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  • Vavrus, S., and D. E. Waliser, 2008: An improved parameterization for simulating Arctic cloud amount in the CCSM3 climate model.. Journal of Climate. In press.

A simple alternative parameterization for predicting cloud fraction in the CCSM3 global climate
model is presented. This formula, dubbed “freeezedry”, is designed to alleviate the bias of
excessive low clouds during polar winter by reducing the cloud amount under very dry
conditions. During winter, freezedry decreases the low cloud amount over the coldest regions in
high latitudes by over 50% locally and more than 30% averaged across the Arctic. The cloud
reduction causes an Arctic-wide drop of 15 W m-2 in surface cloud radiative forcing (CRF)
during winter and about a 50% decrease in mean annual Arctic CRF. Consequently, wintertime
surface temperatures fall by up to 4 K on land and 2-8 K over the Arctic Ocean, thus
significantly reducing the model’s pronounced warm bias. Freezedry also affects CCSM3’s
sensitivity to greenhouse forcing. In a transient-CO2 experiment, the model version with
freezedry warms up to 20% less in the North Polar and South Polar regions (1.5 K and 0.5 K
smaller warming, respectively). Paradoxically, the muted high-latitude response occurs despite a
much larger increase in cloud amount with freezedry during non-summer months (when clouds
warm the surface), apparently because of the colder modern reference climate. While improving
the polar climate simulation in CCSM3, freezedry has virtually no influence outside of very cold
regions and has already been implemented in another climate model (GENESIS1). Furthermore,
the simplicity of this parameterization allows it to be readily incorporated into other GCMs,
many of which also suffer from excessive wintertime polar cloudiness.


Last Updated: 2008-08-26

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