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Journal article

Adjoint sensitivity of global cloud droplet number to aerosol and dynamical parameters

Karydis V, Capps S, Russell A, Nenes A ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 19 (2012) pp. 9041-9055

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We present the development of the adjoint of a comprehensive cloud
droplet formation parameterization for use in aerosol-cloud-climate
interaction studies. The adjoint efficiently and accurately calculates
the sensitivity of cloud droplet number concentration (CDNC) to all
parameterization inputs (e. g., updraft velocity, water uptake
coefficient, aerosol number and hygroscopicity) with a single execution.
The adjoint is then integrated within three dimensional (3-D) aerosol
modeling frameworks to quantify the sensitivity of CDNC formation
globally to each parameter. Sensitivities are computed for year-long
executions of the NASA Global Modeling Initiative (GMI) Chemical
Transport Model (CTM), using wind fields computed with the Goddard
Institute for Space Studies (GISS) Global Circulation Model (GCM) II',
and the GEOS-Chem CTM, driven by meteorological input from the Goddard
Earth Observing System (GEOS) of the NASA Global Modeling and
Assimilation Office (GMAO). We find that over polluted (pristine) areas,
CDNC is more sensitive to updraft velocity and uptake coefficient
(aerosol number and hygroscopicity). Over the oceans of the Northern
Hemisphere, addition of anthropogenic or biomass burning aerosol is
predicted to increase CDNC in contrast to coarse-mode sea salt which
tends to decrease CDNC. Over the Southern Oceans, CDNC is most sensitive
to sea salt, which is the main aerosol component of the region.
Globally, CDNC is predicted to be less sensitive to changes in the
hygroscopicity of the aerosols than in their concentration with the
exception of dust where CDNC is very sensitive to particle
hydrophilicity over arid areas. Regionally, the sensitivities differ
considerably between the two frameworks and quantitatively reveal why
the models differ considerably in their indirect forcing estimates.

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  • V. A. Karydis

  • S. L. Capps

  • A. G. Russell

  • A. Nenes

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