  From STIS observations of Uranus in 2012, we found that the methane volume
mixing ratio declined from about 4% at low latitudes to about 2% at 60 deg N
and beyond. This is similar to that found in the south polar regions in 2002,
in spite of what appears to be strikingly different convective activity in the
two regions. Keck and HST imaging observations close to equinox imply that the
depletions were simultaneously present in 2007, suggesting they are persistent
features. The depletions appear to be mainly restricted to the upper
troposphere, with depth increasing poleward from about 30 deg N, reaching ~4
bars at 45 deg N and perhaps much deeper at 70 deg N. The latitudinal
variations in degree and depth of the depletions are important constraints on
models of meridional circulation. Our observations are qualitatively consistent
with previously suggested circulation cells in which rising methane-rich gas at
low latitudes is dried out by condensation and sedimentation of methane ice
particles as the gas ascends to altitudes above the methane condensation level,
then is transported to high latitudes, where it descends and brings down
methane depleted gas. Since this cell would seem to inhibit formation of
condensation clouds in regions where clouds are actually inferred from spectral
modelling, it suggests that sparse localized convective events may be important
in cloud formation. The small-scale latitudinal variations we found in the
effective methane mixing ratio between 55 deg N and 82 deg N have significant
inverse correlations with zonal mean latitudinal variations in cloud
reflectivity in near-IR Keck images taken before and after the HST
observations. If the CH4/H2 absorption ratio variations are interpreted as
local variations in para fraction instead of methane mixing ratio, we find that
downwelling correlates with reduced cloud reflectivity.
