Journal article

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

Petters J, Jiang H, Feingold G, Rossiter D, Khelif D, Sloan L, Chuang P ...see all

Atmospheric Chemistry and Physics, vol. 13, issue 5 (2013) pp. 2507-2529

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Abstract

The impact of changes in aerosol and cloud droplet concentration (N-a
and N-d) on the radiative forcing of stratocumulus-topped boundary
layers (STBLs) has been widely studied. How these impacts compare to
those due to variations in meteorological context has not been
investigated in a systematic fashion for non-drizzling overcast
stratocumulus. In this study we examine the impact of observed
variations in meteorological context and aerosol state on daytime,
non-drizzling overcast stratiform evolution, and determine how resulting
changes in cloud properties compare.
Using large-eddy simulation (LES) we create a model base case of daytime
southeast Pacific coastal stratocumulus, spanning a portion of the
diurnal cycle (early morning to near noon) and constrained by
observations taken during the VOCALS (VAMOS Ocean-Atmosphere-Land Study)
field campaign. We perturb aerosol and meteorological properties around
this base case to investigate the stratocumulus response. We determine
perturbations in the cloud top jumps in potential temperature theta and
total water mixing ratio q(t) from ECMWF Re-analysis Interim data, and
use a set of N-d values spanning the observable range. To determine the
cloud response to these meteorological and aerosol perturbations, we
compute changes in liquid water path (LWP), bulk optical depth (tau) and
cloud radiative forcing (CRF).
We find that realistic variations in the thermodynamic jump properties
can elicit a response in the cloud properties of tau and shortwave (SW)
CRF that are on the same order of magnitude as the response found due to
realistic changes in aerosol state (i.e N-d). In response to increases
in N-d, the cloud layer in the base case thinned due to increases in
evaporative cooling and entrainment rate. This cloud thinning somewhat
mitigates the increase in tau resulting from increases in N-d. On the
other hand, variations in theta and q(t) jumps did not substantially
modify N-d. The cloud layer thickens in response to an increase in the
theta jump and thins in response to an increase in the q(t) jump, both
resulting in a tau and SW CRF response comparable to those found from
perturbations in Nd. Longwave CRF was not substantially altered by the
perturbations we tested.
We find that realistic variations in meteorological context can elicit a
response in CRF and tau on the same order of magnitude as, and at times
larger than, that response found due to realistic changes in aerosol
state. We estimate the limits on variability of cloud top jump
properties required for accurate observation of aerosol SW radiative
impacts on stratocumulus, and find strict constraints: less than 1 K and
1 gkg(-1) in the early morning hours, and order 0.1 K and 0.1 gkg(-1)
close to solar noon. These constraints suggest that accurately observing
aerosol radiative impacts in stratocumulus may be challenging as
co-variation of meteorological properties may obfuscate aerosol-cloud
interactions.

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Authors

  • J. L. Petters

  • H. Jiang

  • G. Feingold

  • D. L. Rossiter

  • D. Khelif

  • L. C. Sloan

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