Numerical Simulations of Airflow and Cloud Distributions over the Windward Side of the Island of Hawaii. Part II: Nocturnal Flow Regime

Abstract

Numerical experiments are performed with the fifth generation Mesoscale Model to study the evolution of island airflow, thermodynamic fields, and clouds over the island of Hawaii at night. The model has successfully simulated the major observed features associated with the nocturnal flow regime. These include the formation of nocturnal inversion, development, gradual offshore extension and deepening of the katabatic flow, shifting of the overall cloudy areas from the windward slopes to the ocean, and generation of clouds within the simulated offshore convergence zone. Furthermore, it is also shown that rain evaporative cooling affects the depth, strength, and offshore extension of the katabatic flow. In the early evening, the nocturnal cooling provides the land?sea thermal contrasts for the development of the simulated downslope flow on the windward slopes. With continued nocturnal cooling and rain evaporation, the simulated katabatic flow extends toward the coast. The simulated convergence zone and overall cloudy areas also move seaward. Throughout the night, the simulated cold air near the surface moves down the slope to the coast within the katabatic flow layer. The simulated offshore flow deepens and extends farther over the ocean. In the early morning, the simulated offshore extent of the katabatic flow reaches 20 km with a relatively deep (?300 m) offshore flow. Clouds frequently form within the offshore convergence zone. They move westward in the model and weaken over the deep offshore flow before reaching the coast. In the model, the noctural cooling not only affects the near-surface airflow over the slope surface and coastal areas, but also the airflow aloft and upstream. With the development of the descending katabatic flow over the slope surface and coastal areas because of nocturnal cooling, stronger easterly winds are simulated aloft with much weaker incoming trade winds upstream in low levels than without nocturnal cooling. The simulated low-level flow deceleration is most significant early in the morning. At that time, the simulated offshore flow has the largest horizontal extent with a maximum depth.