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The impact of modified rate of precipitation conversion parameter in the convective parameterization scheme of operational weather forecast model (GFS T1534) over Indian summer monsoon region
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  • Malay Ganai,
  • SNEHLATA TIRKEY,
  • R. P. M. Krishna,
  • P. Mukhopadhyay
Malay Ganai
INDIAN INSTITUTE OF TROPICAL METEOROLOGY
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SNEHLATA TIRKEY
Indian Institute of Tropical Meteorology
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R. P. M. Krishna
Indian Institute of Tropical Meteorology
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P. Mukhopadhyay
Indian Institute of Tropical Meteorology

Corresponding Author:[email protected]

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Abstract

The performance of present operational global forecast system (GFS) at T1534 (~12.5 km) horizontal resolution with modified fractional cloud condensate to precipitation conversion parameter in the simplified Arakawa-Schubert (SAS) 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 exponential decreasing function of temperature above the freezing level, whereas below the freezing level, it is constant and 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 continent region as compared to default GFS T1534 (CTRL). The rainfall probability distribution function analysis indicates notable improvement in forecasting moderate and heavier category rainfall in EXPT as compared to CTRL. The improved distribution of total rainfall is found 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 decrease in convective rainfall, which eventually increases the moisture in the upper level through detrainment and hence enhancement in large-scale rainfall. Further, EXPT shows relative improvement in forecasting outgoing longwave radiation, wind circulation, cloud fraction, dynamical-thermodynamical processes and moist-convective feedback through better 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 continental India.
Jan 2021Published in Atmospheric Research volume 248 on pages 105185. 10.1016/j.atmosres.2020.105185