Characteristics of the frequency and distribution of fog events over the Salt Lake and Heber Valleys

The frequency and distribution of fog events over the Salt Lake and Heber Valleys are studied using surface MesoWest observations over 2003-2013. Preliminary statistics indicated that: In the Salt Lake Valley, fog formation is common from November 25 to February 25 with a peak occurrence between December 5 and January 30. On average the nighttime is significantly foggier than the daytime with the peak occurrences near 9pm and 4am local time. Overall, the months of December and January contain a significantly higher number of inversion-based fog events (rather than radiation fog events) when compared to November and February, with an absolute peak in mid January. This can be attributed to a higher instance of snow on the ground, available cold air, and a lower sun angle.

Compared with the Salt Lake Valley, the Heber Valley has a slightly higher ratio of fog events in November and especially in February. During December and January it has a slightly lower ratio of fog events. In effect, its fog season is broader and less peaked than the Salt Lake Valley. The total number of days with inversion-based fog events in the Heber Valley is significantly less than that in the Salt Lake Valley. Most fog is radiation fog. Furthermore, there is a distinct lack of fog in the evening time when compared to the morning hours. We note a strong peak around 6-9 am local time, which coincides with sunrise.

With the knowledge obtained from the above statistics, several typical fog cases are examined in the modeling study in order to evaluate the ability of the mesoscale community Weather Research and Forecasting (WRF) in predicting fog in the Salt Lake and Heber Valleys.