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Study of aerosol direct and indirect effects and auto-conversion processes over the West African monsoon region using a regional climate model

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Abstract

This study assesses the direct and indirect effects of natural and anthropogenic aerosols (e.g., black carbon and sulfate) over West and Central Africa during the West African monsoon (WAM) period (June–July–August). We investigate the impacts of aerosols on the amount of cloudiness, the influences on the precipitation efficiency of clouds, and the associated radiative forcing (direct and indirect). Our study includes the implementation of three new formulations of auto-conversion parameterization [namely, the Beheng (BH), Tripoli and Cotton (TC) and Liu and Daum (R6) schemes] in RegCM4.4.1, besides the default model’s auto-conversion scheme (Kessler). Among the new schemes, BH reduces the precipitation wet bias by more than 50% over West Africa and achieves a bias reduction of around 25% over Central Africa. Results from detailed sensitivity experiments suggest a significant path forward in terms of addressing the long-standing issue of the characteristic wet bias in RegCM. In terms of aerosol-induced radiative forcing, the impact of the various schemes is found to vary considerably (ranging from −5 to −25 W m−2).

摘要

本研究评估了西非季风时段 (6 月-8 月) 非洲西部和中部地区自然和人为 (例如, 黑炭和硫酸盐) 气溶胶的直接和间接效应. 除了针对 RegCM 4.4.1 模式默认的 Kessler 云水自动转换方案进行评估外, 还比对了 Beheng(BH), Tripoli and Cotton (TC) 和 Liu and Daum (R6) 三种新的参数化方案. 通过比对这些方案, 分析了气溶胶对云量的影响, 对云的降水效率的影响及其相应的直接和间接辐射强迫. 在三种新的方案中, BH 方案明显减少了模拟结果中湿度的偏差, 在西非和中非区域的模拟偏差分别降低了 50%和 25%. 敏感性试验表明, 这一方案为解决 RegCM 模式一直存在的湿度偏差问题向前推进了一大步. 另外, 不同的参数化方案模拟的气溶胶辐射效应有很大差别, 在-5 至-25 W m-2浮动.

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Acknowledgements

The authors would like to thank both of the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, for providing the NCEP Reanalysis 2 data on their web site at http://www.esrl.noaa.gov/psd/, and ICTP on their website at http://clima-dods.ictp.it/data/regcm4/NNRP2/.

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Authors and Affiliations

  1. The Egyptian Meteorological Authority, Cairo, 11784, Egypt

    Zeinab Salah, Ahmed Shalaby & Ashraf S. Zakey

  2. University of Michigan, Department of Climate and Space Sciences and Engineering, Ann Arbor, MI, 48109-2143, USA

    Allison L. Steiner

  3. Centre of Studies in Resources Engineering and Interdisciplinary Program in Climate Studies, Indian Institute of Technology Bombay, Bombay, 400076, India

    Ritesh Gautam

  4. Cairo University, Department of Astronomy, Space Science and Meteorology, Giza, 12613, Egypt

    Mohamed M. Abdel Wahab

  5. Environmental Defense Fund, Washington DC, 20009, USA

    Ritesh Gautam

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  1. Zeinab Salah
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  2. Ahmed Shalaby
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Salah, Z., Shalaby, A., Steiner, A.L. et al. Study of aerosol direct and indirect effects and auto-conversion processes over the West African monsoon region using a regional climate model. Adv. Atmos. Sci. 35, 182–194 (2018). https://doi.org/10.1007/s00376-017-7077-3

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