Modelling microphysical and meteorological controls on precipitation and cloud cellular structures in Southeast Pacific stratocumulus

Abstract

Abstract. Microphysical and meteorological controls on the formation of open and closed cellular structures in the Southeast Pacific are explored using model simulations based on aircraft observations during the VAMOS Ocean-CloudAtmosphere-Land Study Regional Experiment (VOCALSREx). The effectiveness of factors such as boundary-layer moisture and temperature perturbations, surface heat and moisture fluxes, large-scale vertical motion and solar heating in promoting drizzle and open cell formation for prescribed aerosol number concentrations is explored. For the case considered, drizzle and subsequent open cell formation over a broad region are more sensitive to the observed boundarylayer moisture and temperature perturbations (+0.9 g kg −1 ; −1 K) than to a five-fold decrease in aerosol number concentration (150 vs. 30 mg −1 ). When embedding the perturbations in closed cells, local drizzle and pockets of open cell (POC) formation respond faster to the aerosol reduction than to the moisture increase, but the latter generates stronger and more persistent drizzle. A local negative perturbation in temperature drives a mesoscale circulation that prevents local drizzle formation but promotes it in a remote area where lower-level horizontal transport of moisture is blocked and converges to enhance liquid water path. This represents a potential mechanism for POC formation in the Southeast Pacific stratocumulus region whereby the circulation is triggered by strong precipitation in adjacent broad regions of open cells. A simulation that attempts to mimic the influence of a coastally induced upsidence wave results in an increaseCorrespondence to: H. Wang (hailong.wang@pnl.gov) in cloud water but this alone is insufficient to initiate drizzle. An increase of surface sensible heat flux is also effective in triggering local drizzle and POC formation.Both open and closed cells simulated with observed initial conditions exhibit distinct diurnal variations in cloud properties. A stratocumulus deck that breaks up due solely to solar heating can recover at night. Precipitation in the opencell cases depletes the aerosol to the extent that cloud formation is significantly suppressed within one diurnal cycle. A replenishment rate of cloud condensation nuclei of order 1 mg −1 h −1 is sufficient to maintain clouds and prevent the boundary layer from collapsing the following day, suggesting that some local and/or remote aerosol sources is necessary for POCs to be able to last for days.

A continuous spectral aerosol-droplet microphysics model

“…While it is often assumed that each evaporated droplet forms one aerosol particle, the size of the newly formed aerosol particle is unknown and must be assumed. Yin et al (2005), Wang et al (2010), and Xue et al (2010) demonstrate the significance of including aerosol regeneration in detailed cloud models, however the actual size of the regenerated particles is not known. The 2-D continuous bin microphysics scheme can be extended to include aqueous-phase chemistry and generate parameterizations for regenerated aerosol sizes.…”

“…The stratified and closed nature of marine stratocumulus presents a good testbed for the 2-D continuous spectral warm-phase microphysics model. Also, marine stratocumulus is a well-studied cloud type via both insitu measurements and modeling studies (e.g., Stevens et al, 1998Stevens et al, , 2003Stevens et al, , 2005Rotstayn and Liu, 2003b;Seinfeld, 2005, 2006;Bretherton et al, 2007;Sandu et al, 2008Sandu et al, , 2009Hill et al, 2008Hill et al, , 2009Ackerman et al, 2009;Shao and Liu, 2009;Wang et al, 2009;Wang and Feingold, 2009b;Wang et al, 2010Wang et al, , 2011.…”

Limited-are a modelling of stratocumulus over South-Eastern Pacific

“…Specific reasons for transitions from closed-to open-cell structure are difficult to determine from observations, and as a result large-eddy simulation (LES) numerical models are often used to investigate the transition (e.g., Savic-Jovcic and Stevens, 2008;Wang and Feingold, 2009a,b;Wang et al, 2010). Model simulations indicate that drizzle can trigger POC formations and subsequently accelerate this process by aerosol depletion, thus pointing to the importance of cloudaerosol interactions.…”

Modelling the effects of gravity waves on stratocumulus clouds observed during VOCALS-UK

“…Nevertheless, it does give some useful insights into the processes that cause the cloud to clear. Wang et al (2010) investigated aerosol perturbations on the generation of POCs using ∼ 100 km 2 -scale model simulations at 300 m horizontal resolution, finding that gradients in aerosol were indeed significant in promoting the development of POCs. This effect has not been investigated here.…”