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A comparative study of the response of modeled non-drizzling stratocumulus to meteorological and aerosol perturbations

by J. L. Petters, H. Jiang, G. Feingold, D. L. Rossiter, D. Khelif, L. C. Sloan, P. Y. Chuang
Atmospheric Chemistry and Physics ()

Abstract

The impact of changes in aerosol and cloud droplet concentration (N-a\nand N-d) on the radiative forcing of stratocumulus-topped boundary\nlayers (STBLs) has been widely studied. How these impacts compare to\nthose due to variations in meteorological context has not been\ninvestigated in a systematic fashion for non-drizzling overcast\nstratocumulus. In this study we examine the impact of observed\nvariations in meteorological context and aerosol state on daytime,\nnon-drizzling overcast stratiform evolution, and determine how resulting\nchanges in cloud properties compare.\nUsing large-eddy simulation (LES) we create a model base case of daytime\nsoutheast Pacific coastal stratocumulus, spanning a portion of the\ndiurnal cycle (early morning to near noon) and constrained by\nobservations taken during the VOCALS (VAMOS Ocean-Atmosphere-Land Study)\nfield campaign. We perturb aerosol and meteorological properties around\nthis base case to investigate the stratocumulus response. We determine\nperturbations in the cloud top jumps in potential temperature theta and\ntotal water mixing ratio q(t) from ECMWF Re-analysis Interim data, and\nuse a set of N-d values spanning the observable range. To determine the\ncloud response to these meteorological and aerosol perturbations, we\ncompute changes in liquid water path (LWP), bulk optical depth (tau) and\ncloud radiative forcing (CRF).\nWe find that realistic variations in the thermodynamic jump properties\ncan elicit a response in the cloud properties of tau and shortwave (SW)\nCRF that are on the same order of magnitude as the response found due to\nrealistic changes in aerosol state (i.e N-d). In response to increases\nin N-d, the cloud layer in the base case thinned due to increases in\nevaporative cooling and entrainment rate. This cloud thinning somewhat\nmitigates the increase in tau resulting from increases in N-d. On the\nother hand, variations in theta and q(t) jumps did not substantially\nmodify N-d. The cloud layer thickens in response to an increase in the\ntheta jump and thins in response to an increase in the q(t) jump, both\nresulting in a tau and SW CRF response comparable to those found from\nperturbations in Nd. Longwave CRF was not substantially altered by the\nperturbations we tested.\nWe find that realistic variations in meteorological context can elicit a\nresponse in CRF and tau on the same order of magnitude as, and at times\nlarger than, that response found due to realistic changes in aerosol\nstate. We estimate the limits on variability of cloud top jump\nproperties required for accurate observation of aerosol SW radiative\nimpacts on stratocumulus, and find strict constraints: less than 1 K and\n1 gkg(-1) in the early morning hours, and order 0.1 K and 0.1 gkg(-1)\nclose to solar noon. These constraints suggest that accurately observing\naerosol radiative impacts in stratocumulus may be challenging as\nco-variation of meteorological properties may obfuscate aerosol-cloud\ninteractions.

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