Abstract
Abstract. Large-eddy simulations of a pocket of open cells (POC) based on VOCALS Regional Experiment (REx) NSF C-130 Research Flight 06 are analyzed and compared with aircraft observations. A doubly-periodic domain 192 km × 24 km with 125 m horizontal and 5 m vertical grid spacing near the capping inversion is used. The POC is realized in the model as a fixed 96 km wide region of reduced cloud droplet number concentration (Nc) based on observed values; initialization and forcing are otherwise uniform across the domain. The model reproduces aircraft-observed differences in boundary-layer structure and precipitation organization between a well-mixed overcast region and a decoupled POC with open-cell precipitating cumuli, although the simulated cloud cover is too large in the POC. A sensitivity study in which Nc is allowed to advect following the turbulent flow gives nearly identical results over the 16 h length of the simulation (which starts at night and goes into the next afternoon). The simulated entrainment rate is nearly a factor of two smaller in the less turbulent POC than in the more turbulent overcast region. However, the inversion rises at a nearly uniform rate across the domain because powerful buoyancy restoring forces counteract horizontal inversion height gradients. A secondary circulation develops in the model that diverts subsiding free-tropospheric air away from the POC into the surrounding overcast region, counterbalancing the weaker entrainment in the POC with locally weaker subsidence.
Highlights
Pockets of open cells (POCs; Bretherton et al, 2004; Stevens et al, 2005) embedded in regions of unbroken stratocumulus are symptomatic of aerosol-cloud-precipitation feedbacks in marine boundary layer (MBL) structure and cloud response, with stark differences in mesoscale organization, dynamics, and microphysics rapidly evolving locally between the pocket of open cells (POC) and the surrounding overcast MBL
We investigate the dynamics that arise at the boundary of the POC and examine the sensitivity of the dynamics to two simplified treatments for the number concentration (Nc) distribution, primarily examining the case in which Nc is held fixed in the POC and overcast at different, observationally representative values
While this framework cannot address the detailed complexities of cloud-aerosol interactions, the dynamics arising from a simple microphysical gradient are a reasonable starting point for analyzing the mesoscale dynamics arising within the POC/overcast system
Summary
Pockets of open cells (POCs; Bretherton et al, 2004; Stevens et al, 2005) embedded in regions of unbroken stratocumulus are symptomatic of aerosol-cloud-precipitation feedbacks in marine boundary layer (MBL) structure and cloud response, with stark differences in mesoscale organization, dynamics, and microphysics rapidly evolving locally between the POC and the surrounding overcast MBL. We investigate the dynamics that arise at the boundary of the POC and examine the sensitivity of the dynamics to two simplified treatments for the Nc distribution, primarily examining the case in which Nc is held fixed in the POC and overcast at different, observationally representative values While this framework cannot address the detailed complexities of cloud-aerosol interactions, the dynamics arising from a simple microphysical gradient are a reasonable starting point for analyzing the mesoscale dynamics arising within the POC/overcast system. We look at entrainment differences and POC-scale circulations between the overcast and open cellular regions
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