Sasaki et al. (2025) Environmental Influences on Deep Convective Upscale Growth Rate in Central Argentina From a Convection‐Permitting Simulation

Identification

• Journal: Journal of Geophysical Research Atmospheres
• Year: 2025
• Date: 2025-12-31
• Authors: Clayton R. S. Sasaki, Angela K. Rowe, Lynn Alison McMurdie, Adam Varble, Zhixiao Zhang
• DOI: 10.1029/2025jd044251

Research Groups

• Department of Atmospheric and Climate Science, University of Washington, Seattle, WA, USA
• Department of Atmospheric and Oceanic Sciences, University of Wisconsin‐Madison, Madison, WI, USA
• Atmospheric, Climate, and Earth Sciences Division, Pacific Northwest National Laboratory, Richland, WA, USA
• Department of Physics, University of Oxford, Oxford, UK

Short Summary

This study investigates environmental conditions influencing the rate of deep convective upscale growth into mesoscale convective systems (MCSs) in central Argentina using a convection-permitting simulation, finding that rapid growth is associated with more favorable thermodynamic environments (higher moisture, instability) and more frequent low-level jets, with wind shear orientation also playing a role near complex terrain.

Objective

• Identify where and when convection grows upscale in central Argentina, particularly relative to the Sierras de Córdoba (SDC).
• Quantify the initial environments in which upscale growth occurs.
• Examine which thermodynamic and kinematic environmental parameters differentiate the rate of initial upscale growth.

Study Configuration

• Spatial Scale: Central Argentina, covering a domain from west of the Andes Mountains to the Atlantic Ocean, including the Sierras de Córdoba (SDC) mountain range. The model used a 3 km horizontal grid spacing with 80 vertical levels (vertical grid spacing ≤ 250 m below 5 km). Environmental conditions were averaged over a 4° by 4° region centered on the "first storms" centroid.
• Temporal Scale: A 6.5-month simulation covering the October 2018 – April 2019 experimental period. Output was saved every 15 minutes for 2D fields and hourly for 3D fields.

Methodology and Data

• Models used: Weather Research and Forecasting (WRF) model (version 4.1.1). Physical parameterizations included the Mellor–Yamada–Nakanishi–Niino (MYNN) level-2.5 eddy diffusivity mass flux scheme, Noah land surface scheme, Eta Similarity surface layer scheme, Rapid Radiative Transfer Model for General Circulation Models (RRTMG) with aerosol interactions, and the Thompson aerosol aware scheme.
• Data sources:
  • Initial and boundary conditions: Fifth-generation global reanalysis (ERA5) from the European Centre for Medium-Range Weather Forecasts.
  • MCS tracking: PyFLEXTRKR software using simulated Top-Of-Atmosphere Infrared Brightness Temperature (TOA IR Tb).
  • Environmental parameters: Calculated from 3D WRF output at the "first storms" stage.
  • Validation: Satellite-observed MCS characteristics, surface radar measurements, and observed South American Low-Level Jet (SALLJ) characteristics.

Main Results

• A total of 137 MCSs were tracked, with "first storms" and "MCS initiation" locations clustered near the southern end of the SDC.
• Convection initiating over the SDC (>500 m MSL) grew upscale into an MCS over a shorter median distance (51 km) compared to convection over lower elevations (76 km).
• The median growth time from "first storms" to "MCS initiation" was 1.5 hours, with a median cold cloud shield area growth rate of 7,700 km² h⁻¹.
• Rapid growth MCSs occurred more frequently during overnight hours (02:00–06:00 UTC), likely linked to the nocturnal acceleration of the SALLJ.
• Rapid growth MCSs were associated with a more favorable thermodynamic environment:
  • A SALLJ was present in 51% of rapid growth MCSs compared to 27% of slow growth MCSs.
  • Median SALLJ maximum wind speed was slightly stronger for rapid growth (15.4 m s⁻¹) than slow growth (14.4 m s⁻¹).
  • Significantly larger 850 hPa specific humidity (median 11.2 g kg⁻¹ for rapid vs. 10.4 g kg⁻¹ for slow).
  • Significantly stronger 850 hPa meridional specific humidity flux (median -49.1 g kg⁻¹ m s⁻¹ for rapid vs. -19.2 g kg⁻¹ m s⁻¹ for slow).
  • Significantly larger conditional instability (median MLCAPE 867 J kg⁻¹ for rapid vs. 468 J kg⁻¹ for slow; MUCAPE 1,202 J kg⁻¹ for rapid vs. 918 J kg⁻¹ for slow).
  • Significantly smaller LFC-LCL height difference, indicating a shallower convective inhibition layer.
• Kinematic environment differences were generally less significant when spatially averaged:
  • The 0–2 km vertical wind shear magnitude was significantly larger for rapid growth MCSs, potentially related to the more frequent presence of the SALLJ.
  • Deep layer (0–6 km) wind shear magnitudes were similar (median ~20.0 m s⁻¹) between slow and rapid growth MCSs.
  • For very rapid growth MCSs (top quartile) compared to very slow growth MCSs (bottom quartile), elevated-layer (2–6 km and 3–10 km) shear magnitudes were significantly smaller.
• When upscale growth occurred near the SDC with a SALLJ present, rapid growth was also supported by a favorable wind shear orientation (e.g., northerly 0–3 km shear vector for ~71% of rapid growth MCSs over SDC).

Contributions

• Provides a unique, long-duration (6.5-month) convection-permitting simulation analysis of deep convective upscale growth environments in central Argentina, a region known for intense convection but less studied at this resolution.
• Quantifies the distinct thermodynamic and kinematic environmental parameters that differentiate rapid from slow upscale growth, emphasizing the critical role of the South American Low-Level Jet (SALLJ) in fostering rapid development through enhanced moisture, instability, and favorable wind shear orientation.
• Demonstrates that while spatially averaged vertical wind shear magnitude is not a universal discriminator, elevated-layer shear magnitude is significantly smaller for the most rapid growth events, highlighting its importance at the extremes of growth rates.
• Underscores the combined influence of the SALLJ and terrain on wind shear orientation, which is crucial for rapid MCS growth near the Sierras de Córdoba, offering regional insights into complex terrain-convection interactions.

Funding

• NSF Grants AGS‐2146708 and AGS‐2146710.
• U.S. Department of Energy Office of Science Biological and Environmental Research, through the Atmospheric System Research program.
• Pacific Northwest National Laboratory (operated by Battelle for the U.S. Department of Energy under Contract DE‐AC05‐76RLO1830).
• Computational and storage resources were provided by the NCAR Computational and Information Systems Laboratory, the University of Utah Center for High Performance Computing, and the National Energy Research Scientific Computing Center (a Department of Energy Office of Science User Facility, supported by the Office of Science of the U.S. Department of Energy under Contract DE‐AC02‐05CH11231).

Citation

@article{Sasaki2025Environmental,
  author = {Sasaki, Clayton R. S. and Rowe, Angela K. and McMurdie, Lynn Alison and Varble, Adam and Zhang, Zhixiao},
  title = {Environmental Influences on Deep Convective Upscale Growth Rate in Central Argentina From a Convection‐Permitting Simulation},
  journal = {Journal of Geophysical Research Atmospheres},
  year = {2025},
  doi = {10.1029/2025jd044251},
  url = {https://doi.org/10.1029/2025jd044251}
}