基于微波辐射计分析低能见度的液态含水量特征

利用广州国家基本气象站2013年11月~2014年4月的能见度（Vis）、相对湿度（RH）、微波辐射计液态含水量（LPR）小时资料和云资料，分析了广州地区能见度的时间变化，以及低能见度时液态含水量变化特征.结果表明广州地区Vis<10km的出现频率为66.37%；Vis ≤ 1km只在RH ≥ 95%情况下出现，1km < Vis ≤ 2km只在RH>90%出现，Vis>5km在RH>95%区间出现频率为0，Vis>15km在RH>90%的出现频率为0；低能见度事件出现的频率高低顺序依次为早 > 晚 > 中，与相应时段低空LPR的平均值高低一致，说明低能见度事件与低空大气中高液态含水量存在一定的正相关；当Vis<10km时，大气液态水含量都处于较高的水平，在3000m以下存在一个大于0.02g/m³的高含水量层，液态含水量出现最大值的高度在1550m左右；当Vis ≥ 10km时，大气液态含水量都处于较低的水平，液态含水量出现最大值的高度大约在2000m，未出现有大于0.02g/m³的情况；广州地面低能见度过程不是低云（或低云接地为雾）过程导致的概率基本在50%以上，Vis ≤ 1km情况下LPR值基本为0（微波辐射计没有监测到液态水），但由于RH>95%接近饱和且出现高浓度的颗粒物，说明广州地区低能见度（Vis ≤ 1km）应为雾霾混合情景.表明只有联合利用微波辐射计的液态含水量、相对湿度与颗粒物浓度等要素变化特征的综合分析才能为雾、雾霾混合与霾现象分辨提供一定的参考.

Characterization of liquid water content during low visibility based on microwave radiometer data

We characterized the temporal variations of visibility (Vis), relative humidity (RH) and liquid water profile (LPR) during low visibility periods based on the visibility, cloud and microwave-radiometer-detected liquid water content data provided by the national basic meteorological station in Guangzhou from November 2013 to April 2014. Results showed that the occurrence frequency of Vis<10km was 66.37%. Vis ≤ 1km happened when RH ≥ 95%, and 1km 90%. Vis>5km did not occur when RH>95% and Vis>15km never happened at RH> 90%. Low visibility events tended to happen more frequently in the morning than in the evening and seldom occurred during noontime. This was consistent with the variations of LPR in the corresponding period, indicating that low visibility events may correlate positively with the high liquid water content in the lower atmosphere. The integral liquid water content of the atmosphere was generally high when Vis < 10km. The high liquid water content exceeding 0.02g/m³ was normally observed in the atmosphere below 3km, with the maximum liquid water approximately appeared at 1550m height. The liquid water content was relatively low in the case of Vis ≥ 10km, which seldom exceeded 0.02g/m³ and the maximum value of which was usually observed at a height of approximately 2km. Less than 50% of the low visibility events in Guangzhou was introduced by the formation process of low cloud (or low cloud reaching the ground to form fog). When Vis ≤ 1km, LPR was observed to be nearly zero as no liquid water content was detected by the microwave radiometer. Simultaneously, it is worth noting that the ground-level aerosol loading was quite high (PM_2.5>88.3μg/m³) when RH > 95% and the atmosphere was approaching saturation condition, suggesting that low visibility events (Vis ≤ 1km) in Guangzhou could be attributed to the combination of fog and haze. This highlights the need for comprehensive analysis of the liquid water content from microwave radiometer measurements, relative humidity, and particle mass concentration in order to further distinguish haze pollution from fog and/or mixed fog and haze phenomena.