Automated Fog and Stratus Forecasts from the Canadian GEM Regional Operational NWP Model

Fog and low stratus, two key weather elements for aviation forecasters, depend on the characteristics of the boundary layer. However, the boundary layer poses particular problems for NWP models. Can the Canadian GEM regional operational forecast model be used to make forecasts of fog and stratus that are useful to operational forecasters?

An automated system to forecast fog (visibility ½ mile or less) and low stratus (ceiling 500 feet or less) has been developed at the Meteorological Service of Canada's Hydrometeorology and Arctic National Lab (HAL) in Edmonton,. It applies a set of subjectively-developed rules for various types of fog to the output of the GEM regional model, over Canada and the adjoining marine waters. The model bulk Richardson number is used to differentiate between fog and stratus. Hourly forecasts, from 1 to 48 hours, are produced based on both 12Z and 00Z data.

Hourly summary charts of land observing stations with fog or low stratus, and of ships with low visibilities in fog, were created and used in the development process.

The following evaluations have been carried out or are planned:

1. subjective verifications during the development process; 2. informal verifications by local operational forecasters who have started to use the product; 3. subjective verifications by forecasters in Halifax; 4. subjective verification for the ship Healy in the Arctic Ocean in the summer of 2009; 5. an objective verification of the forecasts at various land stations (planned).

The subjective verifications agree that the system has a certain degree of skill. Some operational meteorologists (primarily in Halifax and Edmonton) have become familiar with the system and indicate that it is useful as a “first guess” field for fog and low stratus. They also like the hourly availability of the forecasts. The subjective verifications indicate that the system does overforecast fog and stratus to some degree, particularly in the warm season. This is accepted as a price to pay for fewer misses. The authors hope to report in the future on the results of the objective verification for land stations.

The fog and stratus forecast system still has an “experimental” status. It is run locally in Edmonton, and is not supported 24/7. It is hoped that it will eventually be implemented as part of the fully-supported operational runs at the Canadian Meteorological Centre in Montréal.