Statistical Characteristics of Raindrop Size Distribution in the Tibetan Plateau and Southern China

The characteristics of raindrop size distribution （DSD） over the Tibetan Plateau and southern China are studied in this paper, using the DSD data from April to August 2014 collected by HSC-PS32 disdrometers in Nagqu and Yangjiang, com- prising a total of 9430 and 63661-rain raindrop spectra, respectively. The raindrop spectra, characteristics of parameter variations with rainfall rate, and the relationships between reflectivity factor （Z） and rainfall rate （R） are analyzed, as well as their DSD changes with precipitation type and rainfall rate. The results show that the average raindrop spectra appear to be one-peak curves, the number concentration for larger drops increase significantly with rainfall rate, and its value over southern China is much higher, especially in convective rain larger drops, especially for convective rain in southern China. Standardized Gamma distributions better describe DSD for All three Gamma parameters for stratiform precipitation over the Tibetan Plateau are much higher, while its shape parameter （,u） and mass-weighted mean diameter （Dm）, for convective precipitation, are less. In terms of parameter variation with rainfall rate, the normalized intercept parameter （Nw） over the Tibetan Plateau for stratiform rain increases with rainfall rate, which is opposite to the situation in convective rain. The/1 over the Tibetan Plateau for stratiform and convective precipitation types decreases with an increase in rainfall rate, which is opposite to the case for Dm variation. In Z-R relationships, like ＂Z = ARb＇＇, the coefficient A over the Tibetan Plateau is smaller, while its b is higher, when the rain type transfers from stratiform to convective ones. Furthermore, with an increase in rainfall rate, parameters A and b over southern China increase gradually, while A over the Tibetan Plateau decreases sub- stantially, which differs from the findings of previous studies. In terms of geographic location and climate over the Tibetan Plateau and southern China, the precipitation in the pre-flood seasons is dominated by strong convective rain, while weak convective rain occurs frequently in northern Tibet with lower humidity and higher altitude.

DSD参数对双频雷达估测降水的影响研究

双频雷达通过估计降水的雨滴谱分布(Rain Drop Size Distribution,DSD)参数来估测降水,相比多普勒天气雷达利用Z-I关系估测降水的方法,估测结果更加准确。真实的雨滴谱分布随时空不断变化,难以准确描述。本研究的目的是评估将其描述成Gamma分布模型对双频雷达估测降水造成的误差。研究中利用架设在广东龙门和西藏那曲的雨滴谱仪观测资料,计算Ku/Ka双频雷达的反射率因子,利用双频雷达算法反演降水率R和衰减系数k,并将其与雨滴谱仪直接计算的结果进行比较,探讨双频雷达反演算法中DSD模型对反演结果造成的影响,并对比反演误差的地区差异性。研究结果表明:DSD参数对双频雷达反演降水的影响在那曲和龙门地区有一定的差异。从双频技术反演的质量加权的粒子直径(Dm)、Ka波长衰减系数(kKa)、Ku波长衰减系数(kKu)和降水率(R)来看,当Gamma分布的形状因子μ分别取2、3和4时,龙门地区对四个参数的反演都能得到较为稳定准确的结果,相对误差基本<10%。而就kKa和5~40 mm·h^-1的降水率而言,那曲和龙门有很大的不同,在龙门当μ取3时相对误差最小,都在0值附近波动,而那曲则要需要μ值在4和6附近才能使得相对误差达到最小。

Spatial and temporal patterns of drought in Zambia

Drought acutely affects economic sectors, natural habitats and communities. Understanding the past spatial and temporal patterns of drought is crucial because it facilitates the forecasting of future drought occurrences and informs decision-making processes for possible adaptive measures. This is especially important in view of a changing climate. This study employed the World Meteorological Organization(WMO)-recommended standardized precipitation index(SPI) to investigate the spatial and temporal patterns of drought in Zambia from 1960 to 2016. The relationship between the occurrence of consecutive dry days(CDD; consecutive days with less than 1 mm of precipitation) and SPI was also investigated. Horizontal wind vectors at 850 hPa during the core of the rainy season(December–February)were examined to ascertain the patterns of flow during years of extreme and severe drought; and these were contrasted with the patterns of flow in 2007, which was a generally wet year. Pressure vertical velocity was also investigated. Based on the gamma distribution, SPI successfully categorized extremely dry(with a SPI value less than or equal to –2.0) years over Zambia as 1992 and 2015, a severely dry(–1.9 to –1.5) year as 1995, moderately dry(–1.4 to –1.0) years as 1972, 1980, 1987, 1999 and 2005, and 26 near normal years(–0.9 to 0.9). The occurrence of CDD was found to be strongly negatively correlated with SPI with a coefficient of –0.6. Further results suggest that, during wet years, Zambia is influenced by a clockwise circulating low-pressure zone over the south-eastern Angola, a second such zone over the northern and eastern parts, and a third over the Indian Ocean. In stark contrast, years of drought were characterized by an anti-clockwise circulating high-pressure zone over the south-western parts of Zambia,constraining precipitation activities over the country. Further, wet years were characterized by negative pressure vertical velocity anomalies, signifying ascending motion; while drought years were dominated by positive anomalies, signifying descending motion, which suppresses precipitation. These patterns can be used to forecast drought over Zambia and aid in strategic planning to limit the potential damage of drought.

长沙一次强降水过程的雨滴谱特征分析

分析2015年6月21日湖南长沙一次强降水过程的雨滴谱微物理参数特征及其随雨强的变化。结果表明,本次强降水过程主要受到高空低槽、中低层低涡切变和地面冷空气共同影响,雷达反射率因子和雨强随时间的变化基本一致,粒子数浓度与雨强、雷达回波强度与加权平均直径的时间序列之间均是高度正相关。当R≥5mm·h^(-1)时,标准化截距参数随雨强增大而显著增长但形状因子值整体上是变小的,不同雨强间隔中的多峰谱比例进一步提升,lgNw和Dm的分布趋于集中且其值增大,lgN_(w)-R的幂指数关系拟合的相关性较差。各类平均谱基本是单峰谱,随着雨强增大,峰值直径略变大,谱宽逐渐增大,各尺度档粒子浓度值显著增加。适用本次降水的Z-R关系是Z=26.67R^(1.88),尤其当雨强迅速增加时,它更能反映真实降雨分布。

利用激光降水粒子谱仪研究雨滴谱分布特性

利用Parsivel激光降水粒子谱仪2009年3—6月在南京观测获取雨滴谱资料,分类对各次降水过程雨滴谱进行Gamma分布拟合,对比分析了南京地区不同云系降水的雨滴谱分布特征以及Gamm a分布参数相关特性,并且讨论了"标准化"Gamma分布参数随雨强及雷达反射率因子的变化关系。结果表明:用Gamma分布拟合本次试验不同云系降水的雨滴谱分布精度都比较高;"标准化"的Gamma分布参数较Gamma分布参数更具有物理意义,且其参数μ和Dm与雷达反射率因子Z均具有较佳的对应关系,μ随Z的增大依次递减,Dm则随Z的增大呈指数型递增。

近30年黄土高原春季降水特征与春旱变化的关系

利用1980—2014年黄土高原52个气象台站观测的降水资料,计算得到了标准化降水指数(SPI),进而比较了黄土高原不同地区近35年春季干旱的时空变化特征。研究发现:春季SPI与春季降水的时间变化较为一致,二者都能反映出1995年,2000年和2001年的三次较严重的春旱,且二者在1980—2014年间均呈线性减少的趋势。同时,黄土高原春季降水与春季SPI的空间分布却存在一定差别,这主要是因为春季降水的概率密度分布特征在不同站点存在差异。黄土高原北部,一次春季降水量少的发生概率较大,而在高原南部,较大降水概率则对应较多的降水量值。所以,对于黄土高原整体而言,当各区域春季降水偏少,且降水量值相等时,高原南部的春旱更严重;但当各地区春季降水异常量相等时,则在高原北部春旱等级较高。因此,未来黄土高原的春旱不仅受降水量控制,还与春季降水概率密度的分布特征关系密切。

基于微雨雷达的六盘山区地形云降水宏微观特征观测分析

利用2017年6—11月宁夏六盘山区收集的微雨雷达和微波辐射计等探测资料,对比分析了六盘山区山脊和山谷对流云降水、层状云和浅积云降水过程中的云微物理特征及亮带,针对典型层状云降水事件山脊和山谷站的亮带及以下各层的雨滴谱分布特征,探索了亮带以上几层水凝物的分类。结果发现:六盘山区三类降水云山脊的反射率及反射率衰减程度均高于山谷,表明地形强迫使得山脊降水云的物理和动力过程较山谷更剧烈,层状云降水过程中山脊0℃等温线以上的反射率明显高于山谷,表明山脊在0℃等温线以上有更多水凝物。分析一次典型层状云降水过程发现:六盘山区降水液滴自亮带下落的过程中,碰并过程占主导;亮带以下各层DSD符合Gamma分布,山脊站Gamma分布的相关性比山谷站强且拟合优度更优;降水开始的前3 min,推测-4~0℃等温线之间的水凝物主要是霰,此后水凝物主要是雪颗粒和霰,而山谷在降水开始的前5 min,-4~0℃等温线层之间的水凝物主要是雪颗粒和较大的霰,此后水凝物主要是霰。

估算雨滴谱Γ分布三参数的阶矩法研究

利用2014年8月4—23日山西忻州OTT-Parsivel二代激光雨滴谱仪获得的雨滴谱资料,研究了估算雨滴谱T分布三参数的阶矩法。把降水云分为层状云、积状云、积层混合云三种,采用最小二乘法和阶矩法估算雨滴谱,并在现有阶矩法的研究上,提出一种新矩量组合M036与其他矩量一起分1 min、5 min平均和5 mm滑动平均估算雨滴谱样本的均方根误差和误差谱分布,并进行比较,同时研究了不同降水云雨滴谱三参数随雨强R的变化关系。结果表明:阶矩法估算T分布雨滴谱,M036矩量组合代表雨滴数浓度、液态水含量和雷达反射率因子,三个物理量计算方便,物理意义清晰,实际应用中如能通过探测手段获得三个矩量,就能反演得到云中雨滴谱。阶矩法中M036估算最好;估算三种不同时间雨滴谱样本的均方根误差平均值,M036误差最小;分析三种不同时间雨滴谱样本的均方根误差谱分布,M036谱宽最窄,误差集中;估算三种降水云雨滴谱三参数随雨强R的增大而减少;拟合雨滴谱T分布三参数N_0、μ、λ满足二项式分布且相关系数较好。

雨滴谱的变化对降水估计的影响

针对雨滴谱的变化对降水估计的影响,提出根据激光雨滴谱仪上方雷达回波的结构特征将降水过程划分为对流云降水和层状云降水交替分布5个部分,通过基于2种降水类型的第Ⅰ种分类Z-R关系、基于5个部分的第Ⅱ种分类Z-R关系和基于整个降水过程的总体Z-R关系分析雨滴谱的变化对降水估计的影响。结果表明：当对流云降水向层状云降水过渡时,指数谱从无到有、多峰谱比例减小,Nw减小、μ增大、Dm变化不大;Nw与R变化相似,当Z增大时μ和Dm分别是递减和递增的;Z-R关系（Z=aRb）中a值变化范围较大、系数b在1~2波动且与层状云阶段相比,对流云阶段的a和b值较小;利用第Ⅱ种分类Z-R关系反演的雨强与基于雨滴谱仪观测数据计算的雨强最接近;雨滴谱仪在层状云阶段的反演效果明显强于对流云阶段,这与对流云降水中雨滴谱信息变化大且快等因素有关。

春季粤北地区降水微物理特征分析

为揭示春季粤北降水微物理过程,选取2020年2—3月粤北2个站点雨滴谱仪观测资料,统计分析了春季粤北地区层云降水、混合性降水、对流性降水的微物理特征,结果表明:(1)对流性降水过程中大雨滴是影响雨强大小的重要因素;层云降水过程,小雨滴数浓度占比大于90%,平均雨强小于1 mm/h,雨滴数是影响雨强的重要因素。(2)层云性降水雨滴谱呈单峰结构,谱宽小;对流性降水雨滴谱为多峰结构,谱宽最大;混合性降水谱型与对流性降水相似。3类降水峰值均在0.2~0.3 mm范围,均是Gamma分布谱型拟合最优。(3)春季粤北地区对流性降水的Z-I关系为Z=173.9I^(1.452),混合性降水的Z-I关系为Z=72.77I^(1.97),层云性降水过程的Z-I关系为Z=194.3I^(1.296)。