NIWA Model Data Explained

Observed daily fire Indices and codes

The daily fire indices and codes are originally based on the Canadian Fire Weather System and have been adapted to New Zealand by Scion. The daily observed fire weather calculation is always done at 00 UTC and depends on the temperature, relative humidity, winds and 24-hour rain accumulation, and the grass curing value (proportion of grass that is dead material). The fire weather system consists of three fuel moisture codes and three fire behaviour indices. The fuel moisture codes provide an indication of moisture in the fine, medium /sub-surface and heavy/ground fuels. The indices represent rate of spread, fuel available (dry enough) to burn and fire intensity The NZ fire weather system uses about 300 weather stations spread around the country owned by MetService, NIWA, some Councils and Fire And Emergency. These weather stations play a crucial role in providing a real time assessment of the potential fire danger and can be used for modelling of fire behaviour. When there are no values available at 00 UTC at a weather station location for whatever reason, the observed daily values are replaced by interpolated values calculated by looking at available stations within the same region. NB: NZST is 12 hours ahead of UTC and NZDT is 13 hours ahead.

Observed hourly fire indices and codes

The hourly fire indices and codes are basically the same as the daily ones but are done on the hour for every hour. They are dependent on the same weather parameters as the daily ones, but the data is not interpolated if missing. This has some consequences on the 24-hour rain accumulation which may not be able to be accurately calculated if too many data values are missing. When this happens the fire indices and codes cannot be calculated until a full 24-hour rain accumulation can be calculated again. This will show in the hourly tables as an empty row.

Forecast fire indices and codes

The forecast fire indices and codes are done using 2 separate weather models:
  • NZLAM: New Zealand Limited Area Model
    • Horizontal resolution: 4.4 km
    • Forecast Period: 72 hours
    • Forecast interval: hourly
    • Update frequency: every 6 hours (00, 06, 12, 18 UTC)
  • GlobalUM: Global Unified Model
    • Horizontal resolution: ~13 km
    • Forecast period: 144 hours
    • Forecast interval: 3-hourly
    • Updated frequency: twice per day (00 and 12 UTC)
The NZLAM model is the best one to use for short term forecasts (up to 72 hours), while the GlobalUM will provide good guidance beyond the 2 to 3 days. The same calculation method used for the observed fire indices and codes are used for the forecasts. One important aspect of the forecasts is that they are bias corrected. Bias correction is done by comparing the past 16 weeks of observed data against what was forecast to find out how much it differs. If for example the forecast temperature at a specific weather station site is always forecast to be 2 degrees under what was observed, this 2-degree bias is added to the raw forecast value to correct it. This method is called model output statistics (MOS) bias correction. This corrected forecast value is then used for the fire indices and codes calculation. Each time a forecast is produced the biases are recalculated as it can vary from season to season.

General, Forestry, Powerline and Hotwork indices

  • General Fire Danger Class:
    • This index is the same one seen on the road side in New Zealand
    • It is based on the fire danger class (FDC) for the fuel type (i.e. grass, scrub or forest) considered most relevant for the zone that is represented by a particular weather station
  • Forestry:
    • Designed specifically for the forest industry to provide trigger points that guide what activities can be carried out and when additional risk mitigation is needed
    • It uses a combination of the Build Up Index (BUI) and Fire Weather Index (FWI)
  • Powerline:
    • Designed for the electricity transmission lines to help assess the danger during faults. The main concerns are sparks igniting grass typically lying under powerlines.
    • It is based on grass curing (GC), Fine Fuel Moisture Code (FFMC) and wind speed
  • Hotwork:
    • Designed for the industry that works in a warm environment that can potentially start a fire by spark or other means
    • It is based on the grass curing value (GC) and the Fine Fuel Moisture Code (FFMC)

More Info