The Effect of Diurnal Variations on Decadal Trends in PMC Brightness

The Solar Backscatter Ultraviolet (SBUV and SBUV/2) instruments on the NOAA polar-orbiting meteorological satellites have shown that there has been an increase in the average brightness of Polar Mesospheric Clouds (PMC) since 1978 [Thomas et al. 2003]. However, the equator-crossing times of most different satellite platforms have drifted over the life of the satellite, resulting in corresponding variations in the local time of the PMC observations. Measurements from the WINDII instrument on UARS [Shettle et al. 2002], as well several lidar groups have reported diurnal variations in the PMC brightness. This raises the possibility that systematic changes in the local time of the PMC measurements SBUV over the past 25 years have contributed to the reported brightness trends. Each SBUV instrument provides PMC observations at different local times on the ascending and descending nodes of the satellite, and with two or three different instruments in orbit over most of this period there are up to six different local times of measurement on a given day (for each latitude band). We can use these multiple daily measurements to separate the effects of any systematic changes in the local time of the measurements from any long-term trends. Preliminary results indicate that there is still a multi-decadal increase in the average brightness of the PMCs independent of any systematic changes in the local time of the measurements.