Figure 9: Illustration of the relationship between the retrieved values and the MCD prior. The deviance of the synergy from the MCD estimates are calculated as a relative difference such that a value of 0 indicates where the synergy and the MCD are equal, and instances where the synergy yields larger values are positive. The left column shows the ratio of the posterior to the prior full water vapor column, while the right column shows the ratio of the posterior to the prior partitioning indices. The top row visualizes the data on a latitude by Ls grid, while the bottom row distributes the data on a latitude-longitude map. The data has been averaged in bins of 2° latitude, 2° Ls and 4° longitude.
The vertical confinement displays the opposite behavior, with the synergy often finding a PI comparable to the MCD at mid and high latitudes. Note that during the sublimation season the MCD quite accurately reproduces the observations, indicating that the sublimation processes of the NPC are quite well understood in terms of vertical distribution. The PI difference is highest at low and middle latitudes in late summer, when large amounts of water vapor are being transported from the NH and across the equator. At most, the synergy PI at low-latitudes in late summer is almost twice as strong as model predictions. This is in general a fairly dry area, with a CIA of 10-15 pr-μm, where the synergy indicates that roughly 60% of the column is confined below 5 km. The atmospheric behavior in this region is less dominated by temperature and more affected by wind. The details of local air flow patterns are typically known with less certainty than temperature variations, which could explain why the model deviates the most from the observations at low and mid-latitudes.
4.5 Seasonal evolution of water distribution with latitude
The seasonal variations of the CIA and PI can be visualized by zonal averages plotted as a function of latitude. All data points in Figures 11 and 12 illustrate the CIA and PI averaged in bins of 2° latitude and 15° intervals of Ls, and the curves are smoothed using a Savitzky-Golay low-pass filter with a second order polynomial and a window corresponding to 20° latitude. Curves covering the same seasonal periods have identical colors for both hemispheres to aid cross-hemispherical comparison (for example, the red curve corresponds to mid-spring for both hemispheres; Ls=45°-60° in the NH and Ls=225°-240° in the SH).
Both hemispheres are fairly dry from the equator to mid-latitudes during the spring-summer season. The SH displays a smaller spread in seasonal variation and a smaller increase with latitude compared to the NH, remaining at around 10 pr-μm from the equator to 40°S. From there, the water column starts to increase steadily. Overall, the synergy and MCD agree very well in the SH, with the most noticable difference being the degree of seasonal spread, distinguishable at all latitudes in the synergy, while only becoming distinguishable after 40°S for the MCD. All synergistically retrieved seasonal curves show a southern maximum which is migrating poleward with season, matched well by the MCD. The exception is the first seasonal average in mid-spring (Ls=225°-240°) which displays a continuously decreasing curve, with the highest value at equator for the synergy, while the MCD finds a weak maximum of 15 pr-μm at 70°S for the same season. The SH sublimation season maximum occurs during Ls=285°-300°, with a maximum value of 34 pr-μm near 87°S.