積雲參數化中降水方案之改進

An Improved Precipitation Scheme in Cumulus Parameterization

陳毅軒(Yi-Hsuan Chen)

指導教授 ： 陳正平

國立臺灣大學/理學院/大氣科學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.00239

摘要

一般全球模式的水平解析度太粗不足以解析積雲對流，因此必須參數化積雲的影響，稱為積雲參數法。目前的模式評估發現全球模式模擬出合理的降水時空分布，但是高估了低緯處的對流降水量，部分原因可能是因為目前的積雲參數法使用過於簡略的雲微物理過程，且無法反應氣膠的影響。
本研究在深對流參數法Zhang-McFarlane scheme中加入一個雙矩量、暖雲過程的雲微物理參數法診斷雲滴、雨滴的質量混合比與數量濃度，物理過程則考慮了自動轉換、雨滴收集雲滴、雨滴自我收集等暖雲過程，並使用NCAR全球模式第五版的一維氣柱模式與GATE phase III野外觀測資料做初步的測試。和其他積雲參數法相比，本研究的雨滴運動方向由上升氣流速度與終端速度的相對大小決定，雨滴可以向上或向下運動，是一更有物理意義的參數法。由於在積雲參數法中很難得知雨滴是否受到上升氣流的影響，本研究做了兩個敏感度測試：一是假設雨滴相對上升氣流做垂直運動(Rain-updraft)，二是假設雨滴以終端速度相對地面做向下運動(Rain-terminal)。
模擬結果顯示：一、Rain-updraft和Rain-terminal的雲水、雨水垂直分布顯著不同。二、Rain-updraft的對流降水率低於Rain-terminal，且兩者皆低於control run；由於補償作用，層狀降水則增加，平均總降水率與control run差不多。三、Rain-updraft的平均地面太陽短波輻射比Rain-terminal少了五倍，比起control run少了十倍。四、Rain-updraft與Rain-terminal的平均大氣層頂長波輻射彼此差異不大，約比control run小了15%。這些結果顯示積雲參數法中的雲微物理過程會影響降水型態及輻射的模擬結果。在雲滴濃度的敏感度方面，當雲滴濃度較低時，對流降水較多，反應了氣膠的Twomey第二間接效應。不同雲滴濃度對平均大氣層頂長波、地面短波輻射量影響不大，但會影響輻射的時間序列。

關鍵字

積雲參數化 ； 雲微物理參數法 ； 氣膠暖雲效應 ； 一維氣柱模式

並列摘要

Most general circulation models (GCMs) cannot resolve cumulus processes due to the use of coarse horizontal resolution, cumulus effects are therefore represented through cumulus parameterizations. Recent GCMs’ evaluations of precipitation show that most GCMs produce realistic spatial distribution of total precipitation, but overestimate convective precipitation in contrast to stratiform precipitation especially at lower latitudes. Such biases are likely resulted from the highly simplified precipitation treatment in current cumulus parameterizations, including the lack of responses to aerosol.
A two moment, two hydrometeor species (cloud water, rain) warm-rain scheme was incorporated into the Zhang-McFarlane scheme using the single-column model of NCAR Community Atmosphere Model version 5 with GATE phase III field experiment data. This scheme includes the treatment of autoconversion, accretion and raindrop self-collection processes. The raindrop vertical motion depends on either the terminal velocity or updraft velocity, while other schemes assume raindrop instantaneous fallout to the surface. Two sensitivity tests were performed in this paper: one assumes all raindrops move upward with the convective updraft (Rain-updraft); the other assumes raindrops move downward at its terminal velocity (Rain-terminal). The main findings are: 1) Cloud water/rain vertical profiles in convective updraft of “Rain-updraft” show opposite pattern to that of “Rain-terminal”. 2) Convective precipitation rate of “Rain-updraft” is lower than “Rain-terminal” and both are lower than in the control run; due to moisture flux compensation, grid (or stratiform) precipitation increased so that the total precipitation for both tests are close to that of control run. 3) Average surface solar flux in “Rain-updraft” is a factor of 5 less than in “Rain-terminal” and a factor of 10 less than in the control run. 4) Small differences in average outgoing longwave radiation flux between “Rain-updraft” and “Rain-terminal”, both are 15% less than in the control run. These results show that aerosols and cloud microphysics processes in cumulus parameterizations may affect the partition between convective and stratiform precipitation as well as radiation budgets. A lower aerosol amount and thus less cloud drop number concentration results in higher rain water content and more convective precipitation. There are no significant differences in the average surface solar flux and outgoing longwave flux due to variations in cloud drop number concentrations, but it would affect the radiation fluxes time series.

並列關鍵字

cumulus parameterization ； microphysics parameterization ； aerosol warm cloud effect ； single-column model

參考文獻

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