Local Circulation and Its Interaction with Convective Boundary Layer

Sea breezes and valley winds are triggered by the convective boundary layer developed over coastal and mountain areas during the daytime. The horizontal spatial scales of the circulations which belong to meso γ scale are on the order of 10 km while the scale of the dominant large eddies inside the convective boundary layer over land is about 1 km. These multi-scale circulations may play an important role in initiating deep convections. The sea breezes can penetrate farther inland after formation. When sea breeze penetrates inland, it collides with the large eddies and the structures of the two systems of different spatial scales change. The effect of cold advection brought by sea breeze suppresses the thermal convection over land, while the leading edge of the sea breeze circulation undulates due to the interaction with large eddies. On the leading edge of the sea breeze (sea breeze front), low-level moist air from the sea will induce strong moist updrafts on the sea breeze front (Fig 1), and become the mechanism of trigger for deep convections. However if there is terrain inland, the strengths of the sea breeze circulation, the sea breeze front and on-shore wind speed would be affected. If the distance between the coastline and terrain is short, the sea breeze and valley breeze can merge into the sea-valley breeze. When the sea-valley breezes show up, it may become more important triggers to initiate the deep convection than any others. In our idealized numerical study, sea-valley breezes will collide with another valley winds on top of the plateau after sea and valley breezes merged. On the head of sea-valley breezes, it will become a strong upward motion. That's why the head can become the trigger of deep convections. Banta et al. (1993, 1995, 2002) were used to perform an observation experiment （LASBEX, Land–Sea Breeze Experiment） at Monterey Bay of the west coast of the U.S.A. The observations show that local contrast of horizontal temperature will drive the shallow sea breezes, but large scale contrast of temperature comes from the ocean and mountain which is inland. The depth of sea breezes is about 1 km, The speed of wind is about 6 m s-1 under 500 m. They also found another condition to drive sea breeze. In other words, mountains near the coast will has a great impact on the structure of sea breeze. But they did not explain the role of humidity and the mountain slope. In order to understand the nature of sea breezes, we use three-dimensional model to simulate it. We have consistent results with the observations about Atkins (1995). Besides, we use different conditions of humidity to test the sensitivity of intensity of sea and sea-valley winds. The results of simulation show that the humidity of atmosphere will have a huge impact on the intensity of those local circulations. In this three-dimensional simulation, the reason of curative sea breeze front has been understood. Taiwan is surrounded by oceans, according to the distance between the coastline and terrain, the sea-valley breeze may occur in many locations and induce strong deep convection in the mountains. In this study, we use National Taiwan University - Purdue three-dimensional non-hydrostatic model (NTU-Purdue 3D non-hydrostatic model) to simulate the local circulation with a two and three dimensional domain with high resolution (Dx = Dz = 50m). It is free of artificial smoothing used in most numerical models and can still maintain numerical stability. The results show consistency with observations as regards to structures of both sea breeze and inland boundary layer. Our study demonstrates the importance of the advection process, atmospheric humidity, and the terrain slope. We also find that the periodic behavior of the intensity of sea breeze fronts resultes from colliding with large eddies in convective boundary layer. It is expected to understand the internal structure of sea breeze, sea-valley circulations and related physical processes and further predict the weather change in the mountains.