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<p>Climate models show systematic biases in simulating Asian summer monsoon precipitation, and the accurate representation of cloud microphysical processes is one of the key factors affecting model performance. In the double-moment bulk cloud microphysics scheme of the Community Integrated Earth System Model (CIESM), the shape parameter of the ice crystal size distribution is assumed to be 0, which is inconsistent with observations. To address this limitation, this study improves the ice crystal spectral relative dispersion parameterization in the CIESM. Compared with the default scheme, the performance and physical mechanisms of the new scheme in simulating cloud and precipitation over the Asian summer monsoon and adjacent regions are investigated. The results show that, compared with the default scheme, the new scheme notably improves the simulation of June–September mean cloud and precipitation characteristics over the Asian summer monsoon and adjacent regions. Specifically, the new scheme alleviates the underestimation of total cloud fraction simulated by the default scheme in the study region, and reduces both the overestimation of total precipitation over the Arabian Sea and northwestern Indian Ocean and the underestimation of total precipitation over the South Asian region. The underlying physical mechanisms are revealed. For cloud simulations, compared with the default scheme, the new scheme increases high-level cloud fraction by weakening the autoconversion of ice crystals to snow and enhancing the ice crystal deposition process, and increases mid-level cloud fraction by weakening both the autoconversion of ice crystals to snow and the ice crystal sedimentation processes. The new scheme further enhances lower tropospheric stability through radiative effects, promotes the formation of low-level clouds, and ultimately improves total cloud fraction. For precipitation simulations, compared with the default scheme, the new scheme strengthens atmospheric static stability and adjusts regional moisture convergence and circulation features. Specifically, the new scheme weakens the moisture convergence and upward motion over the Arabian Sea and northwestern Indian Ocean, while enhancing the moisture convergence and upward motion over the South Asian region. As a result, the systematic biases in precipitation over the Asian summer monsoon and adjacent regions simulated by the default scheme are effectively reduced. This study provides an important reference for improving Asian summer monsoon cloud and precipitation simulations in climate models.</p>

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Improving Asian summer monsoon cloud and precipitation simulations through an ice crystal spectral relative dispersion parameterization

  • 期刊论文

  • 发表日期：

    2026年5月20日

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  • Xin He,
  • Chunsong Lu,
  • Jian Cao,
  • Wentao Zhang,
  • Hengqi Wang,
  • Lei Zhu,
  • Junjun Li,
  • Xiaoqi Xu,
  • Xiangjun Shi,
  • Yuan Wang

摘要

Climate models show systematic biases in simulating Asian summer monsoon precipitation, and the accurate representation of cloud microphysical processes is one of the key factors affecting model performance. In the double-moment bulk cloud microphysics scheme of the Community Integrated Earth System Model (CIESM), the shape parameter of the ice crystal size distribution is assumed to be 0, which is inconsistent with observations. To address this limitation, this study improves the ice crystal spectral relative dispersion parameterization in the CIESM. Compared with the default scheme, the performance and physical mechanisms of the new scheme in simulating cloud and precipitation over the Asian summer monsoon and adjacent regions are investigated. The results show that, compared with the default scheme, the new scheme notably improves the simulation of June–September mean cloud and precipitation characteristics over the Asian summer monsoon and adjacent regions. Specifically, the new scheme alleviates the underestimation of total cloud fraction simulated by the default scheme in the study region, and reduces both the overestimation of total precipitation over the Arabian Sea and northwestern Indian Ocean and the underestimation of total precipitation over the South Asian region. The underlying physical mechanisms are revealed. For cloud simulations, compared with the default scheme, the new scheme increases high-level cloud fraction by weakening the autoconversion of ice crystals to snow and enhancing the ice crystal deposition process, and increases mid-level cloud fraction by weakening both the autoconversion of ice crystals to snow and the ice crystal sedimentation processes. The new scheme further enhances lower tropospheric stability through radiative effects, promotes the formation of low-level clouds, and ultimately improves total cloud fraction. For precipitation simulations, compared with the default scheme, the new scheme strengthens atmospheric static stability and adjusts regional moisture convergence and circulation features. Specifically, the new scheme weakens the moisture convergence and upward motion over the Arabian Sea and northwestern Indian Ocean, while enhancing the moisture convergence and upward motion over the South Asian region. As a result, the systematic biases in precipitation over the Asian summer monsoon and adjacent regions simulated by the default scheme are effectively reduced. This study provides an important reference for improving Asian summer monsoon cloud and precipitation simulations in climate models.