The Multivariate Modes of Surface Wind and Rainfall Observations during Typhoons Affecting Taiwan
Date Issued
2006
Date
2006
Author(s)
Tsai, Hsiao-Chung
DOI
zh-TW
Abstract
The multivariate relationships between hourly surface wind and rainfall observations during typhoons affecting Taiwan are investigated by Maximum Covariance Analysis (MCA). Historical surface observations from 1987-2004 are used when typhoon centers were located inside the domain of 117-127E, 19-28N. In this study, 17 surface weather stations with hourly rainfall, wind speed, wind direction, air temperature, relative humidity records, and 4 automatic rainfall stations over the Snow Mountain Range (SMR) with hourly precipitation records were chosen. The three leading modes explain 70%, 20.6% and 7.6% of the Squared Covariance Fraction (SCF), and the correlation coefficients are 0.59, 0.48 and 0.49 respectively. Wind directions of the three leading modes are: (1) north-westerly and perpendicular to the SMR; (2) south-westerly with flow convergent at Alishan as well as at the junction of Central Mountain Range (CMR) and the southern SMR; (3) easterly toward the northeastern SMR and the northern CMR. The rainfall patterns of the three principal modes are all bipolar, with the positives (or negatives) occuring on the windward sides and the negatives (or positives) on the leeward sides of the mountain ranges.
Based on the MCA singular vectors and cluster analysis, historical typhoon surface wind patterns are classified into major types. The results show that the three major wind types consist of 53% of the data, which are 25%, 9%, and 19% respectively. The cases of Type 1 are generally located around northeast of Taiwan with the centroid coordinate of (124.3E, 24.5N). Type 2 cases are on the northwest, and the centroid coordinate is (120.5E, 25.1N). The centroid coordinate of type 3 cases is (121.3E, 21.5N). These cases are generally south of Taiwan. Furthermore, the analysis of corresponding surface air temperature, relative humidity, and air pressure also reveal the contrasting patterns between windward and leeward sides.
Take type 1 cases as examples to analyze the differences between “unblocked flow regime” and the “blocked flow regime.” To analyze the foehn phenomenon, the relative humidity and dew point temperature observations at leeward surface stations are used. It is observed that heavy rainfall appears on the windward side, and wind directions are more perpendicular to the SMR if the foehn can be seen on the leeward side. Furthermore, the analysis of wind speed averages on leeward side show that leeward wind speed is higher when flow is over the mountain (i.e. unblocked flow regime). The flow feature of “parallel flow regime” is parallel to the mountains, and the rainfall is relatively small. For type 1 to 3, the parallel flows can be seen over southwestern, eastern, and northwestern Taiwan respectively. The averages of wind and rainfall patterns for type 4 also show the parallel flow and include light rainfall. The wind pattern variations of type 4 cases are examined further, and the RVPCA (Real-Vector Principal Component Analysis) method is applied. The results again confirm that the parallel flow regime can easily happen when the typhoon enters into the NE, SE, and SW sub-domains. The major portions of the type 4 wind pattern variations inside these sub-domains are mainly along the mountains, thus the rainfall amounts are generally small.
The proposed analysis procedures would be useful for diagnosing the rainfall anomalies caused by wind patterns variations. Lastly, correctly categorizing the wind and rainfall data can help to clarify the interaction mechanism among typhoon wind, rainfall and topography. This is an important step to further improve the skill of typhoon rainfall inferences.
Subjects
颱風
颱風降雨
最大協變異數分析法
群集分析法
typhoon
typhoon rainfall
Maximum Covariance Analysis
Cluster Analysis
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-95-D89521012-1.pdf
Size
23.31 KB
Format
Adobe PDF
Checksum
(MD5):aaa2adae098d6b1005d9a60730aa057f