Air Parcel Trajectory Analysis to Identify the Effects of Low Cloud
Formation on High-Latitude Cold Air Outbreaks in Warm Climates
Abstract
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.