Abstract The performance of the present operational global forecast system (GFS) at T1534 (~12.5 km) horizontal resolution with modified fractional cloud condensate to precipitation conversion parameter in the revised simplified Arakawa-Schubert (RSAS) convection scheme is evaluated for the summer monsoon seasons of 2018 and 2019 over the Indian region. The modified parameter has the form of an exponentially decreasing function of temperature above the freezing level. In contrast, below the freezing level, it is constant and is similar to default conversion parameter. The results reveal that the GFS T1534 with modified conversion parameter (EXPT) shows better fidelity in forecasting the mean summer monsoon rainfall over the Indian subcontinent region as compared to default GFS T1534 (CTRL). The rainfall probability distribution function analysis indicates a notable improvement in forecasting moderate and heavier category rainfall in EXPT as compared to CTRL. The improved distribution of total rainfall is found to be contributed by the proper forecasting of convective and large-scale rainfall in EXPT. It is likely that the reduced rate of conversion of cloud condensate to convective precipitation above the freezing level leads to a reduction in the convective rainfall, which eventually increases the moisture in the upper level through detrainment and hence enhances the large-scale precipitation. Further, EXPT shows relative improvement in forecasting outgoing longwave radiation, wind circulation, cloud fraction, dynamical-thermodynamical processes, and moist-convective feedback through improved lower tropospheric moistening over the Indian region. Finally, various skill score analyses suggest that EXPT shows better skill in predicting moderate and heavier category rainfall with longer lead time over the Indian subcontinent region. Considering the large socioeconomic impact of heavy and extreme precipitation over India, the modified conversion parameter can be incorporated in the present operational GFS T1534 model.

中文翻译：

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印度夏季风区业务天气预报模型（GFS T1534）对流参数化方案中降水转换参数修正率的影响

摘要 在 T1534 (~12.5 km) 水平分辨率下，当前业务全球预报系统 (GFS) 的性能在夏季季风季节进行了评估2018 年和 2019 年在印度地区。修改后的参数具有冻结水平以上温度的指数递减函数的形式。相反，在冻结水平以下，它是恒定的，类似于默认转换参数。结果表明，与默认的 GFS T1534 (CTRL) 相比，具有修改转换参数 (EXPT) 的 GFS T1534 在预测印度次大陆地区的平均夏季风降雨量方面表现出更好的保真度。降雨概率分布函数分析表明，与 CTRL 相比，EXPT 在预测中、重类别降雨方面有显着改进。发现总降雨量分布的改善得益于对 EXPT 中对流和大尺度降雨的正确预测。冰点以上凝结云向对流降水的转化率降低可能导致对流降水减少，最终通过排阻增加上层水分，从而增强大尺度降水。此外，EXPT 在预测出射长波辐射、风环流、云量、动力-热力过程、和湿对流反馈通过改善印度地区较低的对流层润湿。最后，各种技能评分分析表明，EXPT 在预测印度次大陆地区的中度和重度降雨方面表现出更好的技能，而且提前期更长。考虑到强降水和极端降水对印度的巨大社会经济影响，可以将修改后的转换参数纳入当前运行的 GFS T1534 模型中。