Sea Breeze Circulations Influencing Radio Frequency System Performance around California Baja Sur

Entrainment layers capping well mixed layers are breeding grounds for radio frequency ducts that trap electromagnetic energy near the surface and alter the performance of coastal ship borne radar. In equation 1, the modified refractivity (M) gradient must be less than zero in order for the entrainment layer to produce a duct. The potential temperature (ć) gradient is positive in a thermally stable entrainment layer and the water vapor mixing ratio (w) in the free atmosphere is typically less than in the mixed layer.

(1)

Measurements during sea breeze events associated with the Wallops 2000 Microwave Propagation Measurement Experiment indicated that dry air from the land at the top of the circulation increased the moisture gradient sufficiently to rapidly produce ducts that significantly altered radio frequency system performance.

This paper will present the results of a study that analyzed the structure of sea breeze circulations around the California Baja Sur in June of 2005 and demonstrate the impact on coastal ship borne radar performance. Data from high resolution rocketsonde launches offshore, the Regional Atmospheric Modeling System (RAMS), and the Advanced Refractivity Effects Prediction System (AREPS) will be employed to demonstrate the spatio-temporal structure of the atmospheric boundary layer and the resulting radar system performance during sea and land breeze events.

The seasonal Pacific northwest high pumped dry air into the free atmosphere down the west coast of North America with water vapor mixing ratio values as low as 0.003 kgkg-1 creating ducts in the entrainment layers during sea and land breezes. This meant that ducting occurred during the entire diurnal cycle unlike that observed during Wallops 2000.

The boundary layer heights during sea breeze events were typically 100m shallower than during the complimentary land breeze resulting in lower duct heights and significantly altering radar system performance.