Performance of McRAS-AC in the GEOS-5 AGCM: aerosol-cloud-microphysics, precipitation, cloud radiative effects, and circulation 1NASA Goddard Space Flight Center, Greenbelt, MD, USA
2Universities Space Research Association, Columbia, MD, USA
3Seoul National University, Seoul, South Korea
4I.M. Systems Group Inc., Rockville, Maryland, USA
5School of Atmospheric and Earth Science, Georgia Tech. Atlanta, Georgia, USA
Received: 11 May 2012 – Published in Geosci. Model Dev. Discuss.: 08 Jun 2012Abstract. A revised version of the Microphysics of clouds with Relaxed
Arakawa-Schubert and Aerosol-Cloud interaction scheme (McRAS-AC) including,
among others, a new ice nucleation parameterization, is implemented in the
GEOS-5 AGCM. Various fields from a 10-yr-long integration of the AGCM with
McRAS-AC are compared with their counterparts from an integration of the
baseline GEOS-5 AGCM, as well as satellite observations. Generally McRAS-AC
simulations have smaller biases in cloud fields and cloud radiative effects
over most of the regions of the Earth than the baseline GEOS-5 AGCM. Two
systematic biases are identified in the McRAS-AC runs: one is
underestimation of cloud particle numbers around 40° S–60° S, and one is overestimate of cloud water path during the
Northern Hemisphere summer over the Gulf Stream and North Pacific.
Sensitivity tests show that these biases potentially originate from biases
in the aerosol input. The first bias is largely eliminated in a test run
using 50% smaller radius of sea-salt aerosol particles, while the second
bias is substantially reduced when interactive aerosol chemistry is turned
on. The main weakness of McRAS-AC is the dearth of low-level marine stratus
clouds, a probable outcome of lack of explicit dry-convection in the cloud
scheme. Nevertheless, McRAS-AC largely simulates realistic clouds and their
optical properties that can be improved further with better aerosol input.
An assessment using the COSP simulator in a 1-yr integration provides
additional perspectives for understanding cloud optical property differences
between the baseline and McRAS-AC simulations and biases against satellite
data. Overall, McRAS-AC physically couples aerosols, the microphysics and
macrophysics of clouds, and their radiative effects and thereby has better
potential to be a valuable tool for climate modeling research.
Revised: 12 Dec 2012 – Accepted: 14 Dec 2012 – Published: 17 Jan 2013
Citation: Sud, Y. C., Lee, D., Oreopoulos, L., Barahona, D., Nenes, A., and Suarez, M. J.: Performance of McRAS-AC in the GEOS-5 AGCM: aerosol-cloud-microphysics, precipitation, cloud radiative effects, and circulation, Geosci. Model Dev., 6, 57-79, doi:10.5194/gmd-6-57-2013, 2013.