS temperature bias for forecast hours 0-168, compared to conventional upper air soundings. Operational GFS on the left and experimental GFS on the right. Scale [-1.0, 1.0] C |
Note the data scales.
![](https://2.bp.blogspot.com/-knEl2Ko9HB8/Vti8dd07JJI/AAAAAAAAAV0/Mx8Cx9iJ3sM/s320/biasmpv_fhr48_T_gglb.png) |
Bias between upper air stations and the 48-hour GFS forecast for the 18Z cycle. Bias statistics computed over ~2,750 observations. |
Now let's look at the same visualizations for the 12Z data analysis/forecast cycle.
![](https://4.bp.blogspot.com/-RW18aTnKf8Q/Vt5J3Fa-tTI/AAAAAAAAAWc/cOjICkQ-x50/s320/biasmp_T_gglb.png) |
12Z analysis cycle GFS temperature bias for forecast hours 0-168, compared to conventional upper air soundings. Operational GFS on the left and experimental GFS on the right. Scale [-0.5, 0.5] C |
Notice that the temperature bias scale is reduced from [-1.0, 1.0] to [-0.5, 0.5] C? The bias reduction is less dramatic than a factor of two.
In the next graph, the scale doesn't change, but the peak of the bias is reduced from ~4.0C to ~2.5C at the tropopause.
![](https://3.bp.blogspot.com/-coLS8zbyw30/Vt5J3GpjkgI/AAAAAAAAAWY/35bWtBx1_4Y/s320/biasmpv_fhr48_T_gglb.png) |
Bias between upper air stations and the 48-hour GFS forecast for the 12Z cycle. Bias statistics computed over ~135,000 observations. |
What causes that difference? The graph on the right gives a clue.
The horizontal scale gives the total number of observations used to compute the statistics. At 18Z, there were ~2,750. At 12Z, there were ~135,000. More radiosondes are released near 12Z than any other time of day (with 0Z coming in a close second.)
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