Mechanisms of marine low cloud sensitivity to idealized climate perturbations: A single‐LES exploration extending the CGILS cases

Climate change sensitivities of subtropical cloud‐topped marine boundary layers are analyzed using large‐eddy simulation (LES) of three CGILS cases of well‐mixed stratocumulus, cumulus under stratocumulus, and shallow cumulus cloud regimes, respectively. For each case, a steadily forced control simulation on a small horizontally doubly periodic domain is run 10–20 days into quasi‐steady state. The LES is rerun to steady state with forcings perturbed by changes in temperature, free‐tropospheric relative humidity (RH), CO2 concentration, subsidence, inversion stability, and wind speed; cloud responses to combined forcings superpose approximately linearly. For all three cloud regimes and 2× CO2 forcing perturbations estimated from the CMIP3 multimodel mean, the LES predicts positive shortwave cloud feedback, like most CMIP3 global climate models. At both stratocumulus locations, the cloud remains overcast but thins in the warmer, moister, CO2‐enhanced climate, due to the combined effects of an increased lower‐tropospheric vertical humidity gradient and an enhanced free‐tropospheric greenhouse effect that reduces the radiative driving of turbulence. Reduced subsidence due to weakening of tropical overturning circulations partly counteracts these two factors by raising the inversion and allowing the cloud layer to deepen. These compensating mechanisms may explain the large scatter in low cloud feedbacks predicted by climate models. CMIP3‐predicted changes in wind speed, inversion stability, and free‐tropospheric RH have lesser impacts on the cloud thickness. In the shallow cumulus regime, precipitation regulates the simulated boundary‐layer depth and vertical structure. Cloud‐droplet (aerosol) concentration limits the boundary‐layer depth and affects the simulated cloud feedbacks.

[1]  D. Randall,et al.  Conditional instability of the first kind upside-down. [in stratocumulus clouds] , 1980 .

[2]  J. Deardorff.  Cloud Top Entrainment Instability , 1980 .

[3]  S. Nicholls,et al.  An observational study of the structure of stratiform cloud sheets: Part II. Entrainment , 1986 .

[4]  S. Klein,et al.  The Seasonal Cycle of Low Stratiform Clouds , 1993 .

[5]  B. Albrecht.  Effects of precipitation on the thermodynamic structure of the trade wind boundary layer , 1993 .

[6]  D. Hartmann.  Global Physical Climatology , 1994 .

[7]  C. Bretherton,et al.  Numerical Simulations and a Conceptual Model of the Stratocumulus to Trade Cumulus Transition , 1997 .

[8]  E. Mlawer,et al.  Radiative transfer for inhomogeneous atmospheres: RRTM, a validated correlated-k model for the longwave , 1997 .

[9]  C. Bretherton,et al.  Moisture Transport, Lower-Tropospheric Stability, and Decoupling of Cloud-Topped Boundary Layers , 1997 .

[10]  M. Khairoutdinov,et al.  A New Cloud Physics Parameterization in a Large-Eddy Simulation Model of Marine Stratocumulus , 2000 .

[11]  B. Kendall.  Nonlinear Dynamics and Chaos , 2001 .

[12]  D. Randall,et al.  Cloud resolving modeling of the ARM summer 1997 IOP: Model formulation, results, uncertainties, and sensitivities , 2003 .

[13]  C. Bretherton,et al.  Evaluation of Large-Eddy Simulations via Observations of Nocturnal Marine Stratocumulus , 2005 .

[14]  S. Bony,et al.  Marine boundary layer clouds at the heart of tropical cloud feedback uncertainties in climate models , 2005 .

[15]  C. Bretherton,et al.  Intercomparison and interpretation of single-column model simulations of a nocturnal stratocumulus-topped marine boundary layer , 2005 .

[16]  C. Bretherton,et al.  On the Relationship between Stratiform Low Cloud Cover and Lower-Tropospheric Stability , 2006 .

[17]  C. Bretherton,et al.  Cloud droplet sedimentation, entrainment efficiency, and subtropical stratocumulus albedo , 2007 .

[18]  G. Vecchi,et al.  Global Warming and the Weakening of the Tropical Circulation , 2007 .

[19]  S. Solomon.  The Physical Science Basis : Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change , 2007 .

[20]  John F. B. Mitchell,et al.  THE WCRP CMIP3 Multimodel Dataset: A New Era in Climate Change Research , 2007 .

[21]  O. Edenhofer,et al.  Mitigation from a cross-sectoral perspective , 2007 .

[22]  B. Stevens,et al.  Understanding macrophysical outcomes of microphysical choices in simulations of shallow cumulus convection , 2008 .

[23]  Dale R. Durran,et al.  Selective monotonicity preservation in scalar advection , 2008, J. Comput. Phys..

[24]  S. Xie,et al.  Muted precipitation increase in global warming simulations: A surface evaporation perspective , 2008 .

[25]  C. Bretherton,et al.  Mechanisms of Low Cloud–Climate Feedback in Idealized Single-Column Simulations with the Community Atmospheric Model, Version 3 (CAM3) , 2008 .

[26]  C. Bretherton,et al.  Understanding Subtropical Low Cloud Response to a Warmer Climate in a Superparameterized Climate Model. , 2008 .

[27]  C. Bretherton,et al.  The sensitivity of stratocumulus-capped mixed layers to cloud droplet concentration: do LES and mixed-layer models agree? , 2009 .

[28]  N. Loeb,et al.  Reply to Comments on “Seasonal Variation of the Physical Properties of Marine Boundary Layer Clouds off the California Coast” , 2010 .

[29]  A. Lock.  Factors influencing cloud area at the capping inversion for shallow cumulus clouds , 2009 .

[30]  C. Bretherton,et al.  Subtropical Low Cloud Response to a Warmer Climate in a Superparameterized Climate Model. Part II: Column Modeling with a Cloud Resolving Model , 2009 .

[31]  C. Bretherton,et al.  Large-Eddy Simulations of a Drizzling, Stratocumulus-Topped Marine Boundary Layer , 2009 .

[32]  M. Cai,et al.  Stabilization of the Atmospheric Boundary Layer and the Muted Global Hydrological Cycle Response to Global Warming , 2009 .

[33]  C. Bretherton,et al.  Response of a Subtropical Stratocumulus-Capped Mixed Layer to Climate and Aerosol Changes , 2009 .

[34]  C. Bretherton,et al.  Subtropical Low Cloud Response to a Warmer Climate in a Superparameterized Climate Model. Part I: Regime Sorting and Physical Mechanisms , 2009 .

[35]  W. Landman.  Climate change 2007: the physical science basis , 2010 .

[36]  C. Bretherton,et al.  Slow Manifolds and Multiple Equilibria in Stratocumulus‐Capped Boundary Layers , 2010 .

[37]  Minghua Zhang,et al.  Cloud-Resolving Simulation of Low-Cloud Feedback to an Increase in Sea Surface Temperature , 2010 .

[38]  Y. Tsushima,et al.  Relative humidity changes in a warmer climate , 2010 .

[39]  B. Stevens,et al.  On the Factors Modulating the Stratocumulus to Cumulus Transitions , 2011 .

[40]  G. Vecchi,et al.  The vertical distribution of cloud feedback in coupled ocean‐atmosphere models , 2011 .

[41]  Chris T. Jones,et al.  Coupled vs. decoupled boundary layers in VOCALS-REx , 2011 .

[42]  B. Stevens,et al.  The influence of wind speed on shallow marine cumulus convection , 2012 .

[43]  D. Chung,et al.  Steady-State Large-Eddy Simulations to Study the Stratocumulus to Shallow Cumulus Cloud Transition , 2012 .

[44]  M. Webb,et al.  Coupling between subtropical cloud feedback and the local hydrological cycle in a climate model , 2013, Climate Dynamics.

[45]  S. Bony,et al.  Interpretation of the positive low-cloud feedback predicted by a climate model under global warming , 2013, Climate Dynamics.

[46]  C. Bretherton,et al.  Fast cloud adjustment to increasing CO2 in a superparameterized climate model , 2012 .

[47]  M. Webb,et al.  Origins of differences in climate sensitivity, forcing and feedback in climate models , 2013, Climate Dynamics.

[48]  B. Stevens,et al.  Marine Boundary Layer Cloud Feedbacks in a Constant Relative Humidity Atmosphere , 2012 .

[49]  C. Bretherton,et al.  Marine low cloud sensitivity to an idealized climate change: The CGILS LES intercomparison , 2013 .

[50]  S. Klein,et al.  CMIP3 Subtropical Stratocumulus Cloud Feedback Interpreted through a Mixed-Layer Model , 2013 .