The eastern Massachusetts sea breeze study

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Thorp, Jennifer E.
This study investigates many different aspects of the sea breeze at Logan Airport in Boston, Massachusetts (KBOS) and along the Massachusetts coastline. Part of the study adapts the method of predicting sea breeze events developed by Miller and Keim (2003) for Portsmouth, New Hampshire (KPSM) to KBOS. An early ten-year dataset of hourly KBOS surface observations (1998-2007) was used to identify 879days when the sea breeze occurred or was likely to occur at the airport. These days were classified as sea breeze, marginal, or non-sea breeze events. Sea breeze events were further classified into fast and slow transitions, with a fast transition identified by a wind shift taking one hour or less to develop, and a slow transition identified by a wind shift taking two hours or more to develop. Marginal events were events that had a duration of 1 hour or less, no clear start or finish, or were interrupted by periods of calm" or "light and variable" winds. Non-events were events in which the background conditions for a sea breeze to occur existed but a sea breeze did not develop. Times of onset and event durations for the sea breeze events (fast slow and marginal) were calculated and used to create seasonal statistics by event type. It was found that seasonal variation did occur with both characteristics but was more evident in the time of onset. Slow events occurred earliest in the day overall while marginal events occurred a bit later and fast events occurred the latest. Slow events had the longest duration overall by definition had the shortest duration. Seasonally similar results were found for both characteristics with a few variations. United States surface analyses for each event at the time of onset (or average time of onset 1500 UTC for non-events) were classified using the seven synoptic classes developed by Miller and Keim (2003) and statistics were developed to evaluate the distribution of synoptic classes amongst the different types of events and various seasons. Composite surface analyses of the different synoptic classes and types of events were then developed. There were significant differences between the composites of each event type within a synoptic class. Wind vector plots created from surface observations using Barnes analysis were used to identify the position of the sea breeze front as the sea breeze air mass penetrated inland. The depth and shape of this front was examined by synoptic class. The prevailing synoptic scale flow was found to limit penetration in expected areas along the coastline. Mesoscale calculations were used to determine the critical balance of the cross-shore temperature gradient (dT/dx) versus the cross-shore geostrophic wind component (uG) at the surface necessary for the occurrence and non-occurrence of the sea breeze. It was found that by stratifying the events by synoptic classes a smaller transition area (containing both sea breeze and non-sea breeze events) could be created. The method was taken further by adding a third variable the 850 hPa geostrophic wind component. The three dimensional plot showed a large transition area and future research may be able to reduce this area by breaking it down by synoptic class. Finally the effect of the sea breeze on convection was analyzed using radar reflectivity data from the Taunton Massachusetts WSR-88D (KBOX) for 2002 through 2007 (562 events). Convection was present in land along the Massachusetts coastline for only 24 of the total 562 events (4%). This small occurrence results from a bias from the methodology used to develop the data set. However when the sea breeze did occur convection developed or was affected by the sea breeze front.