Fast stratocumulus time scale in mixed layer model and large eddy simulation

Abstract

A mixed layer model (MLM) and large eddy simulation (LES) are used to analyze the internal response time scales of a stratocumulus-topped boundary layer (STBL). Three separate time scales are identified: a slow time scale associated with boundary layer deepening (several days), an intermediate thermodynamic time scale (approximately 1 day), and a fast time scale (6-12 h) for cloud water path adjustment associated with an internal entrainment-liquid flux (ELF) feedback. The nocturnal DYCOMSII-RF01 case study is used to establish and interpret the previously unidentified fast STBL adjustment time scale with the MLM. The role of the entrainment closure is investigated by repeating the analysis with several different closures. Nearly every closure considered exhibits a fast time scale. Perturbations are applied to the well-mixed CGILS stratocumulus case in both MLM and LES in order to elicit a short time scale response. Purely radiative perturbations do not project strongly onto the fast scale, while perturbations to the free tropospheric humidity do. A 2K surface and atmospheric temperature perturbation also projects strongly onto the fast scale. We show that the ELF adjustment mechanism behind the fast time scale is responsible for much of the steady state liquid water path response in the perturbed case, acting as a cloud-thinning feedback mechanism in a uniformly warmed climate.

DNS and LES for Simulating Stratocumulus: Better Together

“…When does the mean entrainment velocity change? If local processes at or near the interface are controlling entrainment, this change should have little effect in time scales smaller than those necessary to change the mean properties of the boundary layer, namely, several large‐eddy turnover times (Jones et al, ; van Driel & Jonker, ). If, on the other hand, entrainment is mainly controlled by the kinetic energy of large turbulent eddies, entrainment should increase quicker in response to the increased buoyancy flux.…”

How large-scale subsidence affects stratocumulus transitions

“…3b). This typically causes the cloud layer to be more energetic eventually leading to enhanced entrainment (Nicholls and Turton, 1986;Jones et al, 2014).…”

The Change in Low Cloud Cover in a Warmed Climate Inferred from AIRS, MODIS, and ERA-Interim

“…Here we quantify the dependence of observed subtropical cloud cover on various large-scale predictors utilizing the record afforded to us by remote sensing and reanalysis at 8-day-average time scales. Clouds respond to changes in their environment on a time scale of hours to a week, making this selection of time scale appropriate (Eastman et al 2016;Jones et al 2014;Mauger and Norris 2010). This approach offers a useful comparison to studies utilizing monthly to interannual time Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/ JCLI-D-15-0734.s1.…”

“…If the time and space scales of the observations are too small, the relationships that govern the response of LCC to anthropogenically forced change may not emerge clearly because 1) it takes some time for clouds to respond to large-scale forcing, especially in the boundary layer (Eastman et al 2016;Jones et al 2014;Klein et al 1995;Mauger and Norris 2010), and 2) clouds and boundary layer properties are advected through space by the large-scale horizontal flow. These problems can be partly alleviated by using a Lagrangian analysis that follows the air in the boundary layer, but this only reveals that the boundary layer clouds are in a constant state of adjustment to the changing large-scale conditions and adapting to them (Eastman and Wood 2016).…”