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Air Parcel Trajectory Analysis to Identify the Effects of Low Cloud Formation on High-Latitude Cold Air Outbreaks in Warm Climates
  • Kara Hartig,
  • Eli Tziperman
Kara Hartig
Harvard University

Corresponding Author:[email protected]

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Eli Tziperman
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA.
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In the present-day climate, cold air outbreaks occur when marine air intrudes over high-latitude continental interiors and radiatively cools, producing an abrupt drop in surface air temperature to as low as -40 C. But during the Eocene warm climate period, 55 million years ago, the presence of frost-intolerant species even at high latitudes in the Northern Hemisphere indicates that cold air outbreaks were suppressed. In projected future climate scenarios, relatively high surface temperatures at high latitudes are predicted as part of polar amplification. The lapse rate “feedback”, corresponding to enhanced warming of the lower troposphere, was found to be a major contributor [1]. The suppression of cold air in the Eocene is not well reproduced in global climate models (GCM) and the lapse rate feedback that contributes to polar amplification is still not well understood. Recent work hypothesized that the formation of low clouds as moist air flows from a warm ocean to a cold continental surface could suppress cold air outbreaks in warmer climates. Cronin and Tziperman, 2015, took a one-dimensional Lagrangian column model approach to track cloud formation and surface temperature as an air column migrates from a warm ocean surface to a cold continent [2]. Hu et al, 2018, followed up with an Eulerian analysis of GCM output over a range of cold and warm climates, looking at regional cloudiness, continental interior temperatures, and cold air extremes [3]. But neither approach is complete. The Lagrangian column model does not take into account mixing with surrounding air masses, while the Eulerian analysis does not explicitly follow the formation of clouds and their radiative impact as an air mass moves. In this work, we combine the two perspectives by studying cold air outbreaks in a variety of warm and cold climate scenarios using model output from the Community Atmosphere Model. After identifying cold air outbreaks, we backtrack trajectories for the air parcels that make up the entire cold air column. We then analyze the formation of clouds and the radiative budget to study the effects of clouds along each trajectory. Pithan, F. & Mauritsen, T. (2014). Nat Geo, 7, 181-184. Cronin, T. W. & Tziperman, E. (2015). PNAS, 112(37), 11490-11495. Hu, Z., Cronin, T. W. & Tziperman, E. (2018). JCLI, 31(23), 9625-9640.