Air Parcel Trajectory Analysis to Identify the Effects of Low Clouds on High-latitude Cold Air Formation in Warm Climates

Abstract:

In the present-day climate, cold air outbreaks occur when Arctic air masses intrude over mid-latitude continental interiors, producing surface temperatures as low as –40° C. But observations over the past few decades and model predictions under global warming indicate that Arctic amplification will increase the Arctic mean surface temperature faster than the global average, and the temperature of the coldest days even faster as cold air formation is suppressed [1, 2]. The lapse rate feedback has been identified as one of two major contributors to Arctic amplification, but the mechanism is not completely settled [2, 3]. 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 formation in warmer climates. Cronin and Tziperman, 2015, used 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 [4]. 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 [5]. But neither the Lagrangian nor Eulerian approaches provide a complete picture. The Lagrangian column model does not take into account mixing with surrounding air masses, while the Eulerian analysis cannot establish a cause and effect between cloud formation and surface temperature. We combine the two perspectives by performing an air parcel trajectory analysis on cold air outbreaks. Using output from the Community Atmosphere Model, version 5, over a variety of warm and cold climate scenarios, we examine wintertime cold air outbreaks and 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 and compare across climate scenarios.

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