The effects of the high-frequency variability on the large-scale, including the ISO, is quantified by moisture budget analysis using the outputs of the global nonhydrostatic simulations that marginally resolve mesoscale convective systems. In the composite analysis of short-range (week-long) simulations, upward transport of moisture by the high-frequency variability was of comparable magnitude to that by the low-frequency component, in both the ISO events over the central Indian Ocean (October-December 2011) and the ISO event over the Maritime Continent (December 2015). The high-frequency variability redistributed the bottom-heavy moistening by the low-frequency component on average, and contribute to lower tropospheric moistening in the pre-conditioning phase of the ISO.
During the 2015 ISO event, the high-frequency transport of moisture was nearly in phase with the ISO over ocean, while it was more continuous over land with more bottom-heavy profile. These are indicative of stronger linkage with the ISO for convection over ocean than that over land. In the extended-range (month-long) simulations of the 2015 ISO event, the forecast skill of the ISO was closely related to the route of the maximum low-level westerly and the associated convective envelope. The good cases successfully simulated the southward path of the westerly around the Maritime Continent, whereas the poor cases produced equatorial westerly penetrating through the Maritime Continent in the earlier simulation period, which resulted in too early and weak ISO propagation over the Maritime Continent. Moreover, the simulated behavior of the ISO was quite sensitive to equatorial waves, as well as the local convection over land. Further analysis including ISO cases during the YMC Sumatra 2017 campaign will be also presented.
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