微波遥感海上大气可降水和云液态含水量

根据历史探空资料,对西太平洋海域中有代表性的热带和亚热带地区的云天大气做了l.35cm和8.5mm波段的辐射传输计算,揭示了海上云天辐射传输的某此特征。在此基础上,对原有的一种反演云天大气可降水(Q)及云的垂直积分液态含水量(L)的方法做了相应的改进。 1985年12月至1986年1月用船载双波长微波辐射计取得的西太平洋热带海域中的实际观测结果。着重给出了该海域中Q及L的日变化,对流云Q和L场的水平结构及弱冷锋过境时Q及L的演变等方面的个例。 此外,模式计算及海上观测都表明,8.5mm波长辐射计也可以单独探测海上晴空大气可降水。在此基础上,提出了有可能用两波长辐射计的独立测值进行相互比较来判断视野中是否有云。     

基于Himawari-8气象卫星的东亚夏季冰云云顶特征分布

利用Advanced Himawari Imager(AHI)/Himawari-8(HW8)云产品分析2017 年夏季东亚地区不同下垫面条件对冰云云顶特征的影响,针对不同下垫面条件选择了9 个研究区域.结果表明,冰云最常出现在青藏高原和南亚季风区.冰云的发生频率存在南北递减的趋势,并在日本附近有高值中心.冰云云顶的垂...     

西太平洋热带海域水汽和云中液态水含量的统计特征

利用船载双通道(1.35cm和8.6mm)微波辐射计两次(1985年12月—1986年1月,1986年10月—12月)对西太平洋热带海域的大气可降水和云中液态水含量进行了实地考察,本文根据考察结果,首次分析并给出了该海域大气可降水和云的各种统计特征。     

Remote sensing of cloud liquid water

Summary A method is presented to infer cloud liquid water path (LWP in kg/m²) over the ocean from passive microwave measurements of SSM/I. The algorithm to retrieve LWP is based on simulated satellite observations. They are calculated with a radiative transfer model applied to about 3000 radiosonde ascents over the Atlantic Ocean. Since radiosonde observations do not contain direct information about cloud water and ice, these parameters are parameterized based on relative humidity and temperature using modified adiabatic liquid water density profiles. A multiple linear regression is applied to the simulated radiances and the calculated LWP to derive the algorithm. The retrieval accuracy based on the regression analysis including instrumental noise is 0.03 kg/m². Validation of the LWP-algorithm was pursued through a comparison with measurements of a ground-based 33 GHzmicrowave radiometer on board of R.V. Poseidon during the International Cirrus Experiment 1989 at the North Sea (ICE'89). The LWP values agree within the range of uncertainty caused by the different sampling characteristics of the observing systems. The retrieval accuracy for clear-sky cases determined using colocated METEOSAT data over the North Sea is 0.037 kg/m² and confirms the accuracy estimated from regression analysis for the low liquid water cases.The algorithm was used to derive maps of monthly mean LWP over the Atlantic Ocean. As an example the Octobers of the 5 years 1987–1991 were selected to demonstrate the interannual variability of LWP. The results were compared with the cloud water content produced by the climate model ECHAM-T2 from the Max-Planck-Institut Hamburg.Observations during ICE'89 were used to check the accuracy of the applied radiative transfer model. Brightness temperatures were calculated from radiosonde ascents launched during the overpass of DMSP-F8 in cloud-free situations. The channel-dependent differences range from about –2 to 3 K.The possibility to identify different cloud types using microwave and infrared observations was examined. The main conclusion is that simultaneous microwave and infrared measurements enable the separation of dense cirrus and cirrus with underlying water clouds. A classification of clouds with respect to their top heights and LWP was carried out using a combination of SSM/I derived LWP and simultaneously recorded Meteosat IR-data during ICE'89.With 11 Figures     

热带印度洋夏季水汽输送特征及对南亚季风区降水的影响

利用NCEP/NCAR再分析环流资料、CMAP降水量和NOAA海温资料研究了热带印度洋夏季水汽输送的时空变化特征,并考察其对南亚季风区夏季降水的影响.热带印度洋夏季异常水汽输送第一模态表现为异常水汽从南海向西到达孟加拉湾后分成两支,其中一支继续往西到达印度次大陆和阿拉伯海,对应印度半岛南端和中南半岛的西风水汽输送减弱,导致这些区域降水减少;第二模态表现为异常水汽从赤道东印度洋沿赤道西印度洋、阿拉伯海、印度半岛、中南半岛的反气旋输送,印度和孟加拉湾南部为反气旋异常水汽输送,水汽辐散、降水减少,而印度东北部为气旋性水汽输送,水汽辐合、降水增多.就水汽输送与局地海温的关系而言,水汽输送第一模态与热带印度洋海温整体增暖关系密切,而第二模态与同期印度洋偶极子关系密切.     

Modelling of near-surface ozone over South Asia

Hourly, three-dimensional, fields of tropospheric ozone have been produced for 12 consecutive months on a domain covering South Asia, using the regional Eulerian off-line chemistry transport model MATCH. The results were compared with background observations to investigate diurnal and seasonal variations of near-surface ozone in the region. MATCH reproduced the seasonality of near-surface ozone at most locations in the area. However, the current, and previous, studies indicate that the model consequently overestimate night-time concentrations, while it occasionally underestimates the day-time, near-surface, ozone concentrations. The lowest monthly-mean concentrations of near-surface ozone are typically experienced in June–September, coincident with the rainy season in most areas. The seasonality is not identical across the domain; some locations have a completely different trend. Large areas in Northern India and Nepal show a secondary minimum during the cold winter season (December–January). High concentrations of near-surface ozone are found over the oceans, close to the Indian subcontinent, due to the less efficient dry deposition to water surfaces; over parts of Tibet due to influence of free tropospheric air and little deposition to snow covered surfaces; and along the Gangetic valley due to the large emissions of precursors in this region. Monthly-mean ozone concentrations in the densely populated northern India range from 30–45 ppb(v). The model results were also used to produce maps of AOT40. The results point towards similar levels of AOT40 in India as in Europe: large areas of India show 3-month AOT40 values above 3 ppm(v) hours.

东亚、东南亚、南亚地区降水的年变化和年际变化

本文应用了1951—1980年4—9月月平均和1961—1970年4—9月旬平均降水资料,研究了东亚、南亚和东南亚降水的年变化和年际变化,分析了中国东部、西部降水差异及长江流域夏季旱涝的特征.结果指出,东亚、南亚和东南亚4—9月200mm以上大雨区分布的总趋势呈西南-东北向,它和西南季风的走向近于一致.在这条大雨带之中,包括了三种不同类型的降水.此外,中国江淮地区旬降水量在7—9月存在周期约为20天左右的准周期振荡现象,它和中国西部的降水分布完全不同.中国华南和华中的降水与El Nino现象具有相反的关系.统计结果表明,长江中下游夏季发生持续性多雨和持续性少雨的机率只有23％,大部分年份属于正常降水年份.最后也讨论了影响长江中下游地区夏季旱涝的环流因素.     

Contrasting regional and global climate simulations over South Asia

Climate Dynamics - Two ensembles of climate simulations, one global and one regional, are used to investigate model errors and projected climate change in seasonal mean temperature and...     

Variability of rainfall characteristics over the South Coast region of South Africa

Thirty years of daily rainfall data are analysed for the South Coast region of South Africa, a region which experiences substantial rainfall variability and frequent severe drought and flood events, but whose climate variability has not been much researched. It is found that El Niño–Southern Oscillation (ENSO) exerts an influence since most wet years correspond to mature phase La Niña years. ENSO also influences South Coast rainfall via increases in the number of cut-off lows in southern South Africa during mature phase La Niña years. A statistically significant correlation between the Niño 3.4 index and monthly rainfall totals, and between this index and the frequency of wet days, exists for two summer months and also for June. There are also changes in the heavy rainfall day frequencies from one decade to another. Examination of NCEP re-analyses indicates that wet (dry) years result from an equatorward (poleward) shift in the subtropical jet, cyclonic (anticyclonic) pressure anomalies over the South Atlantic and South Africa, and increased (decreased) density of mid-latitude cyclonic systems.     

Leading Pattern of Spring Drought Variability over East Asia and Associated Drivers

Drought events have become more frequent and intense over East Asia in recent decades, leading to hugesocioeconomic impacts. Although the droughts have been studied extensively by cases or for individual regions, theirleading variability and associated causes remain unclear. Based on the Standardized Precipitation Evapotranspiration Index(SPEI) and ERA5 reanalysis product from 1979 to 2020, this study evealuates the severity of spring droughts in East Asiaand investigates their variations and associated drivers. The results indicate that North China and Mongolia have ex perienced remarkable trends toward dryness during spring in recent decades, while southwestern China has witnessed anopposite trend toward wetness. The first Empirical Orthogonal Function mode of SPEI variability reveals a similarseesawing pattern, with more severe dryness in northwestern China, Mongolia, North China, South Korea, and Japan butincreased wetness in Southwestern China and southeast Asia. Further investigation reveals that the anomalously dry (wet)surface in North (Southwestern) China is significantly associated with anomalously high (low) temperature, less (more)precipitation, and reduced (increased) soil moisture during the previous winter and early spring, regulated by an anomalousanticyclone (cyclone) and thus reduced (increased) water vapor convergence. The spring dry-wet pattern in East Asia isalso linked to cold sea surface temperature anomalies in the central-eastern Pacific. The findings of this study haveimportant implications for improving the prediction of spring drought events in East Asia.     

我国华南3月份降水年代际变化的特征

利用1951～2005年华南3月份降水资料、太平洋年代际振荡(PDO)指数以及NCEP再分析资料,对华南3月份降水年代际变化特征、及其对应的大尺度环流以及与PDO的关系进行了分析。结果表明,华南3月份降水存在显著的年代际变化特征,并且Mann-Kendal突变检验表明华南3月份降水在1978年左右发生年代际突变,从之前的降水偏少转变为降水偏多。我国华南3月份降水与PDO有着显著的相关。进一步研究表明,在年代际降水偏少时期,PDO处于负位相(北太平洋海温偏高,中东太平洋海温偏低),北太平洋海平面气压场和高度场偏高,亚洲大陆海平面气压场和高度场偏低,赤道西太平洋到赤道东印度洋附近的海平面气压场偏低,赤道辐合带附近地区的高度场偏低,东亚对流层大气偏暖,西太平洋副热带高压偏东,东亚高空急流偏北,东亚Hadley环流偏弱。在年代际降水偏多时期,PDO处于正位相,情况则与降水偏少时期相反。     

Intraseasonal oscillations of the monsoon circulation over South Asia

The space–time structure of the three-dimensional circulation over the South Asian monsoon region has been studied using the ERA-40 reanalysis of the European Centre for Medium-Range Weather Forecasts. Applying multi-channel singular spectrum analysis on combined daily values of horizontal winds and pressure vertical velocity at ten vertical levels for the period 1958–2001, two leading intraseasonal nonlinear oscillations were extracted. The first oscillation has an average period of 50?days and propagates northeastward from the Indian Ocean to the Indian subcontinent. The second oscillation has a period of 30?days and propagates northwestward from the West Pacific to the Indian region. Both the oscillations exhibit the oscillatory and propagation features at all vertical levels from 1,000 to 100?hPa. The two oscillations correspond well with similar oscillations found in outgoing longwave radiation and precipitation in earlier studies. The wind oscillations also account for the active and break phases of the Indian monsoon. The vertical structures and propagation of specific humidity and temperature are found to be consistent with those of the winds in each oscillation. The structure and movement of regional Hadley and Walker circulations have also been described. The analyses provide further strong evidence for the existence of two distinct monsoon intraseasonal oscillations.     

Climatic features of cloud water distribution and cycle over China

Analyses of cloud water path(CWP)data over China available from the International Satellite Cloud Climatology Project(ISCCP)are performed for the period 1984-2004.Combined with GPCP precipitation data,cloud water cycle index(CWCI)is also calculated.The climatic distributions of CWP are found to be dependent on large-scale circulation,topographical features,water vapor transport and similar distribution features which are found in CWCI except in the Sichuan Basin.Influenced by the Asia monsoon,CWP over China exhibits very large seasonal variations in different regions.The seasonal cycles of CWCI in different regions are consistent and the largest CWCI occurs in July.The long-term trends of CWP and CWCI are investigated,too.Increasing trends of CWP are found during the period with the largest increase found in winter.The decreasing trends of CWCI dominate most regions of China.The differences in long-term trends between CWP and CWCI suggest that CWP only can influence the variation of CWCI to a certain extent and that other factors need to be involved in cloud water cycle researches.This phenomenon reveals the complexity of the hydrological cycle related to cloud water.     

Spatio-temporal variation of biomass burning sources over South and Southeast Asia

In this study, we have investigated the seasonality and long-term trends of major biomass burning (BB) sources over South and Southeast Asia (S-SE Asia). The activities of BB and related emissions show bi-modal seasonality in S-SE Asia. From January to May period, the BB dominates in the northern hemisphere parts of S-SE Asia. From July to September, the activities shift to the southern hemisphere where the emissions from Indonesian and Malaysian islands make largest contributions. Overall, the activities of BB are lowest during October–December period in S-SE Asia. The seasonality of BB intensity and rain are just opposite in the phase over India. The climatological (1997–2008) emissions of carbon monoxide (CO), oxides of nitrogen (NOx) and non-methane hydrocarbons (NMHCs) show strong spatio-temporal variation. The trends show large inter-annual variations with highest and lowest values during years 1997 and 2000, respectively. In the southern hemisphere parts of S-SE Asia mainly in Indonesia, the intensity of biomass fires has been modulated by the large scale climatic phenomena like El Niño and Southern Oscillation (ENSO). The annual emissions of trace gases in southern hemisphere region during the El Niño years exceed to those for the normal years. The estimates for northern hemisphere region during the La Niña years were significantly higher than those for the normal years. The Model for Ozone And Related Chemical Tracers (MOZART) simulations of columnar CO and NOx tend to capture the prominent features of BB emissions in S-SE Asia. The impacts of extensive fires in Indonesia during El Niño year of 2006 compared to a normal year of 2005 were clearly seen in the MOZART-4 simulations of both CO and NOx.     

南海及周边地区云量分布及低云量与南海海温的关系

利用国际卫星云气候计划提供的月平均云气候资料集,分析了南海及周边地区云量的分布特征,并进一步研究了低云量与南海海温的关系。结果表明:(1)南海及周边地区总云量分布存在显著的季节性差异特征。(2)低云主要分布在南海海区,中云为华南地区,而高云则主要位于靠近赤道区域。(3)低云受海表温度影响较大,而中高云则主要与强对流相对应。低云主要分布于南海海表冷水中心南侧的暖水区内的温度梯度区,其高值区分布与海表温度梯度分布基本一致,海表温度梯度的大小与高值中心的低云量成正比。(4)低云量高值中心位置与水平海温梯度区两侧基本一致,高温暖水受西边界强迫上升在海表层辐合,有利于低云的生成。     

2000-2018年东亚地区云顶高度的时空变化特征

为了了解区域云顶高度对过去气候变化的响应,基于卫星搭载的MODIS传感器提供的2000年3月至2018年2月MOD03_08_v6.0数据,分析了东亚地区云顶高度2000—2018年的时空变化特征,并探讨其长期变化的原因。研究发现,东亚地区云顶高度呈西南高东北低的特征。云顶高度在东亚地区以0.020 km/a的变率增长,其中大陆东部云顶高度的年际变率为0.035 km/a,东部海域年际变率为0.034 km/a。在东部海域地区云顶高度的变化同海表温度的变化相关性较高,相关系数为0.68,这表明云顶高度的变化受下垫面的影响。在东亚地区30°~40°N区域内,年平均云顶高度的增加较为明显。此外,夏季云顶高度在长江中下游盆地、塔里木盆地、吐鲁番盆地以及四川盆地东北部呈-0.03 km/a的减少趋势,这是由于更多低云的形成降低了云顶高度;冬季云顶高度在东亚地区40°N以北呈下降趋势,而在40°N以南呈增加趋势。     

Analysis of ice water path retrieval errors over tropical ocean

Retrieval of multi-layered cloud properties, especially ice water path (IWP), is one of the most perplexing problems in satellite cloud remote sensing. This paper develops a method for improving the IWP retrievals for ice-over-water overlapped cloud systems using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Visible and Infrared Scanner (VIRS) data. A combined microwave, visible and infrared algorithm is used to identify overlapped clouds and estimate IWP separately from liquid water path. The retrieval error of IWP is then evaluated by comparing the IWP to that retrieved from single-layer ice clouds surrounding the observed overlapping systems. The major IWP retrieval errors of overlapped clouds are primarily controlled by the errors in estimating the visible optical depth. Optical depths are overestimated by about 10–40% due to the influence of the underlying cloud. For the ice-over-warm-water cloud systems (cloud water temperature Tw > 273 K), the globally averaged IWP retrieval error is about 10%. This cloud type accounts for about 15% of all high-cloud overlapping cases. Ice-over-super-cooled water clouds are the predominant overlapped cloud system, accounting for 55% of the cases. Their global averaged error is 17.2%. The largest IWP retrieval error results when ice clouds occur over extremely super-cooled water clouds (Tw 6 255 K). Overall, roughly 33% of the VIRS IWP retrievals are overestimated due to the effects of the liquid water clouds beneath the cirrus clouds. To improve the accuracy of the IWP retrievals, correction models are developed and applied to all three types of overlapped clouds. The preliminary results indicate that the correction models reduce part of the retrieval error.     

微波成像仪通道对降水云参数响应的数值模拟研究

由于降水云类型和结构等的差异,将导致微波信号出现不同的响应,因此基于星载微波仪器观测结果对降水云参数进行反演仍是目前国际上的难题之一。为了解层云和对流云等两类降水云的TMI微波信号特点,本文以相应的结构模型为基础,利用微波辐射传输模式MWRT分别模拟了不同下垫面和降水云参数条件下的TMI各通道微波亮温,并分析了各种组合通道信号,包括反映发射信息的极化差D(洋面)及反映散射信息的高频极化校正亮温PCT85(洋面)和高低频率垂直极化差VFD(陆面)的变化情况,以此从理论上探讨可能的降水反演方法。研究结果表明:在洋面,D和PCT85均随降水率的升高而减小,其中D_(10)和PCT85都能很好地反映降水率的变化,但分别受地表状况和云体性质(冻结层高度和霰粒大小)的影响较大;D_(19)和D_(37)虽然对下垫面状况和冻结层高度都不甚敏感,但随降水率的增加,存在极化丧失现象,因此不适合反演降水。在陆面,较高频取85 GHz的VFD与降水率有很好的对应关系,随降水率的增长而增大,其中VFD37-85受地表状况影响最小,但受云体性质影响较大;37 GHz对液态水变化敏感,其发射效应易混淆散射信号,故较高频取37 GHz的VFD10-37和VFD19-37不适合研究降水与微波信号之间的关系;层云与对流云存在有无霰粒的差异,会对微波散射性质造成影响,导致PCT85和VED对层云降水率的响应要高于对流云。