Interpretation of the positive low-cloud feedback predicted by a climate model under global warming
Florent Brient
0
1
Sandrine Bony
0
1
0
F. Brient (&) S. Bony Laboratoire de Meteorologie Dynamique (LMD/IPSL), Universite Pierre et Marie Curie
, CNRS, 4 Place Jussieu, Mailbox 99, 75252 Paris cedex 05,
France
1
F. Brient, S. Bony: Interpretation of positive low-cloud feedback
The response of low-level clouds to climate change has been identified as a major contributor to the uncertainty in climate sensitivity estimates among climate models. By analyzing the behaviour of low-level clouds in a hierarchy of models (coupled ocean-atmosphere model, atmospheric general circulation model, aqua-planet model, single-column model) using the same physical parameterizations, this study proposes an interpretation of the strong positive low-cloud feedback predicted by the IPSL-CM5A climate model under climate change. In a warmer climate, the model predicts an enhanced clear-sky radiative cooling, stronger surface turbulent fluxes, a deepening and a drying of the planetary boundary layer, and a decrease of tropical low-clouds in regimes of weak subsidence. We show that the decrease of low-level clouds critically depends on the change in the vertical advection of moist static energy from the free troposphere to the boundary-layer. This change is dominated by variations in the vertical gradient of moist static energy between the surface and the free troposphere just above the boundary-layer. In a warmer climate, the thermodynamical relationship of Clausius-Clapeyron increases this vertical gradient, and then the import by large-scale subsidence of low moist static energy and dry air into the boundary layer. This results in a decrease of the low-level cloudiness and in a weakening of the radiative cooling of the boundary layer by low-level clouds. The energetic framework proposed in this study might help to interpret inter-model differences in low-cloud feedbacks under climate change.
1 Introduction
As reported by the 4th Assessment Report (AR4) of the
Intergovernmental Panel on Climate Change, current
climate models still exhibit a wide range of climate sensitivity
estimates (Solomon et al. 2007). Inter-model differences in
cloud-climate feedbacks remain the main cause of these
inter-model differences (Soden and Held 2006), with a
large contribution from low-level cloud feedbacks (Bony
and Dufresne 2005; Bony et al. 2006; Webb et al. 2006).
The relative credibility of the different low-cloud
feedbacks predicted by climate models has not been firmly
established so far, although an observational study
combined with an analysis of model simulations suggests some
evidence for a positive low-level cloud feedback (Clement
et al. 2009).
The difficulty of assessing the credibility of low-cloud
feedbacks in climate models stems in part from the large
number of processes and scales potentially involved in
these feedbacks. Identifying and prioritizing better the
primary physical controls of low-cloud feedbacks, at least
in the world of climate models, would help to design
relevant targeted process-oriented observational tests to
assess these feedbacks. With this motivation in mind, the
aim of this study is to analyze the physical mechanisms that
primarily control the low-cloud feedback predicted by the
IPSL-CM5A climate model, a model participating both in
the Coupled Models Intercomparison Project Phase 3
(CMIP3, Meehl et al. 2007) and Phase 5 (CMIP5, Taylor
et al. submitted) and characterized by a strongly positive
cloud feedback (Soden and Held 2006) and a high climate
sensitivity (Randall et al. 2007). The strong cloud feedback
of this model originating mostly from low-latitudes, we
will focus here on the analysis of the model cloud response
to global warming in the tropics.
To identify the physical mechanisms likely to control
low-level cloud feedbacks at first order, one approach
consists in using simple or conceptual models whose
physical characteristics can be readily comprehended (e.g.
Miller 1997; Larson et al. 1999). However this approach
may not necessarily be relevant to understand the cloud
feedbacks that actually operate in climate models. An
in-depth analysis of climate model outputs such as that
undergone by Wyant et al. (2009) may better reveal the
mechanisms at work in complex models. However, there
are so many processes potentially involved in the control of
low-cloud feedbacks in coupled ocean-atmosphere general
circulation models (OAGCMs) that such an analysis
remains difficult.
To facilitate this analysis, our approach consists in
analyzing the response of tropical clouds to external
forcings in several simulations performed with the same set of
physical parameterizations but over a range of
configurations more or less idealized: coupled ocean-atmosphere
simulations run in a realistic configuration,
atmosphereonly simulations, aqua-planet simulations, and
onedimensional simulations. Previous studies have shown the
benefit of such an approach. For instance, by comparing
three-dimensional (3D) atmospheri (...truncated)