This study focuses on the convective activities over the central IO, especially the area around Gan (as illustrated in Figure 2.1). During the convectively active period of the October MJO over this region (15–31 October 2011), convective activity and precipitation tended to occur at quasi-2-day intervals (Zuluaga and Houze 2013) as shown in the time series of average and areal percentage of IRBT, averaged TRMM rain rate and adjusted SMART-R surface rain rate (Figure 3.1a). The IRBT areal percentage in Figure 3.1a was calculated by dividing the number of IRBT grid points in 5-K intervals by the total number of grid points over the 150-km circular region around Gan.
From the time series, seven convective events are identified with quasi-2-day periodicity and convective activity characterized by clouds with a mean IRBT minimum below 240 K and rain rates higher than 1 mm/hr in TRMM or SMART-R over the 150-km region near Gan. The criteria and times of these events are listed in Table 3.1 and indicated in Figure 3.1a. These times of IRBT minimal peaks were used as the 0-hour for the 48-hour window composite analyses. The convective event on 30 October showed an averaged IRBT lower than 240 K; however, it was excluded from the composite analyses due to the fact that the mean rainfall near Gan was considerably less than the other events and the radar reflectivity indicated mostly stratiform clouds over Gan throughout the time of event (Figure 3.2). By way of comparison, the first six of the seven convective events identified by the IRBT and rain rate signatures in our study were included in the 11-case composite of Zuluaga and Houze (2013) based on hourly S-Pol rain accumulation (Table 3.2). However, their composite analysis also includes events on a 4–6 day time scale associated with synoptic-scale waves from the November and December MJOs. Although the sample size in our study is small, there appears to be no preferred time of day for the 0 h occurrence of these events.
Large-scale Convective Features of the Quasi-2-day Convective Events
One of the conspicuous large-scale convective features during DYNAMO SOP was the eastward-moving MJO convective envelope passing over Gan and the DYNAMO ESA from 15 to 31 October 2011 (Figure 1.10 and Figure 1.11). During this convectively active period over the central IO, hereafter referred to as MJO1, seven quasi-2-day convective events occurred at Gan. According to the IRBT areal percentage in Figure 3.1a, during MJO1 the frequency of shallow cloud-tops reached a maximum before the rainfall peaks. The clouds then evolved into a deeper structure and reached maturity when the lowest IRBT peaks occurred. Following the mature stage, deep convective cores gradually dissipated while the frequency of warmer cloud-tops (250–270 K), some of which may be stratiform in nature, maximized (Zuluaga and Houze 2013, Zhang and Yoneyama 2017). These convective transitions repeated at quasi-2-day intervals during MJO1. A second major MJO event occurred over the IO in November with a convectively active phase at Gan from 15 to 30 November 2011 (Figure 1.10 and Figure 1.11), however, the quasi-2-day convective signal was not statistically significant over the central IO region during this period and therefore is not considered in this study (see APPENDIX B). The reasons for this difference between the two periods may be related to the different MJO propagation features over the DYNAMO ESA, but further examination is needed for the full explanation.
Figure 4.1, similar to Figure 1.10 and Figure 1.11, is a time-longitude diagram of IRBT and TRMM rain rate averaged over a 1.5° latitude strip north and south of Gan showing the large-scale zonal features of these quasi-2-day convective events. During DYNAMO, the convective envelope of the October MJO initiated over 60°E–70°E on 14
October, slowly propagated eastward through the DYNAMO ESA (72°E–80°E) during 15–31 October, which then encountered the diurnally excited, westward-propagating convective signal from Sumatra in early November. As observed in many previous studies, although the MJO convective signal over IO propagated eastward, the convective envelope was comprised of a number of westward-propagating features at shorter time scales (Nakazawa 1988, 1995; Hendon and Liebmann 1994; Takayabu 1994a,b;
Takayabu et al. 1996; Chen et al. 1996). The diurnally pulsing, westward-moving convection near the west coast of Sumatra Island (90°E–100°E) may have had some linkages to the convection over the central IO (Kubota et al. 2015); however, such a linkage is not entirely obvious from Figure 4.1.
Focusing on the quasi-2-day convective events over Gan, Figure 4.2 depicts the time-longitude IRBT, TRMM rain rate and 850-hPa total wind composites of the seven events over the DYNAMO ESA. The reference time 0 h, corresponding to the times listed in Table 3.1, is characterized by the lowest averaged IRBT over Gan. In Figure 4.2, the region of convective activity propagated westward from the eastern portion of the ESA, peaking over the area surrounding Gan with dominant cold clouds and heavy rainfall near 0 h. The 850-hPa total wind field shows in general westerlies to the west of Gan and slight easterlies to the eastern ESA. Noted that the zonal wind component shows a low-level convergence prior to the onset of deep convection over Gan, and the westerlies extend toward the east of Gan as the convective disturbance passes through. The ~500–1000 km zonal scale of the area of lower IRBT (< 240 K) suggests the presence of anvil shields and is consistent in size with observations made in TOGA COARE (800–1000 km;
Takayabu et al. 1996). As depicted in the horizontal time evolution of seven-event composite (Figure 4.3), after time 0 h, the convective signal continued to move westward while the peak IRBT gradually warmed at Gan, suggesting (and confirmed later) that
broad areas of stratiform clouds dominated during this period.
The westward propagation speed of the convective signals can also be estimated from Figure 4.2. Considering time-longitude IRBT composites lower than 240 K, the convective signal propagated 7°–8° in longitude in 20 hours, while the rainfall signal showed a similar speed but extended over a greater zonal distance. The propagation speed of the quasi-2-day convective signal is thus approximated to be 10–12 m/s, the same order as that observed over different areas in previous studies, i.e., 10–19 m/s over WPAC during TOGA COARE IOP in 1992–1993 (Haertel and Johnson 1998 as shown in Figure 4.4; Takayabu 1994b) and 15 m/s in Hendon and Liebmann (1994).
While the quasi-2-day convective events over the equatorial IO possess similar propagation speeds as those over the WPAC, the zonal scales are different. From Figure 4.1 to Figure 4.3, the zonal scale of propagation for the 2-day disturbances is ~1000–1500 km, shorter than the ~2000–4000 km scale observed for the WPAC during the TOGA COARE IOP (Chen et al. 1996; Chen and Houze 1997; Clayson et al. 2002). The difference could well be related to differences in the environments between the two ocean basins, but a full explanation is beyond the scope of this study.