Convective self‐aggregation, cold pools, and domain size

Convective self-aggregation, cold pools, and domain size Nadir Jeevanjee, 1,2 and David M. Romps, 1,3 Convective self-aggregation refers to a phenomenon in cloud-resolving simulations wherein the atmosphere spon- taneously develops a circulation with a convecting moist patch and a nonconvecting dry patch. All previous stud- ies have found a sharp transition to aggregated convection when the domain size exceeds a critical threshold, typically in the range of 200–300 km. Here, we show that cold pools are responsible for this sharp transition. When cold pools are inhibited, self-aggregation occurs at all domain sizes. In this case, the aggregation strength decreases smoothly as the domain size L is decreased below about 200–300 km. A streamfunction analysis reveals two distinct sources for the air subsiding into the dry-patch boundary layer: a moist, shallow circulation and a dry, deep circulation. The deep cir- culation scales with L, whereas the shallow circulation does not. At small L, the shallow circulation dominates, thereby weakening the aggregation. Citation: Jeevanjee, N., and D. see Bretherton et al. [2005] and Muller and Held [2012] for details. These two papers also investigated the sensitivity of aggregation to various parameters, one of these being the domain size L. Both studies found that quasi-homogeneous states will only self-aggregate if L & 200–300 km. The goal of this paper is to shed light on this domain-size dependence. This work is motivated by the routine use of CRMs as a benchmark in the evaluation of convective parameter- izations for global climate models. To improve the realism and statistics of CRM simulations, modelers have typically preferred the use of larger domains. Self-aggregation, how- ever, presents a catastrophic failure of CRMs to converge as a function of domain size, casting doubt on the reliability of CRMs as a benchmark. M. Romps (2013), Convective self-aggregation, cold pools, and domain size, Geophys. Res. Lett., 40, doi:10.1002/grl.50204. 2. Numerical Simulations All simulations in this paper were performed using Das Atmospharische Modell (DAM) [Romps, 2008]. DAM is a three-dimensional, fully compressible, nonhy- drostatic CRM that uses the six-class Lin-Lord-Kreuger microphysics scheme. For this paper, we performed runs on both three-dimensional (3-D) square domains as well as effectively two-dimensional (2-D) bowling-alley domains, all of various sizes. All domains were doubly periodic with a horizontal grid spacing of 3 km, and simulations were run over a nondynamical ocean with a fixed sea-surface temperature (SST) of 301 K. This choice of SST is typical of the tropical oceans and is the same as that used in Bretherton et al. [2005]. It is also squarely in the range of SSTs shown to be conducive to aggregation in Khairoutdinov and Emanuel [2010]. Surface enthalpy fluxes were calcu- lated using a bulk aerodynamic formula, and interactive shortwave and longwave radiative fluxes were calculated using RRTM. Each run was 60 days long and was initialized with an aggregated distribution of water vapor similar to that used in Muller and Held [2012]. Here, the lowest-level specific humidity q started at 16 g/kg in the center of the domain and decreased to 8 g/kg at the edge, and this horizontal distribution decreased exponentially with height with a scale height of 3 km. The initial horizon- tal q distribution was a radial function for the square domains and was linear along the long dimension in the bowling- alley domains. The initial temperature profile was obtained from a small-domain (i.e., non-aggregated) sim- ulation 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., 1. Introduction Organized convection, in which the spatial pattern of convection is fixed and persistent over time, has been found in both observations and numerical studies [Houze and Betts, 1981; Grabowski and Moncrieff, 2001; Stephens et al., 2008] and at a wide range of scales. In partic- ular, recent numerical studies [Bretherton et al., 2005; Khairoutdinov and Emanuel (Aggregation of convec- tion and the regulation of tropical climate; preprint: https://ams.confex.com/ams/pdfpapers/168418.pdf); Muller and Held, 2012] with cloud-resolving models (CRMs) have shown that horizontally quasi-homogeneous tropical con- vection can be unstable, yielding spontaneous development of a circulation featuring a moist, convecting patch and a dry, nonconvecting patch. This self-aggregated state has significantly different horizontal-mean properties than the quasi-homogeneous state from which it formed: it is signif- icantly 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 dif- ferences in LRC between moist and dry columns play a crit- ical role in the feedbacks responsible for self-aggregation; 1 Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 2 Department of Physics, UC Berkeley, Berkeley, CA, USA. 3 Department of Earth and Planetary Science, UC Berkeley, Berkeley, CA, USA. Corresponding author: N. Jeevanjee, Department of Physics, University of California at Berkeley, Berkeley, CA 94702, USA. (jeevanje@berkeley.edu)