**Figure 1. Summary of CarbonTracker residuals.**CarbonTracker posterior residuals (simulated minus observed, in μmol mol-1) as a function of time and latitude. Each dot represents the time and location of a CO2 observation that was assimilated in CarbonTracker. Colors represent the difference between the final simulated value and the actual measurement, with warm colors indicating that CarbonTracker simulates too much carbon dioxide compared to observations, and cool colors indicating that CarbonTracker estimates too little CO2. Features of interest include a seasonal cycle in northern mid-latitudes, in which CarbonTracker has a summertime excess of carbon dioxide compared to observations and too little in wintertime; and a persistent underestimate of CO2 simulated in the southern hemisphere.

**Figure 2. Seasonal histograms of northern hemisphere
residuals.** As in Figure 1, posterior residuals of
assimilated CO2 mole
fractions, but only for the northern hemisphere, and
grouped by season. These histograms are intended to
highlight the seasonal cycle of CarbonTracker residuals
in the northern hemisphere.
In the northern hemisphere summer, the mean
CarbonTracker residual is about 0.1 μmol mol-1, and in the winter it is about
-0.3 μmol mol-1. While
these mean seasonal residuals are significant, the
figure also shows the strong contribution of random
error, as evidenced by the wide ranges of the
distributions. The figures shown in blue give the mean
and standard deviation of each set of residuals, and
the blue curves are the Gaussian ("normal")
distributions with equivalent statistical moments.
Notably, summertime random errors are much larger than
those in winter, due in large part to stronger
summertime variability in lower atmosphere carbon
dioxide mole fractions.
The residuals are tightly peaked
in the range of ± 2 μmol mol-1, with long tails of fewer
residuals at much higher values. This

**Figure 3. Summary of aircraft residuals.** Posterior
residuals as in Figures 1 and 2 above, but for aircraft data.
These data, which are mostly
over North America, are grouped by latitude band and by season.
CT2017 is the first release in which these data are assimilated into CarbonTracker.
In previous CarbonTracker releases
(e.g., CT2011_oi),
a marked summertime excess of CO2 was
evident in both latitude bands, with the excess being more closely
confined to the surface in the southern region. While this
summertime near-surface excess of CO2 in
the 25°-45° band has been largely eliminated in CT2019B, there remains a positive model bias throughout the
column in the 45°-70° latitude band. The mostly even
vertical distribution of model bias compared to these
observations is consistent with its source being relatively remote
from North America, since the signal has had time to mix
throughout the troposphere.