Early results from HadGEM2-A CFMIP-2
experiments
M. J. Webb, A.P.Lock, A. Bodas-Salcedo, Y. Tsushima, T.Andrews
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.
2a) Cloud fraction and Relative Humidity [137W,26N] July 1979-2008
3a) AMIP Temperature Tendencies [137W,26N] July 1979-2008
Height (km)
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
2b) Frequency distribution of instantaneous cloud fraction [137W,26N]
AMIP Control July 1979-2008
3b) AMIP +4K Pattern Temperature Tendencies [137W,26N]
Frequency of Cloud Fraction
1c) +4K Patterned SST Net CRE response
and CFMIP time-step output locations
0.23 Wm-2K-1
2c) Frequency distribution of instantaneous cloud fraction [137W,26N]
AMIP + 4K Patterned SST July 1979-2008
cloud fraction
• 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.
• 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.
Frequency of 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)
• 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 separate
the effects of stratocumulus vs trade cumulus feedbacks.
Tendency term analysis can be a useful tool for generating and
selecting hypotheses relating to physical mechanisms.
The suggested role of changes in free tropospheric RH has
implications for CGILS, in which free tropospheric RH is fixed.
References
Met Office Hadley Centre, FitzRoy Road, Exeter, Devon, EX1 3PB United Kingdom
Tel: +44 (0)1392 886905 Fax: +44 (0)1392 885681
Email: mark.webb@metoffice.gov.uk
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Stevens, B. & Brenguier, J.-L. in Clouds in the Perturbed Climate
System: Their Relationship to Energy Balance, Atmospheric Dynamics
and Precipitation (eds Heintzenberg, J. & Charlson, R. J.) Ch. 8 (MIT
Press, 2009)