Aviation Weather and Model-Based Operational Forecasts of Low Visibility and Fog

This article introduces the operational predictions of hazardous weather elements to aviation both en-route and in TAF. Two visibility algorithms and three fog diagnostic schemes used at NCEP are discussed in particular. The frst visibility algorithm is Stoelinga and Warner method. This method requires outputs of four hydrometeors from a model to estimate optical extinction coeffcient and calculate visibility. The second visibility method is an upgrade to the frst one by adding more species of hydrometeor. This method can also estimate visibility in high humidity and haze conditions without knowing hydrometers as well as considering the differences between daytime and nighttime. The concept of airport Runway Visibility Range (RVR) is introduced and the relationship between the RVR and horizontal visibility range (HVR) is given. The fog diagnosis includes the UPS method, multi-rule method and physical-process balanced method. The UPS method uses surface-layer stability index. The multi-rule method uses a combination of visibility, cloud, surface humidity and wind speed to diagnose fog occurrence. These two methods do not predict fog intensity. The third method can diagnose both fog occurrence and intensity. It is based on the turbulence condition for fog formation and persistence as well as the explicit formulation of fog liquid water content suggested by a fog’s physical balance theory. Both multi-rule and physical-process balance methods have been implemented operationally in the NCEP’s regional ensemble forecast systems and showed encouraging results. However, the current skill of low visibility and fog forecasts directly derived from a numerical weather prediction model is generally low comparing to the forecasts of other weather elements such as precipitation. The diffculties are discussed and the ways of improvement are also suggested.