Name

Bart Geerts, Professor, Dept of Atmospheric Science, University of Wyoming

Title

The Organization and Vertical Structure of Shallow Convection in Marine Cold-Air Outbreaks, based on the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE): Observations and Quasi-Lagrangian LES Simulations

Abstract

When cold air blows off boreal continents or the Arctic ice over open water, a well-recognized cloud pattern forms with clouds streets and, further downwind, an open cellular structure. Despite the ubiquity of this cold-air outbreak (CAO) cloud regime over high-latitude oceans, we have a rather poor understanding of its properties, including its macroscale organization. In the 2019-’20 cold season, the COMBLE campaign was conducted along the coast of northern Norway, to examine the relations between surface fluxes, boundary-layer structure, aerosol, cloud and precipitation properties, and mesoscale circulations in marine CAOs. One of the hypotheses driving the COMBLE campaign is that cloud and precipitation processes control mesoscale organization, and transitions in this organization. At the coastal site on Andøya, located over 1000 km from the Arctic ice, an open-cellular structure prevailed, characterized by rather high surface heat fluxes just offshore, pockets of strong updrafts and convective turbulence (alternating with decaying convective cells), occasionally high radar reflectivity with heavy surface precipitation rate, high cloud tops (typically >2-3 km MSL), much liquid water in the updrafts, and broken cloud cover. A less common cloud mode appears to form under weaker surface heat fluxes. This mode is characterized by weak vertical drafts (up- and down-drafts) and turbulence, cloud tops around 2 km MSL, low reflectivity values, light precipitation rate, nearly overcast cloud cover, rather high amounts of supercooled liquid water, and common presence of cloud top generating cells. These cloud modes will be described using data from an array of instruments, including satellite data and profiling and scanning radar data. We will also illustrate the challenge of numerically simulating these mesoscale cloud organizations, using Quasi-Lagrangian Large Eddy Simulations.

Date

Tuesday May 24, 2022, 11:15-12:15   

Place

 C609 Rossbysalen, MISU, Svante Arrhenius väg 16C, 6th floor
 

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