Research Highlights April 2011 : Freidenreich and Ramaswamy
Study by Stuart Freindenreich and V. Ramaswamy on downward
shortwave surface flux in the GFDL CM2.1 General Circulation Model
downward solar flux at the surface is an important component of the
earth’s climate system, being a major factor governing the surface
temperature and hydrologic cycle. The availability of derived flux
climatologies from both ground-based measurements and satellite-based
estimates provide a reference for properly assessing general
circulation model biases, not only in the surface irradiance, but in
the atmospheric factors (aerosols, clouds, and water vapor) affecting
1. The difference in the annual mean all-sky downward surface flux
between CM2.1 and the corresponding climatologies derived from the
Baseline Surface Radiation Network (BSRN) and Global Energy Balance
Archive (GEBA) observations and the International Satellite Cloud
Climatology Project (ISCCP-FD) estimation.
Figure 1 illustrates the
biases in the annual mean all-sky downward shortwave surface flux in
CM2.1 with respect to the climatologies derived from three datasets,
while Figure 2 illustrates the corresponding biases in the total
cloud amount. For land regions, over
Europe, northern and eastern Asia and North America, the model
produces large flux underestimates, with maximum values > 20 W/m2.
These result from the model producing too much cloudiness over
regions where there is frequent cloud cover. For southern Africa,
flux underestimates are an indication that the model can also be too
moist for drier regions as well.
Contrastingly, flux overestimates, associated with too little
cloudiness, occur over equatorial Africa, coastal regions of
Australia, southern Asia, and especially for the Amazon region.
2. The difference in the annual mean total cloud amount (%) between
CM2.1 and ISCCP-FD climatology.
oceanic regions, there are flux underestimates in the subtropical
Pacific and Atlantic, and overestimates near and along the equator.
The large biases just south of the equator are indicative of a double
ITCZ produced by CM2.1. The flux overestimates off the west coast of
the Americas are due to a lack of marine stratocumulus. Overestimates
occur across the southern ocean from around 60oS
southward to coastal Antarctica, due to too much cloudiness. Thus,
analysis of these flux biases aid in the identification of where and
to what degree shortcomings in the model?s simulation of cloudiness
exist, serving as a guide in seeking to improve the surface flux