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  Operational Numerical Modelling

Numerical Models of the Atmosphere

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Operationally, the Met Office runs two configuration of its Unified Model. The global model has a horizontal resolution of 0.8333° longitude (432 columns) and 0.5555° latitude (325 rows) giving an approximate resolution of 60km in mid-latitudes.

The global model is used to provide boundary conditions to the mesoscale model which is a regional model centered on the United Kingdom. This model has a resolution of 0.11° latitude by 0.11° which is approximately 11km. In the mesoscale model there are 146 columns and 182 rows. Both the global and mesoscale models have 38 levels in the vertical.

In the mesoscale model the model North Pole is not located at the geographical North Pole. This is done in order to obtain a fairly uniform horizontal resolution over the area of interest, ie the UK. The mesoscale model has its North Pole situated at 37.5°N 177.5°E.

In both models, the variables are arranged in the same fashion. In the horizontal, an Arakawa C grid is used, the u wind components are east-west staggered from temperature and the v wind components are north-south staggered, and in the vertical Charney-Philips grid staggering is used which means that (potential) temperature, scalars (moisture variables and tracers) and vertical velocity are staggered from (Exner) pressure, density and horizontal wind.

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The Numerical Weather Prediction System

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.

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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.

For 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 National Meteorological Centre (NMC) based in Exeter, Devon. The Met Office also has forecasters based in various outstations both in the UK and overseas.

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Operational Schedule

The global model is run twice a day to produce forecasts for up to 6 days (144 hours) ahead. These main runs are initialised with data valid at 0000Z and 1200Z and are started at approximately 0305Z and 1505Z respectively. The data assimilation scheme uses observations that have been made within 3 hours of the initial data time, i.e. the 1200Z run uses observations made between 0900Z and 1500Z. It takes approximately 70 minutes from the suite starting to produce a forecast for T+144, approximately 20 minutes of that comprises the observation processing and data assimilation schemes.

To maintain the assimilation cycle, there are intermediate runs with data times of 0600Z and 1800Z which are run at approximately 1300Z and 0100Z respectively. These only produce forecasts to 9 hours ahead in readiness for the next main run. It can be seen that these are run with a delay of 7 hours from the initial data time whereas the main runs only have a delay of 3 hours. Therefore, to maintain consistency, forecasts from 0000Z and 1200Z are repeated at 0715Z and 1820Z respectively. These forecasts are only to 9 hours ahead and provide the background for the next intermediate run.

A mesoscale model covering the UK and surrounding areas is also run. The main runs of the mesoscale model are at data times of 0000Z, 0600Z, 1200Z and 1800Z and these are initiated at 0200Z, 0800Z, 1400Z and 2000Z respectively. The mesoscale model uses observations 90 minutes before and after the data time and produces forecasts for upto 48 hours ahead. To maintain the assimilation cycle, intermediate runs are run with data times of 0300Z, 0900Z, 1500Z and 2100Z at 0545Z, 1215Z, 1910Z and 0010Z respectively, each to 4 hours ahead.

In addition to the main atmospheric models, various ocean and wave models are run. The operational suite runs 24 hours a day, 365 days a year.

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'New dynamics'

In August 2002, the Met Office introduced a completely new formulation of its operational models. It was centred around a completely new formulation of the model's dynamical core, the fundamental equations, hence the term 'new dynamics' but there were also many enhancements to the physical parametrizations. A fuller description of the work may be found in the June 2002 edition of NWP Gazette and a factsheet has also been produced.

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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