運用光達參數”Power Ratio”判斷雲底位置與氣膠吸濕性

Abstract

本研究依RCEC光達兩波段(355, 532 nm)的原始訊號比值定義了一個光達參數”Power Ratio (PR)”，並利用散射理論(scattering theory)和柯勒理論(Köhler theory)建立一個簡單的模擬系統來模擬氣膠物理化學性質在不同大氣環境下的PR隨高度分布表現，並試圖擬合觀測結果來判斷雲底位置和氣膠的吸濕參數(hygroscopicity parameter, κ)。 模擬結果顯示PR主要由粒徑分布來主導，其中κ、相對濕度(RH)和初始乾粒徑分布會影響柯勒理論所計算出來的濕粒徑分布，進而改變PR的趨勢變化；整體而言，當κ、RH和乾粒徑分布愈大時，PR也會偏大。此外，折射率(Rrfractive index)則會影響散射理論的計算，虛部愈大(粒子有較強的吸收係數)，PR愈大；實部愈大(粒子有較強的散光係數)，則PR愈小。 在個案分析上，針對2009/1/13、2010/10/31以及2013/10/26三個大氣趨於均勻混合的案例進行PR垂直剖面的模擬。在大氣均勻混合層中，RH隨高度增加，因此模擬結果顯示PR隨著高度上升而變大；而κ則會改變PR隨高度變化的斜率，在較高κ時，PR在較低的RH便能有明顯上升的轉折變化。在給定的乾粒徑分布下，當κ = 0.2時，2009/1/13和2010/10/31案例中模擬結果較貼近觀測結果，而2013/10/26的案例則是κ = 0.4。因此，PR的趨勢變化可以提供κ的判定指標。 根據理論與多組觀測資料比較，當粒徑分布因為成雲而有大幅度改變時，觀測訊號(背向散射比(R)、消偏振率(DP)和PR)也會有明顯的變化，將訊號變化幅度最甚之處定義為雲底位置(cloud base height)，結果顯示PR最為敏感、DP最不適用。另外判定雲底之方法為PR的模擬調整，乃是經由調整κ和RH垂直剖面等來擬合觀測結果，並獲得雲底位置，其中2009/1/13和2013/10/26個案中，兩種方法所找到的雲底位置相近。因此本研究中所定義的PR，除了可以提供κ的訊息外，還可以提供較靈敏的雲底位置的判斷，對後續探討局部渦流形成雲可提供觀測輔助。

In this study, a new parameter of Power Ratio (PR), the ratio of original signal of RCEC Lidar (355, 532 nm), is introduced to illustrate the lidar signal affected by aerosols. PR was simulated at different atmospheric conditions incoporating Köhler theory and scattering theory. The hygroscopicity parameter of aerosols (κ) and cloud base height were determined by the comparision of the simulation and lidar signal. The results show that PR is mainly dependent on the size distribution of ambient aerosols, which were then controlled by dry size distribution, κ, and relative humidity (RH). In general, larger κ, RH and dry size distribution would cause larger PR. In addition, PR is affected by refractive index (m) with scattering theory. A larger imaginary part (absorption part) would result in a larger PR, while a larger real part (scattering part) would result in a smaller PR. For case studies, PR profiles are simulated for three cases: 2009/1/13, 2010/10/31 and 2013/10/26, with well mixed boundary layer. In these cases, RH increases with altitude, so PR increases as RH increases. Furthermore, the rising slope of PR with height will be affected by κ. In cases of 2009/1/13 and 2010/10/31, the results of simulation for a given κ = 0.2 are more consistent with the observation result, while κ = 0.4 for the case of 2013/10/26. In the comparison of simulation and observation, the ambient size distribution of aerosols plays a major role to control PR. As the cloud droplets present, the overall wet size distribution shifts dramatically and cause a significant change in backscattering ratio (R), depolarization ratio (DP) and PR. Such dramatically change was applied to determine the cloud base height, which is more sensitive in PR but less sensitive in DP. To fit the observed PR, κ and RH profile were adjusted in the simulation. Overall, cloud base heights determined in this study using different methods are consistent for cases of 2009/1/13 and 2013/10/26. Therefore, PR defined in this study can be an index of κ and further procide a better indication for the cloud base height, which could be useful to understand the eddies in the boundary layer for the future.