Kroll et al. (2025) Parameterization adaption needed to unlock the benefits of increased resolution for the ITCZ in ICON

⚠️ Warning: This summary was generated from the abstract only, as the full text was not available.

Identification

• Journal: Repository for Publications and Research Data (ETH Zurich)
• Year: 2025
• Date: 2025-11-26
• Authors: Kroll, Clarissa Alicia, Jnglin Wills, Robert, Kornblueh, Luis, Niemeier, Ulrike, Schneidereit, Andrea
• DOI: 10.3929/ethz-c-000789088

Research Groups

Not available from the provided text.

Short Summary

This study investigates the persistent double Inter-Tropical Convergence Zone (ITCZ) bias in climate models across a wide range of resolutions, demonstrating its persistence even with explicitly described deep convection. It identifies insufficient moisture transport from the subtropics to the inner tropics as a key driver of this bias, rather than solely resolution or deep-convective parameterizations.

Objective

• To investigate the persistence and drivers of the double-ITCZ bias in climate models across a resolution hierarchy, from parameterized to explicitly described deep convection, and to assess the impact of resolution and parameterizations on its representation.

Study Configuration

• Spatial Scale: Horizontal resolutions from 160 km to 5 km.
• Temporal Scale: Not explicitly stated, but implied to be climate-scale simulations using specified sea-surface temperatures.

Methodology and Data

• Models used: ICON (XPP resolution hierarchy).
• Data sources: Specified sea-surface temperature simulations.

Main Results

• The double-ITCZ bias persists across horizontal resolutions from 160 km to 5 km in specified sea-surface temperature simulations.
• The persistence of the bias is independent of deep-convective and non-orographic gravity wave parameterizations.
• A key driver of the double-ITCZ bias is insufficient moisture transport from the subtropics to the inner tropics.
• This leads to a dry bias in tropical near-surface moisture, reduced deep convection over the Warm Pool, and a weakened Walker circulation, ultimately culminating in the double-ITCZ feature.
• Changes in the treatment of near-surface wind speed within the turbulence parameterization can reduce the bias, but increasing the near-surface wind speed limiter, while improving tropical near-surface moisture, exacerbates the bias in the moisture source and degrades global circulation, energy balance, and teleconnections.
• Parameter adjustments at low resolution are informative of the response to the same parameter adjustments at high resolution.

Contributions

• Provides a systematic investigation of the double-ITCZ bias across a wide range of resolutions (from parameterized to explicitly described deep convection) within a consistent modeling framework.
• Demonstrates that the double-ITCZ bias persists even at high resolutions (5 km) and with explicit deep convection, challenging the notion that increased resolution or discarded parameterizations alone will resolve it.
• Identifies insufficient moisture transport from the subtropics to the inner tropics as a fundamental driver of the bias, shifting focus from solely deep-convective parameterizations.
• Highlights the complex trade-offs involved in bias reduction strategies, showing that local improvements can lead to global degradation.
• Shows the utility of low-resolution parameter adjustments for informing high-resolution model responses.
• Underlines the continued importance of developing and refining non-discardable parameterizations in climate models.

Funding

Not available from the provided text.

Citation

@article{Kroll2025Parameterization,
  author = {Kroll, Clarissa Alicia and Jnglin Wills, Robert and Kornblueh, Luis and Niemeier, Ulrike and Schneidereit, Andrea},
  title = {Parameterization adaption needed to unlock the benefits of increased resolution for the ITCZ in ICON},
  journal = {Repository for Publications and Research Data (ETH Zurich)},
  year = {2025},
  doi = {10.3929/ethz-c-000789088},
  url = {https://doi.org/10.3929/ethz-c-000789088}
}