Drivers of PMC Variability from SOFIE and the Implications for Long-Term PMC Change from SBUV

Abstract:

Observations of polar mesospheric clouds (PMC), temperature, and water vapor from the Solar Occultation For Ice Experiment (SOFIE) on NASA’s Aeronomy of Ice in the Mesosphere satellite are used to quantify the sensitivity of PMCs to their environment. These results are extended to 35 years of PMC observations by the Solar Backscatter Ultraviolet (SBUV), indicating that the bright PMCs detected by SBUV are ~half as sensitive to environmental changes as the entire PMC population detected by SOFIE. A new approach for retrieving PMC vertical column ice mass abundance (or ice water content, IWC) from SBUV measurements was developed. The method employs a rigorous forward model that accounts for both ice particle size and shape in the interpretation of SBUV observations. The SBUV multi-wavelength measurements (252 - 283 nm) are found to contain information sufficient for inferring ice particle size, and this information is included to self-consistently retrieve IWC from the SBUV data. For the subset of bright PMCs observed by SBUV, SBUV PMC radii are within 23% of SOFIE on average and SBUV IWC is within 8% of SOFIE. The analysis is extended to the 35-year SBUV record of IWC by limiting the time series to between 9 and 15 local solar time, to avoid introducing tidally induced variations to the time-series. SBUV results near 70 degrees North latitude indicate a statistically significant increasing IWC trend of ~4% per decade, and no significant trend in particle size. This IWC trend can be explained in part by a decline in solar Lyman-alpha flux since 1979, and in part by stratospheric forcing.