LLNL-PRES-XXXXXX This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA Lawrence Livermore National Security, LLC April 24, 2013 IM # LLNL-PRES

Lawrence Livermore National Laboratory 2  Daily-mean output from 7 models for control run and 2xCO 2 run (5 years each) from CFMIP1 archive  Cloud optical thickness from ISCCP simulator  Derive cloud-top temperature from atmospheric temperature on model levels  Analyze all clouds below 680mb, with clear-sky above

Lawrence Livermore National Laboratory 3  We see a pattern of increases in optical depth at high latitudes, with opposite, but small, changes in the tropics and subtropics  Similar to pattern reported by Zelinka et al. (2012)

Lawrence Livermore National Laboratory 4  Can models help us understand how and why clouds change as the climate warms?  Can information on the way cloud feedbacks operate in our current climate provide information on how they will affect climate change?  Can climate models replicate the observed relationships between optical thickness and temperature?

Lawrence Livermore National Laboratory 5  The relationship between cloud-top temperature and cloud optical thickness for low clouds, sorted in 15K bins of cloud-top temperature (from Tselioudis et al., 1992)

Lawrence Livermore National Laboratory 6  The relationship between optical depth and cloud-top temperature for low clouds in 7 models (individual dots) and the multi-model average (solid line) in the control climate

Lawrence Livermore National Laboratory 7  By using the following parameterization of optical depth (from Stephens, 1978): we can derive what portion of the feedback comes from changes in cloud water content and cloud physical thickness (r e not an output from any model)

Lawrence Livermore National Laboratory 8  Now for regression with cloud physical thickness. Very little changes, except for warm clouds in the tropics.

Lawrence Livermore National Laboratory 9  Regression of cloud liquid and ice water content with surface temperature. Much of the temperature- optical depth relationship can be explained by adiabatic changes in cloud water.

Lawrence Livermore National Laboratory 10  To understand this as a feedback, we need to compare optical depth with surface air temperature; we see similar relationships at high latitudes, but not a positive feedback in the low latitudes

Lawrence Livermore National Laboratory 11 Cloud liquid water content Cloud physical thickness Cloud optical depth  How do surface- based observations compare

Lawrence Livermore National Laboratory 12  Do feedbacks in the control climate compare to climate change feedbacks?  Take difference in optical depth between 2xCO 2 and control climate for each grid box, then divide by global mean surface temperature change  Next we compare the feedback calculated for the control climate to that for climate change, separately for each region

Lawrence Livermore National Laboratory 13  Value of feedback parameter in the control climate, and between the control and doubled CO 2 for each of the regions and for all regions together (orange dashed line)

Lawrence Livermore National Laboratory 14  The optical depth-cloud temperature relationship for low clouds in models is similar to that from observations  Optical depth variability is primarily driven by increase in cloud water content  Control climate feedback is good proxy for climate change response in models, but does tend to overestimate feedback strength

Thank You This work performed under the auspices of the U.S. Dept. of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA This research was supported by the Regional and Global Climate Program of the Office of Science at the U. S. Department of Energy.

Lawrence Livermore National Laboratory 16  A large portion of the change in physical thickness with cloud temperature can be explained by changes in cloud top.

Lawrence Livermore National Laboratory 17 from Zelinka et al. (2012)

Lawrence Livermore National Laboratory 18  There is a reduction in cloud base height at warmer cloud-top temperatures, but the relationships are less consistent.

Lawrence Livermore National Laboratory 19  Value of cloud water feedback parameter in the control climate, and between the control and doubled CO 2 for each of the regions and for all regions together (orange dashed line)

Lawrence Livermore National Laboratory 20  Change in cloud-top height divided by cloud temperature change  Change in cloud-top height if all temperature change was from lowering of cloud along adiabat