Advanced Applications of Numerical Weather Prediction Models - Case Studies

Abstract

Numerical weather prediction (NWP) models are widely used nowadays in weather forecasting services. The models that are commonly considered include the global models and the mesoscale models with horizontal resolutions in the order of several kilometres to a couple of tens of kilometres. Performance of NWP models with even higher spatial resolutions is studied extensively recently with the objective of making location-specific forecasts. This paper describes some attempts of modelling the weather conditions in Hong Kong, a subtropical, coastal city, with a horizontal resolution of a kilometre or less, and presents the applications of the model results in the forecasting of hazardous weather. The following aspects are included: a. Turbulence forecasts – Turbulence could be hazardous to the aircraft (HKO, IFALPA and GAPAN, 2010). At the Hong Kong International Airport (HKIA), terrain disruption of the prevailing wind is the main case of airflow disturbances experienced by the pilots. Simulations of the wind flow down to a horizontal resolution of 50 m have been tried out to study the possibility of providing an indication of the occurrence of terraininduced turbulence. Moreover, the simulated turbulence intensity is compared with the measurements by sophisticated remote-sensing meteorological instruments, including minisodar, radar wind profilers and Light Detection And Ranging (LIDAR) systems. b. Wind gust forecast – Strong gust could occur in association with the passage of subtropical squall lines. Terrain effect may also bring about gustiness of the wind. A physical-based approach has been attempted in simulating the gusts in intense convective weather and terrain-induced airflow disturbances. The simulations are carried out with a horizontal resolution of 0.2 to 1 km. In the selected case studies, the simulated gusts are comparable with the actual observations by the dense network of ground-based anemometers in Hong Kong. c. Strong wind and heavy rain forecast – High winds associated with tropical cyclones and rainstorms due to summer monsoon are hazardous weather to the general public. This paper also discusses the possibility of improving the forecasting of such weather phenomena by using numerical simulations of high spatial resolutions (1 – 2 km) and sophisticated algorithms of assimilating actual observations into the NWP models. In particular, the inclusion of radar data brings about significant improvement in the forecasting of high winds and heavy rain of tropical cyclones.