Cloud Feedbacks in the climate system: A critical Review Graeme L. Stephens Journal of Climate, vol.18, 2005 Anton Darmenov Apr, 2006 The radiative- convective equilibrium paradigm REC: To first order, the atmosphere exists in a state of quasi balance between radiative cooling and the convective processes that give rise to latent and sensible heating. RCE and cloud feedback • Manabe and Strickler (1964) using simple RCE model pointed out that high cirrus clouds heat the surface by an amount affected by their height and emissivity whereas low clouds cool the surface. • They argued that this heating occurs when the emissivity exceeds about 0.5 (i.e., thicker cirrus). • The profile to the left is the profile of emissivity above which clouds warm the surface relative to the given clear-sky temperature profile (right). Cloud albedo and emission are related producing complicated The critical-blackness cloud experiment balance at the TOA! of Manabe and Strickler. RCE and cloud feedback • Stephens and Webster (1981): when treated in a physically consistent manner, linked via cloud water and ice paths and thus optical depth, then it is not the thicker cirrus that produces a surface heating but rather the thin cirrus. These kinds of experiments had less to do with defining actual cloud feedback and climate sensitivity per se but more to do with demonstrating the potential relevance of cloud–radiation interactions to the climate system. The change in equilibrium surface temperature as a function of cloud LWP and IWP for low, middle and high clouds RCE and cloud-resolving models • The early RCE studies were meant to represent, loosely, a quasi-global-mean state. • These simple models have been replaced by more complex GCM climate models although many of the sensitivities derived from RCE models were broadly replicated in GCMs. • More recently, however, the RCE paradigm has been revisited in a series of equilibrium experiments conducted using cloud-resolving models (CRMs). • The use of CRMs in these experiments offers a more self- consistent treatment of convection and related cloud- radiation processes than is possible with the simple RCE models. • To date, CRM–RCE experiments have been constructed as open systems of fixed SST. RCE and the Earth's hydrological cycle • RCE can also be defined in terms of the atmospheric energy budget. This budget, to first order, occurs as a balance between the radiative cooling of the atmosphere and latent heating associated with precipitation. • Thus RCE implies that the radiation balance of the atmosphere and the planetary hydrological cycle are connected. • If one accepts the simple hypothesis that the hydrological cycle adjusts to changes in the atmospheric radiative cooling, then we have a basis for interpreting how the hydrological cycle might change under global warming. • Both column water vapor and clouds are principal factors that influence the gross radiative budget of the atmosphere. Consequently, changes to clouds and water vapor that induce a change to the column atmospheric cooling will, in turn, produce compensating changes to the hydrological cycle. Regulation of tropical SSTs Tropical SSTs come under the influence of a runaway water vapor greenhouse effect and that a negative feedback must operate to limit that the climatological SSTs to about 30C. • A feedback induced by the coupling of meridional momentum transport, low level winds, and evaporation. • Feedbacks that combine the radiation balance and large-scale dynamics: large-scale dynamics produces a communication between the atmosphere above the warmest waters and deepest convection and the atmosphere above the cooler waters as part of the Walker circulation. • A cloud-radiation feedback involving the relationships between SST, deep convection, and detrained anvil cirrus and solar radiation. • It is practically impossible to verify the simplifying assumptions of the hypotheses in part because we cannot isolate those processes in the real world and in part due to the ambiguous nature of the proxy data used to examine processes. A systems perspective of cloud feedback • System is the entire global climate Input Output system composed of subsystems The system with connecting inputs and R(T; X1, X2,...) outputs. (∆T ) O • Input is the solar energy received from the sun (can vary on many time scales - diurnal,seasonal, and Radiative longer). Transfer Forcing • function The output of the system may also be expressed in a number of ways, ∆Q but usually this is taken to be the Control global temperature T ∆ Action S • Control action is responsible for Control system here is defined as activating the system to modulate an arrangement of connected the output now expressed as a physical components that act as an change T S entire self-regulating unit. Open vs closed system Output Output Input (∆T ) Input (∆T) The system O The system f R(T; X1, X2,...) R(T; X1, X2,...) Radiative Radiative Feedback Transfer Transfer Forcing function Forcing function F(∆T) S ∆Q ∆Q Control Control ∆ ∆ Action Action Open (energy balance) system – the Closed or feedback system - the action triggers control action is independent of output a response that modulates the radiative forcing. and the sensitivity is that which occurs Feedbacks can operate in a series, in parallel or without feedbacks as some combination if both. System identification • What is the system, its component processes and its “control action”? • What is the system output? Feedbacks are only meaningful when defined with respect to a given output. • How do we observe or otherwise quantify open and closed system responses in parallel?