| 摘要: | 研究雲的微物理特性需要雨滴粒徑分布的資訊，但雨滴粒徑分布會隨不同降水型態及地理位置而有所不同，故分析不同地點及不同降水型態下之雨滴粒徑分布特性是重要的課題；且藉由雨滴粒徑分布可以得到液態水含量、雷達回波、降雨率等降雨積分參數，對於雷達估計降水的準確性有幫助。因此本研究使用中央大學光學式雨滴譜儀及台灣北部地區(中央、石門、霞雲、翡翠、南港) 撞擊式雨滴譜儀之觀測資料進行分析探討。中央站資料時間為2002年2月至2006年5月，北部各站資料時間為2004年6月至2006年5月。 統計雨滴譜儀長期觀測資料之結果顯示，從雨滴粒徑分布圖可以看到，隨降雨率增加，大雨滴個數變多，雨滴粒徑分布向右延伸。撞擊式雨滴譜儀相較於光學式雨滴譜儀，在降雨率較大時會低估大雨滴及小雨滴個數。雖然兩種儀器計算得到之Gamma分布參數隨降雨率的變化不同，但對降雨積分參數影響不大。雨滴粒徑分布在空間的變化如下：翡翠水庫的降雨率較小且較偏小雨滴，南港站在小雨滴的部分有明顯較少之情形，石門及霞雲站之降雨率較大且大雨滴較多。 以季節及降水型態來看：冬季及鋒面型態偏小雨滴，降雨率較小。梅雨之雨滴粒徑分布較廣，雨滴顆粒較大。颱風降水偏中大雨滴，雨滴粒徑分布較窄；且當降雨率大於 60 (mm/hr) 時雨滴粒徑分布在大雨滴的部份有內縮情形。午後雷陣雨之降雨率較大且雨滴粒徑最大。不同地區的中值體積直徑(Median Volume Diameter：D0 )對Nw (標準化的N0) 的分布很穩定，但利用D0對Nw的機率分布則可以看出：不同降水型態、同降雨率下D0對Nw的分布雖在同一區塊，但集中的部分不同；另外將此圖與Bringi et al. (2003)之結果相比，發現當降雨率小於10 (mm/hr) 之降水型態偏向層狀性降水，降雨率大於10 (mm/hr) 之降水型態則介於大陸性對流及海洋性對流之間。 在個案討論中，颮線個案中的層狀降水具有較弱的雷達回波強度以及降雨率，且其雨滴粒徑分布較窄，以較小雨滴為主；而對流時段的雷達回波較強，降雨率也很大，相對的雨滴粒徑分布較寬廣，以較大雨滴為主。 艾利颱風個案中，螺旋雨帶通過的地區降雨率較大，雨滴粒徑分布較寬廣，雨滴顆粒較大。而眼牆通過時雨滴粒徑分布變窄，雨滴顆粒反而沒有那麼大。 Characterization of variations in the raindrop size distribution (DSD) is required for microphysical studies. The DSD varies in different rain types and different spatial distribution. And variations in reflectivity-rainfall (Z-R) relations are strongly dependent on DSD variations. Other integral rainfall parameters such as liquid water content, reflectivity, and rain-rate are also functions of DSD. For these reason, it is important to analyze the DSD in different places and rain types. DSD data collected with a Joss-Waldvogel disdrometer (JWD) and a 2d-video disdrometer (2DVD) in NCU from February, 2002 to May, 2006, is used to analyze the variations in gamma parameters of raindrop spectra. Besides, to compare the spatial difference of DSD, we also analyzed JWD data in five stations (i.e. Feitsui, Nankang, NCU, Shiyun and Suiman) from June, 2004 to May, 2006. On average, as the rain-rate increased, more large drops were found in 2DVD and JWD. Although JWD underestimated the small drops, the small drops had little impact on the rain-rate and reflectivity differences between the 2DVD and JWD. For the comparison between five stations in northern Taiwan, the rainfall characteristics of Feitsui were more light rain and more small drops. Nankang had less small drops because small drops were masked by background noise. More heavy rainfall events occurred in Suiman and Shiyun due to its geographical features. And the average raindrop size in Suiman and Shiyun was larger than the other three stations. DSD data from 2DVD and JWD had been analyzed to determine the variability of drop size in different seasons and precipitation types. For winter and front type, the rain-rate was smaller and the raindrops tended to smaller drop size. For Mei-yu front, the raindrop spectra were broader than the other seasons or types except the afternoon thundershower type. And the raindrop size was larger than winter. For typhoon type, the raindrops tended to medium size. As rain-rate greater than 60(mm/hr), the right side of raindrop spectra became narrow and the concentration of large drops decreased. For afternoon thundershower, it had more heavy rainfall events. The drop size was the largest and its spectra were the broadest of all types. The composite statistics based on disdrometer suggested that the median volume diameter and generalized intercept for rain-rate less than 10(mm/hr) lied on a straight line with negative slope, which similar to stratiform rain type discussed by Bringi et al. (2003). And as rain-rate larger than 10(mm/hr), the two parameters lied between “maritime-like” and “continental-like” clusters. In squall line case study, the analysis showed that stratiform precipitation has weaker reflectivity, narrower raindrop spectra, and smaller drop size. For convective precipitation, it had stronger reflectivity, broader raindrop spectra, and larger drop size. In typhoon Aere case, during the spiral band passage, the raindrop spectra became broader and the raindrop size was larger compared to the raindrop size during the passage of eye wall. |
