Abstract
AbstractIdealized explicit convection simulations of the Met Office Unified Model exhibit spontaneous self‐aggregation in radiative‐convective equilibrium, as seen in other models in previous studies. This self‐aggregation is linked to feedbacks between radiation, surface fluxes, and convection, and the organization is intimately related to the evolution of the column water vapor field. Analysis of the budget of the spatial variance of column‐integrated frozen moist static energy (MSE), following Wing and Emanuel (2014), reveals that the direct radiative feedback (including significant cloud longwave effects) is dominant in both the initial development of self‐aggregation and the maintenance of an aggregated state. A low‐level circulation at intermediate stages of aggregation does appear to transport MSE from drier to moister regions, but this circulation is mostly balanced by other advective effects of opposite sign and is forced by horizontal anomalies of convective heating (not radiation). Sensitivity studies with either fixed prescribed radiative cooling, fixed prescribed surface fluxes, or both do not show full self‐aggregation from homogeneous initial conditions, though fixed surface fluxes do not disaggregate an initialized aggregated state. A sensitivity study in which rain evaporation is turned off shows more rapid self‐aggregation, while a run with this change plus fixed radiative cooling still shows strong self‐aggregation, supporting a “moisture‐memory” effect found in Muller and Bony (2015). Interestingly, self‐aggregation occurs even in simulations with sea surface temperatures (SSTs) of 295 and 290 K, with direct radiative feedbacks dominating the budget of MSE variance, in contrast to results in some previous studies.
Highlights
Self-aggregation in idealized models of radiative-convective equilibrium (RCE) has been studied extensively since at least Held et al [1993], who found that 2-D cloud-system resolving model (CRM) simulations of RCE organized into a single convective region surrounded by subsidence and suppressed conditions
Using the Met Office Unified Model at 4 km grid spacing in idealized RCE mode, we reproduce several findings of previous studies [e.g., Bretherton et al, 2005; Muller and Held, 2012; Wing and Emanuel, 2014] regarding the evolution of self-aggregation in idealized models, including the reduction of domain-mean column water vapor (CWV) with increasing aggregation, the increase of domainmean outgoing longwave radiation (OLR), the lack of change in reflected shortwave radiation, and the increase of Interquartile Range (IQR)
Selfaggregation has been found in several other models including the DAM [Jeevanjee and Romps, 2013] and RAMS [Stephens et al, 2008], but the present study attempts to make close comparisons to several recent studies, regarding mechanisms involved in self-aggregation
Summary
Self-aggregation in idealized models of radiative-convective equilibrium (RCE) has been studied extensively since at least Held et al [1993], who found that 2-D cloud-system resolving model (CRM) simulations of RCE organized into a single convective region surrounded by subsidence and suppressed conditions. Studies of nonrotating RCE with 3-D CRMs found that homogeneous initial conditions evolved into band-like convective structures, which contained larger amounts of tropospheric water vapor than in the surrounding subsidence bands, and this organization was sensitive to interactions between convection, surface fluxes, and radiation [Tompkins and Craig, 1998; Tompkins, 2001]. Bretherton et al [2005] found that homogenizing either surface fluxes or radiative heating suppressed the self-aggregation process They argued that a low-level circulation, forced by anomalous radiative cooling in the lower troposphere in drier regions, leads to an import of moist static energy into already moist regions, amplifying aggregation. Stephens et al [2008] found that interactive radiation was necessary for self-aggregation in large ‘‘bowling alley’’ CRM RCE runs, but they argued that interaction between upper-level cloud radiative warming and convection was the key feedback
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