华北地区降水过程分析研究

利用NCEP/NCAR再分析资料,对2012年7月21～22日华北降水过程进行了系统分析,结果发现:NCEP/NCAR再分析资料的降水低于站点观测值,从空间分布上来看,华北、东北以及西南部分地区降水为明显的正异常;这次降水500hPa环流形势为三高并存的状态(阻塞高压,日本海高压以及青海高压),925hPa有明显的超低空南风急流,700hPa存在南北向华北切变线,200hPa副热带西风急流在40°～50°N之间,分为两个中心,分别位于高原以北以及华北北部地区,因此降水区域表现为明显的低层辐合以及高层辐散;降水区为一致的上升运动,在副热带西风急流以北,对流层下层有明显的经向垂直正环流,而在副热带西风急流入口区到南亚高压东部脊线以南对流层中上层有明显的经向垂直反环流,经向垂直环流使得上升运动强烈。     

阿坝州一次自北向南降雪天气过程分析

以2017年5月3～6日阿坝州一次自东北向西南降雪天气过程为例,主要利用2.5°×2.5°NCEP再分析资料和台站降水观测资料对此次降雪天气过程的影响因素进行了分析。结果显示:此次系统性降雪天气过程为乌山低槽东移型,阿坝州自东北向西南的降雪天气过程主要由槽后强冷空气配合高原短波槽东移并伴有高原低涡共同作用造成。过程中600 hPa高原低涡与700 hPa低槽配合较好,同时,较强的垂直速度与充沛的水汽输送,是此次降雪天气持续时间长、覆盖面积广的主要影响因子。     

Decomposition dynamics of coarse woody debris of three important central European tree species

Background:Coarse woody debris(CWD)is an important element of forest structure that needs to be considered when managing forests for biodiversity,carbon storage or bioenergy.To manage it effectively,dynamics of CWD decomposition should be known.Methods:Using a chronosequence approach,we assessed the decomposition rates of downed CWD of Fagus sylvatica,Picea abies and Pinus sylvestris,which was sampled from three different years of tree fall and three different initial diameter classes(10 –≤ 20 cm,20 –≤40 cm,40 cm).Samples originating from wind throws in 1999 were collected along a temperature and precipitation gradient.Based on the decay class and associated wood densities,log volumes were converted into CWD mass and C content.Log fragmentation was assessed over one year for log segments of intermediate diameters(20 – 40 cm)after 8 and 18 years of decomposition.Results:Significantly higher decomposition constants(k)were found in logs of F.sylvatica(0.054 year~(-1))than in P.abies(0.033 year~(-1))and P.sylvestris(0.032 year~(-1)).However,mass loss of P.sylvestris occurred mainly in sapwood and hence k for the whole wood may be overestimated.Decomposition rates generally decreased with increasing log diameter class except for smaller dimensions in P.abies.About 74 % of the variation in mass remaining could be explained by decomposition time(27 %),tree species(11 %),diameter(17 %),the interactive effects between tree species and diameter(4 %)as well as between decomposition time and tree species(3 %)and a random factor(site and tree; 9.5 %),whereas temperature explained only 2 %.Wood fragmentation may play a more important role than previously thought.Here,between 14 % and 30 % of the decomposition rates(for the first 18 years)were attributable to this process.Carbon(C)density(mg C · cm~(-3)),which was initially highest for F.sylvatica,followed by P.sylvestris and P.abies,decreased with increasing decay stage to similar values for all species.Conclusions:The apparent lack of climate effects on decomposition of logs in the field indicates that regional decomposition models for CWD may be developed on the basis of information on decomposition time,tree species and dimension only.These can then be used to predict C dynamics in CWD as input for C accounting models and for habitat management.