FRAM-S Freezing Fog Project: Emphasis On Visibility Forecasting

The purpose of this work is 1) summarize the microphysical observations and 2) compare visibility (Vis) measurements to a numerical model forecast runs to better understand issues related to low visibility conditions. The ground-based observations were collected during the Fog Remote Sensing and Modeling Project in St. John's (FRAM-S), which took place in March-April 2009 and focused on freezing fog conditions. The instruments used to measure Vis were 1) the Biral HSS VPF-730 Combined Visibility & Present Weather Sensor, 2) Sentry VIS sensor, 3) Vaisala FD12P present weather sensor, 4) Vaisala PWD12 sensor (two of them), and 5) Belfort Vis sensor. Human-based Vis observations were also available during the project and used for validations. Validations were also performed by estimating Vis from the particle measurements of a) CAP (Climatronic Aerosol Profiler) sensor (for aerosols and droplets; 0.3-10 micron), b) DMT Fog Measuring Device (FMD; 2-50 micron), and c) DMT Ground Cloud Imaging Probe (GCIP; 7.5-900 micron). Results representing various low Vis conditions because of freezing fog and warm fog cases are presented and the previous Vis probability curves developed using only FD12P measurements are discussed. Possible applications of the results are offered for nowcasting issues. 8-10-2009-->