Meso-Convective Complex that triggers flash foods
The next type of storm is indicated by the spatial structure of convective clouds which form clusters called Meso-Convective Complex (MCC) and occur on a wider scale (~100- 1,000 km) compared to bow-echo and squall-lines. MCC has more detailed characteristics, because the spatial pattern of the cloud is spherical, with almost perfect eccentricity, and has two layers — the core and the MCC blanket characterized by the lowest temperature of the cold cloud it forms.
The formation of this cloud cluster does not occur suddenly, but rather through a multiphase system. A convective cloud is formed from a single cumulonimbus cloud which continues to grow into the so-called Deep Convective Cloud (DCC). The combination of several DCCs then forms MCC, and several MCCs can combine to form a Mesoscale Convective System (MCS).
Both MCC and MCS are phenomena commonly found in mid-latitude or subtropical countries, because they are associated with tornadoes and other forms of severe weather. However, the characteristics of MCC in BMI have been frequently studied and even been linked to the occurrence of strong convection over the Java Sea, which triggered fooding in Greater Jakarta in 2015.
Research that examines the ability of weather prediction models to simulate MCC in BMI is important, given the frequency of MCC events, which often occur during the rainy season (December-February). Weather prediction models also need to be tested for their ability to capture MCC phenomena during the peak dry season (June-August).
The twin MCCs in northern and central Sulawesi on July 11–13, 2021 triggered a sudden increase in the amount of normal rainfall at night, causing catastrophic flash foods in North Luwu, Masamba, Sulawesi which killed 38, injured 58, left 40 people missing and damaged 14,483 structures.
Research to simulate this incident shows that the twin MCCs had a long life cycle (more than 10 hours) with slow propagation from east to west over Sulawesi, triggered by anticyclonic vortex circulation in the Makassar Sea. The research also found the central role of sea surface temperatures in shifting the location of widespread extreme rain from the Banda Sea east of Sulawesi to landfall.
The role of sea surface temperature can be seen from model simulations that compare weather prediction models that change their sea surface temperature input every six hours, and those where the sea surface temperature input remains constant. These show that the rain prediction results have improved accuracy in terms of the onset time of sudden increase in rain, intensity and location, and the crucial central role of sea surface temperature, in creating strong convective storms like the MCC over Sulawesi even though they occur during the peak of the dry season.
Cyclonic Storms trigger catastrophic disasters
On the broadest scale, the type of storm that can form is a tropical storm or cyclone (~1,000-100,000 km). Unlike the types of storms mentioned previously, this one is characterized by rotating movements (vortices), both cyclonic (clockwise) and anticyclonic (counterclockwise), over a very wide area of thousands, even hundreds of thousands of kilometers. Thus, tropical storms can be identified using three main parameters — cloud, wind and vortex.
For the cloud parameter, on a tropical storm scale, the visual appearance of clouds takes the form of MCS formed from a combination of several MCCs that can reach 5-10 times that of a single MCC. This is supported by continuous and persistent strengthening of wind from a precondition to mature stages. The second parameter is the vorticity anomaly, which differs in contrast between the core and the outer layer of the storm, thus allowing the storm to rotate cyclically for regions in the Southern Hemisphere and anti-cyclically for those in the Northern Hemisphere.
With regard to these rotating motions or vortices, conventional theory suggests that the forces are determined by absolute and relative vortices. Absolute vorticity depends on the Coriolis force, which is determined by the magnitude of the latitude of the place. This means that the further it is from the equator, the greater the Coriolis force becomes. This explains why the formation of tropical storms cannot occur in regions near the equator, considering that the Coriolis force is close to zero. However, there is still a contribution from relative vorticity, which can provide energy for the storm.
So, how can relative vorticity be formed in regions close to the equator? Along the equator there is a phenomenon of trapped equatorial tropical waves called “Convective-Coupled Equatorial Waves” (CCEW). This can form throughout the season, interacting with the current and triggering the formation of a vortex near the equator, which can continue until it morphs into a tropical storm and further into a tropical cyclone, as happened in the case of tropical cyclone Lotus.
It is worth bearing in mind that before a tropical storm forms, there is a stage called “vortex storm”. When it is still in this stage, there is a rotating movement of relative vorticity, while the wind strength is still weak, with a limited radius (2-200 km). As the vortex grows bigger and bigger, it turns into a tropical depression-tropical storm-tropical cyclone. A “tropical cyclone” is categorized on a scale of 1-4, with levels 3-4 usually called a “super typhoon” or “severe tropical cyclone”.
In the case of tropical cyclone Seroja, in the precondition stage twin vortices were formed to the north and south of the equator, indicating the active movement of the Rossby equatorial current forming over the Banda-Maluku Sea. It then interacts with MJO phase 4 in eastern Indonesia. This played a role in providing continuous support for the persistence of easterlies (westerlies), feeding the vortex that occurs south of the equator.
Additionally, the MJO plays a role in creating a low-pressure system that extends across eastern Indonesia, thereby providing support for high humidity for convective accumulation to increase, from a vortex to a tropical cyclone seed. Other findings show that the twin vortices that formed in the Banda Sea were preceded by an extreme heat phenomenon on the surface of the sea called a “marine heatwave” (MHW).