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

by H. Wang, G. Feingold, R. Wood, J. Kazil
Atmospheric Chemistry and Physics ()
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Microphysical and meteorological controls on the formation of open and\nclosed cellular structures in the Southeast Pacific are explored using\nmodel simulations based on aircraft observations during the VAMOS\nOcean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). The\neffectiveness of factors such as boundary-layer moisture and temperature\nperturbations, surface heat and moisture fluxes, large-scale vertical\nmotion and solar heating in promoting drizzle and open cell formation\nfor prescribed aerosol number concentrations is explored. For the case\nconsidered, drizzle and subsequent open cell formation over a broad\nregion are more sensitive to the observed boundary-layer moisture and\ntemperature perturbations (+0.9 g kg(-1); -1 K) than to a five-fold\ndecrease in aerosol number concentration (150 vs. 30 mg(-1)). When\nembedding the perturbations in closed cells, local drizzle and pockets\nof open cell (POC) formation respond faster to the aerosol reduction\nthan to the moisture increase, but the latter generates stronger and\nmore persistent drizzle. A local negative perturbation in temperature\ndrives a mesoscale circulation that prevents local drizzle formation but\npromotes it in a remote area where lower-level horizontal transport of\nmoisture is blocked and converges to enhance liquid water path. This\nrepresents a potential mechanism for POC formation in the Southeast\nPacific stratocumulus region whereby the circulation is triggered by\nstrong precipitation in adjacent broad regions of open cells. A\nsimulation that attempts to mimic the influence of a coastally induced\nupsidence wave results in an increase in cloud water but this alone is\ninsufficient to initiate drizzle. An increase of surface sensible heat\nflux is also effective in triggering local drizzle and POC formation.\nBoth open and closed cells simulated with observed initial conditions\nexhibit distinct diurnal variations in cloud properties. A stratocumulus\ndeck that breaks up due solely to solar heating can recover at night.\nPrecipitation in the open-cell cases depletes the aerosol to the extent\nthat cloud formation is significantly suppressed within one diurnal\ncycle. A replenishment rate of cloud condensation nuclei of order 1\nmg(-1) h(-1) is sufficient to maintain clouds and prevent the boundary\nlayer from collapsing the following day, suggesting that some local\nand/or remote aerosol sources is necessary for POCs to be able to last\nfor days.

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