Thermal-Driven Graupel Generation Process to Explain Dry-Season Convective Vigor over the Amazon
Abstract. Large-eddy simulations (LESs) are conducted for each day of the intensive observation periods (IOPs) of the Green Ocean Amazon (GoAmazon) field campaign to characterize the updrafts and microphysics within deep convective cores while contrasting those properties between Amazon wet and dry seasons. Mean Doppler velocity (Vdop) simulated using LESs are compared with 2-year measurements from a Radar Wind Profiler (RWP) as viewed by statistical composites separated according to wet and dry season conditions. In the observed RWP and simulated LES Vdop composites, we find more intense low-level updraft velocity, vigorous graupel generation, and intense surface rain during the dry periods than the wet periods. To investigate coupled updraft-microphysical processes further, single-day golden cases are selected from the wet and dry periods to conduct detailed cumulus thermal tracking analysis. Tracking analysis reveals that simulated dry-season environments generate more droplet-loaded low-level thermals than wet-season environments. This tendency correlates with seasonal contrasts in buoyancy and vertical moisture advection profiles in large-scale forcing. Employing a normalized time series of mean thermal microphysics, the simulated cumulus thermals appear to be the primary generator of cloud droplets. At the same time, ice crystals tend to be generated in inactive parts of clouds. Time series shows that thermals, however, entrain ice crystals and enhance riming due to large concentrations of droplets in the thermal core. This appears to be a production pathway of graupel/hail particles within simulated deep convective cores. In addition, less-diluted dry-case thermals tend to be elevated higher, and graupel grows further during sedimentation after spilling out from thermals. Therefore, greater concentrations of low-level moist thermals likely result in more graupel/hail production and associated dry-season convective vigor.
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