New science upgrades to the Met Office's high resolution models over the UK
July 2017 - Significant upgrades have been made this month to the UK high resolution Numerical Weather Prediction (NWP) models run at the Met Office.
The changes were mainly targeted at getting the most out of the latest weather observations to improve predictions for a few hours ahead. The way the NWP models keep track of all the atmospheric moisture in our weather systems has also been improved, with dividends for predictions in the one or two day time frame.
These latest upgrades have taken further advantage of the new supercomputer, using twice as much computational resource as the previous operational system.
Hourly cycling and NWP nowcasting
Previously the UK system ingested new observations and produced a new forecast every three hours. The new system has moved to an hourly cycle so that model predictions can exploit the very latest data and quickly feed into a variety of downstream systems generating products for customers.
To keep within the new tight schedule, the UK system can now only afford to wait up to 45 minutes past the hour to collect new observations, a ‘cut-off’ time that is 30 minutes tighter than before. So some data arrives too late to be used but, to offset that loss, we now accept more frequent observations from radar and satellites within each one-hour window.
The hourly model predictions are supplied out to a minimum forecast range of 12 hours. The combination of an hourly cycle, a short cut-off for data collection and more powerful supercomputer resource means that the NWP data reaching the product systems is typically based on information that is 2 hours ‘fresher’ than before. Which is good news for improving accuracy over the first few hours of the forecast. This is the realm of ‘NWP nowcasting’. An illustration of improved rainfall prediction from the new system can be seen below.
Radar image at 02UTC on Saturday 27th May 2017 (right), with precipitation rate forecasts from 18UTC on 26th May valid for the same time, using the previous operational model (left) and the new version (middle). Forward convection line moving NEwards through SW England apparent in the new operational version but absent in the previous model.
4-dimensional variational data assimilation – an Olympic legacy
Each new observation tells us something about the error in our latest prediction at the place and time it was made. We need to transform that information into corrections at nearby locations and times before and after the measurement time itself.
Our previous operational system handled just the spatial part of that error translation, by a technique known as 3-dimensional variational assimilation or ‘3D-Var’. The new system goes further in treating the time dimension.
In this ‘4D-Var’ approach, the evolution of a correction at one time is predicted for earlier and later times by a simplified version of the whole NWP model. It’s more expensive in computer time but gives better initial conditions for the new forecast.
4D-Var has been around in operational global models for about 20 years, but is less common in operational high-resolution convective-scale models with grid-lengths of 1-2km. The Met Office previously tested it on this scale during the ‘Nowcasting Demonstration Project’ run for the London 2012 Olympics. The new implementation builds on the success of that project.
Keeping track of the moisture
Another key ingredient of the July 2017 package is an improved treatment of moisture conservation. This prevents spurious sources and sinks of atmospheric moisture from arising in the model as we calculate the movement of water vapour and cloud droplets by the wind field. One benefit is that it leads to fewer predictions of unrealistically high rain rates at isolated locations. Another impact has been detected on cloud cover forecasts - an example of the new package giving a significantly improved forecast can be seen below.
Visible satellite image at 11am on Thursday 16th April 2017 (left), with cloud forecasts from Wednesday evening for the same time, using the previous operational model (middle) and the new version (right).
Summary of UK model upgrades