Mixed-Phase Convective Clouds in the Polar Marine Boundary Layer

Principal Investigator(s):
Bart Geerts, University of Wyoming

The cold-air outbreak (CAO) cloud regime is commonly encountered over high-latitude oceans and adjacent coastal regions when a cold air mass becomes exposed to a sufficient fetch of open water. Cold-air outbreak clouds are generally convective, mixed-phase, and even though they are rather shallow, they can produce heavy snowfall. Their structure is fetch-dependent, evolving from narrow bands into open or closed cells as the marine boundary layer (MBL) deepens. Cold-air outbreak convection and associated marine boundary layer circulations effectively transfer heat into an otherwise highly stratified environment and involve interactions between sea-surface, boundary layer, cloud microphysical, and radiative processes. These interactions remain poorly understood given the hostile environment in which these clouds occur, both offshore and near-shore.

This proposal, a collaboration between two universities, firstly aims to describe the high-latitude cold-air outbreak cloud regime using the array of instruments at the NSA (Barrow, Alaska) and at two mobile Atmospheric Radiation Measurement (ARM) sites (MARCUS 2017-18, and COMBLE 2020). These ARM data, and the multi-sensor variables we propose to derive, allow detailed descriptions of mesoscale organization of precipitation, thermodynamic profiles, vertical velocity, cloud depth, and cloud and precipitation properties. These observations are key for our second goal, which is to use output from validated cloud- and eddy-resolving simulations to gain insights into the linkages between microphysical processes, the marine boundary layer vertical structure, and the marine boundary layer dynamics that control cloud macrostructure. Specifically, the two linked objectives of this proposal are:

  1. To document the cold-air outbreak cloud regime in three regions with DOE ARM facilities, in particular the vertical structure of stability and shear, vertical velocity, clouds, and precipitation, in the context of observed surface fluxes and cloud macrostructure; and
  2. To explore the role of clouds and precipitation on the boundary-layer circulations that control the cloud macrostructure, through high-resolution model simulations for specific cases in the three regions, specifically focusing on the feedbacks between microphysical processes and dynamics through the novel piggybacking technique.

Intellectual merit: Both dynamical and microphysical processes controlling shallow convection incold air masses are fundamentally different from those in warm marine boundary layer clouds, which are relatively well-documented. The cold-air outbreak cloud regime is not adequately represented in weather and climate models because the dominant scale of vertical motion and precipitation growth is unresolved. The proposed study is deeply anchored in observations thanks to the rich array of sensors at the NSA site and at the two mobile ARM sites. These data, combined with well-constrained numerical simulations, enable, for the first time, the testing of the hypothesis that cloud microphysical processes, especially precipitation, control the mesoscale organization of the cold-air outbreak cloud regime.

Broader impact: Cold-air outbreak clouds are ubiquitous in high-latitude regions in the cold season. This cloud regime (and thus the processes controlling it) matter, given their significance in the global climate system, and, more generally, given the potential role that the polar regions (especially the Arctic) play in amplifying global climate change, and interacting with weather and climate in the lower latitudes (NRC 2014).