This study analyzes data obtained by intensive observation during a pilot field campaign of the Years of the Maritime Continent Project (Pre-YMC) to investigate the diurnal cycle of precipitation in the western coastal area of Sumatra Island. The diurnal cycle during the campaign period (November–December 2015) is found to have a number of similarities with statistical behavior of the diurnal cycle as revealed by previous studies, such as afternoon precipitation over land, nighttime offshore migration of the precipitation zone, and dependency on Madden–Julian oscillation (MJO) phase. Composite analyses of radiosonde soundings from the Research Vessel (R/V) Mirai, deployed about 50 km off the coast, demonstrate that the lower free troposphere starts cooling in late afternoon (a couple of hours earlier than the cooling in the boundary layer), making the lower troposphere more unstable just before precipitation starts to increase. As the nighttime offshore precipitation tends to be more vigorous on days when the cooling in the lower free troposphere is larger, it is possible that the destabilization due to the cooling contributes to the offshore migration of the precipitation zone via enhancement of convective activity. Comparison of potential temperature and water vapor mixing ratio tendencies suggests that this cooling is substantially due to vertical advection by an ascent motion, which is possibly a component of shallow gravity waves. These results support the idea that gravity waves emanating from convective systems over land play a significant role in the offshore migration of the precipitation zone.

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This paper describes an analysis of multiyear satellite datasets to characterize the modulations of convective versus stratiform rain, rain system size, and column water vapor by convectively coupled equatorial waves. Composites are built around space–time filtered equatorial-belt data from the Tropical Rainfall Measuring Mission (TRMM) 3B42 rainfall product, while TRMM Precipitation Radar (PR) and passive microwave data are the composited variables. The results are consistent with the more reanalysis-dependent findings in Part I, indicating that higher-frequency (or more divergent) waves, such as Kelvin and inertia–gravity families, modulate mesoscale convective systems and stratiform rain relatively more, whereas slower (or more rotational) types such as Rossby, mixed Rossby–gravity, and tropical depression (TD) or “easterly” waves primarily modulate convective rain and smaller-sized precipitation systems. Column water vapor composites indicate that the more rotational wave types modulate the moisture field more pronouncedly than do the divergent waves, leading the authors to speculate that the slow/rotational versus fast/wavelike distinction in precipitation characteristics may correspond to the different balances of two main convective coupling mechanisms: moisture control of cumulus cells versus convective inhibition control (via low-level density waves) of mesoscale convective systems. The Madden–Julian oscillation (MJO) is unique in that it exhibits prominent modulation of both stratiform precipitation (like the fast divergent waves) and small-sized precipitation features, convective rainfall, and moisture (like the other low-frequency, rotational waves). A composite of other waves’ amplitudes as a function of MJO amplitude and phase shows that divergent waves are more active in the developing phase and rotational waves are more active in the decaying rather than developing phase of the MJO.

Precipitation-related differences in different types of convectively coupled equatorial waves are examined here and in a companion paper. Here the authors show spectra and cross-spectra among tropical-belt time sections of satellite-derived surface rain, infrared brightness temperature T b , precipitable water (PW), and Japan Meteorological Agency reanalysis of divergence and PW.Cross-spectra between rain and divergence at 1000-and 200-hPa levels show significant coherence peaks oriented along the dispersion curves of Kelvin, n 5 1 equatorial Rossby (ERn1), mixed Rossby-gravity (MRG), n 5 0 eastward inertial gravity (EIGn0), and n 5 1 and n 5 2 westward inertial gravity (WIG) waves, as well as the spectral signatures of the Madden-Julian oscillation (MJO) and tropical depression (TD)-type disturbances. Middle-troposphere divergence (indicative of stratiform rain and half-depth convection involvement in the coupling) is coherent with rain for the higher-frequency and more divergent wave types (Kelvin, EIGn0, WIG) but shows little coherence with rain for more rotational disturbance types (ERn1, MRG, TD). These two broad families also exhibit different rain-PW phase lags, a result supportive of the notion that stratiform rain (which occurs in dry conditions after peak PW and rain) is more involved in the more divergent wave types.