Investigating boundary layer processes in tropical cyclones in the Met Office operational forecasting model

This Project has been filled

Investigating boundary layer processes in tropical cyclones in the Met Office operational forecasting model

Juliane Schwendike and Andrew Ross (School of Earth and Environment)

Adrian Lock and John Edwards (Met Office)

Jeff Kepert (Bureau of Meteorology, Melbourne, Australia)

Contact email:j.schwendike@leeds.ac.uk

Potential CASE award

 

Project description / Motivation

Each year tropical cyclones cause enormous amounts of damage due to their destructive winds, heavy precipitation and their effects on the sea, e.g. storm surges. The 2017 tropical cyclones season in the Atlantic was a particularly destructive one. Hurricane Irma (Fig. 1) reached wind speeds of 180 mph (285 km/h), and caused damage of around US$ 64.76 billion as well a 44 deaths directly and 85 indirectly in several Caribbean Islands and Florida. Hurricane Maria reached a maximum wind speed of 175 mph (280 km/h) and caused 112 fatalities and US $91.61 billion in damages. Hurricane Maria moved directly over Puerto Rico and left considerable destruction in its wake. Being able to accurately forecast the track and intensity of these high impact weather systems is crucial to enable people to take appropriate action in time to minimise the damage to livelihood, property and economy.

 

Fig. 1:Satellite image of Hurricane Irma on 08 September 2017. Image Courtesy: NASA (https://www.nasa.gov/image-feature/geocolor-image-of-hurricane-irma).

 

The boundary layer is the part of the atmosphere directly influenced by the earth’s surface within about an hour or less. These surface influences include frictional drag, evaporation and transpiration, heat transfer and pollution emission. The depth of the boundary layer ranges from a few hundred meters to kilometres, and is variable in time and space (Stull, 1997). Turbulence in the boundary layer plays a crucial role in modulating the weather we experience at the surface and so there is a strong need to understand the processes and characteristics of this part of the atmosphere. Of particular relevance to this project, the boundary layer also plays an important role during the intensification of tropical cyclones. Correctly capturing this intensification is one of the main challenges of tropical cyclone research, and accurate representation of the boundary layer in tropical cyclones is a key aspects of this.

 

Objectives

The scientific objectives of the PhD project are:

  1. Conduct a detailed investigation into the representation of the boundary-layer structure and processes in simulations of tropical cyclones;
  2. Develop an understanding of the model’s limitations and investigate ways to improve it.

 

To do this the student will:

  • Investigate the representation of the boundary layer structure of selected tropical cyclones in Met Office Unified Model (MetUM) simulations at different horizontal resolutions.
  • Compare the modelled boundary-layer structure with flight-level (e.g. wind, temperature, radar), and dropsonde (wind, temperature, moisture) observations from the Hurricane Research Division (HRD) to identify potential model biases.
  • Investigate the three-dimensional thermal and wind structure in the boundary-layer for selected storms and compare them to the results from idealised model simulations.
  • Develop ways to improve the representation of the boundary-layer in the model and to minimise the identified model biases.
  • Investigate the distribution of vertical velocity and clouds in the storm.
  • Investigate the link between the boundary layer and the free-troposphere.
  • Compare the MetUM set up at the Met Office to the tropical cyclones system at the Bureau of Meteorology for the same Atlantic tropical cyclone. Do different data assimilation systems have an impact of the representation of boundary-layer processes?

 

Potential for high-impact outcome

The Met Office has close working relationships with several weather forecasting agencies in countries directly affected by intense tropical cyclones (e.g. the Bureau of Meteorology in Australia, and other countries in SE Asia). Forecasting the track and intensity of these storms is a key challenge for numerical weather prediction models such as the MetUM, in both global and regional configurations. This project provides a way to investigate in detail how tropical cyclones are represented in the model. Close collaboration with the Met Office and the Bureau of Meteorology will ensure the results will feed into model development, with the aim of improving our ability to forecast these storms more accurately, ultimately benefiting people at risk. We will focus on tropical cyclones in the Atlantic, as flight observations from the HRD are available. Inner-core tropical cyclones dynamics are the same in each ocean basin, which is why the results of the project will be useful for the SE Asia region too. The outcome of this study may also provide guidance for forecasters. The project will generate results for several papers, with at least one being suitable for submission to a high impact journal.

 

Training

The student will work under the supervision of Dr Juliane Schwendike and Dr Andrew Ross from the University of Leeds, Dr Adrian Lock and John Edwards from the Met Office, and Dr Jeff Kepert from the Bureau of Meteorology, Australia. The supervisory team includes expertise in tropical cyclone and boundary layer meteorology, as well as expertise in boundary layer parameterisation. Co-supervision will involve regular meetings between all partners and regular visits to the Met Office in Exeterto work closely with the Met Office supervisors as part of the CASE Met Office Academic Partnership. Additionally, the student might have the opportunity to visit the Bureau of Meteorologyand Monash University in Melbourne, Australia. The successful PhD student will have access to a broad spectrum of training in numerical modelling, through to managing your degree or to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/). This project provides a high level of specialist scientific training in:

  • Numerical modelling and use of cutting-edge supercomputers;
  • Analysis of in-situ measurements from aircraft and ground-based monitoring sites;
  • State-of-the-science application and analysis of global atmospheric reanalysis data;
  • A computer programming language (e.g. Python) to perform complex analysis techniques;
  • Effective written and oral communication skills.

The student will be part of the Dynamics and Clouds Group, which is embedded in the Institute for Climate and Atmospheric Science (ICAS) within the School of Earth and Environment. The Dynamics and Clouds Group is a large and active group of people working on a range of problems, with particular interests in the tropics and in convection. The group meets regularly and these group meeting provide an excellent opportunity to discuss your work as well as to learn more about what others are working on. We encourage you to be an active member of the Dynamics and Clouds Group.

 

Further reading

Kepert, J. D., J. Schwendike, and H. Ramsay, 2016: Why is the tropical cyclone boundary layer not “well-mixed”?.  Journal of the Atmospheric Sciences.  73, pp. 957-973

Schwendike, J., and J. D. Kepert, 2008: The boundary layer winds in hurricanes Danielle (1998) and Isabel (2003).  Monthly Weather Review.  136, pp. 3168-3192.

Stull, R.,1997: Introduction to boundary layer meteorology, Kluwer Academic Publishers.