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<p>This study examines the efficacy of the National Center for Medium-Range Weather Forecasting (NCMRWF) global coupled unified model (CNCUM) hindcasts in depicting seasonal biases, focusing specifically on the Indian summer monsoon (ISM). Using 23 years (1993–2015) of coupled hindcasts driven by the UK Met Office GloSea5 seasonal prediction system, we analyze model biases in the mean state and sub-seasonal variability. High-resolution observations from, a key instrument aboard the Tropical Rainfall Measuring Mission (TRMM) satellite (hereafter referred to as TMI) SST and rainfall, along with GPCP rainfall data, are used for validation. The results indicate that while the CNCUM hindcasts satisfactorily portray key aspects of boreal summer monsoon over the Indian subcontinent and surrounding oceanic regions, systematic biases still persist. Notably, there is a significant wet bias (~ 6–8&#xa0;mm/day) during July and August over the western Indian Ocean, and dry biases dominate large regions of the Indian subcontinent. The annual cycles of SST and rainfall over the equatorial Indian Ocean are relatively weak compared to other monsoon regions. Additionally, the model exhibits a leading nature of SST anomalies (~ 5–7 days) during active-break monsoon periods, which is encouraging, but also reveals inconsistencies in the SST-rainfall relationship over the Bay of Bengal. A novel contribution of this study is the identification of a strong association between rainfall biases and free tropospheric (700 − 400&#xa0;hPa) moisture distribution. This finding highlights the need for a deeper understanding of moist processes within the CNCUM modelling system. By highlighting both the strengths and caveats in the model’s performance, this research provides valuable insights and suggests pathways for future model development and improvement.</p>

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Fidelity of coupled hindcast simulations in representing intraseasonal biases over South Asia

  • 期刊论文

  • 发表日期：

    2025年5月7日

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  • D. Nagarjuna Rao,
  • T. S. Mohan,
  • Ankur Gupta,
  • Kondapalli Niranjan Kumar,
  • Akhilesh Mishra,
  • Imranali M. Momin,
  • Saji Mohandas,
  • Ashis K. Mitra,
  • V. S. Prasad

摘要

This study examines the efficacy of the National Center for Medium-Range Weather Forecasting (NCMRWF) global coupled unified model (CNCUM) hindcasts in depicting seasonal biases, focusing specifically on the Indian summer monsoon (ISM). Using 23 years (1993–2015) of coupled hindcasts driven by the UK Met Office GloSea5 seasonal prediction system, we analyze model biases in the mean state and sub-seasonal variability. High-resolution observations from, a key instrument aboard the Tropical Rainfall Measuring Mission (TRMM) satellite (hereafter referred to as TMI) SST and rainfall, along with GPCP rainfall data, are used for validation. The results indicate that while the CNCUM hindcasts satisfactorily portray key aspects of boreal summer monsoon over the Indian subcontinent and surrounding oceanic regions, systematic biases still persist. Notably, there is a significant wet bias (~ 6–8 mm/day) during July and August over the western Indian Ocean, and dry biases dominate large regions of the Indian subcontinent. The annual cycles of SST and rainfall over the equatorial Indian Ocean are relatively weak compared to other monsoon regions. Additionally, the model exhibits a leading nature of SST anomalies (~ 5–7 days) during active-break monsoon periods, which is encouraging, but also reveals inconsistencies in the SST-rainfall relationship over the Bay of Bengal. A novel contribution of this study is the identification of a strong association between rainfall biases and free tropospheric (700 − 400 hPa) moisture distribution. This finding highlights the need for a deeper understanding of moist processes within the CNCUM modelling system. By highlighting both the strengths and caveats in the model’s performance, this research provides valuable insights and suggests pathways for future model development and improvement.