Michael Johnston

PhD Year 3

PhD Research Project: Island Convection and its Interaction with Larger Scales

It is well known that convection regularly occurs over islands of all sizes and topography. Differential heating between an island and the surrounding sea forces a local circulation response, this then sets up favoured regions of ascent (where clouds are more likely to form), and descent (where clouds are more suppressed). On small flat islands, the flow carries heated air downwind before strong enough circulations can form locally. Instead, an ascending plume of warmer air extends downwind and becomes cloud topped when it is sufficiently humid (cloud trails). On larger islands (or small islands with light or no background flow), organised seabreeze fronts can form that move inland toward the centre of the island and trigger organised convective systems. Meanwhile, topography acts as an obstacle to the impinging flow - clouds can form where sufficiently humid flow ascends over topography.

We investigate cloud trails, a band or several bands of convective cloud that are anchored to a small flat island and can extend over 100 km downwind. Cloud trails are a case in which small scale surface inhomogeneities trigger larger scale flow perturbations. A climatological review of cloud trail activity at Bermuda is conducted to add to our understanding of the conditions under which they form, and how they behave. High resolution idealised simulations are then conducted to focus on the mechanisms through which they form, are maintained, transition from shallow to deep convective states, and eventually decay. Finally, the implications of these findings for convective parameterisation schemes are then investigated.

Research Interests:

  • Convection, transitions between convective states
  • Numerical Modelling at fine spatial resolution
  • Tropical Cyclones
  • Remote Sensing

Associated Projects:

  • Parameterisation of Convection : A five-year programme of work jointly funded between NERC and the Met Office with the aim of significantly improving the representation of convection across model scales from 1-100km. (ParaCon)


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