Breakout Summary Report

ARM/ASR User and PI Meeting

Breakout Description

For many years, climate models have struggled to simulate correctly top-of-atmosphere radiative fluxes and the surface energy budget of the SO. These radiative errors influence local and global atmospheric and oceanic circulations, as well as global climate sensitivity. The radiative biases are smaller on average in the current generation of climate models (specifically, those participating in theCloud Model Intercomparison Project phase 6 relative to phase 5). Nonetheless significant radiative bias remain, and if anything, concerns have grown as to how well models capture cloud responses to warming (feedbacks) and aerosol-cloud-precipitation interactions, especially in low clouds. Understanding low-cloud feedbacks and aerosol-cloud-precipitation interactions in mixed-phase clouds and in a pristine region remains of critical importance for climate change.

Main Discussion

2:00–2:30 Cape-K overview: Roj Marchand, Heath Powers, Tom Hill

Cape-K starts April 2024 with an ARM mobile facility deployment in the northwest corner of Tasmania, at the Kennaook Cape Grim Baseline Air Pollution Station.

Why?

-The Southern Ocean is a region where low-cloud feedbacks are large and highly uncertain.

-There is a strong seasonal cycle in aerosols that modulate cloud (and precip?) properties across the entire Southern Hemisphere mid-latitudes.

-KCG station has produced a long and consequential record of Southern Hemisphere aerosol and gas-phase chemistry; however, extensive cloud and precipitation measurements have not been collected at this site.

-KCG provides an accessible location for a comprehensive deployment of modern ground-based remote sensors that can sample clouds in pristine (baseline) conditions.

Australian collaborators include CSIRO, Australian Bureau of Meteorology (BoM), and several Australian universities.

AMF deployment will include:

-Ka and W-band radar, MPL, microwave radiometer, surface precip, and radiation.

-AMF AOS shelter (for guest instrumentation).

-Augmenting the KCG station aerosol instrumentation: w/APS, SP2, UHSAS, INP filters.

-BoM is lending an autosonde launcher.

-Full details can be found in the science plan.

A formal request for guest instrument proposals will be released by ARM sometime soon.

Individuals interested in joining the science team (or just getting on the email list) should contact Roger Marchand (rojmarch@uw.edu)

2:30-2:45 – Important ice processes are missed by global climate models in Southern Ocean mixed-phase clouds: Insight from comparisons with observations (remote presentation). Liu, X, Zhao, X, Burrows, S M, DeMott, P J, Diao, M, McFarquhar, G, Patade, S, Phillips, V, Roberts, G, Sanchez, K J, Shi, Y, Zhang, M

In GCMs, LWP is too high and IWP is too low at high latitudes.
Models underestimates INP (observations indicate representation of dust INP + marine organic aerosol together is needed) but this (by itself) does not explain water path bias.

Inclusion of secondary ice processes (including ice-ice collisions) can bring CESM2 into good agreement with observational estimates (for zonal mean LWP and IWP).

Doing so decreasing equilibrium climate sensitivity from 5.5 to 4.4 K.

2:45-3:00 – Investigating Southern Ocean Aerosol-Cloud Interactions and their Representations in the Community Atmosphere Model Version 6. McCluskey, C S, Gettelman, A, Nusbaumer, J, Hannay, C, Riihimaki, L D, Niu, Q, McFarquhar, G, Medeiros, B

Biases (underprediction) in SO low cloud cover and IWP remain problematic in CAM6/CESM2.

Increases in primary INP resulting from improved parameterization of marine organics (McCluskey et al. 2018) and Mineral dust (Demott et al. 2015) does little to improve the situation.

The low cloud droplet number (Nd) bias in CAM6 is consistent with a low bias CCN number concentration.

Simulated Southern Ocean accumulation-mode sulfate aerosol are underpredicted by CAM6 at all altitudes.

3:00-3:15 – Evaluating Precipitation and Cloud Occurrence in the GISS-ModelE3 ESM against MICRE Observations. McKenna, S

Precipitation efficiency modulates shortwave cloud feedback.

GISS-ModelE3 produces a too small cloud occurrence frequency compared to MICRE obs (57% versus 73%). This is opposite of what one finds based on CloudSat retrievals, and likely due to CloudSat under-detecting the occurrence of light precipitation.

Partially supercooled layers (top<0 oC, Bottom>o0 C) are precipitating a large fraction of time both in observations and in the GISS model (97 versus 93%).

Regardless of “Z” threshold, warm and fully supercooled clouds precipitation LESS frequently in the GISS model than observed (65 versus 53%, 63 versus 50% respectively).

GISS E3 does, however, capture well the precipitation rate trends as a function of geometric thickness.

3:15-3:30 – Dependence of Cloud Macrophysical Properties and Phase Distributions on Different Environmental Conditions over the North Atlantic and Southern Ocean: Results from COMBLE and MARCUS. Xia, Z, McFarquhar, G

Cold Air Outbreaks (CAOs) are common in both the North Atlantic and Southern Ocean but …

COMBLE features more intense CAOs (M > 5 K), lower cloud-top temperatures, thicker clouds, and greater precipitation (occurrence?) than MARCUS.

MARCUS features stronger BL inversions and lower surface temperatures than COMBLE.

3:30-3:45 – Closure Study of Southern Ocean Sea Spray Aerosol. Brian, E, *Kreidenweis, S, DeMott, P J, Schnaiter, M

MARCUS aerosol observational are used for closure studies (using aerosol optical properties to estimate CCN and INP) and parameterization testing.

-Submicron optical closure achieved

-Evidence suggest there is a loss of supermicron aerosol è sampling efficiencies

-Should be examined

Merged aerosol size distributions support latitudinal gradients discussed in Humphries et al. (2021) on basis of CCN data.

CCN(0.2%) were estimated within a factor of 2 from bsp, for Angstrom > 0.5 as high latitudes, but need a separate relationship to represent “open ocean” sea spray in mid-latitudes.

Aerosol surface area can be well approximated using light scattering and assumed value for Q of 2.2.

3:45-4:00 – The Island Effect: Topographic Influence on Clouds at Macquarie Island as Seen from MODIS. Tansey, E, Marchand, R, Chand, D

Cloud droplet number concentration and effective radius are found to be enhanced to the east of Macquarie Island. Why?

Increased (more or larger) aerosols?

-MODIS aerosol OD compares well with that estimated by CIMEL sun photometer during MICRE. (Not shown: MODIS Angstrom Exponents do appear to be biased high, consistent with other studies of MODIS data for marine regions with low aerosol OD).

-We find no relationship between cloud enhancement and MODIS aerosols to east of island. MODIS aerosol OD and distribution east and west of island are statistically indistinguishable from each other.

Rather, enhancement appears to be result of orographically forced lee waves that occur during westerly winds (which are the climatological norm).

-The hypothesis is that the newly formed cloud generated by lee waves have not had time for collision-coalescence processes to increase cloud effective radius and decrease drop concentrations.

-Enhancement is stronger when the MBL has higher LTS and EIS, and boundary layer is shallower.

Work is ongoing, but these lee waves do not seem to be influencing cloud occurrence or cloud microphysical properties over the isthmus near the northern tip of the island (where the MICRE ground site was located).

Key Findings

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Issues

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Needs

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Decisions

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Future Plans

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Action Items

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