A 38 year record of UV cloud albedo from SBUV and OMPS instruments: inter-satellite calibration, trends and preliminary comparison with climate models

Black-sky Cloud Albedo (BCA) equatorward of 45^o is derived from satellite UV radiance (340nm) observations from NOAA and NASA instruments to infer long-term (1980-2018) short-wave (SW) radiation changes induced by volcanic eruptions and the El Niño-Southern Oscillation (ENSO). The derived BCA assumes either a C-1 water cloud or an ice and water cloud model with varying cloud optical depths and a Cox-Munk surface BRDF over ocean. The UV radiance measurements are calibrated consistently over the East Antarctic Plateau and Greenland during summer.

This record merges seven drifting NOAA SBUV satellites with the stable orbit Nimbus-7 SBUV and the Suomi NPPOzone M appingP rofiler S uite Nadir mapper. When temporally overlapping instruments observe the same location, there are large differences if the instruments measure at different local times. We significantly reduce these inter-satellite disparities by implementing an adjustment method that accounts for the diurnal cycle of clouds based on the BCA frequency distributions. We estimate an 'overlap uncertainty' based on the remaining disparities between overlapping instruments.

We explore uncertainties in our BCA retrieval algorithm caused by errors in assumed cloud height, effective droplet radius or cloud phase. Mischaracterizing the cloud phase contributes uncertainty, but is still within the range of our 'overlap uncertainty'.

We estimate a peak SW forcing reduction of 4 and 6 Wm^-2 for Pinatubo and El Chichon respectively. A linear fit of our SW cloud albedo forcing with global surface temperature yields a slope of -0.65 (.4) Wm^-2K^-1over 45^oS to 45^oN.

We present a geographic map of trends in SW cloud albedo forcing over the 38-year data record, during which the global mean surface temperature warmed about +0.5 C, and compare it with trends from recent CMIP-5/-6 climate models.