Comparisons between Mesoscale Model Terrain Sensitivity Studies and Doppler Lidar Measurements of the Sea Breeze at Monterey Bay

Abstract

A NOAA/Environmental Technology Laboratory Doppler lidar measured the life cycle of the land- and sea-breeze system at Monterey Bay, California, in 1987, during the Land?Sea Breeze Experiment (LASBEX). On days with offshore synoptic flow, the transition to onshore flow (the sea breeze) was a distinct process easily detected by lidar. Finescale lidar measurements showed the reversal from offshore to onshore flow near the coast, its gradual vertical and horizontal expansion, and a dual structure to the sea-breeze flow in its early formative stages. Initially, a shallow (<500 m) sea breeze formed that later became embedded in a weaker onshore flow that was ?1 km deep. Eventually these two flows blended together to form a mature sea breeze about 1 km deep. Regional Atmospheric Modeling System (RAMS) two-dimensional simulations successfully simulated this dual structure of the sea-breeze flow when both the coastal mountain range just east of Monterey Bay and the Sierra Nevada range, peaking 300 km east of the shore, were included in the domain. Various sensitivity simulations were conducted to isolate the roles played by the land?water contrast, the coastal mountain range, and the Sierra Nevada range. Notable results included the following: 1) the Sierra Nevada range greatly affected the winds above 1500 m at the shore, even though the peak of the mountain range was 300 km east of the shore; 2) the winds at the shore, below 1500 m, were most affected by the land?sea contrast and the coastal mountain range; and 3) the presence of the coastal mountain range enhanced the depth of the sea-breeze flow but not necessarily its speed. A factor separation method was employed to further isolate the contributions of the terrain and land?water contrast to the vertical structure of the modeled u component of the wind. When both mountains were included in the domain, the interaction of the slope flows generated by these mountains acted to strongly enhance onshore flow early in the morning. In contrast, the interaction of flows generated by the land?water contrast and the sloping terrain had its strongest effect late in the afternoon and early evening, working to oppose the sea-breeze flow. The triple interaction of the flows generated by the coastal mountain, inland mountain, and the land?water contrast enhanced the sea-breeze flow from the surface to 500 m above the sea level throughout the day.