The CACTI Field Campaign: Improving Understanding of Convective Cloud Processes in Complex Terrain

The US DOE ARM Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign involved deployment of over 50 surface-based instruments to the Sierras de Córdoba range in central Argentina, which is a north-south oriented ridgeline that rises 2000 m above surrounding plains. CACTI sought to study the processes that influence convective cloud life cycles from shallow cumulus through deep convective initiation to mesoscale convective organization by utilizing the high frequency of these phenomena in this location throughout the wet season. Instrumentation included scanning C-, X-, and Ka-band radars, several lidars, and frequent radiosonde launches that nearly continuously monitored surface, boundary layer, aerosol, cloud, precipitation, and radiative flux conditions between October 2018 and April 2019. The G-1 aircraft measured in situ aerosol and cloud conditions with 22 flights during the IOP period in November-December 2018 that coincided with the NSF-led RELAMPAGO field campaign.

A range of terrain-influenced phenomena were observed and are currently being studied including frequent topographically modulated deep convective initiation, frequent hail, extreme convective cells penetrating above 20 km above sea level, and frequent mesoscale organization. Convection was often decoupled from the surface and warm rain formation was common, especially at night, despite the mid-continental location. Several periods of significant rainfall resulted in extremely low condensation nuclei concentrations in stable, moist air masses trapped against the mountains. During these and many other times, lightly drizzling fog was observed, at times beneath elevated convective precipitation. Orographic cumulus clouds formed on most days of the experiment but varied in location depending on thermodynamic profiles, frequently expanding horizontally into stratocumulus cloud decks tied to the mountains later in the day.

Taking advantage of the large number and variability of convective cells observed in close proximity to comprehensive environmental measurements, we developed a database of spatially and temporally tracked cells using 15-minute C-band PPI volume scans as the cells initiated, grew, merged, split, and decayed. A range of radar and satellite retrieved properties were saved for each cell, and high-resolution hemispheric RHIs from the X- and C-band radars performed every 30 azimuthal degrees are used to characterize fine-scale convective circulations. We use the database to examine the impacts of convective cell life cycle stage, merging, and ambient thermodynamic, kinematic, and aerosol conditions on cloud microphysical and macrophysical evolution. Results are then used to statistically evaluate the same relationships in convection-permitting model output.