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Severe weather over Southeast Asia

The Maritime Continent in Southeast Asia (predominately Malaysia, Indonesia and Papua New Guinea) plays a key role in the global weather and climate system. Its complex island geography and position among the warmest oceans on Earth lead to severe convective weather systems that produce intense rainfall, leading to disasters such as flooding and landslides. In turn, the large amounts of latent heat released from these weather systems force a global atmospheric response, which affects weather and climate across the Earth.

The diurnal cycle is the fundamental building block of organised convection over the Maritime Continent. Conceptual models suggest that convection is initiated inland of the coastlines during the late afternoon due to upslope mountain winds and sea breeze convergence. During the night, a combination of downslope mountain winds, the land breeze and gravity waves causes the convection to propagate offshore and become more organised, producing a distinct diurnal cycle that is common to many of the islands.

There are major knowledge gaps in our understanding of the processes within this multi-scale system due to difficulties modelling the tropical atmosphere and ocean over such complex geography, and a dearth of suitable in-situ observations against which to evaluate models. This lack of understanding is a significant limitation to weather forecasting and climate projections in the region as well as, given the large-scale imprint, on the rest of the tropics and the extra-tropics.

The international initiative “Years of the Maritime Continent” (YMC) is currently underway, including the UK contribution through a 5-year NERC large grant “TerraMaris”, which includes air- and ground-based fieldwork in 2022 (Covid-19 dependent) and extensive modelling. This PhD project provides a unique opportunity for a student to be involved in the international project and make use of satellite products, extensive and unique in-situ observations already made through YMC, and TerraMaris modelling. There will be an opportunity to take part in research aircraft flights or ground-based fieldwork in Indonesia if Covid-19 allows.

Specifically, the project will:

  • Evaluate rainfall retrievals from the Integrated Multi-satellitE Retrievals for GPM (IMERG) over sub-regions of the Maritime Continent using ground-based radar observations available through YMC.
  • Use satellite rainfall retrievals and IR brightness temperature data from geostationary satellites (specifically Himawari-8 in this region), ground-based radar and in-situ observations to study the atmospheric processes that lead to the initiation and propagation of severe convective storms (Fig. 1). Processes include the land-sea breeze, storm outflows, cloud formation and gravity waves.
  • Investigate how low-frequency modulations of the background state (e.g. variability in wind, stability and humidity through the annual cycle and Madden-Julian Oscillation) impact the initiation and propagation of Maritime Continent convection.
  • Evaluate how numerical weather models of varying complexity represent severe storm formation and use these models to study key processes beyond the limitations of the observations.
  • Provide research that will ultimately lead to improved weather forecast models and weather forecasting practises.

The Met Office CASE award will allow the student access to a suite of state-of-the-art model products and computing facilities as well as staff expertise in remote sensing and in-situ observations, modelling and understanding of atmospheric processes such as convection. The student will be based and registered at the University of Leeds.



Figure 1: Squall line along the west coast of Sumatra in a Himawari satellite image of brightness temperature on 10 December 2015 at 0920 UTC. The IMERG precipitation is shown in contours. The horizontal wind at 850 hPa and 200 hPa is shown in blue and green arrows, respectively, and based on ERA5 reanalysis data. Image courtesy: Simon Peatman.