Breakout Summary Report

ARM/ASR User and PI Meeting

Breakout Description

The TRacking Aerosol Convection interactions Experiment (TRACER) took place in the Houston, Texas region from October 2021 through September 2022 with an intensive operational period from June through September 2022. A comprehensive, high-quality data set was collected including detailed information on cloud, aerosol, precipitation, radiation, and meteorology characteristics from baseline ARM measurements, guest instrumentation, ASR PI deployments, and interagency partners. This session focused on building collaborations around addressing some of the pressing TRACER-related science and technical questions to facilitate accelerated progress. Topics included: (1) A TRACER model intercomparison project (facilitated by the Aerosol, Cloud, Precipitation and Climate Initiative) on aerosol invigoration of convection, (2) quantification of the regional variability of aerosol and thermodynamic conditions, (3) sea-breeze-boundary-layer interaction and impacts on cloud and aerosol life cycle and (4) convective updraft microphysical properties.  

Main Discussion

The session focused on building collaborations towards addressing some of the pressing TRACER-related science and technical issues to facilitate accelerated progress towards TRACER science goals. Four invited speakers shared ongoing analysis and modeling activities, areas in need of collaboration, and plans for future activities to seed discussions towards identifying opportunities and partnerships.

Milind Sharma (Texas A&M): Regional variability of thermodynamics/sea breeze

Kelcy Brunner (Texas Tech U.): Convective updraft microphysical properties

Stephen Saleeby (Colorado State U.): TRACER - Model Intercomparison Project

Maria Zawadowicz (BNL): Variability of aerosol/CCN properties

Key Findings

Some key findings from the preliminary work presented:

• Mesoscale boundaries (e.g. sea and bay breeze fronts, convective outflow) with variable thermodynamic gradients were identified at the fixed and mobile sites. These boundaries, in combination with anvil shading and urban heat island effects, often result in a complex diurnal evolution of the boundary layer.


• The timing, depth, and inland penetration of the sea-breeze circulation varies greatly from case to case.


• Approximately 60 sea-breeze cases identified at the AMF1 site during the TRACER IOP. Sea breezes tend to occur more frequently when surface wind is southwesterly or northeasterly.


• Measurements from Doppler lidar confirm that southwesterly sea breezes promote stronger, deeper, and wider updrafts, compared to northeasterly sea breezes.


• The diurnal evolution of the boundary-layer height, mixing ratio, and temperature profiles exhibits significant variability when comparing three sites situated in close proximity to the coast, downtown, and north of the Houston metropolitan region.


• Lightning observations provide information on variability of precipitation microphysical properties that is not captured by radar polarimetry.


• Analysis of aerosol observations at the AMF1 site shows a strong seasonal control on aerosol loading and characteristics. The summer months were generally cleaner than the other months and air masses with marine characteristics were most frequent. The summertime marine air masses were associated with more hygroscopic particles.


• Ice nucleating particle concentrations (T >= 20^oC) were similar at the main and ancillary sites with similar contributions from biological and heat-stable organic INPs.

Issues

N/A

Needs

• TRACER studies of aerosol-convection interactions will need to consider the often significant spatiotemporal heterogeneity across the sea and bay-breeze fronts.


• The covariability of environmental stability and aerosol size distributions (and composition) during active sea breeze days needs further investigation.


• Spatiotemporal characteristics of aerosol will need to be considered through a combination of fixed and mobile measurements, AQ measurement networks, aircraft observations (when available), satellite observations, and model reanalysis.


• For the TRACER MIP, several important observational data needs were identified: (1) Aerosol (accumulation mode and ultrafine) number, size distribution, hygroscopicity, and fractional solubility, as well as vertical profiles for aerosols in the urban and marine air, (2) “Climatological” maximum and minimum aerosol concentrations to bound sensitivity experiments, (3) precipitation amount, radar reflectivity, cloud-top height vertical velocity for case study days, and (4) large-scale meteorological properties, sea breeze analysis, and boundary-layer characteristics for case study days.

Decisions

• Have identified six potential cases for a TRACER Model Intercomparison Project (June 2, 17, 21; August 7; Sept 17, 18) with the June 17 and August 7 cases as the current first-choice candidates.


• June 17 shows a distinct sea-breeze boundary with associated convection in addition to some larger-scale convection. This date overlaps with NSF-ESCAPE aircraft flights. The main TRACER site experiences mainly “clean” marine air on this date.


• August 7 shows a consistent onshore flow with the early formation of a sea-breeze front. Isolated convection occurs with longer-lived cells later in the day, some of which were tracked by the CSAPR radar for up to 1.5 hours. At the main TRACER site, aerosol concentrations were high earlier in the day, followed by much cleaner air after the sea-breeze frontal passage.

Future Plans

Investigate the role of aerosols on lightning frequency and intensity, radar observables, and estimated precipitation.

Look at processes that drive lightning formation with and without polarimetric signals.

TRACER Model-Intercomparison Project (led by Steve Saleeby and Jiwen Fan) as a follow-on to the ACPC MIP (Marinescu et al.).

Outside of TRACER MIP, consider model ensemble runs at lower resolution with perturbed forcing and physics for evaluation of convective processes and aerosol-convection interactions.

Action Items

N/A