輻射參數化模式之測試比較

Intercomparison of Radiation Parameterization Models

馮欽賜(Chin-Tzu Feng)

《大氣科學》 20卷4期 (1992/12) Pp. 387-404

摘要

正確的輻射參數化模式無論對氣候模擬或數值天氣預報都是相當重要的。本文參照國際間輻射模式比較計劃（Intercomparision of Radiation Codes Used in Climate Models, ICRCCM）設定的方法，測試分析兩版不同的輻射參數化模式，一是中央氣象局現行全球作業模式中的輻射模式（簡稱舊版，CWB1），一是第二代全球模式將採用的新版輻射模式（CWB2）。藉由最精確單線模式或次精確頻段模式的結果為標準值，本文選用熱帶、中緯度夏季和副極區冬季三種標準大氣，並設定在晴天條件下，分別計算新舊版長波和短波的垂直大氣加熱／冷卻率以及邊界輻射通量值（包括地表與大氣層頂），比較兩者相對於標準值的差異。為了進一步瞭解垂直解析度與輻射計算精確性之關聯，本文同時分析新版模式於不同垂直分層下結果（CWB2L為新版含20層，而CWB1及CWB2含9層）。比較結果如下：（一）長波方面－CWB1忽略臭氧吸收因而高估300mb以上大氣的冷卻率，另一方面未考慮e-type continuum吸收效應，導致低估溫暖潮溼大氣之中低對流層冷卻率與高估地表淨輻射通量。CWB2則無上述CWB1的缺失，垂直大氣冷卻率分佈的誤差都幾乎小0.5℃/day，並且輻射通量計算值小於5%的誤差。其次，加大CWB2的垂直解析度至20層得到CWB2L的結果，發現溫暖潮溼大氣的長波冷卻率與垂直解析度有密切相關，CWB2L明顯縮小CWB2因垂直解析度不夠出現在熱帶大氣低對流層長波冷卻率高估的現象。（二）短波方面－CWB1簡化的設計，導致明顯低估大氣的加熱率和吸收量。CWB2相較CWB1雖然略見改善，但仍低估兩者，不過大氣加熱率的偏差尚保持在0.5℃/day以內。CWB1與CWB2則相對地高估到達地面的向下輻射通量，其次，垂直20層的CWB2L除了改進平流層解析度而增加臭氧的短波吸收量外，其餘結果類似CWB2，垂直解析度對短波輻射的計算並不敏感。針對新版（CWB2L）短波輻射低估大氣吸收量的問題，經由進一步測試，發現與計算水氣有效吸收光程時所使用的非均勻修正函數的誤差有關。

關鍵字

輻射參數化模式 ； 長波 ； 短波

並列摘要

Two versions of radiation parameterization are examined following the procedure of Intercomparision of Radiation Codes Used in Climate Models (ICRCCM). The old version (CWB1) is the one used in the CWB operational global model, the new version (CWB2) is the one that will be used in the future second generation global model. Intercomparisions of the radiative transfer computed for the clear sky of the standard atmospheres by the two versions are made concerning longwave and shortwave radiative fluxes at vertical boundaries, and heating/cooling rates at each model levels with more detailed line-by-line or narrowband models used as bench-marks. In addition, higher vertical resolution (20 levels) is tested in the new version (CWB2L) for vertical resolution sensitivity. The conclusions are:1. For longwave, the cooling rates in the lower troposphere of the tropical and midlatitude summer atmosphere are underestimated in CWB1 because of neglecting e-type continuum absorption. As a result, net long-wave fluxes at the surface are overestimated in CWB1. The error ranges in CWB2 are within 5% for fluxes at vertical boundaries, and within 0.5℃/day for vertical cooling profiles. The results of CWB2L show that the vertical resolution is rather sensitive for the humid lower troposphere in tropics.2. For shortwave, the total absorption of the solar flux by the atmosphere is underestimated both in CWB1 and CWB2. The problem is more serious in CWB1 because of its simpler design. However, both schemes lead to overestimate downward shortwave fluxes at the surface. Besides increasing ozone absorption in the stratosphere, the results of CWB2L are similar to those of CWB2L and are insensitive to the vertical resolution.More detailed analyses on absorption in CWB2L show that the discrepancy on shortwave absorption comes from the calculation of the effective water vapor optical depth.

並列關鍵字

radiation parameterization model ； longwave ； shortwave