Comparison of a global-climate model to a cloud-system resolving model for the long-term response of thin stratocumulus clouds to preindustrial and present-day aerosol conditions Department of Atmospheric, Oceanic, and Space Science, University of Michigan, Ann Arbor, MI, USA
Received: 23 Aug 2009 – Published in Atmos. Chem. Phys. Discuss.: 09 Oct 2009 Abstract. The response of a case of thin, warm marine-boundary-layer (MBL) clouds to
preindustrial (PI) and present-day (PD) conditions is simulated by a
cloud-system resolving model (CSRM). Here, both the aerosol conditions and
environmental conditions match those of a general circulation model (GCM).
The environmental conditions are characterized by the initial condition and
the large-scale forcings of humidity and temperature, as well as the surface
fluxes. The response of the CSRM is compared to that simulated by the GCM.
Revised: 23 Jun 2010 – Accepted: 30 Jun 2010 – Published: 14 Jul 2010
The percentage increase of liquid-water path (LWP) due to a change from the
PI to PD conditions is ~3 times larger in the CSRM than that in the
GCM due to the formation of cumulus clouds. The formation of cumulus clouds
is controlled by a larger increase in the surface latent-heat (LH) flux in
the PD environment than in the PI environment rather than by the change in
aerosols. However, the aerosol increase from the PI to PD level determines
the LWP response in the stratocumulus clouds, while the impacts of changes
in environmental conditions are negligible for stratocumulus clouds. The
conversion of cloud liquid to rain through autoconversion and accretion
plays a negligible role in the CSRM in the response to aerosols, whereas it
plays a role that is as important as condensation in the GCM. Also, it is
notable that the explicit simulation of microphysics in the CSRM leads to a
smaller LWP in the CSRM than that in the GCM using heavily parameterized
microphysics for stratocumulus clouds. The smaller LWP in the CSRM is closer
to an observed LWP than the LWP in the GCM for stratocumulus clouds.
Supplementary simulations show that increasing aerosols increase the
sensitivity of the cloud responses to the PI and PD environmental
conditions. They also show that aerosol effects on clouds depend on the
cloud type. The LWP of warm cumulus clouds is more sensitive to aerosols
than that of stratocumulus clouds.
Citation: Lee, S. S. and Penner, J. E.: Comparison of a global-climate model to a cloud-system resolving model for the long-term response of thin stratocumulus clouds to preindustrial and present-day aerosol conditions, Atmos. Chem. Phys., 10, 6371-6389, doi:10.5194/acp-10-6371-2010, 2010.