Preliminary Calibration of GPS Signals and Its Effects on Soil Moisture Estimation

In recent years, Global Navigation Satellite Systems Reflectometry (GNSS-R) is developed to estimate soil moisture content (SMC) as a new remote sensing tool. Signal error of Global Positioning System (GPS) bistatic radar is an important factor that affects the accuracy of SMC estimation. In this paper, two methods of GPS signal calibration involving both the direct and reflected signals are introduced, and a detailed explanation of the theoretical basis for such methods is given. An improved SMC estimation model utilizing calibrated GPS L-band signals is proposed, and the estimation accuracy is validated using the airborne GPS data from the Soil Moisture Experiment in 2002 (SMEX02). We choose 21 sites with soybean and corn in the Walnut Creek region of the US for validation. The sites are divided into three categories according to their vegetation cover: bare soil, mid-vegetation cover (Mid-Veg), and high-vegetation cover (High-Veg). The accuracy of SMC estimation is 11.17% for bare soil and 8.12% for Mid-Veg sites, much better than that of the traditional model. For High-Veg sites, the effect of signal attenuation due to vegetation cover is preliminarily taken into consideration and a linear model related to Normalized Difference Vegetation Indices (NDVI) is adopted to obtain a factor for rectifying the "over-calibration", and the error for High-Veg sites is finally reduced to 3.81%.     

Potential impacts of northeastern Eurasian snow cover on generation of dust storms in northwestern China during spring

The effects of the northeastern Eurasian snow cover on the frequency of spring dust storms in northwestern China have been examined for the period 1979–2007. Averaged over all 43 stations in northwestern China, a statistically significant relationship has been found between dust-storm frequency (DSF) and Eurasian snow-water equivalent (SWE) during spring: mean DSF of 7.4 and 3.3 days for years of high- and low SWE, respectively. Further analyses reveal that positive SWE anomalies enhance the meridional gradients of the lower tropospheric temperatures and geopotential heights, thereby strengthening westerly jets and zonal wind shear over northwestern China and western Inner Mongolia of China, the regions of major dust sources. The anomalous atmospheric circulation corresponding to the Eurasian SWE anomalies either reinforces or weakens atmospheric baroclinicity and cyclogenesis, according to the sign of the anomaly, to affect the spring DSF. This study suggests that Eurasian SWE anomalies can be an influential factor of spring DSF in northwestern China and western Inner Mongolia of China.     

“8.8”舟曲特大山洪泥石流灾害天气特征分析

利用常规气象资料以及卫星、雷达资料,分析了“8.8”舟曲特大山洪泥石流灾害.结果表明,这次强降水是在高空冷平流入侵,与700 hPa切变线配合,由地面冷空气扩散触发对流不稳定能量释放,造成的强对流天气.由于地形等原因形成的一系列γ中尺度气旋造成降水的分配极不均匀;强降水与亮温有很好的对应关系,但亮温并非唯一影响因素.多普勒雷达的径向速度对γ中尺度气旋具有很好的指示意义,表现在旋转速度可以指示涡旋强度;降水与γ中尺度气旋有很好的正相关关系.     

“12.7.21”西南涡极端强降雨的成因分析

利用常规观测资料、ECMWF分析场、区域自动站、多普勒雷达及SWAN系统产品等资料对2012年7月21日西南涡暴雨过程及盘龙极端强降雨进行分析。分析发现：此次过程是“北槽南涡”形势下,地面冷空气触发西南涡其东侧辐合上升运动强烈发展,高层强辐散,因而产生了对流性暴雨天气过程;冷空气从西侧侵入西南涡是925 hPa “S”形冷锋形成的直接原因,也是地面辐合线形成的重要因素;极端短时强降雨就发生在西南涡东侧中尺度雨带的中部偏北区域,有地面辐合线相配合,降雨最强时MCC冷云中心TBB达最低值。雷达回波表明：西南涡两侧冷暖空气的交绥促进了β中尺度气旋式环流的形成;偏南风低空急流为强降雨提供了充足的水汽,增强了中低层的垂直风切变,有利于强降水超级单体风暴的发展和维持;盘龙的极端短时强降雨是β中尺度气旋式环流中,伴随有深厚中气旋的强降水超级单体风暴在环流中心附近持续发展的结果。     

辐射板强化喷管换热与红外抑制效果试验研究

试验研究了在发动机喷管中加装金属辐射板前后,喷管壁面温度、热喷流温度与喷管红外辐射特征的变化。结果表明,加装金属辐射板后,热喷流与喷管壁面之间的热量传递显著增强,热喷流中心温度降低,壁面温度明显升高,在90°方向上,热喷流3～5μm波段的红外辐射强度降低了38.5%。文中从热喷流、喷管壁面以及金属辐射板等相关部件的温度变化情况对红外辐射强度的变化原因进行了解释。      

2010年6月中下旬南方暴雨过程变形场作用分析

利用常规原始报文资料,采用基于Lanczos窗口滤波器的合成分析方法,对2010年6月13-27日的形势场、物理量进行合成分析,计算并分析了低空合成风场的总变形.结果表明:500 hPa大陆中、高纬地区存在一“Ω”形类阻塞高压,西北冷空气从其东部源源不断向长江及以南地区输送,与副热带高压西北侧的西南暖湿气流在我国南方地区交汇.700、850 hPa大陆高压与西太平洋副热带高压之间形成的鞍型场,位于长江以南地区,鞍型场鞍点附近及其东部膨胀轴是暴雨发生的主要区域.在南方地区上空200 hPa西北气流辐散产生的抽吸作用,对这次长时间、高强度的降水起了重要作用.发生在鞍型场和切变线上的低空中尺度气旋,是造成暴雨的重要天气系统.鞍型场膨胀轴附近,总变形最大,其位置、走向均与雨带基本一致,有利于锋生和中尺度气旋形成.低空冷暖气流交汇区域形成低空强θse梯度,在垂直剖面图上,等θse线呈现十分倾斜状态,该区域强θse水平梯度与低空总变形、正涡度区一致,可能是特大暴雨发生的重要因素.     

2012年初春粤北一次少见高架雷暴过程的分析

利用常规观测资料、地面中尺度自动气象站资料以及NCEP再分析资料,分析了2012年2月27日广东发生的一次罕见高架雷暴天气过程的特点及成因。结果表明,这次出现在低层冷高压控制下地面冷锋后部并伴有短时强降水、雷电和冰雹的强对流天气过程,是一次典型的"高架雷暴"过程。与从地面发展的普通雷暴不同的是,强对流天气发生时,近地面层大气层结稳定,低空有逆温层存在,暖湿空气是从逆温层以上开始抬升的,强对流天气落区与850 hPa切变线有较好的对应关系。中高层西风槽东移南压,850 hPa偏南急流的建立和显著增强,为此次高架雷暴过程提供了有利的环流背景。近地面层冷空气补充南下迫使低层暖湿空气抬升,配合高空槽前辐散气流的抽吸作用以及广东上空有利的大气动力、热力不稳定条件,是此次高架雷暴过程形成的原因。     

区域极端降水事件阈值计算方法比较分析

根据南京站1951—2010年逐日降水资料,采用2种传统的百分位法以及3种正态变换方法,探讨了确定区域极端降水事件阈值的最佳方法。结果表明,由于降水量的实际概率分布是一种明显的偏态分布,而传统的百分位法是在假设降水量遵从均匀分布条件下进行的,计算结果的稳定性较差。正态变换的3种方法是在降水量实际概率分布下采用百分位法计算阈值的,结果稳定性较好。其中以方法4效果最佳。为消除气候变化的影响,可以将研究时段按降水量变化的不同趋势分为几个气候阶段分别计算阈值。或者采用滑动气候阶段处理整个研究时段,并以各个滑动气候阶段阈值的平均值作为整个研究时段的阈值。     

Ensemble data assimilation in a simple coupled climate model: The role of ocean-atmosphere interaction

A conceptual coupled ocean-atmosphere model was used to study coupled ensemble data assimilation schemes with a focus on the role of ocean-atmosphere interaction in the assimilation. The optimal scheme was the fully coupled data assimilation scheme that employs the coupled covariance matrix and assimilates observations in both the atmosphere and ocean. The assimilation of synoptic atmospheric variability that captures the temporal fluctuation of the weather noise was found to be critical for the estimation of not only the atmospheric, but also oceanic states. The synoptic atmosphere observation was especially important in the mid-latitude system, where oceanic variability is driven by weather noise. The assimilation of synoptic atmospheric variability in the coupled model improved the atmospheric variability in the analysis and the subsequent forecasts, reducing error in the surface forcing and, in turn, in the ocean state. Atmospheric observation was able to further improve the oceanic state estimation directly through the coupled covariance between the atmosphere and ocean states. Relative to the mid-latitude system, the tropical system was influenced more by ocean-atmosphere interaction and, thus, the assimilation of oceanic observation becomes more important for the estimation of the ocean and atmosphere.     

共线SPDC下剩余脉冲泵浦光滤光特性研究

从宽带脉冲泵浦光的光谱及滤光片的滤光特性参数出发,分析了自发参量下转换（SPDC）产生纠缠光源的系统中各类滤光片对剩余脉冲泵浦光的滤除效率。分析结果表明,适当组合不同类型的滤光片可以有效地滤除剩余脉冲泵浦光,并从实验上验证了分析结果的正确性。该研究为脉冲泵浦光条件下SPDC过程产生的纠缠光源的有效测量提供了理论指导和实验验证。