Definition-dependent diurnal cycle variations in rainfall over the Maritime Continent

Abstract:

The diurnal cycle of convection over the Maritime Continent (MC) islands and nearby ocean interacts strongly with larger scale circulations like the Madden-Julian Oscillation (MJO) and convectively coupled Kelvin waves (CCKWs). However, characterizing diurnal variations in rainfall during the passage of these events depends on the definition of the diurnal amplitude used. In this study, the diurnal maximum amplitude is defined as the maximum hourly rainfall minus the daily mean rainfall, which is then normalized by the daily mean. This definition is used because tropical diurnal convection is not simply sinusoidal but pulse like and the normalization makes a more fair comparison of diurnal peakedness regardless of overall daily rainfall amounts. We apply this definition to TRMM/GPM IMERG precipitation observations from 2011-2019 over the Maritime Continent, which experiences 40 MJO events and nearly 300 CCKWs during the nine-year period. We find that the diurnal amplitude of rainfall is weakened (enhanced) during the active (suppressed) MJO over both land and water, while the diurnal phase of rainfall does not show significant changes. These results change if different diurnal amplitude definitions are used. For example, a non-normalized diurnal amplitude usually sees a synchronized change in the diurnal amplitude and the daily mean rainfall and when represented by the first harmonic, the diurnal amplitude is reduced and the peak time is delayed for about 1 hour over land. Similar comparisons will be made in relation to diurnal rainfall changes over the MC during the passage of CCKWs. As almost every MJO event is accompanied by one or more CCKW, changes in the MC diurnal precipitation during an MJO may also be affected by CCKWs. Three-way interactions between the diurnal cycle, MJO and CCKWs will be further investigated.