The role of orography in enhancing frontal rainA.N.Ross@leeds.ac.uk
Frontal rain accounts for a significant fraction of the total precipitation in the mid-latitudes. The enhancement of such rainfall due to hills and mountains can be significant, both on a seasonal timescale, as shown by the rainfall climatology of the UK (Fig 1a), and for individual events (Fig 1b). Severe flooding is often associated with stationary frontal systems over mountain ranges that result in large localised rainfall totals (e.g. Cockermouth floods of 2009, Sibley (2010); Storm Desmond and the Boxing Day floods in December 2015, Burt et al (2016)). A number of different mechanisms can contribute to orographic enhancement of rainfall including the seeder-feeder process; triggering of embedded convection; flow blocking; upwind moisture transport by low level jets and modulation by gravity waves (see e.g. Houze, 2012). There are important questions about how these mechanisms operate in particular UK events, what is their relative importance and how they interact with each other. In high resolution models, many of these processes should be captured providing the underlying orography is adequately resolved. Although the operational 1.5km model over the UK for example often does a reasonable job in identifying the location and timing of enhanced precipitation, it still struggles to accurately forecast the peak precipitation amounts observed. However in global weather and climate models much of the orography is sub-grid and therefore the orographic enhancement of precipitation is significantly under-predicted.
Figure 1: (a) Climatological annual rainfall over the UK (1981-2100) and (b) 48 hour rainfall radar loop for Storm Desmond 5th - 6th December 2015.
The proposed project will be concerned with orographic enhancement of frontal rain over the UK. The focus will be on understanding the dominant physical processes; mainly the role of stratified processes (gravity waves, upwind blocking), moist stability and latent heating effects, including the impact of cloud microphysics. A combination of observations and a state-of-the-art numerical model will be available for the project. These processes are all known to play a role, yet there are few systematic studies of their relative importance in orographic enhancement of frontal rain. It is expected that high resolution Unified Model (UM) simulations that include the latest microphysics scheme (called CASIM) will be performed for past case studies to identify the dominant physical mechanisms responsible for enhancement in the different cases. Use will also be made of a wide range of observation data both operational (rain gauge network, radar) and research (e.g. high density range gauge and NCAS radar data from COPE, Leon et al. 2016, and other projects). Further idealised simulations will be conducted to investigate how the answers depend on the upstream conditions and the orography. In recent years several simplified theoretical models have been put forward to explain aspects of orographic precipitation and the student would also potentially develop these in light of the modelling and observational studies.
Key questions will include
How do flow blocking and gravity waves affect the enhancement of frontal rain over mountains?
How does moisture alter the gravity waves and flow blocking?
Are interactions between the stratified flow and cloud processes important in controlling the location and intensity of orographic precipitation?
Why do even high resolution (1.5km) forecast models struggle to capture the peak rainfall observed over mountains?
Can a combination of observations, theory and state-of-the-art high resolution simulations be used to reduce model uncertainty and improve predictability of severe flooding events?
Answering these questions will lead to a better physical and theoretical understanding of processes controlling orographic enhancement. This understanding will help to identify key features of the flow which need to be accurately modelled in high resolution simulations in order to accurately forecast orographic enhancement of precipitation. Improved physical understanding and theoretical models are also required for future parametrization development. Working closely with the Met Office will ensure that improvements in understanding are tested as parametrization schemes in research versions of the model in collaboration with those working on improving the operational model.
Potential for high impact outcome
Recent flooding has highlighted the importance of orographic enhancement of precipitation and the need to represent it better in models – both for operational forecasting and to predict how extreme rainfall will change with climate change. This, together with the possibility of high resolution simulations including the new CASIM microphysics scheme, make this a timely project. It is therefore expected that the project will lead to publications in high impact journals. There is also the expectation that the research will inform and direct future improvements in the representation of orographic enhancement to precipitation in the Met Office Unified Model. Finally, the public awareness of flooding following recent events means there are significant opportunities for outreach and public engagement around the science of forecasting heavy rainfall and flooding.
The student will work under Dr Andrew Ross and Prof Alan Blyth in the Dynamics and Clouds research group within ICAS. This project provides a high level of specialist scientific training in: (i) State-of-the-science application and analysis of high resolution numerical weather prediction (NWP) models; (ii) analysis of in-situ and remote sensing measurements of precipitation and atmospheric structure from a range of observational platforms; (iii) numerical modelling and use of cutting-edge supercomputers. Co-supervision will involve regular meetings between all partners and extended visits for the student to Exeter, where they will work alongside researchers in the UK Met Office, under the joint supervision of Dr Simon Vosper and Prof Paul Field. The successful PhD student will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of training workshops in numerical modelling, through to managing your degree, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).
The student should have a keen interest in the challenges of understanding and modelling the weather and a strong background in a relevant quantitative science (meteorology, maths, physics, engineering, environmental sciences). Experience of scientific programming / data analysis is desirable, but not essential.
The proposal has been agreed as a “Partnership Project” (a potential CASE project) with the UK Met Office providing extra funding additional to the NERC student stipend. Leeds has a strong record of close collaboration with the Met Office and is one of only 4 universities in the Met Office Academic Partnership. The project aligns with existing collaborations between Leeds and the Met Office on orographic flows, convection and precipitation. The successful student will spend time working with the CASE supervisors at the Met Office
Burt S. et al (2016) Cumbrian floods, 5/6 December 2015. Weather 71 36-37. doi: 10.1002/wea.2704
Houze, R.A. (2012) Orographic effects on precipitating clouds. Rev. Geophys. 50 RG1001, doi: 10.1029/2011RG000365
Leon et al (2016) The Convective Precipitation Experiment (COPE): Investigating the origins of heavy precipitation in the southwestern United Kingdom. Bull. Am. Meteorol. Soc. 97 1003-1020. doi: 10.1175/BAMS-D-14-00157.1
Sibley, A. (2010) Analysis of extreme rainfall and flooding in Cumbria 18-20 November 2009. Weather 65 287-292. doi: 10.1002/wea.672
Related undergraduate subjects:
- Environmental science