A 38 year record of UV cloud albedo from SBUV, TOMS and OMPS instruments: inter-satellite calibration, trends and response in cloudiness during El Nino events
Abstract
UV sensors have unique sensitivity to atmospheric clouds that reveal a morphology different from IR and Vis sensors. How the UV-sensible cloud albedo changes in the past decades has an important implication for understanding climate variability and radiative forcing. To quantify global cloud variability, we derive a long-term (1980-2016) inter-calibrated record of Hemispheric Cloud Albedo (HCA) from the suite of NOAA and NASA UV (331-380nm) sensing satellite instruments. Our derived HCA assumes a C-1 water cloud at varying cloud optical depths, a Cox-Munk surface BRDF over ocean and a lambertian surface over land.
The satellite observed radiances, prior to HCA calculation, are calibrated over the East Antarctic Plateau during summer. Here we assume there has been no long-term change in the ice albedo since 1980 and adjust the radiances accordingly. While the Antarctic-adjusted intensities over the Greenland ice cap show no long-term trend, there are sometimes small reductions in radiances in specific years as seen by multiple instruments. This suggests there are geophysical events that darken an ice sheet and temporarily reduce the intensities. We infer an uncertainty of +/-0.5% in the observed intensities over both ice sheets which is equivalent to +/-1.8% in global mean HCA, and cloud albedo forcing uncertainty of 0.77 Watts m-2 . Over the course of the 38-year record five El Nino events occur and global mean surface temperatures warm about +0.5 C. So each successive El Nino event occurs under warmer background conditions. During the five El Nino events (1982, 1986, 1998, 2010, 2016) there was a marked positive response in HCA over the pool of warmer ocean water in the Eastern Tropical Pacific (180oW - 100oW, 15o S- 15oN) and the timing is in agreement with the Multi-variate ENSO index. However, there was an equally compensating negative HCA response in the Western Tropical Pacific. The result is that a time-series of HCA over larger areas (30oS - 30oN) shows no HCA response during El Nino conditions; instead, the only anomalies are from the Pinatubo and El Chichon volcanic eruptions. Over ocean there is also no detectable HCA trend from -30oS to 30oN over the 38-year record despite the warming surface temperatures. We have less confidence in our land product but do see a long-term decrease in cloudiness.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A11I2336W
- Keywords:
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- 0319 Cloud optics;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0320 Cloud physics and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSESDE: 3360 Remote sensing;
- ATMOSPHERIC PROCESSES