Figure 6: Seasonal and geographical variations in normalized
total column abundances of water vapor. Panel A: Seasonal distribution
of all qualified retrievals averaged in bins of 2°x2° Ls and latitude.
Panels B-E: Geographical distribution of qualified retrievals for each
seasonal interval, averaged in bins of 2°x4° latitude and longitude.
Given the variation of the sampled local times, it is assumed that
seasonal variations of water vapor column abundances dominate over any
diurnal variability. Although the temporal and spatial coverage is
incomplete, known trends of the water cycle appear unambiguously. The
spring is overall a dry season (Figure 6 panel B), with abundances
rarely higher than 10 pr-μm. The few instances with larger CIA occur
late in spring at the edge of the NPC. The early northern summer season
(Ls = 90° –135°) is characterized by large latitudinal contrasts with
high water content in the polar regions which decrease monotonically
southward, this is particularly prominent in Panel C. At around Ls=115°,
a Northern maximum of ∼60 pr-μm is visible poleward of 70°N, while at
the same time only around 13 pr-μm is measured south of 30°N (in panel
A). The northern hemisphere fall season shown in panel D displays a
north polar region now devoid of water vapor, most of which having been
transported to the mid and low latitudes and across the equator. The
south polar maximum occurs around Ls=285° and reaches an average total
column abundance of 40 pr-μm, as seen in Figure 6 panel E. A global dust
storm occurred in MY 28, which degraded the quality of the measurements
for some time, causing the number of qualified retrievals in MY 28 to be
low even though many observations were conducted in this period. The
drier patch around Ls=300° in the southern hemisphere is constructed
almost entirely from observations in MY 28, when the dust storm is
thought to aid transport of water vapor from the lower atmosphere to
higher altitudes
(Fedorova et al.,
2018).
4.2 Synergy compared to single domain retrievals and MCD
Numerous studies of the climatology of water vapor have been made using
the PFS and SPICAM instruments individually. As this is the first time
observations from both are used in synergy, a direct comparison has been
made between them and the MCD. In Figure 7, synergy retrievals and MCD
prior values satisfying the adjusted criteria as described in Section
4.1 are plotted, along with single spectral domain retrievals for SPICAM
and PFS. No criteria have been imposed on the single domain retrievals
other than unphysically high abundances have been filtered out. CIAs are
averaged across the 15°-45°N latitude band, the region which contains
the longest continuous coverage, and in intervals of 5° Ls.
The selected time period covers the early northern summer, the polar cap
sublimation season and continues into late summer of MY 27. In general,
the MCD predicts a much higher water vapor abundance than what is
obtained with either of the retrieval approaches (except during
Ls=120°-140°). The MCD agrees well with the observations only at the
very beginning and end of the time period shown here, which corresponds
to before the onset of the sublimation season, and after the water vapor
has been transported beyond the area of focus. This might suggest that
the transportation mechanisms dominant in the summer mid-latitudes are
currently not fully understood. Another factor which could impact this
discrepancy is the large MCD sublimation peak, which might then
propagate southward. The difference in the CIA, as well as the vertical
partitioning, predicted by the MCD and the values retrieved by the
synergy is further elaborated upon in section 4.3.
The synergy and the single spectral domain retrievals with PFS/TIR are
overall in good agreement, with the synergy on occasion yielding
slightly lower values. SPICAM/NIR also agrees well with the synergy and
PFS, albeit with slightly larger abundances. The general seasonal
behavior displayed by the three retrieval approaches is similar; an
increasing trend in the early summer, peaking at around Ls=135°, when
water vapor from the North Polar Cap (NPC) has sublimed and been
transported to mid-latitudes. The MCD predicts a much more rapid
increase of the sublimed water, with CIA values a factor of 2.5 higher
than the synergy at Ls=100°. The decreasing CIA found after Ls=140° by
both the synergy and the single spectral domain retrievals as well as
the MCD is expected, as the water is successively transported across the
equator. The “double-hump” shape of the MCD abundances (also evident
in Figure 10) are not clearly distinguishable from either the retrieval
techniques.