Variation of Radar-Observed Precipitation Characteristics in Relation to the Simultaneous Passages of a Madden-Julian Oscillation Event and Convectively Coupled Equatorial Waves during the Years of the Maritime Continent Pilot Study

In this study, we examined the variations of precipitation morphology and rainfall in relation to the simultaneous passages of aMadden-Julian oscillation (MJO) event and convectively coupled equatorial waves (CCEWs) observed during the Years of the Maritime Continent pilot study. We utilized globally merged infrared brightness temperature data and the radiosonde and radar data observed aboard the Research Vessel Mirai at 4 degrees 4'S, 101 degrees 54'E. As well as the observed MJO event, equatorial Rossby waves (ERWs), Kelvin waves (KWs), and mixed Rossby-gravity waves (MRGWs) were identified. The radar data exhibited high-frequency variation, mainly caused by KWs and MRGWs, and low-frequency variation, mainly caused by the MJO and ERWs. The MRGWs predominantly modulated convective echo areas and both convective and stratiform volumetric rainfall. In contrast, the MJO event had little influence on the variance of convective echoes. Moreover, stratiform echo areas and volumetric rainfall were more strongly modulated by the combined effects of the MJO, ERWs, KWs, and MRGWs than their convective counterparts. The intense development of stratiform echo areas and volumetric rainfall was coherent with the superimposition of the active phases of the MJO event and all the analyzed CCEWs. The strongest development and a significant reduction of convective echo-top heights before and after the peak MJO date, respectively, were coherent with the passages of ERWs and MRGWs, which were the dominant wave types in modulating echo-top heights. Thus, it appears that the superimposition of the CCEWs on the MJO event exerted complex modulations on the convective activities within the MJO event. SIGNIFICANCE STATEMENT: To improve the understanding of the variation characteristics of precipitation associated with the Madden-Julian oscillation (MJO) superimposed by equatorial waves, we examined variations of precipitation morphology in relation to the simultaneous passages of an MJO event and equatorial Rossby, Kelvin, and mixed Rossby-gravity waves, which were observed near the west coast of Sumatra Island during the Years of the Maritime Continent pilot study. We found that the multiple time-scale variability of convective and stratiform precipitation was mainly associated with one or two types of equatorial waves, rather than the MJO event. This highlights that equatorial waves can be key factors in modulating the development and variability of convective activities within an MJO event.

Automated summary

The study found that equatorial waves can be key factors in modulating the development and variability of convective activities within an MJO event. The multiple time-scale variability of convective and stratiform precipitation was mainly associated with one or two types of equatorial waves, rather than the MJO event. This highlights the complex modulations that the superimposition of CCEWs on the MJO event can have on convective activities.