Use-case: National Weather Service operational forecasting for severe weather warning service

A national weather service provides weather forecasts on a routine basis. This use case describes the data inputs & outputs within the process chain required to deliver the end-user service.

1) Observation collection:
The forecasting centre continually receives observations of varied types from a number of sources in order to establish an accurate starting condition for the numerical weather prediction process. Quality control procedures are applied to the collected data to ensure that data from poorly calibrated or faulty equipment does not adversely impact the weather forecast.

-- Satellite information collection [representative]:
  -- Platform: polar orbiting satellites (NOAA-15, NASA EOS Aqua [http://aqua.nasa.gov/about/], EUMETSAT MetOp [http://en.wikipedia.org/wiki/Metop], US Defense Meteorological Satellite Programme 'DMSP' [http://nsidc.org/data/docs/daac/f17_platform.gd.html])
    -- Instrument: Advanced TIROS Operational Vertical Sounder (ATOVS)
      -- datasets: 3-dimensional representation of atmospheric temperature & humidity
    -- Instrument: Advanced Microwave Sounding Unit (AMSU) [http://en.wikipedia.org/wiki/Advanced_Microwave_Sounding_Unit]
      -- datasets: temperature profile (surface to 45 km altitude), water vapour profiles (surface to 15 km) altitude, snow & ice coverage, cloud liquid water, rain-rate
    -- Instrument: Atmospheric Infrared Sounder (AIRS) [http://aqua.nasa.gov/about/instrument_airs.php]
      -- datasets: temperature profiles, humidity profiles, cloud-cover, surface temperature
    -- Instrument: Advanced Microwave Scanning Radiometer for EOS (AMSR-E) [http://aqua.nasa.gov/about/instrument_amsr.php] 
      -- datasets: precipitation rate, cloud water, water vapour, sea surface winds, sea surface temperature, ice, snow & soil moisture
    -- Instrument: Infrared Atmospheric Sounding Interferometer (IASI)
      -- datasets: atmosphere temperature soundings accurate to 1°C and relative humidity soundings accurate to 10% at 1 km vertical resolution
    -- Instrument: Special Sensor Microwave Imager/Sounder (SSM/I)
      -- datasets: 25 km resolution polar sea ice concentrations
    -- Advanced Scatterometer (ASCAT) [http://www.esa.int/esaLP/SEMBWEG23IE_LPmetop_0.html]
      -- datasets: sea surface wind speed & direction, land- and sea-ice, snow cover, soil moisture
  -- Platform: geostationary satellites (Meteosat 2nd Generation 'MSG' [http://www.esa.int/esaMI/MSG/SEMQSCULWFE_0.html], NOAA GOES East & West [http://www.goes.noaa.gov/], Multifunctional Transport Satellites 'MTSAT' [http://www.jma.go.jp/jma/jma-eng/satellite/])
      -- datasets: visible imagery (reflected sunlight from the earth's surface & clouds), thermal infrared imagery (temperature of clouds, land, sea surface plus water vapour, carbon dioxide & ozone within the atmosphere)

   Data from the satellite platforms are often collected and processed at designated land-stations then distributed globally to subscribers; for example via the NOAA Satellite and Information Service (NESDIS) [http://www.nesdis.noaa.gov/] or EUMETSAT's EUMETCAST [http://www.eumetsat.int/HOME/Main/What_We_Do/EUMETCast/index.htm].

-- Radar information collection:
    A weather radar is used to locate precipitation, calculate its motion & estimate the precipitation type (rain, snow, hail etc.) ... [http://en.wikipedia.org/wiki/Weather_radar]

  Data from weather radar networks is often collated within a single operational 'hub' in order to develop a composite picture derived from many individual weather radars. The Operational Programme for the Exchange of weather RAdar information (OPERA) [http://www.knmi.nl/opera/] provides an example of such a facility.

-- Upper air information collection:
  -- Radiosonde [http://en.wikipedia.org/wiki/Radiosonde] - worldwide there are more than 800 radiosonde launch sites. Nearly all radiosonde launches occur at 0000 UTC or 1200 UTC to provide an instantaneous 'snapshot' of the atmosphere from all locations where a radiosonde was launched.

  -- Aircraft Meteorological Data Relay (AMDAR) [http://en.wikipedia.org/wiki/Aircraft_Meteorological_Data_Relay] - data are collected from commercial aircraft navigation systems and the onboard standard temperature and static pressure probes. The data from the aircraft are then processed before transmission to ground stations and onward distribution to subscribers.

  -- Wind Profilers [http://en.wikipedia.org/wiki/Wind_profiler] - detection of wind-speed and direction at various elevations above the ground creating a vertical profile

  -- GPS Water Vapour - using the delay in signal propagation from GPS satellites to ground receivers one can derive estimates of the total column integrated water vapour content of the atmosphere with temporal resolution of 6-minutes.

  -- SFERICS - each lightning flash creates an identifiable radio-frequency 'signature'. With a sufficiently distributed network of receivers, one can use the arrival time difference of a specific radio-frequency pulse (matched using the 'signature' of the flash) at each receiver to triangulate the origin of the signal and hence the location of the lightning flash. These 'sferics' observations are used to correlate the location of storms. It is estimated that the frequency of lightning flashes is approximately 44 times a second, with 80% being cloud-to-cloud and the remaining 20% cloud-to-ground.

-- Terrestrial observations
  -- Automatic Weather Stations - atmospheric pressure, visibility, cloud-cover/height/type, wind-speed & direction, air temperature, humidity, dew-point, precipitation, hourly rain-accumulations.  Traditionally, surface observations were collected manually at synchronized times throughout the globe in order to create an instantaneous 'snapshot' of the state of the surface-level atmospheric conditions at regular intervals. The WMO governs a strict protocol for the collection of these 'synoptic' observations (SYNOPS). As weather stations have become automated and the atmosphere in the vicinity of the instrument is continuously sampled, it has become routine to collect information more frequently, perhaps on 10-minute or 5-minute intervals, in order to develop a near-real time appreciation of the observed conditions. [The British Atmospheric Data Centre provides more information on surface data collected by the UK Met Office here: http://badc.nerc.ac.uk/data/surface/ukmo_guide.html]

  -- Drifting Buoys - satellite-tracked surface drifting buoys observations of currents, sea surface temperature, atmospheric pressure, wind-speed & direction and salinity (wave / swell height & period?). See NOAA's global drifter programme for examples [http://www.aoml.noaa.gov/phod/dac/gdp.html]

  -- Moored Buoys - as above, but deployed to a fixed location.

2) Data Assimilation & merge-back
The forecasting centre will receive around half a million observations each day, recording the atmospheric conditions around the world. However, even with this many observations, the forecasting centre does not have enough information to establish the state of the atmosphere at all points on or above the Earth's surface. 

There are large areas of ocean, inaccessible regions on land and remote levels in the atmosphere where there are very few observations, or none at all. To fill in these 'gaps' the observations are combined with a forecast of what the expected conditions should be using a process known as 'data assimilation'.  A 4-dimensional variational analysis (4DVAR) is used to position each observation in space and time so that a hybrid estimate of the atmospheric conditions can be derived from a previous forecast.

Because the assimilation accurately models the processes at work in the atmosphere, the assimilation process is also used to apply further quality control to observed phenomena that are beyond expected tolerances within the model. These additional quality control fields are 'merged-back' into the observations databases.

The output from the assimilation (the 'analysis') is exported from the model for archive & may be provided to other numerical weather prediction centers for initialization of their global models. 

Parameters include: [[to be added]]

Assimilation and merge-back occurs 2-times daily to initialize the global model and at additional times to support regional models.

3) Global weather prediction
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 period s of time (time steps) to then give us numerical equations that are executed on a computer. 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 required to represent weather features, in the same was that a digital image needs more than one pixel to represent something like a face.

The 'global model' is run twice daily, providing forecast guidance for up to six days ahead (T+144 hours). This provides the best estimate of the forecast state of the atmosphere at a resolution of ~40 km at 50 vertical levels to a ceiling of ~63 km above the Earth's surface and 3- to 6-hour time-steps.

These 'main' runs are initialized with data valid at 0000 UTC and 1200 UTC respectively, starting around 0240 UTC and 1440 UTC. It takes approximately 70 minutes to produce a five-day forecast. Parameters include: [[to be added]]

The global model runs provide boundary conditions for 'mesoscale' (or limited-area) models that cover discrete regions of the Earth's surface at finer resolutions.

4) Mesoscale weather prediction
Much of the 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 global model is designed to capture large-scale features of the dynamic atmosphere, accurately modeling the troposphere and stratosphere away from the Earth's surface. However, at ~40 km resolution,the global model is far too coarse to capture detail on this convective scale where a large thunderstorm may be about 10 km across, with a very strong active core less than 1 km across.

In order to model the potentially damaging weather phenomena and those phenomena that most influence the surface conditions, one must increase the resolution of the numerical model. 

The forecasting centre operates two nested mesoscale models, that are 'forced' at their boundaries by the enclosing forecast conditions. The 12 km model is 'forced' by the global model and has 38 vertical levels (up to ~39 km altitude) and runs at hourly intervals to T+48 hours. The 4 km resolution model is 'forced' by the 12 km model and has 70 vertical levels (up to ~40 km altitude) and runs at hourly intervals to T+36 hours. Both 12 km and 4 km mesoscale models are run four-times daily.

Parameters include: [[to be added]]

The observations collected has assimilated into the higher-resolution mesoscale models as for the global model above.

5) Probablistic forecasting
A forecast is an estimate of the future state of the atmosphere. It is created by estimating the current state of the atmosphere using observations, and then calculating how this state will evolve in time using a numerical weather prediction model. As the atmosphere is a chaotic system, very small errors in its initial state can lead to large errors in the forecast. This means that it is not possible to create the perfect forecast because one cannot observe every detail of the atmosphere's initial state.

To test how these small differences in the initial conditions may affect the outcome of the forecast, an ensemble system can be used to produce many forecasts.

Instead of running a single forecast model, the forecasting centre runs the model multiple times, each with slightly different 'perturbed' boundary conditions. The complete set of forecasts is referred to as an ensemble, whilst the individual forecasts within are the ensemble members. The perturbation of the initial conditions is designed so that each member of the ensemble should be equally likely.

As the errors in the forecast of each ensemble member are amplified, so the potential spread of potential future states can be established. For example, where all the results from all ensemble members 'cluster' together because the variation in initial conditions has little affect on the outcome, there is likely to be significant certainty that the 'correct' forecast is well represented (a low probability of error). However, if the forecasts diverge, this indicates uncertainty in the forecast. Depending on the size of the divergence, this may indicate a high probability of error. Forecasters can use the statistical results derived from the ensemble forecast to indicate the level of confidence with the forecast.

The forecasting centre runs a 24-member global ensemble at ~90 km resolution with 38 vertical levels and a 24-member regional (mesoscale) ensemble at ~24 km resolution with 38 vertical levels. Each ensemble is run twice-daily (?) for two-days ahead to T+48 hours in order to produce uncertainty information for short-range forecasts. The focus is on aiding the forecasting of rapid storm development, wind, rain, snow and fog.

Parameters include: [[to be added]]

6) Nowcasting
More than six hours ahead, numerical model forecasts gain an advantage over other forecasting techniques. However, 'nowcasting' is a technique for very short-range forecasting that maps current weather, then uses an estimate of its speed and direction of movement to forecast the weather a short period ahead - assuming the weather will move without significant changes. Since it takes time to gather and map the weather observations, a short forecast is needed to even know what the weather is 'now'.

Rainfall and associated severe weather, such as hail and lightning, are the most widespread and advanced applications of nowcasting.

The advantage of the nowcasting technique is that it can be applied quickly, meaning it is possible to update forecasts frequently. The forecasting centre executes the nowcast every hour for six-hours ahead (T+6). The observed weather from observations, including radar and satellite, are blended with the high-resolution 2 km forecast that has been 'downscaled' from the 4 km mesoscale model using ancillary information such as orography.

7) Statistical correction
The forecasting centre further fine-tunes the forecast situation by applying a site-specific modeling algorithm at specific locations where terrestrial observations are collected. The forecast values are 'verified' against the observed values in order to develop a statistical corrections to manage systematic biases for given locations in given weather situations.

8) Severe Weather Warning Service
Ultimately, severe weather has the potential to cause human, economic and environmental impact. One of the core missions of the national weather service is to limit those impacts by providing warnings of severe or hazardous weather which could cause problems, ranging from widespread disruption of communications to conditions resulting in transport difficulties or threatening lives and property.

The operational forecasters visualize all the observation and forecast information provided to them (as described above) using their forecaster workstation applications. Their focus is on interpreting the forecasts provided by the numerical weather prediction models to provide a 'consumer-oriented' commentary that clearly indicates the level and nature of the threat and provides guidance for the appropriate response.

These warnings alert recipients to a forecast severe/extreme weather event and allows for planning of actions needed to mitigate or reduce the impact of weather.

This support is provided in three ways: to the public at large via the broadcast media, to the civil emergency authorities and to the military when conditions become sufficiently severe to warrant military aid.