Bow-Echo triggers cyclone
On a micro or meso-gamma scale (less than 2 km), a tornado is a type of extreme weather that occurs for a very short duration, making it very difficult to observe, let alone predict. However, a strong tornado (56 km/h) on the Beaufort 8 scale is equivalent to an early phase tornado, such as the one that damaged 305 houses and injured five residents in Cimeunyan, West Java, on March 28, 2021.
Through high-resolution X-band rain radar data recorded every two minutes, it can be seen that these damaging strong winds which lasted 32 minutes are closely related to bow-echo storms, which can be identified spatially from rain with a boomerang-like pattern. Uniquely, storms are not produced by widespread synoptic-scale weather disturbances in the surrounding area, but are formed from the combination of several isolated convective systems from the mountainous region south of Bandung.
This is possible because there is a large meso-convergent area in West Java centered in the Bandung Basin triggering the growth of single convective cells, which then merge with each other to form a bow-echo pattern. In this boomerang-like pattern, sustained strong winds form in the tapering center, and occur first in the afternoon before the formation of clouds that cause heavy rain in the late afternoon.
By using a very high resolution (0.5 km) numerical weather prediction model, which was carried out for the first time in Indonesia, the mechanism for the formation of two meso-vortices in the northern and southern parts of the bow-echo pattern was discovered. These two meso-vortices are what triggered the tornado, as seen in a video recorded by one of the residents in the area around Dago, north of Bandung. Research on waterspouts using this model simulation also shows the ability of a 0.5 km resolution model which can simulate bow-echo storms in terms of pattern, location and intensity, with a time difference of around 30 minutes faster than observations using X-band rain radar.
Given that the bow-echo storm pattern is one that often occurs in BMI region in all seasons, long-term projections of bow-echo associated with strong or extreme winds need to be studied further in the future, as part of climate change mitigation measures.
Squall-Lines that trigger tidal foods
On a wider medium scale (~20– 100 km), a type of convective storm can also be identified from the spatial pattern of rain in a long line called the squall-line. As with bow-echo, the squall-line is also a mesoscale storm phenomenon, one that often occurs in the BMI region but is still rarely studied due to limited long-term data with high spatial resolution that can cover the entire territory of Indonesia.
Both bow-echo and squall-line are difficult to capture by global satellite data, because the resolution is low, making it difficult to record this phenomenon, except for squall-lines which occur on a wide scale, have a long duration (more than 6 hours), and are generated by a movement system of large-scale cyclonic circulation that produces squall lines.
The squall-line phenomenon with a long life cycle occurred on May 20, 2020 and triggered a tidal food disaster that affected the entire southern and northern coasts of Java and Bali, from May 26 to June 3, 2020; it was recorded as the worst tidal food in Indonesian history. The squall-line storm first formed over central Sumatra, triggered by a cyclonic vortex in the Indian Ocean, south of the equator near Sumatra.
Other than atmospheric disturbances, there is also Kelvin wave activity in the sea, as indicated by the rise in sea levels along the equator towards the southern coasts of Sumatra and Java. The interaction between Kelvin and the squall-line has triggered severe tidal foods which occur not only on the southern but also northern coasts of Java and Bali.
Since there no scientific documentation has been created on the Sumatran squall-line which is propagating towards the south (Java), this research specifically investigates the mechanism propagating the squall-line with a long life cycle over a distance of thousands of kilometers, extending from Sumatra to Bali.
By analyzing radar data, satellite imagery, and high-resolution (3 km) weather prediction models, we discovered the squall-line mechanism that developed from several multicell storms, coalescing into giant storms called supercells when crossing the Sunda Strait, because they received a significant boost from steam and moisture above the sea, resulting from the convergence and transport of moisture from the Java Sea and the Indian Ocean.
In addition, the role of horizontal wind shear, with a strong west-southeast orientation has played a role in accelerating propagation which moves at a speed of 13.8 m/sec, taking six hours from Sumatra to reach western Java. This research provides a scientific record regarding the ability of weather prediction models to capture squall-line patterns but has differences in location and slower propagation duration compared to data from radar and satellites with around a three-hour lag.