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  Operational Numerical Weather Prediction

Atmospheric Numerical Models

model areas

Operationally, the Met Office runs a number of configurations of its Unified Model. These range from the global model, with a mid-latitude resolution of ~40 km, down to a high resolution 4 km UK model.

A similar formulation is used for all Met Office forecasts, and is described in a factsheet.

The global model runs provide boundary conditions to each of the operational regional models and the NAE provides the boundary conditions for the 4 km UK model.

Main Operational Model Configurations

  Global NAE UK 4km
Resolution 0.5625° x 0.375°
(~40 km in mid-lats).
0.11° x 0.11°
~12 km
0.036° x 0.036°
~4 km
model size 640 x 481 600 x 360 288 x 360
Model Levels 50
lid ~63 km
lid ~39 km
lid ~39 km
Forecast length 144 hrs 48 hrs 36 hrs

The Met Office also has the capability to rapidly relocate regional models to any area of interest worldwide.
These Crisis Area Models (CAMs) are run in support of allied military operations and disaster relief.

The Numerical Weather Prediction System

Each of the forecast models run within a system known as the operational suite. Once initiated, the operational suite will perform all tasks needed to produce the forecast with no further manual intervention. There are, however, facilities to manually override tasks should any problems occur.

The operational suite embraces all the individual tasks that are required to produce a forecast. Most of the software within the operational suite has been written in-house. The suite itself is controlled by what is known as the suite control system (SCS). The SCS can be used to select which tasks are run, how they are run and when they are run.

The first task is the observation processing to extract all the observations that have been received, to quality control them and finally reformat them into a form ready for use by the model.

For certain runs of the model, a reconfiguration then occurs. This is a procedure to incorporate data fields from external files into the model. This is required to update fields that have their own standalone analysis, such as the sea-surface temperature, or to update a climatological field.

The data assimilation scheme is then run. This adjusts the model background field, which is a forecast from a previous model run, towards the new data received from the observations.

The main forecast is then run, the length of which varies according to the particular run of the model, more details are given below.

The forecast data are written into files known as fieldsfiles. Using these, various plotted charts and maps are produced which forecasters then use to produce the weather forecast. It is important that the charts are available at the earliest possible time and therefore fieldsfiles are produced that cover a 24-hour period only. This enables charts for say T+24 to be plotted and made available even though the forecast is continuing.

Once the forecast has completed various ancillary tasks are run. These include archiving data, monitoring the observations and verifying previous forecasts that were for the current time.

There are user friendly graphical user interfaces to set up the suite and many of the individual tasks such as the observation processing, the data assimilation, the verification and the forecast model itself. This enables an user to set up and run their own experiment as if it were the operational suite.

Producing Forecasts for our Customers

Running a numerical weather prediction model is only part of the process in producing a weather forecast. Before a forecast is issued, the output from the model is studied by a forecaster. For short ranges, the forecaster is able to compare a model field against actual observations and identify any possible errors so that appropriate allowances may be made.

The forecaster may be able to add extra detail to the model forecast. An example of this is in the forecasting of summer showers which are often on a scale too small for the model to resolve adequately. The forecaster is also able to respond quickly and amend a forecast should the situation warrant it.

NWP ProcessFor medium-range forecasts, the forecaster is able to compare the results from our model with those from other centres such as ECMWF (European Centre for Medium-Range Weather Forecasts), NCEP (National Centers for Environmental Prediction) and DWD (Deutscher Wetterdienst). If all models are producing approximately the same solution confidence in the forecast would be high. If, however, all were showing different evolutions the confidence would diminish rapidly. Another factor that influences confidence is the consistency between model runs. If the model consistency follows the same evolution confidence may be high but if it suddenly changes then confidence falls rapidly, and in these situations the solutions of other models may be crucial as to the forecast issued. Sometimes, alternative forecasts may be issued with probabilities assigned.

This human-machine partnership is very important in producing accurate weather forecasts.

The main forecasting facility of the Met Office is the Operations Centre based in Exeter, Devon. The Met Office also has forecasters based in various outstations both in the UK and overseas.

Operational Schedule

model areas

The Met Office operational forecast suite is run 24 hours a day, 365 days a year. The operational suite is made up of the aforementioned atmospheric configurations and various ocean and wave models. Each model is routinely run within a given time slot, within the operational schedule.

For example, the NAE model is run 4 times a day providing model forecast guidance, at a 12 km resolution for up to 2 days ahead. The NAE boundary data is supplied by the previous global model run. NAE runs are initialised with data valid at 00z, 06z, 12z and 18z, and each run starts approximately 1 hour 25 mins later, allowing time for observations to be gathered for the data assimilation scheme to provide the initial condition. An NAE forecast slot is 65 minutes long, 20 minutes of which is used by observation processing and data assimilation.

The global model is run twice daily providing forecast guidance for up to 6 days ahead. These 'main' runs are initialised with data valid at 0000z and 1200z respectively, starting around 0240z and 1440z, taking approximately 70 minutes to produce a forecast out to T+144, again approximately 20 minutes of the run time is used by observation processing and data assimilation.

Short global runs are also run at 06z and 18z to supply further lateral boundary conditions for the regional models in the operational suite.

Operational Products

The Met Office disseminates various products from the operational numerical weather prediction models in GRIB and GRID formats. In this section there is a description of the products that are available.

More about operational products

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Operational models
NWP system
Producing forecasts
Operational schedule
Operational products
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