Classification Of Summertime West Coast Fog And Stratus Events And The Development Of Fog And Stratus Forecast Techniques

The fog and stratus that frequently plagues the West Coast in the summer months is responsible for a variety of impacts on everyday life, the greatest impact on aviation. Many flight delays and cancellations that are experienced around the Pacific Rim are attributed to the development and evolution of the fog and stratus on the U.S. West Coast. This thesis studies the evolution of the fog and stratus events during the summer of 2000 through the use of geostationary, GOES-10, visual satellite imagery to develop a classification scheme. The synoptic-scale weather patterns as well as the mesoscale coastal regime were then associated with a type of stratus evolution. The Navy's mesoscale model, coupled ocean/atmosphere mesoscale prediction system (COAMPS), provided detailed simulation of 11 events to highlight the boundary layer evolution and its relationship to fog and stratus evolution. The fog and stratus classification scheme produced several consistent synoptic and mesoscale signals associated with stratus evolution. These relationships provide some forecasting techniques that should aid forecasters with predicting the evolution of fog and status events.

In the low visibility range, forecasts during the summer period along the west coast of California are presently not made with any degree of accuracy. Modeling sequences associated with the nonfrontal fog formations during the summer period offer the possibility of improving fog diagnosis. Such sequences have been in use in Southern California for some time. This study uses a synoptic approach, focusing on sequences observed in the non-diurnal aspects of coastal fog. A development model is presented in order to delineate patterns of the fog phenomenon along the Central California coast. Actually observed fog situations are presented in order to evaluate the model and determine if day-to-day changes in specific non-diurnal indices represent trends which can aid forecasters. Results show that, although the model is general in nature, a correlation between the stages of the observed fog exists. The relationship of the time of occurrence of dense fog and the trends in the height of the inversion base and daily maximum temperatures at the top of the inversion and the inland valley are pointed out. (Author).

;Contents: Summary of marine fog occurrence off the West Coast; The importance of organized vertical motions in coastal fog; Bay fog (marine fog formation through continental influences); Observations of a mesoscale organization of fog occurrence off the West Coast.

The method is a success in producing forecasts for ceiling and visibility criteria that had never previously been examined. It is suggested that the 15 OWS incorporate this methodology into their operational forecasting routine. Ceiling forecasts at Dover AFB and McGuire AFB show improvements over conditional climatology ranging from 1-51% with an average improvement of 19.2% when verified against an independent data set. McGuire AFB visibility forecasts show an average improvement over conditional climatology of 3%. These findings are of particular importance to the Air Force in general and specifically to the 15th Operational Weather Squadron (15 OWS) who produces forecasts for these airfields. Demonstrating a method superior to conditional climatology is expected to provide improved forecasts and flight operations in this region. The two forecasts for Andrews AFB show relatively low mean square errors, but are unable to consistently improve on conditional climatology, demonstrating an average decrease in forecasting skill of 42%. Small samples of data could be the reason for the decrease in skill. The Dover visibility forecast also shows negative forecast skill, with an average decrease of 39%. The method is a success in producing forecasts for ceiling and visibility criteria that had never previously been examined. Further research into the forecasts could produce a powerful tool consistently able to defeat conditional climatology. It is suggested that the 15 OWS incorporate this methodology into their operational forecasting routine.

Marine fog forecasts during the summer period in the North Pacific are not made presently with any acceptable degree of accuracy. Objective fog development models exist and are used with some success for localized coastal regions of the western U.S.; scarcity of accurate data has hindered creation of a reliable open-ocean model. The Eulerian single-station approach, utilizing a segment of the complete accurate data of Ocean Weather Station Papa (50N, 145W) is applied in this study to an objective marine fog forecasting model. The time-series study of significant atmospheric variables at OWS Papa, when coupled with a chronological synoptic overview, delineates accurately fog/no fog sequences in the summer months of 1973 and 1977. Actual observed fog situations are evaluated by the general model and presented in relation to open-ocean fog indices, NOAA 5 satellite coverage and synoptic history. The open-ocean forecast model is tested on an independent data set for the month of July 1975 at OWS Papa, with favorable results. The research delineates four required indices that must all be positive to forecast fog. These indices, when plotted daily in the region of OWS Papa allow a single station to predict, with some confidence out to twenty-four hours, the occurrence of advection fog. (Author).

Aircraft measurements made offshore during a coastal fog sequence by R.A. Markson are analyzed. Fog occurrence and areal extent are determined using aircraft, ship and shore station observations obtained during the Cooperative Experiment in West Coast Oceanography and Meteorology--1976 along with analyzed satellite visual and infra-red imagery. The offshore conditions are compared with those at the shore stations using selected fog indices. Photographs of special features observed are included. The offshore conditions are classified into phases by assigning limits within the specified fog indices used in the modified Leipper fog model. The sequence of observed fog events is compared to the ideal sequence. The trends in the sequence are analyzed and a general relationship between the phase sequence and the local offshore flow is indicated. The Leipper indices, the San Diego raob and the sequential fog model appear to be useful in fog prediction for the nearshore oceanic region from San Diego the Point Conception. (Author).