地面雙頻微波輻射偵測大氣中水氣含量及溫度剖線

Ground-Based Dual-Channel Radiometric Sensing of Water Vapor and Temperature Profiles

劉說安(Yuei-An Liou) ； 張銓倫(Chuan-Lun Chang)

《大氣科學》 28卷1期 (2000/03) Pp. 17-26

摘要

本研究探討微波輻射計(Water Vapor Radiometer;WVR)觀測大氣中水氣含量及溫度剖線，於民國八十七年三月十一日到五月二十五日間在台北氣象站進行觀測實驗，蒐集23.8及31.4 GHz兩種頻率的亮度溫度(brightness temperature;亮溫)，並由統計迴歸估得大氣中水氣含量及溫度剖線。統計迴歸係數係根據民國七十七年到八十六年間，在中央氣象局台北氣象站蒐集的探空資料做分析所得。爲探究該迴歸公式品質之良窳，比較迴歸所得大氣之溫、濕剖線與相應的探空觀測，發現加權的均方根差(weighted root mean squared error;weighted RMSE)分別爲2.93K、1.50g/立方公尺（以水氣含量爲權重）。加入地面溫、濕、壓觀測當約束條件後，迴歸品質則顯著改進，加權的(weighted)RMSE分別降到1.62K、0.83g/立方公尺。基本上而言，迴歸大氣最低層之溫、濕與相應的探空觀測比較，兩者間差異幾乎爲零。依據地面氣象觀測當約束條件方式，本研究中挑選兩種不同大氣剖線，說明雙頻輻射觀測大氣中水氣含量及溫度剖線之技術。第一種個案中，大氣溫、濕度隨高度單調性地遞減。第二種個案中，大氣溫、濕剖線隨高度非單調性地遞減，即剖線存有隨高程快速變化之逆溫、逆濕高頻訊號。雖然如預期般地，WVR觀測掌握不住高頻訊號，但是對單調性遞減之大氣溫、濕剖線，WVR觀測則與相應的探空觀測相當吻合。

關鍵字

水氣含量 ； 亮溫 ； 輻射偵測

並列摘要

A ground-based water vapor radiometer(WVR)operating at 23.8 and 31.4 GHz was utilized to observe sky brightness temperatures at the Taipei weather station from March 11 to May 25,1998.Theobserved sky brightnesses are used to investigate radiometric sensing of water vapor and temperature profiles through a statistical regression method whose coefficients are derived based on radiosonde soundings collected at the same site every March starting from the year 1988 to 1997.The regression method is evaluated by a self-consistent test by comparing its retrievals of water vapor and temperature profiles with those observed by radiosonde observations(ground truth).While the weighted root mean square error(RMSE)between the retrievals and ground truth is somewhat large(about 1.5g/m^3)for the water vapor, the RMSE decreases exponentially with altitude. In contrast, the RMSE for the temperature is on the range of 2.2 to 3 K. To improve the retrieval accuracy, surface observations of temperature, water vapor density, and pressure are used as constraints. We found the RMSE for both water vapor and temperature retrievals are significantly decreased near the surface. Essentially, zero differences between the retrieved profiles and the corresponding radiosonde observations are acquired near the surface for both variables.
With the constraints of surface meteorological measurements, the regression scheme is applied to derive water vapor and temperature profiles from WVR-observed sky microwave emissions. Two extreme cases are chosen for the current study, one with an atmosphere of monotonically decreasing water vapor and temperature profile and the other with a non-monotonically decreasing (inversion) profile. While WVR does not capture a low-elevation inversion (high-frequency signals) in both temperature and water vapor profiles for the latter case, it does perform reasonably well in determining the profiles for the former case.

並列關鍵字

Water vapor ； Brightness temperature ； Radiometric sensing