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We have run a full set of CFMIP-2 experiments with HadGEM2-A, an atmosphere-only version of the HadGEM2-ES Earth System Model. These include an AMIP control forced with observed SSTs, a 4xCO2 perturbation experiment with observed SSTs, uniform and patterned +4K SST perturbation experiments, and aquaplanet equivalents. All include CFMIP Observation Simulator Package outputs, temperature, humidity and cloud increments and time-step outputs at 119 locations. Here we show some preliminary results to illustrate the use of increments and time-step outputs to investigate feedback mechanisms.
• Fig 1a shows that the cloud component of the CO2 forcing is consistent with what would be expected from cloud masking in the global mean (roughly -0.5 Wm-2), but there is a suggestion of positive cloud adjustment over land and the trade wind regions, and a negative adjustment over mid-latitude oceans and some stratocumulus to cumulus transition regions.
• The SST forced experiments show strong positive cloud feedbacks in the subtropical stratocumulus regions. Negative cloud feedbacks are seen off the edge of the sea ice in the Southern Ocean. The patterned SST experiment has a less positive feedback overall because of a less positive feedback poleward of 40oS and a less extensive area of positive feedback in the south-east Tropical Pacific.
• Instantaneous time-step data have been saved from all of the experiments at the 119 locations shown on Fig 1c. We focus on [137W,26N] (location shown with a square) which shows a strong positive feedback in patterned and uniform +4K experiments.
1a) CFMIP-2 4CO2 response in Net Cloud Radiative Effect (CRE) 30 year annual mean Global 2CO2 Equivalent = -0.44 Wm-2
1b) +4K Uniform SST Net CRE response 30 year annual mean. Global Cloud Feedback = 0.31 Wm-2K-1
1c) +4K Patterned SST Net CRE response 0.23 Wm-2K-1
and CFMIP time-step output locations 2c) Frequency distribution of instantaneous cloud fraction [137W,26N] AMIP + 4K Patterned SST July 1979-2008
2b) Frequency distribution of instantaneous cloud fraction [137W,26N] AMIP Control July 1979-2008
Hei
ght
(km
)
Frequency of Cloud Fraction
cloud fraction
• Fig 2a shows a substantial reduction in boundary layer cloud fraction at 1km in the +4K patterned SST experiment at [137W,26N]. Relative humidity is reduced by 5% at 1km. Free tropospheric relative humidity increases, presumably due to remote processes such as deep convection.
• Although the long term mean cloud fraction is less than 0.5 (Fig 2a), a histogram of instantaneous time-step outputs (Fig 2b) shows that instantaneous cloud fractions are mostly either close to unity (stratocumulus diagnosed by the large scale cloud scheme) or less than 0.2.
• In the +4K Patterned SST experiment, the frequency of occurrence of stratocumulus is greatly reduced, contributing to the strongly positive cloud feedback in this location. (Fig 2c)
3a) AMIP Temperature Tendencies [137W,26N] July 1979-2008
3b) AMIP +4K Pattern Temperature Tendencies [137W,26N]
• Fig 3a shows temperature tendencies at the same location for the AMIP run. At 1km (the level of maximum cloud) the longwave radiative cooling is balanced mainly by heating from the boundary layer and large scale cloud schemes, and also shortwave absorption. The boundary layer scheme heats the cloud layer by mixing warm air from below and by entraining warm air from above, while the cloud scheme heats it via latent heat release due to cloud condensation.
• Fig 3b shows the tendencies for the patterned SST perturbation experiment. We see large reductions in longwave cooling and in the heating from the boundary layer and cloud schemes at 1km.
• We have considered a number of physical mechanisms and ruled out several which are inconsistent with temperature and humidity tendencies changes – for example changes in shallow convection.
• Our results are consistent with a mechanism proposed by Stevens and Brenguier (2009) whereby increases in free tropospheric humidity inhibit cloud top cooling, breaking up stratocumulus clouds. We plan to test this hypothesis by repeating our experiments with fixed free tropospheric relative humidity.
Conclusions
High frequency instantaneous model outputs can be used to separatethe effects of stratocumulus vs trade cumulus feedbacks.
Tendency term analysis can be a useful tool for generating andselecting hypotheses relating to physical mechanisms.
The suggested role of changes in free tropospheric RH hasimplications for CGILS, in which free tropospheric RH is fixed.
References
Stevens, B. & Brenguier, J.-L. in Clouds in the Perturbed ClimateSystem: Their Relationship to Energy Balance, Atmospheric Dynamicsand Precipitation (eds Heintzenberg, J. & Charlson, R. J.) Ch. 8 (MITPress, 2009)
Frequency of Cloud Fraction
2a) Cloud fraction and Relative Humidity [137W,26N] July 1979-2008
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Early results from HadGEM2-A CFMIP-2 experimentsM. J. Webb, A.P.Lock, A. Bodas-Salcedo, Y. Tsushima, T.Andrews
