Latitude-dependent long-term variations in polar mesospheric clouds from SBUV version 3 PMC data

[1] Previous studies have suggested that there should be secular trends in polar mesospheric cloud (PMC) occurrence frequency and brightness on decadal timescales and that those trends would be strongest at the lowest latitudes of the PMC existence region. We have analyzed the 27-year PMC data set created from Solar Backscatter Ultraviolet (SBUV, SBUV/2) satellite instruments for long-term variations in albedo using three latitude bands (50 degrees- 64 degrees, 64 degrees- 74 degrees, 74 degrees- 82 degrees). The improved version 3 data set includes revisions to the PMC detection algorithm to produce more consistent results in all measurement conditions. A detailed error analysis yields an approximate uncertainty of 1 - 2% for seasonally averaged 252-nm albedo values. Adjustments for local time variations in PMC brightness between different satellite data sets were derived to ensure accurate trend calculations. Multiple linear regression fits show that albedo variations are anticorrelated with solar activity in all latitude bands, with a stronger response at high latitudes. The albedo increase from solar maximum to solar minimum ranges from + 2% at 50 degrees- 64 degrees, to + 17% at 74 degrees- 82 degrees N. Secular trends in albedo are positive, with long-term changes over 27 years ranging from + 12% to + 20% depending on hemisphere and latitude. The derived long-term trend in PMC albedo at 50 degrees- 64 degrees is smaller than that of higher latitudes. This result contradicts previous suggestions that PMC brightness changes might be most rapid at low latitudes. The albedo response to solar variations is larger in the Northern Hemisphere, while long-term trends are approximately the same in both hemispheres.