Breakout Summary Report

ARM/ASR User and PI Meeting

19 - 23 March 2018

Session Title: COMBLE
Session Date: 21 March 2018
Session Time: 10:30 AM - 12:30 PM
Number of Attendees: 25
Summary Authors: Bart Geerts, Greg McFarquhar, Pavlos Kollias, Mike Jensen, and Mikhail Ovchinnikov

Breakout Description

Some presentations, followed by a discussion on observational and modelling needs to improve understanding of the CAO cloud regime, and its numerical representation in weather and climate models, with a focus on the COMBLE campaign.

Main Discussion

Ann Fridlind (NASA GISS) started the session with a presentation (abstract below) discussing microphysical process uncertainties in cold-air outbreak (CAO) clouds using LES simulations of a CAO event observed by A-train satellites. This presentation (summarized below) finds that while a ~1 km resolution WRF simulation can capture the marine boundary-layer (MBL) depth and essential mesoscale organization of shallow convection, the cloud-resolving LES simulations reveal many possible cloud microphysical outcomes, and these are inadequately constrained by observations. For instance, free tropospheric and surface sources of aerosol are unknown, owing to lack of observations that COMBLE could provide, and therefore neglected in the LES presented by Dr. Fridlind. And although these simulations are on a domain too small to represent mesoscale organization, case studies with strong observational constraints are well suited to the study of the relatively complex CAO organization evolution across stratocumulus to cumulus regimes.

Greg McFarquhar (OU/CIMMS) gave a quick summary of the just-completed MARCUS campaign across the Southern Ocean, and gave a preliminary analysis of the statistical distributions of macrophysical cloud and aerosol properties over the Southern Ocean during MARCUS. He examined the impacts of environmental conditions by looking at air parcel back-trajectories during MARCUS, to determine airmass source. Of particular interest to COMBLE are those cases with trajectories originating over the Antarctic ice. Histograms and scatterplots (LWP, SST, M value, MWACR cloud top, CEIL cloud base…) were presented.

James Booth (CUNY) compared synoptic circulations and low-cloud properties during cold air outbreaks over the Gulf Stream off the mid-Atlantic US east coast, the ARM ENA site, and Bear Island, using reanalysis data, satellite imagery, and ENA radar data. He noted that the transition from linear to cellular convection is quite common. Some events are clearly post-frontal while other events result from persistent cold-air advection under otherwise benign synoptic conditions.

Roel Neggers (U. Cologne) reported on LES simulations of a CAO event in the Fram Strait during a recent AC3 event with aircraft observations, both close to the ice edge and further downwind. His simulations capture the BL growth and mesoscale organization of clouds well, but simulated cloud properties remain highly uncertain.

Bart Geerts (U. Wyoming) presented an overview of the COMBLE campaign (objectives, platforms, and instruments) and highlighted the links with synergistic campaigns, especially CAESAR (a large campaign proposal to NSF, deploying the NCAR C-130, currently pending), MOSAiC, and the AC3 (Arctic Amplification) campaign, all around the COMBLE timeframe (Jan – May 2020). Essentially the measurements aboard the Polarstern (MOSAiC), at Ny Alesund (AC3), and at Bear Island and Andenes (COMBLE), constitute a meridional transect.

Discussion. During the remaining 30 minutes, we discussed observational and modelling needs to improve understanding of the CAO cloud regime, and its numerical representation in weather and climate models, with a focus on the COMBLE campaign. On the modelling side, much can be learned from Lagrangian modelling over a domain stretching from Andenes in the south across the sea ice edge to the Polarstern during warm intrusions into the Arctic, and vice versa in CAO conditions when the MOSAiC instrument array may be upstream of the COMBLE sites. MOSAiC will have 4 daily soundings, with data transferred in real time to GTS. The COMBLE sites have 4 radiosondes/day by default (Met Norway), with plans calling for these to be supplemented by DOE to yield three-hourly sondes during CAO events, all transferred to GTS. The remaining discussion items regarded COMBLE instrumentation, in particular at the ARM satellite site on Bear Island, located in the middle of the climatological CAO maximum. The surface flux estimation from ECOR data collected on Bear Island may not be representative, since the flow will be contaminated by the island. Therefore it was recommended to explore the existence of offshore buoys in the area. Also, it was noted that Doppler plus Raman lidar allows measurement of large-eddy fluxes below cloud base, and that these BL fluxes could be used to constrain surface flux estimation. Another discussion item regarded scanning radar data on Bear Island. Finally, the lack of scanning radar on Bear island was discussed. Met Norway has a mobile scanning X-band radar. We requested deployment of the SELEX radar on Bear Island, but the request was denied. Ewan O’Connor (U. Helsinki) mentioned that he has a Ka-band mobile scanning radar that has been used mainly in profiling mode, but could be used for scanning also. Total weight is 2 tons, which matters because of the lack of a loading dock on Bear Island. The scanning radar coverage on Bear Island is much desired, as it is the only way in which mesoscale precipitation structure can be monitored during COMBLE, so we concluded that we ‘d explore further possibilities.