評估西北太平洋季風指數與臺北及高雄夏季氣溫之關聯性

Assessment on Association between Western North Pacific Monsoon Index and Air Temperatures during summers in Taipei and Kaohsiung Cities

林尚農(SHANG-NONG LIN)

指導教授 ： 王玉純

中原大學/工學院/環境工程學系/碩士(2022年)

https://doi.org/10.6840/cycu202201456

摘要

大氣環境溫度變化對於人體健康是關鍵的指標，近年隨都市化程度發展，探討季風盛行區域下都市中溫度為重要研究課題之一。本研究為探討西北太平洋季風 (western north pacific monsoon, WNPM)作用下，臺北與高雄測站的氣象因子之變化，並以機器學習建模進行溫度預測評估。
本研究蒐集並彙整交通部中央氣象局局屬氣象測站 (Central Weather Bureau, MOTC) 以及國際太平洋研究中心 (International Pacific Research Center, IPRC) 1980年至2015年間的局屬氣象測站資料，以及西北太平洋季風指數資料，使用氣象參數包含臺北站 (編號：466920) 及高雄站 (編號：467440) 的平均溫度、最高溫度、最低溫度、日較差溫度、相對濕度、降雨量、降雨時數和季風指數。首先進行資料彙整與勘誤，接續透過敘述性統計 (descriptive statistics) 了解氣象參數基本特性與長期趨勢，再以主成分分析 (principal component analysis, PCA) 將多氣象變數之間的關聯性聚合成一個特徵特性，以觀察數據之相關性。建立1980年至2015年氣象因子與西北太平洋季風指數夏季 (6月至9月) 的歷史資料非延遲天數的A (自變數：每日平均溫)、B (自變數：每日平均溫與每日西北太平洋季風指數)、C (自變數：每日西北太平洋季風指數與每日其他氣象因子)、D (每日其他氣象因子) 組合之模型並進行成效評估；建立1980年至2015年氣象因子與西北太平洋季風指數夏季的歷史資料延遲一天至三十天 (延遲為兩個相關事件或現象在發生時的時間點有所不同，如前置時間的概念，夏季延遲約為25-35天，因此本研究設定最大延遲天數為30天) 的E (自變數：每日平均溫)、F (自變數：每日最高溫與最低溫)、G (自變數：每日西北太平洋季風指數) 組合之模型並進行成效評估；為了更深入了解季風指數的影響，從A、B、C、D之組合篩選出季風指數具有最高變數重要性 (對此模型貢獻度最大的參數) 之模型並進行成效評估。A至G組合輸出的參數皆為平均氣溫。先將氣象因子與西北太平洋季風指數依照時間排序分為訓練資料與測試資料 (以資料總量之7:3分配)，並採用隨機森林 (random forest, RF) 與 XBGoost (extreme gradient boosting)模擬氣象資料研究的機器學習算法，隨機森林與XGBoost皆使用迴歸作為預測進行建模，並調控機器學習中的最大深度，使機器學習找出最佳模型與最高變數重要性之參數。最後，藉由平均絕對誤差(mean absolute error, MAE)、均方誤差(mean square error, MSE)、平均絕對百分比誤差(mean absolute percentage error, MAPE) 和對稱性平均絕對百分比誤差 (symmetric mean absolute percentage error, SMAPE) 評估隨機森林與XGBoost的成效。
1980至2015年氣象參數敘述性統計結果顯示，整體溫度呈現上升之趨勢，測站之最低溫度中上升趨勢最為顯著的測站為高雄測站。臺北與高雄測站的相對濕度皆呈現下降趨勢，此結果是因為夜晚最低溫度的上升導致相對濕度的下降。透過主成分分析結果，高雄測站其平均溫度、最高溫度、最低溫度皆與季風指數呈現正相關，而臺北其平均溫度、最高溫度、最低溫度皆與季風指數呈現負相關，由此可知在臺北與高雄測站季風指數對溫度變化影響較其他氣象因子大。

使用隨機森林迴歸進行預測時，臺北與高雄測站從A、B、C、D之組合篩選出季風指數具有最高變數重要性之組合位於C，最佳模型之參數依序為季風指數、日較差、降雨量最後是降雨時數；臺北測站非延遲天數預測在C與D組合成效較好，延遲天數預測則在E組合較好；高雄測站非延遲天數預測在A與B組合成效較好，延遲天數預測則在E組合較好。
使用XGBoost迴歸進行預測時，臺北與高雄測站從A、B、C、D之組合篩選出季風指數具有最高變數重要性之組合位於C，最佳模型之參數依序為季風指數、日較差、降雨量最後是降雨時數；臺北測站非延遲天數預測在A與B組合成效較好，延遲天數預測則在E組合較好；高雄測站非延遲天數預測在A與B組合成效較好，延遲天數預測則在E組合較好。
本研究透過敘述性統計分析結果發現近年來平均溫度以及最低溫度呈上升趨勢，而相對濕度則呈下降趨勢，此結果代表都市高溫伴隨都市化程度的上升而有更顯著的提升；主成分分析結果發現溫度相關因子與季風指數呈現正相關，並在夏季時兩者的關聯會更強烈，此結果代表西北太平洋季風槽加深會對臺灣都市的溫度上升產生影響；機器學習法評估A、B、C與D組合結果發現，溫度相關因子皆有最高變數重要性，此結果代表溫度預測溫度在非延遲天數預測上具有更好的成效，且XGBoost的成效評估相較隨機森林來的更加精準；機器學習法評估E、F與G組合結果發現，雖然E與F組合的精準度較高但與G組合相比不超過2%，而又因E、F組合的延遲天數 (皆為一天) 較低，G組合的延遲天數 (十二天以上) 較高，此結果代表在應用於提前預警夏季高溫之情況下，季風指數可以獲得比溫度相關因子更前期的成效。建議後續研究可新增2015年後的氣象資料以及納入其他類型的氣象資料進行預測，並可使用延遲天數三十天以上的參數進行預測以觀測，亦可使用不同地區的季風指數資料，或是使用更適合研究地區之機器學習算法以更貼近研究地區特性。

關鍵字

氣象測站 ； 都市氣溫 ； 氣象因子 ； 季風指數 ； 主成分分析 ； 隨機森林 ； XGBoost

並列摘要

With increasing urbanization and economic developments in Taiwan, the issue of the urban high temperature is gradually being emphasized in recent years. The study of urban temperature in the monsoon region is one of the most important issues nowadays. The objective of this study is to investigate the changes of meteorological factors in Taipei and Kaohsiung stations under the western north pacific monsoon (WNPM), and project the long-term air temperature (AT) using machine learning modelings.
This study collected and compiled data from the Central Weather Bureau (MOTC) and the International Pacific Research Center (IPRC) from 1980 to 2015, as well as the western north pacific monsoon index (WNPMI). The meteorological parameters used, include mean AT, maximum AT, minimum AT, diurnal AT range, relative humidity, precipitation, rain hours recording in the Taipei (No. 466920) and Kaohsiung (No. 467440) stations, and daily monsoon index. The data were firstly compiled and harmonized, followed by descriptive statistics was used to observe the basic characteristics and long-term trends of meteorological parameters. Further, principal component analysis was used to aggregate the correlations among multiple meteorological variables into a characteristic feature to observe the correlation of the data. Models A (independent variable: daily mean AT), B (independent variable: daily mean AT and daily WNPMI), C (independent variable: daily WNPMI and daily other meteorological factors), and D (daily other meteorological factors) were evaluated for the non-lagged days using the historical data of meteorological factors and WNPMI during summers (June to September) from 1980 to 2015. And models E (independent variable: daily mean AT), F (independent variable: daily maximum and minimum AT), and G (independent variable: daily WNPMI) for the period 1980 to 2015 with a lagged of one day to 30 days (the lagged is the difference in time point between two related events or phenomena, such as the concept of leading time, the lagged in summer is about 25-35 days, so the maximum lagged is set to 30 days in this study) were developed and evaluated for AT projection effectiveness as well. To better understand the impact of the monsoon index, the models with the highest importance of variables (The parameters majorly contributed the variance of models) were selected from the combinations of A, B, C, and D and evaluated for their effectiveness. The output parameters of combinations A to G are the mean AT. The temporal meteorological factors and the WNPMI were divided into training data and testing data (7:3 distribution of the total data), and the machine learning algorithms of random forest (RF) and extreme gradient boosting (XBGoost) were used to simulate the meteorological data. Both RF and XGBoost used regression as a prediction for modeling and modulating the maximum depth in machine learning to identify the best model setting and the parameters with the highest variable importance. Finally, the mean absolute error (MAE), mean square error (MSE), mean absolute percentage error (MAPE), and symmetric mean absolute percentage error (SMAPE) was used to evaluate the effectiveness of RF and XGBoost.
The descriptive statistics of meteorological parameters from 1980 to 2015 showed an increasing trends of overall AT variables with the most significant increasing trend for daily minimum AT in Kaohsiung station. The relative humidity of both Taipei and Kaohsiung stations showed a decreasing trend that resulted from the increase in nighttime minimum AT. The results of principal component analysis showed that, the mean, maximum, and minimum ATs of Kaohsiung station were all correlated with the WNPMI, while the mean, maximum, and minimum ATs of Taipei were all negatively correlated with the WNPMI indicating the monsoon index has a greater influence on the AT change than on other weather variables in Taipei and Kaohsiung stations.
Assessing the projection performance using RF regression, the Model C was identified the best model with the highest variable importance from the combinations of Modles A, B, C, and D in the Taipei and Kaohsiung stations, and the parameters of the best model were in the order of WNPMI, diurnal AT change, precipitation, and rain hours. The prediction of non-lagged days in Taipei station was better in Models C and D, while the prediction of lagged days was better in Model E. The prediction of non-lagged days in Kaohsiung station was better in Models A and B, while the prediction of delayed days was better in Model E.
Assessing the projection performance using XGBoost regression, the Model C was identified the best model with the highest variable importance from the combinations of Modles A, B, C, and D in the Taipei and Kaohsiung stations, and the parameters of the best model were in the order of WNPMI, diurnal AT change, precipitation, and rain hours. The prediction of non-lagged days in Taipei station was better in Models A and B, while the prediction of lagged days was better in Model E. The prediction of non-lagged days in Kaohsiung station was better in Models A and B, while the prediction of delayed days was better in Model E.
This study summarized that descriptive statistical analysis revealed that the mean AT and minimum AT have been increasing in recent years, while the relative humidity has been decreasing that means that the urban high AT has been increasing more significantly with the increase of urbanization. The principal component analysis showed that the AT correlation factor and WNPMI are correlated and the correlations are stronger in summers that means that The deepening of the western north pacific monsoon trough had an impact on AT rise in Taiwan's cities. The machine learning method evaluated the results of Models A, B, C, and D. It was found that the AT-related factors all had the highest variable importance that means that a better AT prediction in non-lagged day’s models is to use ATs as inpact parameter. Moreover, this study found evaluation effectiveness of XGBoost was more accurate in compare to RF. This study found the accuracy of the Models E and F are higher than Model G with a difference of predicting accuracy less than 2% , but the best lagged day setting is 1 day for Models E and F and more than 12 days for Model G. This result implying the WNPMI can be more effective than the AT-related factors in operating the warning system for summer high ATs. This study suggests that subsequent studies can extend meteorological data after 2015 and incorporate other types of meteorological data for AT forecasting, and evaluate the performance of weather parameters with lags more than 30 days for AT forecasting. Futhermore, assessment on various monsoon index data from different regions, or use machine learning algorithms that are more suitable for the study area is recommended.

並列關鍵字

meteorological stations ； urban air temperature ； meteorological factors ； monsoon index ； principal component analysis ； random forest ； XGBoost

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