Book Description

To develop improved techniques for predicting advection fog, fog droplet spectra data were collected during several advection fog episodes at Otis AFB. The data were collected with two PMS forward scatter probes (FSSP-100) and two optical array probes (OAP-200X). These two probes cover the ranges from 0.5- to 47-micrometers and 20- to 300-micrometer diameter, respectively. The probes were mounted at 5 m and 30 m above the ground. Drop size data were obtained in nine fogs over a total period of 70 h. Other data that were collected continuously during the fog episodes include extinction coefficient, temperature, dew point, wind direction, and speed at the 5-, 15-, 30-, 45-, and 60-m levels. The droplet data show three basic types of spectra, all of which have a high droplet count below 2.5-micrometer diameter. Some spectra show as many as three modes while others show no modes within the size range of the instruments. The shape of the droplet spectra and the number concentration vary considerably from one fog to another and during a fog episode. The total droplet concentration appears to be a function, primarily, of the past trajectory of the air mass; lower counts being observed in air masses with long over water trajectories. The drop-size data were used to parameterize the extinction coefficient and the mean droplet terminal velocity in terms of liquid water content and droplet concentration for use in numerical prediction models. The new parameterizations will result in the prediction of lower liquid-water contents and higher visibilities than with previous parameterization used in fog prediction models.