Start Date/Time: Friday, February 05, 2010, 3:30 PM
Ending Date/Time: Friday, February 05, 2010, 4:50 PM
Location: JHN 075
***ATMOSPHERIC SCIENCES COLLOQUIUM, ATM S 520,
Chris Bretherton, Coordinator***
Speaker: Jennifer Kay, Project Scientist, CGD, NCAR
Title: The Boundary Layer Response to Sea Ice Loss and Implications for High-Latitude Climate Feedbacks
co-sponsored by the Program on Climate Change
Recent observations and modeling experiments demonstrate the importance of the boundary layer to projected Arctic climate change. My work uses observationally constrained atmospheric forecasts to evaluate the boundary layer and net radiation response to prescribed 2007 sea ice conditions in the Community Atmosphere Model version 3.5 (CAM3.5). While July 2007 boundary layer temperatures and humidities are minimally affected by sea ice loss, the September 2007 boundary layer is more turbulent, unstable, warm, and moist over newly open water. Low clouds increase over newly open water in both the July 2007 and the September 2007 forecasts. In response to the 2007 sea ice loss, net surface energy budgets change by +19.4 Wm-2 and -17.9 Wm-2 in the July 2007 and September 2007 forecasts respectively.
While many aspects of the forecasted response to sea ice loss are consistent with physical expectations and observations (e.g., Kay and Gettelman (2009)), CAM3.5's ubiquitous July cloud increases over newly open water are not. In coupled model runs, these unrealistic cloud increases would dampen projected Arctic sea ice loss and warming. The unrealistic cloud response results from the global application of parameterization designed to diagnose stratus clouds based on lower tropospheric stability. Because Arctic boundary layers are often not well mixed, an assumption underlying the implemented stratus parameterization is violated. A physically motivated change to the stratus parameterization improves the modeled cloud response as compared to observations, and increases surface energy budgets by +11 Wm-2 in the July 2007 forecasts. Of importance to projected Arctic change and high-latitude climate feedbacks, the cloud response to the 2007 sea ice loss depends on the model atmospheric state. Unrealistic cloud compensation for sea ice loss only occurs when dry and stable atmospheric conditions exist, such as those that led to the record-breaking 2007 Arctic sea ice loss.