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Characteristics of Surface Airflow and Pressure Patterns over the Island of Taiwan during TAMEX

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  • 1 Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii
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Abstract

Principal component (PC) analysis was used to study the wind and sea level pressure patterns over the island of Taiwan during the early summer rainy season. An eight-point, running-mean, low-pass filter was applied to the 3-h surface winds and sea level pressure deviations from its areal average to remove signals with a time scale of 1 day or less. The first and the second PC modes of the low-pass-filtered wind data account for 66% and 13% of the total variance, respectively. The first, the second, and the third PC modes of the low-pass-filtered pressure deviations account for 63%, 22%, and 10% of the total variance, respectively.

Composite analysis of low-pass-filtered surface variables was made during a 3-day period for each TAMEX (Taiwan Area Mesoscale Experiment) frontal passage episode, using the timing of the maximum PC scores of the dominant PC modes to arrive at a general picture of the evolution of surface airflow and pressure patterns. The prefrontal southwesterly flow reached the maximum intensity when the large-scale surface pressure trough arrived. At this time, the pressure pattern over Taiwan was dominated by a windward ridge along the southwestern coast and a leeside trough. A case study of the low-level airflow at this stage was made based on the wind data from aircraft and soundings. Below the 1000-m level, the airflow moved around the topography. The northern branch of the splitting flow had a cross-isobar wind component down the windward pressure ridge and accelerated downstream. Notable westerly flow crossing the southern portion of the central mountain range was observed above the 1200-m level. As an upper-level trough approached, the mesolow over southeastern Taiwan developed. It reached the maximum intensity during the time the westerly wind aloft was the largest. With the arrival of the surface front and the decrease of the westerly wind aloft, the mesolow weakened. After the frontal passage, northeasterlies prevailed over the Taiwan area with a increasing easterly wind component. The shallow cold northeasterlies interacted with the central mountain range, resulting in a pressure ridge along the northeastern coast and a Iceside trough. This pressure pattern was most significant after the migratory high moved off the southeastern China coast.

Principal component analysis was also applied to the bandpass-filtered diurnal winds and sea level pressure. The land—sea-breeze circulations were more pronounced over the southwestern plain with a slightly larger amplitude in diurnal sea level pressure than most areas. There were no significant differences in the diurnal cycle between the southwest and northeast flow regimes. In general, the winds turned clockwise (counterclockwise) during the diurnal cycle for the stations along the western (eastern) coast. The mesolow over southeastern Taiwan was slightly stronger during the day than at night.

Abstract

Principal component (PC) analysis was used to study the wind and sea level pressure patterns over the island of Taiwan during the early summer rainy season. An eight-point, running-mean, low-pass filter was applied to the 3-h surface winds and sea level pressure deviations from its areal average to remove signals with a time scale of 1 day or less. The first and the second PC modes of the low-pass-filtered wind data account for 66% and 13% of the total variance, respectively. The first, the second, and the third PC modes of the low-pass-filtered pressure deviations account for 63%, 22%, and 10% of the total variance, respectively.

Composite analysis of low-pass-filtered surface variables was made during a 3-day period for each TAMEX (Taiwan Area Mesoscale Experiment) frontal passage episode, using the timing of the maximum PC scores of the dominant PC modes to arrive at a general picture of the evolution of surface airflow and pressure patterns. The prefrontal southwesterly flow reached the maximum intensity when the large-scale surface pressure trough arrived. At this time, the pressure pattern over Taiwan was dominated by a windward ridge along the southwestern coast and a leeside trough. A case study of the low-level airflow at this stage was made based on the wind data from aircraft and soundings. Below the 1000-m level, the airflow moved around the topography. The northern branch of the splitting flow had a cross-isobar wind component down the windward pressure ridge and accelerated downstream. Notable westerly flow crossing the southern portion of the central mountain range was observed above the 1200-m level. As an upper-level trough approached, the mesolow over southeastern Taiwan developed. It reached the maximum intensity during the time the westerly wind aloft was the largest. With the arrival of the surface front and the decrease of the westerly wind aloft, the mesolow weakened. After the frontal passage, northeasterlies prevailed over the Taiwan area with a increasing easterly wind component. The shallow cold northeasterlies interacted with the central mountain range, resulting in a pressure ridge along the northeastern coast and a Iceside trough. This pressure pattern was most significant after the migratory high moved off the southeastern China coast.

Principal component analysis was also applied to the bandpass-filtered diurnal winds and sea level pressure. The land—sea-breeze circulations were more pronounced over the southwestern plain with a slightly larger amplitude in diurnal sea level pressure than most areas. There were no significant differences in the diurnal cycle between the southwest and northeast flow regimes. In general, the winds turned clockwise (counterclockwise) during the diurnal cycle for the stations along the western (eastern) coast. The mesolow over southeastern Taiwan was slightly stronger during the day than at night.

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