Site Characterization Using Solar Hα Images

A Multi-Application Solar Telescope (MAST) is proposed to be installed at the lake site (Lake Fatehsagar) of Udaipur Solar Observatory (USO) in India. The lake site Observatory of USO is located on a small island in the middle of the lake. To determine the optimum size of the MAST (for use with an adaptive optics system), it was decided to quantify the seeing conditions prevailing at the lake site during the different months of the year. For this purpose, we have used short-exposure (3 ms) high-resolution Hα (6563 Å) images (spatial scale of ～0.55 arc sec per pixel) of the Sun taken in burst mode with the 15-cm refractor Spar telescope located at the lake site of USO. Spectral ratio technique as reported by von der Lühe (1984, J. Opt. Soc. Am. A1, 510) has been used to estimate the Fried’s parameter (r ₀) at this site, which gives the quantitative measure of astronomical seeing. This study has been carried out daily on an hourly basis during 4:30?–?10:30 UT over the months January?–?June of the years 2005 and 2006 to understand the diurnal and seasonal variations in r ₀ at this site. It is noteworthy that the lake was almost dry during the observing period in 2005, while it overflowed during our observations in 2006 because of abundant monsoon rains. The seeing in the presence of water shows improvement in r ₀ by about 1.0 cm with respect to the previous year’s dry condition and mean r ₀ varies between 4.0 and 4.5 cm as evident from the data obtained between January and June, 2006.     

On transient electric potential variations in a standing tree and atmospheric electricity

Transient electric potential variations have been observed in a standing poplar tree equipped with electrodes up to a height of 10.5 m. The simultaneous signals at all electrodes have the same shape and their amplitude grows linearly with height, up to values of 10 to 50 mV. This corresponds to an electric current through the tree of the order of a few μA. The frequency of appearance of the signals does not depend on the season or on the time of the day. It is suggested that the potential variations are caused by the passage of thunderstorm clouds, of little activity, whose electrically charged base could induce charges in the ground and give rise to a current flowing through the tree and discharging at its top by point discharge.     

青藏高原及铁路沿线地表温度变化趋势预测

青藏高原及其铁路沿线各站的年地表温度具有很好的互相关性,特别是各站10年滑动平均温度互相关系数达到0．92,以此建立了1961-2003年青藏铁路沿线平均地表温度序列。研究表明：青藏高原地表温度的升高是明显的,40年来升高1．1～1．5℃,其升温率为0．44℃／10a。大气CO2浓度的增加有利于青藏高原地表温度的升高,而太阳黑子周期长度（SCL）的变长则起相反作用。地表温度对人气CO2浓度和SCL的最好响应约滞后10年。若根据SCL的变化和IPCC第三次评估报告给出的新的温室气体排放情景SRES-B1预测,目前青藏高原地表温度的升温到2010年前后达到最强,此后可能会出现一个明显的降温过程,到2030年前后可能低于20世纪70～90年代的平均值。新一轮的升温开始于2040年代。若综合考虑CO2和SCL两者的共同影响预测,未来50年平均最低、最高和年地表温度与1971-2000年的平均比较,分别升高0．2,1．0和0．6℃。     

东亚副热带季风雨带建立特征及其降水性质分析

利用1961—2006年NCEP/NCAR再分析数据集和TRMM、CMAP多年平均逐候降水资料,分析了中国东部副热带季风雨季的起始时间、建立特征及其降水性质。结果表明,第16—18候,在中国江南南部和华南北部地区(25°30°N)日降水率达到6 mm/d,且范围较大,在低层该雨带的水汽主要来源于西太平洋副热带高压南侧转向的西南水汽输送,其源地即为西太平洋副热带季风雨季开始。雨带建立同时,东亚副热带地区中东太平洋的纬向海平面气压梯度首先在中纬度发生反转,即西低东高(相应于西暖东冷)。中国东部副热带地区出现加热中心并伴有上升运动,强度逐渐增强,并伸展至对流层顶,其强度及对流高度与热带地区相当,对流层中低层大气呈对流不稳定,降水已具有对流性降水性质。与此同时,南海西太平洋地区仍在副热带高压控制之下,盛行下沉运动,无降水产生,南海夏季风及其相应的水汽输送尚未建立。东亚副热带季风雨带的建立(3月底4月初)早于热带夏季风雨带,两雨带分别具有独立的热源中心和上升运动。南海夏季风即将爆发之际,赤道地区加热中心快速北移至南海地区,与副热带地区热源相互作用。     

南京地区大气颗粒物影响近地面臭氧的个例研究

通过对2008年4月2～7日南京地区地面气象观测数据以及两个站点空气质量(O₃、NO_x、PM10)监测资料的分析, 发现O₃和PM10之间存在一定程度的反相关。利用一个光化学箱模式对该个例中大气颗粒物影响近地面臭氧的过程进行模拟, 结果发现大气颗粒物浓度的升高使得气溶胶光学厚度增加20％～40%, 导致NO₂和O₃近地面光解率下降20%～30%, OH和HO₂自由基浓度分别减少20%～50%, 造成O₃净生成率下降30%～40%。研究表明, 颗粒物对光化学过程的抑制造成了大气氧化能力的降低, 是近地面臭氧浓度减少的可能原因。     

AI方法在地面气温观测资料质量控制中的应用

提出一种基于自回归与反距离加权的空间质量控制方法——AI方法,该方法能够在时间维度和空间维度对气象资料进行质量控制。选择不同地区4个地面气象观测站(南京58238站,连云港58044站,无锡58353站,徐州58027站)2007年逐时气温观测数据作为被控对象,检验该方法在气温资料质量控制中的适用性。通过对添加的随机人为误差的检验发现,该方法能够有效地标记出存疑数据,相对于反距离加权和空间回归,该方法具有更好的检验效果,稳定性高、适应性强,适用于平原或丘陵地带。     

波流相互作用的大气系统中振荡模态的分析

把经验模态分解(EMD)方法用于波流相互作用的大气动力系统,对不同系统状态的流甬数解序列求其本征模态函数(IMF)分量,分析系统状态与IMF之间的关系,揭示流甬数的振荡结构.结果表明:随着系统的复杂性增大,构成解序列的IMF分量的个数相应增多.对于系统的定常态(Hadley流),每个流函数ψ分量的各个IMF分量都趋于零均值线;对于周期态(Rosssby流),ψ分量由1个周期性的高频IMF分量构成;对于振荡态,分量主要由2-3个较高频IMF分量构成;对于非周期流,ψ分量由多个非周期的IMF分量构成;对于周期-非周期状态,其本征模态有的有周期性,有的没有周期性.低频IMF分量表现ψ序列的长时间尺度的变化.     

Moisture structure of the quasi-biweekly mode revealed by AIRS in western Pacific

Using Atmospheric Infrared Sounder (AIRS) humidity profiles, rainfall from the Tropical Rainfall Mea- suring Mission (TRMM) Global Precipitation Index (GPI), Quick Scatterometer (QSCAT) satellite-observed surface winds, and SST from the Advanced Microwave Scanning Radiometer for NASA's Earth Observing System (AMSR E), we analyzed the structure of the summer quasi-biweekly mode (QBM) over the western Pacific in 2003-2004. We find that the signal of 10-20-day oscillations in the western Pacific originates fro...     

The CLIVAR C20C project: selected twentieth century climate events

We use a simple methodology to test whether a set of atmospheric climate models with prescribed radiative forcings and ocean surface conditions can reproduce twentieth century climate variability. Globally, rapid land surface warming since the 1970s is reproduced by some models but others warm too slowly. In the tropics, air-sea coupling allows models to reproduce the Southern Oscillation but its strength varies between models. We find a strong relationship between the Southern Oscillation in global temperature and the rate of global warming, which could in principle be used to identify models with realistic climate sensitivity. This relationship and a weak response to ENSO suggests weak sensitivity to changes in sea surface temperature in some of the models used here. In the tropics, most models reproduce part of the observed Sahel drought. In the extratropics, models do not reproduce the observed increase in the North Atlantic Oscillation in response to forcings, through internal variability, or as a combination of both.     

A high-resolution 43-year atmospheric hindcast for South America generated with the MPI regional model

An evaluation of the present-day climate in South America simulated by the MPI atmospheric limited area model, REMO, is made. The model dataset was generated by dynamical downscaling from the ECMWF-ERA40 reanalysis and compared to in-situ observations. The model is able to reproduce the low-level summer monsoon circulation but it has some deficiencies in representing the South American Low-Level Jet structure. At upper levels, summer circulation features like the Bolivian High and the associated subtropical jet are well simulated by the model. Sea-level pressure fields are in general well represented by REMO. The model exhibits reasonable skill in representing the general features of the mean seasonal cycle of precipitation. Nevertheless, there is a systematic overestimation of precipitation in both tropical and subtropical regions. Differences between observed and modeled temperature are smaller than 1.5°C over most of the continent, excepting during spring when those differences are quite large. Results also show that the dynamical downscaling performed using REMO introduces some enhancement of the global reanalysis especially in temperature at the tropical regions during the warm season and in precipitation in both the subtropics and extratropics. It is then concluded that REMO can be a useful tool for regional downscaling of global simulations of present and future climates.