2008 年 SoWMEX 期間雷達折射率 之特徵及應用;The Characteristics and Application of Radar Refractivity During 2008 SoWMEX

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题名:	2008 年 SoWMEX 期間雷達折射率之特徵及應用;The Characteristics and Application of Radar Refractivity During 2008 SoWMEX
作者:	許修維;Hsu, Hsiu-Wei
贡献者:	大氣科學學系
关键词:	折射率;邊界層;對流肇始;refractivity;boundary layer;convection initiation
日期:	2019-08-20
上传时间:	2019-09-03 12:20:47 (UTC+8)
出版者:	國立中央大學
摘要:	近地表的水氣資訊對於研究對流肇始及邊界層發展相當重要，但是過去對於水氣的觀測僅限於地面觀測點或線上的方式，高時間空間解析度的水氣資訊仍相當缺乏。折射率值的變化會受到溫度、壓力、濕度的影響，從地面測站的資料顯示折射率的變化有79%以上歸因於水氣的變化，因此本研究透過2008年SoWMEX(Southwest Monsoon Experiment)期間，NCAR S-Pol雷達所反演出的折射率及地面測站資料，來評估高時間空間解析度的水氣變化特徵與天氣現象之間的關聯。結果顯示，雷達折射率與地面測站和探空的觀測結果具有相當高的一致性，表示其能夠代表近地表水氣資訊。在實驗期間，雷達折射率的變化主要受到不同天氣系統下的水氣影響。在弱綜觀條件下，雷達折射率的變化能夠清楚地顯示海陸風環流的發展，且由於午夜時風速減小造成的水氣累積以及接近清晨時較乾燥的陸風，內陸測站在一天當中折射率的最大(小)值會發生在午夜(清晨)。分析結果也顯示早上富含水氣的海風鋒面移入，對於下午的對流提供更有利於其生長的環境。伴隨降雨造成的的蒸發冷卻效應影響，雷達回波與折射率的梯度分布相當一致，同時降雨造成的冷濕空氣亦會被大環境風場影響而逐漸消散。本研究成功地利用氣象雷達來反演折射率，其能夠補足地面測站及其他儀器在空間上的不足，提供在Meso-γ尺度下更細微的水氣變化，並捕捉山區對流肇始的關鍵因子，未來進一步使用折射率研究邊界層發展，對流降雨或是應用至資料同化亦具有相當大的潛力。 ;Near-surface high temporal and spatial moisture information is important for research of convection initiation(CI) and boundary layer evolution, but the in-situ water vapor observation is limited to the surface station points and quite deficient. The change of refractivity is caused by variations of the pressure, temperature, and particularly humidity. The result of fixed stations shows that the variation of refractivity is mainly from water vapor contribution. Therefore, this study combines the refractivity retrieved from NCAR S-band dual-Polarization Doppler radar (S-Pol) and surface station data during 2008 SoWMEX(Southwest Monsoon Experiment) to study the relationship between the characteristics of moisture variation with high resolution and different weather systems. The comparison shows that the refractivity results between the radar retrieval and in-situ observation, like surface stations and radiosonde, have high correlation. Thus, the radar data derived refractivity is representative of low-level atmospheric refractivity. During 2008 SoWMEX, it is shown that the variation of refractivity is mainly influenced by water vapor in different weather systems. Under weak-synoptic condition, we find that radar-retrieved refractivity field can clearly reveal the development of land sea breeze circulation. The maximum (minimum) of refractivity takes place around the midnight (dawn), as a result of the air mass accumulation caused by wind speed reduction near midnight and the dryer land breeze near dawn. The intrusion of sea breeze front with abundant water vapor in the morning might set up a favorable environment for afternoon convection initiation. Besides, due to evaporation cooling caused by rainfall, the distribution of sharp refractivity gradient and reflectivity over 30dBZ show high consistency. The cold and wet air is reduced gradually owing to dryer downslope wind from typhoon circulation. This study demonstrates that refractivity inferred by weather radar can compensate the limited data coverage from fixed surface stations and provide the low-level water vapor information with high resolution under Meso-γ scale. It is also feasible to use refractivity to study the development of the boundary layer and convective storms, or be applied to data assimilation.
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