12:00 PM - 12:15 PM
[ACG30-06] Why does convection weaken over Sumatra Island in an active phase of the MJO?
Keywords:Diurnal Convection, Land-sea Interaction, Madden–Julian Oscillation, Pre-YMC 2015
This study examined the diurnal cycle of convection over Sumatra Island in an active phase of the Madden-Julian Oscillation (MJO) during the pre-Years of the Maritime Continent (YMC) observation campaign in December 2015 based on observations and convection-permitting numerical model. Satellite observation indicates that, prior to an active phase of the MJO in early December, convection frequently occurred over the island in the afternoon. In contrast, during the active phase of the MJO in mid-December, afternoon convection over the island was suppressed, and a gap in convection aligned with the island was evident during the early morning hours. On the contrary, convective activities were observed over the offshore region within the whole day, especially during the daytime that was much weaker before.
Our numerical experiments successfully replicated the main features of the observed modulation of the diurnal convection. Results suggest that, during the active phase of the MJO, although the moisture was accumulated within the boundary layer over the mountainous areas as before, the afternoon convection was suppressed and delayed. Moisture budget analysis demonstrates that vertical moisture advection over the island, the dominating factor of diurnal convection, was weakened and delayed during the MJO active phase. It further became negative within the lower-to-mid troposphere from midnight to the next morning, indicating that the convection over the island was suppressed by the subsidence. Our results showed that such subsidence was highly related to the enhanced convective activities over the offshore region. On the other hand, our results also suggest that the strong westerlies played a secondary role in weakening and delaying the diurnal convection over the island, which induced the negative horizontal moisture advection within the mid-troposphere.
Overall, this study suggests that the modulated diurnal cycle over Sumatra Island was likely induced by the enhanced convective activities over the offshore region during the active phase of the MJO. Detailed results and further analyses would be shown in our presentation.
Our numerical experiments successfully replicated the main features of the observed modulation of the diurnal convection. Results suggest that, during the active phase of the MJO, although the moisture was accumulated within the boundary layer over the mountainous areas as before, the afternoon convection was suppressed and delayed. Moisture budget analysis demonstrates that vertical moisture advection over the island, the dominating factor of diurnal convection, was weakened and delayed during the MJO active phase. It further became negative within the lower-to-mid troposphere from midnight to the next morning, indicating that the convection over the island was suppressed by the subsidence. Our results showed that such subsidence was highly related to the enhanced convective activities over the offshore region. On the other hand, our results also suggest that the strong westerlies played a secondary role in weakening and delaying the diurnal convection over the island, which induced the negative horizontal moisture advection within the mid-troposphere.
Overall, this study suggests that the modulated diurnal cycle over Sumatra Island was likely induced by the enhanced convective activities over the offshore region during the active phase of the MJO. Detailed results and further analyses would be shown in our presentation.