CR2025_13 Understanding km and sub-km scale model forecasts of orographically enhanced stratiform precipitation in the UK

Lead Supervisor: Chris Westbrook, Department of Meteorology, University of Reading

Email: c.d.westbrook@reading.ac.uk

Co-supervisors: Humphrey Lean, Met Office; Samantha Smith, Met Office

It has long been known that the precipitation locally on and around hills and mountains can be much heavier than in surrounding areas of flatter terrain. This effect is a major source of flooding events in the UK, such as the extreme Cockermouth flood in Cumbria in 2009, and hence is an important forecasting problem (Lean and Browning 2013). As warm, moist air is lifted from low-levels over the hills, dense clouds of water droplets form at the hill tops. These droplets acts as “food” for raindrops or snowflakes falling from higher “seeding” clouds (typically from rainbands associated with extratropical cyclones), allowing them to grow very quickly over a short fall distance, before reaching the ground close to the mountain peaks. The image shown (from a recent study by Cuckow et al 2024) shows the rainfall during the passage of a high-profile extratropical cyclone.  The very heavy precipitation evident over the hills and mountains in Wales, and across the Pennines, are caused by this “seeder-feeder” mechanism.

Despite advances in the resolution and complexity of atmospheric models, this kind of orographic precipitation often suffers from poor performance in numerical weather prediction models at the current operational gridlength of 1.5km, with either insufficient water falling to the ground, or the placement of that water being incorrect (which has knock-on effects for the subsequent hydrological impacts). Possible reasons for these problems include:

• insufficient resolution (either of the orography itself, or the fine-scale atmospheric motions produced in response to it)
• missing / inaccurately represented large scale dynamical features, such as a the fast moving stream of warm moist air commonly found ahead of cold fronts
• inaccurate representation of the microphysical processes in the clouds and precipitation (for example: incorrect diagnosis of liquid vs ice precipitation, persistence of liquid water cloud at cold temperatures, unrealistic representation of how the precipitation size spectrum evolves during the fall through the feeder cloud)

In this PhD project, the student will explore these challenges using high-resolution numerical weather prediction models (from 1.5km which is currently used for operational forecasting over the UK, down to experimental models at 100m grid lengths), and observational data sets (encompassing data from a variety of sensors and field projects).

Using the numerical model, we can explore the effect of improving the detail of the resolved orography and air flow at higher resolution; we can also investigate how the details of the simulation results are controlled by the representation of the physical processes (e.g. using the new CASIM microphysics scheme) or small changes in the initial conditions of the forecast (e.g. making use of ensemble forecasts).

However in addition to modelling, a key facet to this project is the use of observations to understand what is going on in the real atmosphere, and use that to evaluate the realism of the model simulations and understand how and why they go wrong.

• Using research weather radar measurements from field projects in Cumbria and/or around the mountains of northern Scotland, we can investigate the strength, orientation and vertical depth/location of low-level moist airflows onto the hills. We can also use these measurements to quantify the precipitation falling from the seeder clouds, and observe it being modified by the feeder cloud. Finally the radar measurements can also tell us about the cloud phase and microphysics. This information can be supplemented using raingauge data across the area of interest
• Another dataset we can exploit is from the UK’s research aircraft with measurements sampled through a system where this process is believed to have been active.
• A third option is to explore a long-term field project on Bodmin Moor in Cornwall where a dense network of raingauges and surface meteorology was collected, which can be synthesised with weather radar observations.

This project is an opportunity to work closely with the Met Office on an exciting and important scientific problem which is relevant to improving important model forecasts for these high impact events.

Training opportunities:

The project includes a placement at the Met Office’s headquarters in Exeter.

Student profile:

This project would be suitable for students with a degree in a physical or environmental science

Co-Sponsorship details:

The project will receive a CASE award from the Met Office.