The Leibniz Centre for Tropical Marine Research (ZMT) in Bremen is a member of the Leibniz Association, which is supported by the German Federal and State Governments. Through its research, ZMT contributes to developing science-based strategies for sustainable use of tropical coastal systems.

Master thesis project

Understanding the spatial and temporal organisation of precipitation on tropical islands:

Role of land/sea contrast, topography, and wind direction

 

There is typically more precipitation over tropical islands than over their ocean surroundings. This “island effect” occurs for example over the Maritime Continent (Qian et al 2008). This general effect holds well, except perhaps for very small and very large islands (Robinson et al 2011, Ulrich and Bellon 2019). The optimal island size which most enhances precipitation seems to be around 20km (Robinson et al 2008, Cronin et al 2014). Previous studies have essentially focused on the effect of island size on island precipitation intensity, which can be understood by the thermodynamic and dynamic consequences of surface heat flux heterogeneities. But other factors such as topography and position with respect to the prevailing winds (e.g., leeward side vs. windward side), also seem to play an important role (Specq et al 2020). Even though large-scale climate modes and convectively coupled equatorial waves may also significantly influence island precipitation (Hopuare et al 2018, Sui et al 2020), we leave these large-scale effects aside for now, as we wish to focus on the influence of convection itself.

The overall precipitation intensity over islands is a critical feature, but so is the spatial organisation of precipitation over and around the islands. The centre of tropical islands is usually where most rain occurs (Ulrich and Bellon 2019). And around islands, there is night-time offshore propagation of precipitation (Vincent and Lane 2016, Coppin and Bellon 2019a,b). Moreover, the timing of precipitation is also a key feature to understand. For example, Cronin et al 2014 showed that for flat idealised islands with no winds, the bigger the island, the later the precipitation peaks in the diurnal cycle. Here we would like to extend these studies by analysing slightly less idealised island situations, by including topography (Coppin and Bellon 2019b) and mean wind (representing the Trade winds).

By analysing idealised simulations of tropical islands, we can test ideas to explain the relative effects of island size, island topography, and prevailing wind direction, on precipitation intensity as well as on the spatial and temporal organisation of precipitation over and around islands. Relevant questions may be: (1) How much can topography influence precipitation intensity and timing, compared with island size? (2) How do island size and topography affect the spatial organisation of precipitation? (3) What is the minimum wind speed necessary to have a significant difference between the leeward and the windward sides of the island? These results should have implications to help authorities determine flood risks and plan for more resilient tropical islands.

Intended learning outcomes: To become familiar with the state-of-the-art modelling of tropical deep convection, including the thermodynamics and fluid dynamics around tropical islands. To understand concepts of spatial organisation and temporal memory of precipitation. And to contribute to an exciting and timely research topic with the potential for publication in a peer-reviewed research journal.

We offer: You will be part of a dynamic, international research group (Complexity & Climate), have the opportunity to collaborate with PhD and postdoctoral researchers and will be able to present your research output at an international conference. Given conclusive results, you will be able to submit them to a peer-reviewed journal.

Candidates (gn) should have an interest in tropical deep convection, in tropical island climate, and/or in discovering dominant physical processes in Nature. A quantitative background is an advantage, e.g. physics, applied maths, meteorology or engineering. The project start would be as soon as possible.

To apply please contact:

Jan O. Haerter, WG Complexity & Climate (This email address is being protected from spambots. You need JavaScript enabled to view it.) and just explain your interests and background within a short email, as well as your timeframe.

References                                             

  • Coppin, D., & Bellon, G. (2019a). Physical mechanisms controlling the offshore propagation of convection in the tropics: 1. Flat island. Journal of Advances in Modeling Earth Systems, 11, 3042–3056. https://doi.org/10.1029/2019MS001793
  • Coppin, D., & Bellon, G. (2019b). Physical mechanisms controlling the offshore propagation of convection in the tropics: 2. Influence of topography. Journal of Advances in Modeling Earth Systems, 11, 3251–3264. https://doi.org/10.1029/2019MS001794
  • Cronin, T. and Emanuel, K. and Molnar, P. (2014). Island precipitation enhancement and the diurnal cycle in radiative-convective equilibrium.
  • Hopuare, M. and Guglielmino, M. and Ortega, P. (2018). Interactions between intraseasonal and diurnal variability of precipitation in the South Central Pacific: The case of a small high island, Tahiti, French Polynesia
  • Qian, J-H (2008). Why Precipitation Is Mostly Concentrated over Islands in the Maritime Continent
  • Robinson, F. and Sherwood, S. and Li, Y (2008). Resonant Response of Deep Convection to Surface Hot Spots
  • Robinson, F. and Sherwood, S. and Gerstle, D and Liu, C. and Kirshbaum, D. (2011). Exploring the Land–Ocean Contrast in Convective Vigor Using Islands
  • Specq, D. and Bellon, G. and Peltier, A. and Lefevre, J. and Menkes, C. (2020): Influence of Subseasonal Variability on the Diurnal Cycle of Precipitation on a Mountainous Island: The Case of New Caledonia
  • Sui, C.-H., P.-H. Lin, W.-T. Chen, S. Jan, C.-Y. Liu, Y. J. Yang, C.-H. Liu, J.-M. Chen, M.-J. Yang, J.-S. Hong, L.-H. Hsu, and L.-S. Tseng (2020): The South China Sea Two Islands Monsoon Experiment for studying convection and subseasonal to seasonal variability. Terr. Atmos. Ocean. Sci., 31, 103-129, doi: 10.3319/TAO.2019.11.29.02
  • Ulrich, M., & Bellon, G. (2019). Superenhancement of precipitation at the center of tropical islands. Geophysical Research Letters, 46 , 14,872–14,880. https://doi.org/10.1029/2019GL084947
  • Vincent, C. and Lane, T. (2016). Evolution of the Diurnal Precipitation Cycle with the Passage of a Madden–Julian Oscillation Event through the Maritime Continent

Leibniz Centre for Tropical Marine Research (ZMT), Fahrenheitstraße 6, D-28359 Bremen.