2014年7月大气环流和天气分析

提2014年7月环流特征如下：北半球高纬地区为单一极涡,中高纬地区呈5波型分布,巴尔喀什湖附近低槽和东亚大槽强度均较常年偏强;西北太平洋副热带高压带呈东西带状分布,强度与常年同期相当。7月全国平均降水量115．0 mm,较常年同期（120．6 mm）偏少4．6％;全国平均气温为22．3℃,较常年同期（21．9℃）偏高0．4℃。月内共出现8次强降水过程,多站出现极端日降水量。7月共有5个热带气旋（风力8级以上）在西北太平洋和南海活动,生成个数较常年偏多,并有“威马逊”、“麦德姆”2个热带气旋登陆。华南、江南等地出现持续高温天气,全国87个气象观测站发生极端高温事件,74站发生极端日降水量事件。     

2013年夏季我国南方区域性高温天气的极端性分析

提利用1960-2013年我国南方10省（市）733个站点的日最高、最低气温和日平均气温资料,对2013年夏季我国南方高温天气的极端性进行了系统的分析。分析结果显示：2013年夏季我国南方高温天气具有显著的群发性特征,覆盖了长江中下游以及重庆等八个省、两个直辖市;也具有以高温天气过程重现构成的持续性特征,主要经历了4次高温天气过程,其中,7月22日至8月21日的第三次高温天气过程,强度最强、范围最广。重点围绕区域性高温在历史上的极端性做进一步分析,结果表明：所研究高温区域的夏季平均气温、平均最高气温、平均最低气温均破历史纪录,为近50年新高;平均高温日数和强度也超过了历年平均高温日数和强度的极值,属历史罕见;高温日数和高温强度的高值区域范围比历年向北扩展,且高值中心值超过历史最高纪录,极端性突出;2013年极端高温事件的发生次数突破了历史纪录,其中8月的极端高温事件十分突出。     

气象要素对太阳能电池板温度的影响

温度升高会引起光伏电池发电效率下降,电池板温度是确定温度折减系数的必要条件,目前我国尚没有充足的外场实测电池板温度数据。该文基于北京南郊太阳能试验站2010年全年逐时电池板温度、气温、地表温度、斜面和水平辐照度实测数据,分析了电池板温度随时间的变化及其与各气象要素的关系。结果表明:电池板温度与气温和斜面辐照度的综合相关或与地表温度的线性相关最好,但实测数据不易获得;电池板温度与气温的线性相关较好,数据较易获得且质量有保证,从现实可行性考虑,是推算电池板温度最实用的相关方程;电池板温度与气温和水平辐照度的综合相关可以作为辅助方程,用于推算气温较高情况下的电池板温度。基于2010年电池板温度实测数据和加权计算的方法,得到北京地区年平均光伏发电温度折减系数约为2%,最高可达到13.3%。     

北京城区近地面比湿和风场时空分布特征

利用2008—2012年北京城区平均5 km的高密度自动气象站逐时观测数据,分析了北京城区近地面比湿、风向和风速的时空精细分布特征,初步探究了城市下垫面对局地气象要素的影响机制。研究表明:夏季白天北京城区为干岛,冬季城区表现为弱湿岛特征。受城市效应的影响,北京城区与郊区比湿日变化有明显差异。近地面10 m风受到地形、城市和季节性盛行风的共同影响。当气流经过城区时有明显的绕流现象。夏季05:00—10:00 (北京时,下同),受山风、弱的夏季偏南风和城市热岛共同作用,气流向城市中心辐合。冬季15:00—19:00,受季节盛行风偏北气流和谷风偏南气流的共同作用,在城区形成一条西北—东南走向的辐合线。对风速研究发现:城市粗糙下垫面使北京城区风速减小,二环路和三环路之间存在一条“n”状的风速小值带。由此可见,除已开展较多研究的城市热岛效应外,北京城市效应对近地面湿度和风场亦有显著影响。     

孤立金属尖端的电晕触发阈值

为了得到金属尖端在发生电晕放电时尖端处的电场强度,该文首先采用实验室实验得到不同高度、不同形状、不同材质的金属尖端发生电晕放电时的环境电场阈值;再采用有限元法计算二维泊松方程,得到尖端处电晕触发阈值,由此得出以下结论:环境电场阈值随金属尖端高度的增大基本呈线性减小趋势,随着尖端越来越尖,环境电场阈值呈先减小后增大的变化趋势;高度、形状对金属物尖端处电晕触发阈值无影响,尖端处电晕阈值为定值;给出尖端处电晕触发阈值为158.75 kV·m-1与空间分辨率的拟合公式,可为今后电晕放电数值模拟中判断电晕放电的起始时刻提供参考。     

Soil respiration mapped by exclusively use of MODIS data for forest landscapes of Saskatchewan,Canada

Soil respiration (R_s) is of great importance to the global carbon balance. Remote sensing of R_s is challenging because of (1) the lack of long-term R_s data for model development and (2) limited knowledge of using satellite-based products to estimate R_s. Using 8-years (2002–2009) of continuous R_s measurements with nonsteady-state automated chamber systems at a Canadian boreal black spruce stand (SK-OBS), we found that R_s was strongly correlated with the product of the normalized difference vegetation index (NDVI) and the nighttime land surface temperature (LSTn) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. The coefficients of the linear regression equation of this correlation between R_s and NDVI × LSTn could be further calibrated using the MODIS leaf area index (LAI) product, resulting in an algorithm that is driven solely by remote sensing observations. Modeled R_s closely tracked the seasonal patterns of measured R_s and explained 74–92% of the variance in R_s with a root mean square error (RMSE) less than 1.0 g C/m²/d. Further validation of the model from SK-OBS site at another two independent sites (SK-OA and SK-OJP, old aspen and old jack pine, respectively) showed that the algorithm can produce good estimates of R_s with an overall R² of 0.78 (p < 0.001) for data of these two sites. Consequently, we mapped R_s of forest landscapes of Saskatchewan using entirely MODIS observations for 2003 and spatial and temporal patterns of R_s were well modeled. These results point to a strong relationship between the soil respiratory process and canopy photosynthesis as indicated from the greenness index (i.e., NDVI), thereby implying the potential of remote sensing data for detecting variations in R_s. A combination of both biological and environmental variables estimated from remote sensing in this analysis may be valuable in future investigations of spatial and temporal characteristics of R_s.     

Effects of green space spatial pattern on land surface temperature: Implications for sustainable urban planning and climate change adaptation

The urban heat island (UHI) refers to the phenomenon of higher atmospheric and surface temperatures occurring in urban areas than in the surrounding rural areas. Mitigation of the UHI effects via the configuration of green spaces and sustainable design of urban environments has become an issue of increasing concern under changing climate. In this paper, the effects of the composition and configuration of green space on land surface temperatures (LST) were explored using landscape metrics including percentage of landscape (PLAND), edge density (ED) and patch density (PD). An oasis city of Aksu in Northwestern China was used as a case study. The metrics were calculated by moving window method based on a green space map derived from Landsat Thematic Mapper (TM) imagery, and LST data were retrieved from Landsat TM thermal band. A normalized mutual information measure was employed to investigate the relationship between LST and the spatial pattern of green space. The results showed that while the PLAND is the most important variable that elicits LST dynamics, spatial configuration of green space also has significant effect on LST. Though, the highest normalized mutual information measure was with the PLAND (0.71), it was found that the ED and PD combination is the most deterministic factors of LST than the unique effects of a single variable or the joint effects of PLAND and PD or PLAND and ED. Normalized mutual information measure estimations between LST and PLAND and ED, PLAND and PD and ED and PD were 0.7679, 0.7650 and 0.7832, respectively. A combination of the three factors PLAND, PD and ED explained much of the variance of LST with a normalized mutual information measure of 0.8694. Results from this study can expand our understanding of the relationship between LST and street trees and vegetation, and provide insights for sustainable urban planning and management under changing climate.     

Regional-scale surface air temperature and East Asian summer monsoon changes during the last millennium simulated by the FGOALS-gl climate system model

The spatial patterns and regional-scale surface air temperature （SAT） changes during the last millennium,as well as the variability of the East Asian summer monsoon （EASM） were simulated with a low-resolution version of Flexible Global Ocean-Atmosphere-Land-Sea-ice （FGOALS-gl） model.The model was driven by both natural and anthropogenic forcing agents.Major features of the simulated past millennial Northern Hemisphere （NH） mean SAT variations,including the Medieval Climate Anomaly （MCA）,the Little Ice Age （LIA） and the 20th Century Warming （20CW）,were generally consistent with the reconstructions.The simulated MCA showed a global cooling pattern with reference to the 1961-90 mean conditions,indicating the 20CW to be unprecedented over the last millennium in the simulation.The LIA was characterized by pronounced coldness over the continental extratropical NH in both the reconstruction and the simulation.The simulated global mean SAT difference between the MCA and LIA was 0.14°C,with enhanced warming over high-latitude NH continental regions.Consistencies between the simulation and the reconstruction on regional scales were lower than those on hemispheric scales.The major features agreed well between the simulated and reconstructed SAT variations over the Chinese domain,despite some inconsistency in details among different reconstructions.The EASM circulation during the MCA was stronger than that during the LIA The corresponding rainfall anomalies exhibited excessive rainfall in the north but deficient rainfall in the south.Both the zonal and meridional thermal contrast were enhanced during the MCA.This temperature anomaly pattern favored a stronger monsoon circulation.     

Effects of meridional sea surface temperature changes on stratospheric temperature and circulation

Using a state-of-the-art chemistry-climate model,we analyzed the atmospheric responses to increases in sea surface temperature （SST）.The results showed that increases in SST and the SST meridional gradient could intensify the subtropical westerly jets and significantly weaken the northern polar vortex.In the model runs,global uniform SST increases produced a more significant impact on the southern stratosphere than the northern stratosphere,while SST gradient increases produced a more significant impact on the northern stratosphere.The asymmetric responses of the northern and southern polar stratosphere to SST meridional gradient changes were found to be mainly due to different wave properties and transmissions in the northern and southern atmosphere.Although SST increases may give rise to stronger waves,the results showed that the effect of SST increases on the vertical propagation of tropospheric waves into the stratosphere will vary with height and latitude and be sensitive to SST meridional gradient changes.Both uniform and non-uniform SST increases accelerated the large-scale Brewer-Dobson circulation （BDC）,but the gradient increases of SST between 60°S and 60°N resulted in younger mean age-of-air in the stratosphere and a larger increase in tropical upwelling,with a much higher tropopause than from a global uniform 1.0 K SST increase.     

Climatology of Lightning Activity over the Indian Seas

The climatology of lightning activity over the Indian seas (Arabian Sea (AS) and Bay of Bengal (BoB)) has been studied using monthly satellite-based lightning flash count grid (0.5°?×?0.5°) data from 1998 to 2007. These data have been used to investigate the annual and seasonal variations in lightning activity over the Indian seas. It was found that annual variations in flash rate density and sea surface temperature (SST) show a bimodal pattern with the first peak occurring in May and the second in October. The correlation coefficients between flash rate density and SSTs are 0.76 and 0.65 for the AS and BoB, respectively. Further, the relationship between flash rate density and a low pressure system (LPS) over the BoB shows that the formation of severe tropical cyclonic storms starts during April with the maximum number of storms forming during August. The performance of monsoon on a seasonal and monthly basis depends on the total number of lows, the formation of a depression in the monsoon trough, and the number of days with an LPS. Secular decreases in the number of lows and monsoon depressions were observed in 2000, 2002, and 2004. Overall, results indicate that the peaks in SST during April and September/October over the AS and the BoB may be responsible for advancing the onset of the southwest and northeast monsoon by 30–40 days.