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Tuesday, 15 September 2015
11 a.m.
Mesa Lab, Main Seminar Room
NCAR, 1850 Table Mesa Drive
Kevin Trenberth, CGD/CAS
Co-variability of top-of-atmosphere
radiation, temperature and
precipitation: observations and
CESM
Using correlations and regressions a detailed examination is made of monthly mean atmospheric
temperatures throughout the atmosphere and water vapor from ERA-Interim reanalyses, top-of-atmosphere
outgoing longwave (OLR), absorbed shortwave (ASR) and net radiation (RT = ASR-OLR) from CERES, and
precipitation from GPCP. Both global and local relationships related to feedbacks and climate sensitivity are
explored. As well as observations the relationships are evaluated in NCAR’s CESM large ensemble (30
members) and an AMIP run where the surface SSTs are specified. There is a lot more high frequency variability
in radiative fluxes than in temperatures, highlighting the role of clouds and transient weather systems in the
radiation statistics, with implications for climate sensitivity. The model prefers to have positive anomalies in
precipitation with positive SST anomalies, but this is not true in some ocean areas in nature. The relationship
between water vapor and temperatures in subsiding air in the tropics is not replicated well. Over the tropical
western Pacific warm pool and Indian oceans, where non-local effects from the Walker circulation driven by
the ENSO events are important, several related biases emerge: in response to high SST anomalies there is
more precipitation, water vapor and cloud, and less ASR and OLR in the model than in the real world. There is
too much ASR over the southern oceans and not enough in the tropics, and ENSO is too large in amplitude in
this version of the model. However, the co-variability of monthly mean anomalies in the NCAR CESM runs,
both in fully coupled mode and for AMIP, reproduces remarkably well most of the observed relationships
examined. In the observations, tropospheric vertically-averaged temperatures (surface-150 hPa) are
negatively correlated globally with net radiation (-0.57), implying 2.18±0.10 W m-2 extra net radiation to
space for 1°C increase in temperature. There is a stronger radiative damping for temperatures in the
observations than implied by the model versions. This could imply that the model sensitivity is too high but it
needs to be confirmed with longer datasets that avoid volcanic eruptions.
Seminars are live webcast: http://www.fin.ucar.edu/it/mms/ml-live.htm
For more information, contact Gaylynn Potemkin, email potemkin@ucar.edu, phone: 303.497.1618