以CALMET模式模擬台灣地區混合層高度

Abstract

環境中混合層高度是影響大氣中空氣污染物質的擴散與混合能力的主要因素之一，當混合層高度越高，大氣層越能提供稀釋其中空氣污染物質的空間，使空氣污染的程度降低。因此對於空氣污染防制上，混合層高度的推估是一個重要參考依據。過去對於混合層高度的推估大多利用探空溫剖配合地面溫度依乾絕熱遞減率線與其相交之高度，定為混合層高度。但此方法缺點在於如果地表溫度的些微波動，或是探空溫剖因氣候影響無法呈現正常遞減的現象，都將導致所推估的混合層高度不穩定且偏向低估的情況。然而，造成混合層高度發展的機制必須同時考慮因日夜不同所產生的熱浮力紊流與機械紊流兩種方法。熱浮力紊流與日間太陽輻射的加熱效應所導致，而當夜晚來臨，機械紊流則由近地層風切機械力決定。本研究利用CALMET氣象前處理模式模擬2003年台灣混合層高度發展，其模式採用地表能量收支平衡法以及引用摩擦風速量值來進行模擬，而模擬結果則以實測資料所推算的混合層高度來加以比對其合理性，實測資料包括環保署北中南三地的逆溫儀即時溫剖以及台灣南部混合層高度實場觀測計畫。 根據本研究結果發現，CALMET與實測資料兩者所得混合層高度結果具有高度的線性相關，並且在時間上發展趨勢相同。以地表上的混合層高度而言，其混合層高度變化與對應之可感熱通量呈正相關，而中午時分為一日內混合層高度發展最高的階段，約為一千公尺左右，夜間的混合層高度變化則與摩擦風速大小有關，分為平地區域偏低為兩百公尺左右，山區與海陸交界處則有較高的混合層高度發展。至於海面上混合層高度則不分日夜一致穩定，主要由海風所形成的摩擦風速影響其高度變化。而CALMET模擬之日間地表上混合層高度於空間上的變化，在同一時刻下，太陽輻射加熱效應一定時，模擬區域內的雲量越多的地方會導致混合層高偏低，而在同一時間同一雲量區域內，其區域內地表參數之鮑溫率越高的地方所獲得之混合層高度發展越快且高度較高。

Mixing layer height(MH) is very important for that the air pollutants in the atmosphere can be transferred vertically and diluted. As MH develops much higher, the signification deterioration episodes happen much fewer. That is the reason the air pollution science focus the MH in the environment discussions. As so far, the MH can be judged by the upper air vertical temperature structure and measured surface temperature. The MH is set on where the surface temperature decreasing on the adiabatic lapse rate along the altitude meets the upper air temperature. Unfortunately the method often makes the mistake as MH as the unstable surface temperature and not very well for the lapse of upper air temperature slope. It should take more consideration for the MH development. Generally the mechanisms of MH development are different. For the daytime MH, heat buoyant turbulence production determines the vertical extent of dispersion because of on unstable situation, on the contrary, in the stable MH at nighttime, the mechanism is mechanical turbulence. In this study, the CALMET model with the characteristic MH simulations of above mechanisms is used to simulate Taiwan MHs in 2003. In order to confirm results of simulations, they are compared with the MH calculated by real measured data including MTP5 and observations project of MH in south Taiwan. From the result of the study, the daytime overland MH is influenced by overland sensible heat flux, in the other side, nighttime overland MH and oversea MH get the friction wind speed to judge. But when daytime overland MH get more cloud above, MH gets developed slowly. If the same cloud above somewhere on the same moment, the higher Bowen Ratio of the section gets higher MH and speeds MH development.