《Table 1.Normalized gamma DSD–fit parameters for several rain categories during active and inactive

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《Determination of Intraseasonal Variation of Precipitation Microphysics in the Southern Indian Ocean from Joss–Waldvogel Disdrometer Observation during the CINDY Field Campaign》

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Thompson et al.（2015）argued that the Z–R relationship should not change as a function of the MJO.The intraseasonal variation of the Z–R relationship may be ignored for light and moderate rains because the difference in DSDs is very small（Fig.4）.However，under heavy rain，the intraseasonal variation of DSD may influence the accuracy of radar data conversion.For example，if we employ the DSD-based Z–Rs during active MJO phases，i.e.，Z=272R1.25，Z=271R1.27and Z=255R1.40，to convert a measured Z value of 40 d BZ to rainfall rate，the following rainfall rates are obtained:17.9，17.1 and 13.8 mm h-1，respectively.These values are greater than the value obtained by the Marshall–Palmer model（11.5 mm h-1）.On the other hand，the DSD-based Z–Rs during inactive MJO phases，such as Z=250R1.21，Z=250R1.23and Z=245R1.40，yield the following rainfall rates:21.1，20.1 and 14.1 mm h-1.The difference in the results of Z–R conversion becomes larger when the measured value of Z is greater，such as that measured under highly intense convective conditions.For a measured Z of 50 dBZ，the Z–R conversion using the DSD-based Z–Rs during active MJO phases provides rainfall rates of 112.8，105.0 and 71.2 mm h-1.On the other hand，the Z–R relation obtained during the inactive MJO phase provides rainfall rates of 141.4，130.5 and 73.3 mm h-1.These conversions are rough examples，and the accuracy of rainfall estimates using weather radar is determined not only by the selection of the Z–R relation，but also by multiple sources of error（Marzuki et al.，2018）.Nevertheless，the effect of the intraseasonal variation of DSD on rainfall estimates can be appreciated.The same Z translates to a greater R in the inactive MJO phase compared with that in the active MJO phase.