An Energy-Balance Analysis of Deep Convective Self-Aggregation above Uniform SST

Abstract

The spatial organization of deep moist convection in radiative-convective equilibrium over a constant sea surface temperature is studied. A 100-day simulation is performed with a three-dimensional cloud-resolving model over a (576 km) 2 domain with no ambient rotation and no mean wind. The convection self-aggregates within 10 days into quasi-stationary mesoscale patches of dry, subsiding and moist, rainy air columns. The patches ultimately merge into a single intensely convecting moist patch surrounded by a broad region of very dry subsiding air.The self-aggregation is analyzed as an instability of a horizontally homogeneous convecting atmosphere driven by convection-water vapor-radiation feedbacks that systematically dry the drier air columns and moisten the moister air columns. Column-integrated heat, water, and moist static energy budgets over (72 km) 2 horizontal blocks show that this instability is primarily initiated by the reduced radiative cooling of air columns in which there is extensive anvil cirrus, augmented by enhanced surface latent and sensible heat fluxes under convectively active regions due to storm-induced gustiness. Mesoscale circulations intensify the later stages of self-aggregation by fluxing moist static energy from the dry to the moist regions. A simple mathematical model of the initial phase of self-aggregation is proposed based on the simulations.In accordance with this model, the self-aggregation can be suppressed by horizontally homogenizing the radiative cooling or surface fluxes. Lower-tropospheric wind shear leads to slightly slower and less pronounced self-aggregation into bands aligned along the shear vector. Self-aggregation is sensitive to the ice microphysical parameterization, which affects the location and extent of cirrus clouds and their radiative forcing. Self-aggregation is also sensitive to ambient Coriolis parameter f, and can induce spontaneous tropical cyclogenesis for large f. Inclusion of an interactive mixed-layer ocean slows but does not prevent self-aggregation.

Radiative convective equilibrium as a framework for studying the interaction between convection and its large‐scale environment

“…One clear sign of convective aggregation is an abnormally dry mean atmospheric state ( 20% RH between 2 and 10 km in Bretherton et al [2005] (Figure 4a, Tobin et al, 2012). This behavior is not pronounced in the present simulations (Figure 5), rather the relative humidity profiles are comparable to the tropical mean profiles of comprehensive GCM simulations.…”

“…Specific limitations of modeled clouds include both features which differ from what one observes (e.g., ''too few, too bright,'' [Webb et al, 2001;Nam et al, 2012]) as well as features which are inadequately understood (e.g., controls on cloud vertical structure [Nuijens et al, 2015], and organization [Bretherton et al, 2005;Muller and Held, 2012;Wing and Emanuel, 2014]). An important component of these limitations is the delicacy of the interactions between clouds and circulations Sherwood et al, 2014].…”

“…So doing allowed assumptions about the structure of the atmosphere, convection, and clouds to be relaxed as more atmospheric processes could be resolved by the simulations. These pioneering studies have come back into focus with the realization that convective aggregation arises spontaneously from simulations with an explicit representation of convection [e.g., Nakajima and Matsuno, 1988;Held et al, 1993;Tompkins and Craig, 1998;Bretherton et al, 2005;Muller and Held, 2012], in ways that depend on temperature , thereby raising the question as to its importance for the climate system Mauritsen and Stevens, 2015;Coppin and Bony, 2015].…”

An Observational View of Relationships Between Moisture Aggregation, Cloud, and Radiative Heating Profiles

“…It has been known for more than 20 years, however, that convection in numerical model simulations of radiative-convective equilibrium frequently gathers itself together, increasing in spatial scale until, under many circumstances, the entire domain contains only a single region of convection (Held et al 1993;Bretherton et al 2005;Stephens et al 2008;Wing and Emanuel 2014;Tompkins and Craig 1998). The phenomenon of convective 'self-aggregation' was originally noted in cloud-resolving models in which deep convection is explicit (Held et al 1993;Bretherton et al 2005;Stephens et al 2008;Wing and Emanuel 2014) but it also appears in global models (Bony et al 2016;Shi and Bretherton 2014;Reed et al 2015;Coppin and Bony 2015) in which convection is parameterized.…”

Precipitation distributions for explicit versus parametrized convection in a large‐domain high‐resolution tropical case study

“…Many studies have found a strong relationship between tropospheric water vapour and precipitation in observations (Bretherton et al, 2004;Peters and Neelin, 2006;Holloway and Neelin, 2009) and CSRMs (Grabowski, 2003;Derbyshire et al, 2004;Bretherton et al, 2005). However, parametrized convection usually lacks this strong relationship; in particular, models with parametrized convection tend to have too much light rain falling in relatively dry tropospheric conditions (Thayer-Calder and Randall, 2009, their figure 4).…”