Large-Scale Meteorological Drivers of Extreme Precipitation Event and
Devastating Floods of Early February 2021 in Semarang, Indonesia
Abstract
Unusually long duration and heavy rainfall on 5-6 February 2021 causes
devastating floods in Semarang. The heavy rainfall is produced by two
mesoscale convective systems (MCSs). The first MCS develops at 13Z on 5
February 2021 over the southern coast of Sumatra and propagates towards
Semarang. The second MCS develops over the north coast of Semarang at
18Z on 5 February 2021, which later led to the first peak of
precipitation at 21Z on 5 February 2021. These two MCSs eventually merge
into single MCS, producing the second peak of precipitation at 00Z on 6
February 2021. Analysis of moisture transport indicates that the strong
and persistent northwesterly wind near the surface induced by CENS prior
to and during the event, creates an intensive meridional (southward)
tropospheric moisture transport from the South China Sea towards
Semarang. In addition, the westerly flow induced by low-frequency
variability associated with La-Nina and the tropical depression
associated with tropical cyclone formation over the North of Australia,
produces an intensive zonal (eastward) tropospheric moisture transport
from the Indian Ocean towards Semarang. The combined effects of the
zonal and meridional moisture transports provide favorable conditions
for the development of MCSs, and hence, extreme rainfall over Semarang.