Mesoscale modelling

Forecasting with high-resolution models

A system called numerical weather prediction (NWP) forms the basis of modern weather forecasting. The system uses a mathematical model of the atmosphere which has been derived from the laws of physics. This model provides a set of equations to solve in order to predict the future weather. These equations are solved by averaging over 'chunks' of the atmosphere (grid boxes) and short periods of time (time steps) to then give us numerical equations which are put into the supercomputer.

Model resolution

The size of a chunk is called the 'resolution' of the model, similar to the resolution of an image from a digital camera. More than one grid box is needed to represent weather features in the much the same way as a digital image needs more than one pixel to represent something like a face. The number of equations which have to be solved depends on the total number of grid boxes. As the equations have to be solved long before the weather happens, the size of grid boxes (given the forecast area) is limited by how quickly they can be solved.

The current global forecast model has a horizontal resolution of about 40 km over the UK, meaning 160 million equations have to be solved just to step the atmosphere 15 minutes in time. This resolution is very good for information about the general weather conditions over the UK, and the Met Office's current computer power means a 5-day forecast can be produced in a few hours. At shorter range (1-2 days) a higher-resolution model (about 12.5 km) is used, because it provides more regional detail.

Convective clouds

Much of the UK's most damaging weather involves clouds which 'bubble up' from near the surface when a layer of cool air lies above a layer of relatively hot, moist air. These are called convective clouds, and the most energetic are often thunderstorms which may produce torrential rain, snow, damaging hail, flash flooding and strong winds, including tornados.

The resolution of current models is unable to represent individual convective clouds. A large thunderstorm may be about 10 km across, with a very strong core less than 1 km across, so the current models completely average over even the largest thunderstorm. While the forecast might state that a particular region is at risk from thunderstorms and torrential rain, it is currently impossible to know where individual thunderstorms might form.

Convective-scale NWP

As computer power increases it will be possible to reduce the grid box size in the models. In preparation, the Met Office is already experimenting with models which make a major jump in resolution. Instead of completely averaging over thunderstorms, models are being developed which actually allow thunderstorms to form. So far there have been good results for large storms from a resolution of around 1 km. This is known as 'convective-scale' NWP. This resolution is also very useful for other aspects of weather, such as major areas of fog and detailed features of the wind around ranges of hills.

The animation on the right is an example forecast from an NWP model running at 1.5 km resolution, showing a representation of cloud (grey) and rainfall (colours).

Convective-scale NWP in action

We have tested our experimental system using a wide variety of recent events over the UK. For example, the major flash flood in High Wycombe on 3 August 2004.