SoMAS - Aerosol cloud precipitation interactions in Northwest Atlantic cold air outbreak

Dr. Florian Tornow, an Associate Research Scientist at Columbia's Center for Climate Systems Research working in collaboration with NASA GISS, speaks to SoMAS at the Topics in Atmospheric and Oceanic Sciences seminar on the topic "Aerosol-cloud-precipitation interactions in Northwest Atlantic cold-air outbreaks".
More about Florian can be found here: ccsr.columbia.....
Florian is interested studying the aerosol-cloud interactions by means of field campaigns and application of large eddy simulations (LES) cloud-resolving models.
Abstract:
The multi-year NASA research campaign ACTIVATE deployed two coordinated aircraft over the Northwest Atlantic, measuring cloud, aerosol, and precipitation properties using in-situ and remote sensing probes. During the winter and shoulder seasons between 2019 and 2022, ACTIVATE focused on marine boundary layer (MBL) clouds in the postfrontal sector of extra-tropical cyclones, in particular cold-air outbreaks (CAOs). Over the NW Atlantic, CAOs commonly show MBL clouds of steadily increasing cloud condensate until the onset of substantial precipitation triggers a transition from closed to open cell conditions, which may be very rapid or more gradual.
To better understand the MBL cloud regime transition and the concomitant interactions between aerosol, clouds, and precipitation, we use large-eddy simulations (LES) that translate with the MBL wind. The LES are initialized with tri-modal aerosol profiles derived from aircraft observations. Aerosol, two-moment mixed-phase microphysics, and sea spray are treated prognostically. In addition to field observations, we use satellite retrievals as observational constraints, including imager-based cloud properties (cloud cover, cloud optical depth, cloud-top temperature, and cloud droplet number concentration) and total liquid water path retrievals from microwave radiometry.
Results shed light on interactions between the MBL and the overlying free tropospheric (FT) air that is often shaped by dry intrusion events during CAO events. First, results from multiple CAO events indicate that the relatively clean FT air acts to dilute the MBL when entrained, explaining the reduction in droplet number concentration with fetch offshore prior to the onset of substantial precipitation. Second, the meteorological spatial pattern in FT subsidence and humidity is probed in one CAO event in order to understand a pattern in MBL cloud transitions. There we find that large-scale subsidence exerts a controlling influence on liquid water path. In LES sensitivity tests, we also find that frozen hydrometeors accelerate cloud regime transitions via riming. Lastly, we investigate the potential of nucleation and Aitken mode aerosol, which likely arise from upwind new particle formation events, to slow cloud transitions or even recover from them.
In addition to use of LES to understand factors controlling CAO albedo, our case studies are designed to bridge the gap to earth system model physics. Namely, we use the observation-constrained LES as a benchmark for the NASA GISS climate model in single-column model (SCM) mode. The SCM receives identical initial and boundary conditions as LES, thus providing a testbed for evaluating and improving the climate model's basic physics performance. Here use of observation-constrained LES provides a stronger basis than observations alone since all process rates and causal mechanisms can be probed in the LES framework, better enabling physics improvement in SCM mode.
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