Text S3.
Understanding the “Faster than FAT” Response in the Tropics of IPSL
To determine if the “Faster than FAT” response in IPSL may be an
effect of the MISR discretization we investigate how a “thin”
distribution of high clouds appears to move upward given the MISR grid
discretization. Specifically, we assume a gaussian distribution for the
vertical profile of high clouds (Figure S5 ). We find that if
the width (standard deviation) of the underlying CTH distribution is 700
m or larger, then the WCTH captures the increase in CTH well. However,
if the width is less than 700 m (400 m is used in the example inFigure S5 ), the MISR discretization poorly captures the slow
rise in the peak position of the distribution (left panel) and WCTH can
appear to rise faster or slower than the true peak depending on whether
the true peak is above or below the bin mid-point; and in the limit
where there is no width in the distribution (meaning all CTH occurs at a
single fixed height), the WCTH would appear as a flat except for a
single step of 2 km (the size of the MISR vertical bins) as the CTH
moves from one MISR bin to the next. In Figure S5 a gaussian
distribution of CTH with a width of 400 m is slowly lifted with the
position of the peak moving from 12 to 14 km (top right panel) and the
resulting MISR CTH distribution and WCTH are shown in the top left panel
and bottom left panels, respectively. The bottom right plot is designed
to mimic the results produced by tropical cloud in IPSL by adding white
noise to the position of the Gaussian peak. In summary, IPSL has a
narrow high cloud distribution in the tropics, the rapid rise in WCTH
seen in IPSL after 2040, and slow down after 2060, as well as the
increased variability between 2040 and 2060 (as seen in Figure
7 of the main text) are consistent with how a narrow cloud distribution
would be seen by the MISR simulator. It is noted that if the
distribution of CTH in the real world were as narrow as suggested by the
IPSL model, the WCTH based on the MISR observations would have this same
undesirable property. However, that does not appear to be the case (as
can be seen in Figure S1 , as observed high cloud is spread
across several MISR height bins). Of course, it is worth noting that the
MISR retrievals have some random error associated with them (largely due
to uncertainty in wind correction, see Marchand et al. 2010), which will
cause the observed distribution to be broader than the true
distribution. Given the potential for artifacts, it might be wise to
increase the resolution of the MISR CTH-OD product and simulated output,
in the future.