Detailed Investigation of the Self-Aggregation of Convection in Cloud-Resolving Simulations

Abstract

In models of radiative-convective equilibrium it is known that convection can spontaneously aggregate into one single localized moist region if the domain is large enough. The large changes in the mean climate state and radiative fluxes accompanying this self-aggregation raise questions as to what simulations at lower resolutions with parameterized convection, in similar homogeneous geometries, should be expected to produce to be considered successful in mimicking a cloud-resolving model.The authors investigate this self-aggregation in a nonrotating, three-dimensional cloud-resolving model on a square domain without large-scale forcing. It is found that self-aggregation is sensitive not only to the domain size, but also to the horizontal resolution. With horizontally homogeneous initial conditions, convective aggregation only occurs on domains larger than about 200 km and with resolutions coarser than about 2 km in the model examined. The system exhibits hysteresis, so that with aggregated initial conditions, convection remains aggregated even at our finest resolution, 500 m, as long as the domain is greater than 200-300 km.The sensitivity of self-aggregation to resolution and domain size in this model is due to the sensitivity of the distribution of low clouds to these two parameters. Indeed, the mechanism responsible for the aggregation of convection is the dynamical response to the longwave radiative cooling from low clouds. Strong longwave cooling near cloud top in dry regions forces downward motion, which by continuity generates inflow near cloud top and near-surface outflow from dry regions. This circulation results in the net export of moist static energy from regions with low moist static energy, yielding a positive feedback.

Convective self‐aggregation, cold pools, and domain size

“…Figure 1 shows x-y snapshots of specific humidity q on day 60 at z = 500 m for various domain sizes L. Aggregation, with its trademark dry patches in regions of steady-state subsidence, is clearly evident for domain sizes larger than L 300 km. This is in rough agreement with the critical domain size of 200-300 km found in Bretherton et al [2005] and Muller and Held [2012].…”

“…The initial temperature profile was obtained from a small-domain (i.e., non-aggregated) simulation of radiative-convective equilibrium. We turned off large-scale dynamical forcings, set initial horizontal winds to zero, and nudged the horizontal-mean winds to zero over a time scale of two hours, both for consistency with previous work [Muller and Held, 2012;Bretherton et al, 2005] and because we found these adjustments conducive to aggregation.…”

“…The streamfunction may be naturally divided into a shallow circulation (confined below z 3500 m) and a deep circulation (extending above z 3500 m). Note that a secondary shallow circulation was also observed in Bretherton et al [2005] and Muller and Held [2012]. The contour spacings for each plot in Figure 4 are chosen so that each plot has the same number of contour lines.…”

“…This self-aggregated state has significantly different horizontal-mean properties than the quasi-homogeneous state from which it formed: it is significantly drier in the mean, for instance, and hence exhibits much stronger mean longwave radiative cooling (LRC). Horizontal variations in LRC are important as well, as differences in LRC between moist and dry columns play a critical role in the feedbacks responsible for selfaggregation; see Bretherton et al [2005] and Muller and Held [2012] for details.…”

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

“…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].…”