Wednesday, September 26, 2012

New paper finds clouds over the Pacific have a strong negative-feedback cooling effect

A new paper published in Geophysical Research Letters finds from satellite observations that "Clouds over the Pacific warm pool have a longwave cooling effect for increases in sea surface temperatures." Thus, clouds changes act as a negative feedback to sea surface warming. Climate models, however, erroneously assume clouds and water vapor act as positive feedbacks. The authors find a "strong" increase in outgoing longwave radiation [OLR] to space of 15.72 W/m2 for each 1C change in sea surface temperature. By way of comparison, the IPCC alleges a doubling of CO2 will decrease or "trap" OLR by 3.7 W/m2, a factor of 4 less. According to the authors, "This atmospheric cooling effect is found to be primarily associated with reduced areal coverage of clouds (a 14.4% decrease in cloud cover per 1C change in sea surface temperature).

GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L18802, 6 PP., 2012
doi:10.1029/2012GL052700
Key Points
  • Cloud's noise effect on SST hinders estimation of the radiative feedback
  • Clear-sky SST helps meaningful estimation of the longwave response to SST change
  • Clouds over the Pacific warm pool have longwave cooling effect for increased SST
Heeje Cho
Computational Science and Technology, Seoul National University, Seoul, South Korea
Chang-Hoi Ho
Computational Science and Technology, Seoul National University, Seoul, South Korea
School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea
Center for Climate/Environment Change Prediction Research, Ewha Womans University, Seoul, South Korea
Yong-Sang Choi
Center for Climate/Environment Change Prediction Research, Ewha Womans University, Seoul, South Korea
Department of Environmental Science and Engineering, Ewha Womans University, Seoul, South Korea
This study investigated variations in outgoing longwave radiation (OLR) in response to changes in sea surface temperature (SST) over the Pacific warm pool area (20°N–20°S, 130°E–170°W). OLR values were obtained from recent (January 2008–June 2010) geostationary window channel imagery at hourly resolution, which resolves processes associated with tropical convective clouds. We used linear regression analysis with the domain-averaged OLR and SST anomalies (i.e., ΔOLR, ΔSST; deviations from their 90-day moving averages). Results show that the regression slope appears to be significant only with SST least-affected by cloud radiative forcing, for which SST needs to be obtained as daily average over cloud-free regions (ΔSSTclear). The estimated value of ΔOLRSSTclear is 15.72 W m−2 K−1, indicating the presence of strong outgoing longwave radiation in response to surface warming. This atmospheric cooling effect is found to be primarily associated with reduced areal coverage of clouds (−14.4% K−1).

1 comment:

  1. another paper showing a negative feedback cooling effect of clouds, which is underestimated by models:

    http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00370.1?af=R

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