Extended Abstract (1.3M)16.7
The influence of Arctic sea ice extent on Polar cloud fraction and vertical structure and implications for regional climate
Stephen P. Palm, SSAI, Greenbelt, MD
Over the last 20 years and especially during the last decade, Arctic sea ice has decreased in both extent and thickness. In fact, the last 2 years have seen the smallest areal coverage of summertime Arctic sea ice in recorded history. If this rather alarming rate of decrease continues, the Arctic may be ice free in summer by the mid part of this century. Since the surface is a boundary condition for the atmosphere, the presence or absence of sea ice has a large impact on the atmosphere and polar climate. For instance, the high albedo of thick ice reflects a large percentage of shortwave radiation and the ice effectively traps the oceanic heat content, shutting off or greatly reducing the latent and sensible heat flux from the ocean to the atmosphere. In the absence of ice, the albedo is much lower, shortwave radiation is readily absorbed by the surface, and there is a much larger transfer of sensible and latent heat to the atmosphere. Thus, it appears that the reduction in ice cover will result in a warming of the polar atmosphere. However, an obvious consequence of increased moisture and heat flux associated with ice free areas is an increase in cloud cover. Increased cloud cover will complicate the atmospheric radiative balance since clouds will increase the albedo (over that of water) while trapping more of the emitted longwave radiation. The overall radiative effect of clouds will depend on their fractional coverage, height, geometric and optical thickness, vertical structure and water phase. Until recently, accurate remote sensing measurement of polar clouds was extremely difficult, especially over ice. Since 2003, orbiting lidar instruments have greatly increased our ability to measure cloud amount, vertical structure and water phase. This study will utilize satellite data from GLAS and CALIPSO to ascertain if the cloud cover is increasing in the Arctic as summertime sea ice extent decreases, and further to quantify the cloud fraction, vertical structure and optical thickness in ice free versus ice covered areas. Additionally, we will use these findings to infer what effect the reduced ice cover in the Arctic has on the radiative balance and Polar climate.
Session 16, Atmosphere-Ocean-Sea Ice Interactions
Thursday, 21 May 2009, 10:30 AM-12:30 PM, Capitol Ballroom AB
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