Search Results (2)

This study performs idealized simulations of rainfall in Taiwan in a winter monsoon regime under different wind direction (W_d; every 15° from 0° to 90°), speed (W_s; 12, 8, and 4 m s⁻¹), and relative humidity (RH; 80, 70, and 60%) combinations at low levels, based on a simplified mean sounding profile from observations during the event over 20–24 November 2020. Thus, at a horizontal grid size of 2 km, a total of 7 × 3 × 3 = 63 runs are performed and the aim is to investigate the response in daily rainfall in northern Taiwan to these three parameters. The model results from the closest combination are verified against the observed rainfall both during the reference event and in several historical cases with reasonable agreement, indicating the usefulness of the approach, albeit with some limitations. From the experiments, our main findings can be summarized as below. First and foremost, with W_s = 12 m s⁻¹ and RH = 80%, when the prescribed W_d changes from 0° (northerly) to 90° (easterly) gradually, the main rainfall areas shift from northern to northeastern Taiwan, with local maxima at the northern tip, northeastern tip of Taiwan, and near Suao (at the end of the Central Mountain Range) in response, indicating topographic uplifting for rainfall production. At a larger impinging angle, the Suao area tends to receive the most daily rainfall and can exceed 300 mm at W_d of about 75°–80°. Second, when W_s decreases to 8 m s⁻¹, the general rainfall regions often remain similar but the amounts become lower, especially at local maxima. The peak amount near Suao is only about 100 mm. At weak wind of W_s = 4 m s⁻¹, only moderate rainfall of 20 mm or below can be produced in Taiwan, and the local centers become not discernable. Third, when RH is lowered, the rainfall in northern Taiwan decreases significantly, especially along and near the coast under weaker winds coming from smaller angles. At RH = 70%, a higher accumulation (≥100 mm or so) near Suao is only possible with W_d ≥ about 55° at W_s = 12 m s⁻¹ or when W_d ≥ 70° at W_s = 8 m s⁻¹. At RH = 60%, the rainfall in northern Taiwan (and on the entire island) further decreases, again more evidently in cases with smaller W_d and W_s values. Full article

In the South China Sea (SCS), 17% of tropical cyclones (TCs) formed in the late season (November−January) were associated with a strong northeasterly monsoon. This study explores the effects of northeasterly strength on TC formation over the SCS. The Weather Research and Forecasting (WRF) model is used to simulate the disturbances that develop into TCs (formation cases) and those that do not (non-formation cases). Two formation (29W on 18 November 2001 and Vamei on 26 December 2001) and two non-formation (30 December 2002 and 9 January 2003) cases are simulated. To address the importance of upstream low-level northeasterly strength to TC formation, two types of sensitivity experiments are performed: formation cases with increased northeasterly flow and non-formation cases with decreased northeasterly flow. If the strength of the northeasterly is increased for the formation case, the stronger cold advection reduces the convective instability around the disturbance center, leading to the weakening of the simulated disturbance. If the strength of the northeasterly is decreased for the non-formation case, the simulated disturbance can develop further into a TC. In summary, strength of the upstream low-level northeasterly flow does affect the environmental conditions around the disturbance center, resulting in the change of TC formation probability over the SCS in the late season. Full article