| 摘要: | 本研究在中國文化大學華林校區(24o53’N, 121o34’E)佈署了一台光學式雨滴譜儀(Parsivel, Particle Size and Velocity disdrometer)，利用這部儀器可同時測量水滴的粒徑與落速分布的特性，分析在2010年06月至2011年6月期間總降雨時數裡的水滴粒徑與落速之觀測特徵。同時搭配華林站內高解析度的三維超音波風速計觀測，藉以評估風場對水滴落速的影響。
分析結果顯示，小(大)水滴的落速傾向比理想終端速度來的較快(較慢)。對於較小顆水滴(D < 0.9 mm)而言，較強降雨環境時水滴落速與終端速度的偏差情形更為顯著，其表示著有可能是受到水滴間的合併與破裂交互過程的影響。在弱降雨強度和弱水平風場環境條件下，伴隨著向下(向上)空氣運動時的水滴落速相對於平均落速有正(負)速度偏差情形。而在較小顆水滴(D < 1.1 mm)時，尤其處於弱降雨強度和弱向下空氣運動環境下，當水平風強度增強時相對於平均落速之速度偏差更為顯著。此特徵很可能與周圍的環境風作用於水滴上時，使其托曳力調節有所相關。整體而言，向下空氣運動、較強水平風場及較強降雨率的降水環境裡皆使觀測到的水滴落速傾向有較快的情形。
With a Particle Size and Velocity (PARSIVEL) optical disdrometer deployed at the Hua-Lin site in Taipei (24o53’N, 121o34’E), which can provide joint size and fall speed distributions of raindrops, this study investigates the characteristics of DFS as a function of drop size for all rain events collected from June in 2010 to June in 2011. Temporally high-resolution wind measurements from a 3-Axis Ultrasonic Anemometer available at the Hua-Lin site were also used in this study to evaluate the possible impact of winds on influencing the behavior of DFS. Preliminary analyses indicate that small (big) raindrops tend to have superterminal (subterminal) velocities. For smaller drop sizes (D < 0.9 mm), the departure of DFS from mean DFS appears to be more significant at higher rainfall rates, which implies the possible impact of the raindrop coalescence-breakup-relaxation processes. Under environmental conditions with low rainfall rates and weak horizontal winds, the velocity deviation from the mean DFS was observed to be positive (negative) for the downward (upward) air motions. For smaller drop sizes (D < 1.1 mm), the velocity departure from the mean DFS increasing with the intensity of horizontal winds was evident, particularly under conditions of low rainfall rates and weak downward air motions. This characteristic would be probably related to the modulations of drag force exerting on a drop by ambient winds. It was generally found that the downward air motions, stronger horizontal winds, and higher rainfall rates tended to speed the DFS observed in natural rainfall events. |
