Abstract:LMDZ4,a variable-resolution global atmospheric general circulation model,is assessed against observed data in Southeast Asia before its further utilization in climate change downscaling studies.The model is developed in the Laboratoire de Météorologie Dynamique in Paris,as the atmospheric component of the ocean-atmosphere coupled system IPSL-CM5A.In order to investigate the ability of LMDZ4 in simulating East Asian summer precipitation,temperature and circulation,this paper analyzes the causes of simulation deviation and some of the uncertainties in this model.The study is expected to lay a foundation for further forecasting of future climate changes.In this study,LMDZ4 has its domain centered over Southeast Asia[domain center at (30°N,110°E);coverage: (5-55°N,85-135°E)],and is driven by ERA-Interim reanalysis data from 1979 to 2009.It is actually used as a traditional limited-area model with the whole globe as the buffer zone that receives the ERA-Interim data.The nudged variables include zonal wind,meridional wind,temperature and specific humidity,with a time interval of 6 h.There are 121×120 (latitude×longitude)grids and 19 layers in the vertical direction.The spatial resolution inside the domain is approximately 0.6°×0.6°.The driving forcing is added through a relaxation procedure.We impose a long (10 days,but not infinite,to be comparable with other simulations using this historical set-up)relaxation time scale inside the domain,while the relaxation time scale outside the domain is 1.5 h.The use of two distinct relaxation time scales inside and outside the domain enables the model to run almost freely inside the domain,whereas it completely follows the forcing outside the domain.For the purpose of model assessment,daily temperature and rainfall at a few meteorological stations in China are used.These two datasets are available with a resolution of 0.5°×0.5°.Results show that the mean state of the major atmospheric general circulation systems can be simulated well,including the South Asian High,westerly jet,western Pacific subtropical high,and moisture transport.The main characteristics of rainfall and surface air temperature are also successfully reproduced.Nevertheless,the simulation shows some biases.Wet and cold biases are found over southern coastal areas,warm and dry biases over the Huaihe river basin,and warm and wet biases over northeast regions,which are related to the simulation deviations of mid-upper level circulations.For example,the simulated strengths of the South Asian high and westerly jet stream are too weak,while the water vapor transport from the Indian Ocean is stronger,leading to more rainfall over southern coastal areas.The missed rainfall belt along the Yangtze River Valley is due to a lack of moisture brought by southeast winds that are too weak,and is directly related to the weak western Pacific subtropical high,which leads to less moisture over western Pacific regions.The deviations mentioned above may largely be attributable to the cloud parameterization scheme in LMDZ4,which yields insufficient cloud cover.In addition,the coarse vertical resolution in this model version is also thought to be a factor contributing to some of the imperfections in upper-level circulation,and to model biases.As we use only one regional climate model driven by one set of reanalysis data,we cannot fully explore the uncertainties of this model in terms of climate changes for precipitation and temperature.It is clear that more research is needed to assess these uncertainties.We need to include as many different types of downscaling model (e.g.,dynamical and statistical downscaling)as possible when generating climate-change information at the local scale.

Abstract:The interdecadal variability of precipitation in eastern China is studied based on data from 66 meteorological stations for the period 1880-2010.Combined with the summer Pacific Decadal Oscillation (PDO) during 1900-2010 and NCEP/NCAR data for the period 1961-2010,summer precipitation in eastern China is analyzed corresponding to the different phases of the PDO,and the possible underlying physical mechanism is discussed.In order to resolve the spatial distribution differences,empirical orthogonal function (EOF) analysis is carried out on the summer precipitation in the near 130-year period.Conventional synthesis analysis,the t-test of statistical significance,and sliding correlation analysis are also used.Decadal features are obtained using Fourier filtering,which filtered out less than 10 years.The work presented in this paper is useful in improving our understanding of the decadal characteristics of summer precipitation in eastern China,especially in the context of promoting the forecasting ability of summer precipitation.The results show that the interdecadal variability of precipitation in eastern China is significant,with obvious differences in North China,the Jianghuai region,and South China.Three change-points occur in North China (in the early 1920s,1940s and early 1970s),four in the Jianghuai region (in the early 1920s,1940,late 1950s and 1970s),and three in South China (in the late 1900s,1940s and early 1970s).A dipole (negative-positive or positive-negative) and tripole (negative-positive-negative or positive-negative-positive) distribution are the two main modes of summer precipitation in eastern China,as determined by EOF analysis.In 1929-1944 (positive PDO),summer rainfall in eastern China presents a negative-positive dipole distribution,which indicates the main rain belt to be located in South China,with the negative anomaly in the Jianghuai basin.During 1945-1977 (negative PDO),the meridional distribution of summer rainfall in eastern China is characterized by a positive-negative-positive tripole pattern,with the main rain belt in North China and South China.In another positive phase of the PDO (1978-2005),the spatial structure of precipitation follows a negative-positive dipole distribution-opposite to that of the previous positive PDO phase-with the main rain belt in eastern China located in the Jianghuai region and the negative anomaly in North China and South China.We also find through sliding correlation analysis that summer precipitation in different areas of eastern China show response characteristics according to the different phases of the PDO.It is apparent that the location of the main rain belt is not only affected by the phase of the PDO,but also by the main circulation system (e.g.the western Pacific subtropical high,East Asia teleconnection and East Asia blocking high).The interdecadal components of the 500-hPa height field and Pacific SST in the summer,via canonical correlation analysis,indicates that the positive phase of the PDO can trigger a negative wave train of the Pacific-Japan teleconnection.This negative teleconnection wave train can lead to more precipitation over the middle and lower reaches of the Yangtze river,less precipitation over North China,and vice versa.

Abstract:ENSO is the strongest interannual variability in our climate system.Recently,it has been found to vary as two types in spatial terms;the Central Pacific (CP) type,whose SST anomalies (SSTAs) are strongest in the central Pacific;and the Eastern Pacific (EP) type,characterized by SSTAs being strongest in the eastern equatorial Pacific.These two types of El Niño involve different teleconnections and climatic impacts,as the intensity and location of their associated SST-induced heating are different.Many studies have pointed out that CP El Niño events have increased in frequency in recent decades.Several kinds of indices have been designed for different types of El Niño events.For instance,the El Niño Modoki index (EMI) captures the second EOF pattern of equatorial Pacific SST anomalies.Another somewhat complicated index pair called the Central and Eastern Pacific Index (CPI/EPI),which is also an EOF-based index but with the SSTA related to the Niño1+2 or Niño4 index removed,has also been put forward.The most striking difference between the two types of El Niño is the longitudinal displacement of maximum anomalous SST along the equator in their mature stages.Motivated by this method of classification,we redefined two indices of SST anomalies,i.e.,I_CP and I_EP,for describing CP and EP El Niño/La Niña events and their interactions during boreal winter,by using both NCEP/NCAR reanalysis data and SST data from the Hadley Centre,for the period 1963-2013.This index pair was reconstructed based on the function for the CP pattern that changes with longitude and is perpendicular to the function for the EP pattern.Compared to other El Niño indices proposed previously,we found that the new indices can be used for better separating EP-type ENSO events from CP-type events,both spatially and temporally.The major features of air-sea coupling in the tropical Pacific related to CP-and EP-type events were investigated using the two new indices.It was found that these two distinct types of ENSO events demonstrate different periodic variation.Specifically,although they both exhibit 2-7- and 10-15-year oscillations in the tropical SSTA field,the decadal (10-15-year) variations for CP-type ENSO events are more dominant.It was also found that the anomalous warming center of CP-type El Niño appears in the central equatorial Pacific as expected,which is quite different from canonical EP-type El Niño events with their largest warm centers located in the eastern equatorial Pacific.Consequently,during CP-type events,the ascending branch of Walker circulation shifts farther west than normal,resulting in rainfall intensification mainly over the central tropical Pacific.However,when EP-type El Niño events occur,anomalous ascending motion is observed in the eastern equatorial Pacific,along with increased rainfall.Interestingly,during boreal winter,both types of El Niño can affect the Maritime Continent,but the CP type more strongly.Specifically,the Maritime Continent receives less than normal rainfall.Both of the reconstructed SSTA indices developed in this study are expected to be highly useful for monitoring ENSO events,and helpful in improving our understanding of the similarities and differences in the influence of CP-and EP-type ENSO events on climate variation at the global and regional (particularly East Asia) scale.

Abstract:This paper investigates the variability of abnormally wet and dry conditions during the past 60 years in the eastern part of northwestern China (ENWC).Daily precipitation,temperature and compound index (CI) of meteorological drought data provided by the China National Climate Center are used,along with monthly reanalysis data from NCEP/NCAR,for the period 1951-2012.The methods of rotated empirical orthogonal function (REOF),correlation analysis,composite analysis and dynamical diagnosis are employed.Additionally,the possible underlying physical mechanism of extreme summer drought events is revealed through analys is of atmospheric circulation anomalies,such as the East Asian summer monsoon,as well as vapor transportation and vertical motion.In particular,the relationship between the upper-level subtropical jet and the upper-level anomalous divergence field is explored.The results can be summarized as follows:The trend of change in CI index is similar to that of precipitation;there are obvious interannual and interdecadal variations of CI index and precipitation in summer over ENWC.Meanwhile,a positive anomaly of summer temperature has continuously maintained over recent decades.More seriously,an obvious decrease in precipitation after the mid-1990s is apparent;and consequently,a warmer and dryer trend began and continues today over ENWC.The extreme drought centers in summer are mainly located in the Hetao Plain and the southeast of ENWC,representing a southwest-northeast trend.Moreover,this spatial distribution tendency and associated features are almost the same as those of wet events over ENWC,and are very similar to those of the northern marginal zone of the East Asian summer monsoon in the same period.Evidence shows that the CI over ENWC is significantly and positively correlated with the 500-hPa geopotential height anomaly in the Ural Mountains,and significantly and negatively correlated with that over Lake Baikal and the north Okhotsk Sea.It is also found that the upper-level subtropical jet's axis tilts (it is flat during extreme wet events),and,as a result,causes large-scale anomalous convergence during extreme summer drought events.At the same time,due to the weak intensity of the upper-level subtropical jet,abnormal easterly flows exist in the upper levels over ENWC,indirectly bringing about a weak circulation on the right of the subtropical jet exit area,which is not conducive to the monsoon moving northward.Moreover,the low-level anomalous southwest winds from Northwest China and the plateau regions indicate the East Asian summer monsoon is weak.Besides,the variation in the location of the ridge line and westward extent of the subtropical high over the western Pacific Ocean is weak,and one of the important reasons for extreme summer drought may result from the divergence of water vapor and descending motion in summer over ENWC.Previous studies have also confirmed that the changes in dry and wet conditions over ENWC are opposite to those over the middle and lower reaches of the Yangtze River.In summary,the main cause of extreme drought over ENWC in summer is the responses to the tilt of the upper-level subtropical jet's axis and its weak intensity.Specifically,a wide range of convergence in circulation may appear,which in turn causes abnormal divergence in circulation in the low levels,and thus a deficit in the water vapor budget throughout the whole troposphere.In addition,the East Asian summer monsoon is weak.Therefore,the combination of the above higher and lower level systems and the abnormal atmospheric circulation is unfavorable for precipitation,leading to extreme drought.

Abstract:Continuous autumn rain is long in duration,meaning it can easily cause crop mold and other adverse effects relating to daily life and industrial and agricultural production.In order to mitigate such losses caused by continuous autumn rain,it is important to study in depth the climate characteristics of continuous autumn rain and its causes.The present study does this based on the daily rainfall and sunshine data of 753 meteorological stations in China in autumn (September-November) from 1961 to 2010,along with NCEP/NCAR reanalysis data for the period 1948-2010.After defining continuous autumn rain in China,its climate characteristics for different levels (5-7 d,8-10 d,and 11 d and above) are analyzed.Based on a general survey of regions where continuous autumn rain was found to happen often during 1961-2010,China is divided into four regions of continuous autumn rain;namely,North China,Southwest China,the Changjiang-Huaihe basin,and South China,where representative stations are Xingtai (Hebei Province),Wufeng (Hubei Province),Gushi (Henan Province) and Yongan (Fujian Province),respectively.The circulation characteristics of continuous autumn rain and continuous autumn rain of different levels in North China are also analyzed.The results show that: (1)The accumulated frequency of continuous autumn rain and continuous autumn rain of different levels mainly concentrates in the mid-upper reaches of the Yangtze River during 1961-2010,and is very small in the areas north to the Yellow River.With an increase in the level of rain,the high-frequency areas move from southeast to southwest.The frequency standard deviation is similar to the frequency of continuous autumn rain and continuous autumn rain of different levels. (2)During the continuous autumn rain and continuous autumn rain of different levels in North China,in the 500 hPa geopotential height field,the northern cold air is guided to the south by northerly flow after the East Asian trough,and southern warm air is guided to the north by southerly flow on the west side of the subtropical high.The cold and warm air masses meet over North China,which is beneficial to the formation of the continuous rain weather.Positive height anomalies are located over North China,but negative height anomalies are located in the east and west sides of North China. (3)The circulation patterns feature a number of differences among the different levels of continuous autumn rain.The East Asian trough and Urals ridge are the strongest and located farthest east for the 8-10 d level,followed by the 11 d and above level,and finally the 5-7 d level.The trough is most obvious over Lake Balkhash for the 8-10 d level. (4)In the 850-hPa wind field,the enhanced southern and northern air streams meet over North China,as do the anomalous southerlies and northerlies.However,the wind field features differ among the different levels of continuous autumn rain:with an increase in the level of continuous rain,the wind speed converging over North China increases. (5)In the sea level pressure field,the blocking high between Lake Balkhash and Lake Baikal strengthens,and North China is located in the peripheral region of the blocking high.Positive sea level pressure anomalies are located over North China.However,among the different levels of continuous autumn rain,the blocking high strength shows some differences,as does the extent of the enhancement of the blocking high,which is maximum for the 11 d and above level,followed by the 5-7 d level,and is minimum for the 8-10 d level.

Abstract:Diurnal variations of precipitation are caused jointly by different thermal and dynamic forcings on the surface.The mechanisms and characteristics involved are extraordinarily complex.This paper,based on hourly rain-gauge data from 77 stations in Hubei Province,for the period 2001-2014,provided by Hubei Meteorological Information and Technical Support Center,analyzes the diurnal variation characteristics of precipitation in summer (June-August) in Hubei Province by dividingit into five different regions (Northwest Hubei,Southeast Hubei,Jianghan Plain,Northeast Hubei and Southeast Hubei),and dividing the precipitation into three levels (0-20 mmh^-1,≥20 mmh^-1,and ≥50 mmh^-1).The diurnal variation characteristics of summer precipitation in Hubei Province are highly remarkable.The diurnal variation curve of precipitation is of double-peak structure,showing characteristics of semi-diurnal circulation.The peaks of precipitation occur at 0800 and 1700 BST,respectively,and the precipitation levels in the two time periods are the same.The valley values occur at 1200 and 2300 BST.The diurnal variation curves of both precipitation frequency and intensity are of double-peak structure,with one weaker than the other.This is mainly related to eastward-moving weather systems from the Tibetan Plateau and the effects of local thermal forcing.The precipitation intensity during 1500-1800 BST shows an obvious interannual increasingtrend for the past 14 years.Regionally,the diurnal variation characteristics of precipitation in Hubei Province are noticeably distinct,with the precipitation peak or valley values in different regions occurring at different times.The primary or secondary peak of precipitation amount occurs in the evening or at midnight in Northwest Hubei;however,whilst this is also the case in Southwest Hubei,the peak amount is not subdivided into a primary or secondary value.Also,the valley values of precipitation in both areas occur at midday.Meanwhile,the primary or secondary peak of precipitation amount occurs in the morning or in the evening in Jianghan Plain,and the valley value of precipitation occurs at midnight.The situation in Northeast Hubei issimilar to that of Jianghan Plain.The diurnal variation curve in Southeast Hubei shows a smooth double-peak structure,and the primary or secondary peak value occurs in the evening or in the morning,which is opposite to that in Northeast Hubei and Jianghan Plain.The valley value of precipitation occurs at midnight.The diurnal variation curve of precipitation frequency and precipitation intensity is similar to that of precipitation amount.The most active 0-20 mm·h^-1 precipitation region is the western mountain area of Hubei Province.The short duration heavy precipitation frequency in Hefeng and Xuan'en is over 13%,while the precipitation frequency in the eastern plain and hills is lower.The precipitation frequency in Jianghan Plain and Northeast Hubei is about 8%,and in Southeast Hubei it is about 10%.The trend of 0-20 mm·h^-1 precipitation is a decreasing one (stations with a decreasing trend account for 67.53% of the total),andthe precipitation frequency increases mainly in western and Southeast Hubei.Opposite to the frequency distribution of 0-20 mm·h^-1 precipitation,the most active regions with precipitation frequenciesof≥20 mm·h^-1 and≥50 mm·h^-1 are mainly in Jianghan Plain and Eastern Hubei.In general,the frequency of short-durationheavy precipitation is larger in the east than in the west,and larger in the plain than in mountain areas.Thosestations with an increasing trend of short-durationheavy precipitation account for 53.24% of total stations,increasing at 0-2.5 %· (10yr)^-1.These stations are mainly distributed in Southwest Hubei,Jianghan Plain and Eastern Hubei.The diurnal variation curve of 0-20 mm·h^-1 precipitation is of double-peak structure,with one weaker than the other.The primary or secondary peak value occurs in the morning or in the evening.The precipitation frequency at 1200 and 2400 BST is the lowest.The peak value of short-durationheavy precipitation (≥20 mm·h^-1) occurs at 1700 BST,and the valley value occurs at 1200 BST.Short-durationcatastrophic precipitation (≥50 mm·h^-1) occurs at 1500-2000 BST,and rarely occurs during 0300-1400 BST.

Abstract:Frost injury has great influence for tropical and subtropical crops in Fujian.Such crops possess poor ability to resist cold air,and so studying their frost injury characteristics is extremely important for disaster prevention and reduction.Firstly,based on daily meteorological observational data at 68 stations over Fujian from January to April and from October to December (1981-2011),an index of frost injury in Fujian is determined,by combining the lowest ground surface temperature,weather phenomena,and lowest temperature.Frost injury is defined by any one of these three conditions as follows:the lowest ground surface temperature is lower than or equal to 0 ℃;frost or freezing is found on the ground surface;and the lowest temperature is lower than or equal to 2 ℃.The climatic characteristics are investigated and discussed.The basic conclusions are as follows: (1)The annual average number of frost injury days,the first and last frost injury date,and the number of frost injury-free days in Fujian demonstrate obvious regional differences,with northeast-southwest-trending contours that correlate well with metrological station elevations.The whole province of Fujian is divided into three frost injury regions.The western and northern regions and high elevation mountain area belong to region I,where frost injury occurs easily,begins earlier,ends later,and there are fewer frost injury-free days.Region III encompasses hilly landscapes in coastal areas,seldom effected by frost injury.The regional averages of the first and last frost injury date in regions I and II,respectively,show delayed and advanced trends,while the number of frost injury-free days increase. (2)Frost injury over most areas of Fujian with short duration is uncommon,and its occurrence frequency becomes lower and lower. (3)By comparison of frost injury with different intensity,it shows that the stronger frost injury takes more widespread damage,occurs later,and finishes earlier,with less number of frost injury days.The circulation background leading to frost injury in Fujian is also discussed,based on NCEP/NCAR four-times-daily reanalysis data during the same period.It is found that the meridional circulation increases at the level of 500 hPa,the line of 568 dagpm goes through the south of Zhejiang Province,and the upper cold center intensity at -36 ℃ stays behind the East Asian trough,with strong cold advection.The line of 0 ℃ on the 850 hPa layer moves southwards close to Fujian Province.On the surface layer,most areas of East China are controlled by cold high pressure at 1 029 hPa.In this circulation background,the weather is relatively fine in Fujian,but then the temperature plummets owing to surface radiation at night,so frost injury occurs easily.Through analysis of the circulation characteristics at 0800 BT on the day of beginning and the next day of ending with different frost injury intensity,it is found that the position of the index lines,such as the lines of 568 dagpm (500 hPa),0 ℃ (850 hPa),1 025 hPa (surface layer) and so on,is closely related to the frost injury intensity.Frost injury strengthens gradually along with the index lines moving southward,and vice versa.The index lines move less significantly in the period from medium-strength to high-strength,as compared to during the period from low-strength to medium-strength.

Abstract:The ensemble square-root filter (EnSRF) is a kind of deterministic ensemble-based data assimilation method,and is used in a growing number of research fields and applications.Ensemble methods,compared with variational methods,require little expert knowledge for the development of tangent linear and adjoint versions of models and forward observation operators,the background-error covariances are flow dependent,and they can be combined with the ensemble forecast.At the same time,EnSRF based on the traditional ensemble Kalman filter update equation,ameliorates the impacts of sampling errors introduced by adding random perturbations to the observations.Furthermore,the computational cost of the method is relatively lower compared with that of other deterministic ensemble methods;plus,EnSRF is easy to code and implement.Because of these advantages,EnSRF has become a hot topic in research and applications related to data assimilation.Owing to its high temporal and spatial resolution,Doppler weather radar has become the most effective method in monitoring and providing warnings for severe convective weather.The assimilation of Doppler radar data is therefore important for the improvement of storm-scale numerical weather prediction.To retrieve dynamically consistent wind,thermodynamic and microphysical fields from radar radial velocity and reflectivity,advanced data assimilation methods are required.According to operational needs,a WRF-EnSRF system for storm-scale assimilation was constructed in previous work.The study involved developing key assimilation techniques of the WRF-EnSRF system,and introduced adaptive localization and adaptive covariance inflation error correction algorithms to help filters to tolerate errors from many sources,including sampling errors,model errors and fundamental inconsistencies between the filter assumptions and reality,which lead to insufficient variance in ensemble state estimates.During the ideal storm tests,the results showed the characteristics and a good performance level of the adaptive algorithms developed,and a better assimilation scheme was obtained.During the real tests of assimilating Doppler radar data,the adaptive localization and adaptive covariance inflation introduced demonstrated it was possible to take into consideration many complex factors of influence.In the present study,based on the assumption that a multi-scheme ensemble forecast that combines different microphysical parameterization schemes may significantly improve the performance of EnKF,as opposed to using a single scheme,the WRF-EnSRF system is examined to assimilate the simulated radar data of a typical super storm that occurred on 20 May 1977 in Oklahoma city,USA.Based on the self-developed WRF-EnSRF data assimilation system,this study assimilates the simulated Doppler radar data and discusses the impact of microphysical schemes and the uncertainty of their parameters on the performance of EnSRF data assimilation,and uses the improved scheme in a series of comparison tests involving the assimilation of simulated radar data.Different mixes of microphysical schemes and perturbations of microphysical parameters are involved in the experiments.The overall goal of the research was to develop an EnSRF data assimilation system and to investigate its ability in radar data assimilation for storm-scale numerical weather prediction.The results show that,in the absence of model error,using a single microphysical scheme with its parameters perturbed,retreives the main features of the storm better than without the perturbed parameters.This difference,especially for the spatial distribution of most variables in the analysis,becomes more significant in the presence of model error.In this case,synchronouly involving a mix of microphysical schemes and the perturbation of their parameters produces convective clouds in the analysis that are better than without any one of these two approaches.With both approaches,data assimilation produces the best result among all the experiments with model errors;the main features of the storm are reasonably retrieved.Meanwhile,results also show that the range of parameter perturbation has to be small enough to produce an optimal analysis.

Abstract:Mass and energy exchanges in the urban canopy directly affect urban atmospheric thermodynamic and dynamic processes,and thus affect the structure of the urban boundary layer.Urban canopy parameterization is an indispensable physical process in the numerical simulation of atmospheric motion.In the present work,simulations were conducted for the urban thermal environment of Nanjing under typical summer weather conditions (sunny days with weak wind),for the period 1-3 August 2010,using the WRF model.By selecting a parameterization that considered the urban canopy structure (UCM experiment),and one that did not (NOUCM experiment),the boundary layer characteristics over different underlying surfaces were examined.The results showed that: (1) The UCM simulation results agreed with the observational data very well.The simulation of 2-m temperature was improved considerably compared to that of NOUCM.Moreover,the drag of buildings to near ground wind speed was better reflected;the simulation of 10-m wind speed was substantially improved.According to observational data analyses,the heat island intensity reached its maximum at 2100 BST,and was about 4 ℃.The UCM scheme simulated a value of around 3 ℃,while the NOUCM scheme simulation yielded a value of around 1.5 ℃. (2) The UCM experiment simulated the 3D heat island distribution favorably.Owing to the retention of radiation,the heat island at 1400 BST was relatively strong,and it covered a wide range at about 2 m above the ground (~120 km²),with an intensity of 2 ℃.The heat island intensity in the NOUCM scheme simulation was 1 ℃.Meanwhile,due to the existence of urban structures,the turbulent kinetic energy was greater and the upward vertical diffusion increased.At 20 m above the ground,an obvious heat island effect could still be found,and the intensity produced using the UCM scheme was 1.5 ℃.The heat island range in the NOUCM experiment was substantially narrower,and the intensity was 0.8 ℃.At around 55 m above the ground,the heat island range simulated using UCM narrowed,and the intensity was 1.1 ℃;meanwhile,the intensity of the NOUCM scheme was only 0.4 ℃.Based on the UCM simulation,the ground heat diffusion effect during the day reached 143 m,the heat island at 02:00 BST and 2 m above the ground reached a maximum of 2 ℃,and there was a detectable heat island impact above 70 m.Based on the NOUCM simulation,however,the range of the simulated heat island shrank rapidly with height,and a heat island effect could not be found with an obvious inversion phenomenon. (3) Different underlying surfaces produce distinct boundary layer features,with cities having distinct impacts on surrounding underlying boundary structures.During the day,turbulent mixing was enhanced at 14:00 BST in urban areas,and the urban boundary layer height increased to about 100 m.At night,the stable inversion stratification of the suburbs cropped the underlying surface,and the Purple Mountain vegetation underlying surface decreased significantly at 02:00 BST because of the presence of the urban canopy structure.For water,near-surface potential temperature and air temperature were mainly affected by the water.Cities have a limited impact on near-surface water;however,they appear to gradually influence surface water after a certain height.

Abstract:With the improvement of weather forecasting and the development of technology,the measurements of meteorological satellites have gradually become an important supplement to conventional observations.Owing to its advantage of all-day observation,Advanced Microwave Scanning Radiometer (AMSR-E) measurements have been widely used in research on global environmental change.The AMSR-E data record is very important in climate change monitoring and data assimilation in numerical weather prediction,but man-made radiative signals are also received by the microwave instrument and interfere with the natural thermal emissions of Earth.This phenomenon of satellite observations being mixed with signals from active microwave transmitters is referred to as Radio-Frequency Interference (RFI).RFI causes severe contamination of passive and active microwave sensing observations and corresponding retrieval products over some continents.The presence of RFI signals will reduce the scientific value of satellite measurements,so RFI signals should be detected and filtered before applying the microwave data in retrieval and data assimilation.With the long-term AMSR-E data being used in climate research,the characteristics of RFI signals also need to be analyzed.This paper focuses on the characteristics of RFI signals from AMSR-E data and their trend from September 2002 to September 2011.In this study,firstly,the spectral difference method and double principal component analysis (DPCA) method are used to obtain the spatial distribution of RFI signals.Compared to the spectral difference method,the DPCA method can detect RFI signals even over snow-covered areas,by taking advantage of the correlation of different channels for natural and snow radiation,and the de-correlation caused by RFI,and it is more robust and suitable for application worldwide.Then,the DPCA method is chosen to detect the RFI signals from the AMSR-E data over land from September 2002 to September 2011,and the trend of RFI signals with time is analyzed.The results show that the DPCA method can identify the RFI signals from brightness temperature of AMSR-E over land effectively,and can avoid the scattering effect of snow surfaces.The RFI signals from AMSR-E detected by the DPCA method are distributed mainly over the United States,East Asia and the India-Arabia Peninsula on C-band channels,and over Europe and East Asia on X-band.Strong RFI signals are mainly concentrated in populated cities.The locations of RFI signals are almost the same for horizontal polarization and vertical polarization channels;and RFI for horizontal polarization is stronger than that for vertical polarization.The areas and positions of RFI signals change with time,and their intensities are also not constant.The variation of RFI signals is probably due to changes in the human utilization of the radio spectrum,and obvious position changes may be related to the replacement of the infrastructure.The number of RFI signals detected by the DPCA method also varies with the seasons,being high in summer and low in winter in different regions except East Asia.From September 2002 to September 2011,the number of RFI signals decreased with time on C-band channels and increases on X-band.

Abstract:Particular matter (PM) is one of the major pollutants in China.The radiation balance,visibility and human health can be strongly influenced by high concentrations of PM.Also,roduction rates of relevant heterogeneous reactions can be dramatically enhanced by significant levels of PM.Due to local anthropogenic emissions and the effect of regional transport,haze events occur frequently in the Yangtze River Delta (YRD),severely impacting upon the everyday and working lives of citizens.In this study,an intensive field campaign was conducted in December 2013 at a regional background site in Nanjing referred to as SORPES (Station for Observing Regional Processes of the Earth System) and,based on the data collected,the formation mechanism of haze pollution in wintertime was discussed and analyzed with a focus on the aerosol chemical composition and synoptic pattern.An online composition analysis instrument named MARGA (Monitoring Instrument for Aerosols and Gases) was used to measure water-soluble ions such as NO₃^-,SO₄^2-,NH⁺₄,Cl^-,K⁺,Na⁺,Ca²⁺ and Mg²⁺ in the fine PM (PM_2.5) fraction.Meanwhile,the gases SO₂,NO_x and O₃ were measured by a trace gas instrument manufactured by Thermo Fisher Scientific (Massachusetts,USA).In order to study the synoptic pattern during hazy days,weather systems were analyzed using data from NECP.Moreover,the HYSPLIT model was used to simulate the atmospheric transport and dispersion of pollutant plumes originating from various air mass sources.HYSPLIT has capabilities ranging from simple trajectories to complex dispersion and deposition calculations for both puffs and particles.It can simulate dispersion based on input variables (wind,temperature,pressure etc.) from various meteorological models.These fields can then be integrated both in space and time to with respect to particles' positions in order to simulate the dispersive nature of the atmosphere.The results showed that the monthly average concentration of PM_2.5 was 142 μg·m^-3 and the water soluble ions (NO₃^-,SO₄^2-,NH₄⁺,Cl^-,K⁺,Na⁺,Ca²⁺and Mg²⁺) contributed about 62% to the total mass concentration of PM_2.5.Two heavy haze events were observed during the campagin.The concentration of PM_2.5 reached 267 μg·m^-3 in the first event and 150 μg·m^-3in the second event,during which three major inorganic ions (sulfate,nitrate and ammonium)accounted for 63.6% and 66.4%,respectively.Both haze events lasted for several continuous days with very low visibility-only 1.9 km in the first event and 4.7 km in the second.Aerosols in the first event were mostly secondarily formed,with a stable weather system and lower mixing layer.The higher temperature and humidity compared to the second event provided appropriate conditions for the heterogeneous transformation of particles in the surface layer.HYSPLIT showed that the air masses of the first event mainly originated from local Nanjing and the surrounding YRD.However,the second event was largely influenced by long-range transport from the North China Plain.The formation of sulfate and nitrate in daytime was observed during both haze events,which mainly came from homogeneous reactions.Meanwhile,nighttime heterogeneous or aqueous-phase reactions contributed to the first event only.The catalytic oxidation of metal ions or liquid-phase oxidation of NO₂ may have been important mechanisms through which sulfate formed at night in first haze event.

Abstract:Based on meteorological data and land-use data,statistical research methods,the genesis of haze in Hebei Province,as well as the spatial distribution and variation of haze during 1980-2013 and its long-term trend,are analyzed in this paper.The statistical analysis results can be summarized as follows: (1)There is a gradual decreasing trend in the multi-year average number of haze days from the southwest to the northwest.It is more than 5 days in most areas south of the Great Wall in Hebei Province,and more than 10 days in the Taihang Mountain area and Piedmont Plain area,with a maximum of 30 days or more in Shijiazhuang,West Xingtai and North Handan.The spatial distribution of visibility on haze days depends on the season.Visibility is better on haze days in winter than in spring,summer and autumn,with a decreasing trend from high values in the southwest to low values in the northeast in summer,autumn and winter,while a distribution of low values in the southeast to high values in the northwest in spring. (2)In recent decades,the number of haze days has an increasing trend (slope:0.45),especially since 1981.Furthermore,for years after 2001,there is a brief decreasing trend after a rapid rise in the number of haze days in Hebei Province.However,the number of low-visibility days does not increase as the number of haze days increases,mainly due to the decrease in fog days. (3)The number of haze days and meteorological factors are significantly correlated.The contribution rate of humidity is higher than that of temperature,meaning we can reduce or control fog and haze by increasing the air humidity to a certain extent.With the economic growth in Hebei Province,the number of fog and haze days shows an increasing trend,and the visibility shows a decreasing trend.Haze days,visibility and GDP are significantly correlated.There is positive correlation between haze and GDP,population growth,and energy consumption,but negative correlation between visibility and GDP and energy consumption in Hebei.The increase in the number of motor vehicles affects the number of haze days in Hebei Province very weakly.Therefore,to alleviate haze in Hebei Province,effective methods include industrial restructuring,changing the traditional heating system,as well as adjusting the structure of the fuel.

Abstract:Based on monthly data of precipitation and air temperature at 160 stations for the period 1951-2012 provided by the National Climate Center of China,the Standardized Precipitation Evapotranspiration Index (SPEI),which characterizes drought,was calculated.Then,the Extreme-Point Symmetric Mode Decomposition (ESMD) of this index was performed to investigate the interannual and interdecadal variations of summer drought in Northeast China.The results showed that summer drought in Northeast China during the study period had distinct characteristics of interannual and interdecadal variations.Significant quasi-biennial,5-yr and 7-yr oscillations occurred in the interannual variations of the SPEI in Northeast China,while quasi-17-yr and 22-yr periods occurred in the interdecadal variations.Further investigations showed that two phases (1975-84 and 1994-2008) were relatively dry,with severe drought having occurred during the latter period (1994-2008).Meanwhile,three phases (1953-1975,1984-1994 and 2009-2012) were relatively wet.Based on the Mann-Kendal test,1975 and 1994 were found to be the change-points of the drought and flood trends for the period 1951-2012.

Abstract:The statistical downscaling technique based on large-scale numerical forecasting productions is an effective method for fine-scale forecasting.In China,researchers use interpolation methods such as bilinear interpolation and inverse distance interpolation to produce a downscaled forecast.In recent years,the Kalman filter-type self-adapting decaying average downscaling technique has been designed overseas for forecast downscaling,which is better than the MOS method.Based on a daily surface temperature dataset of 752 weather stations for the period 2000 to 2010 in China,a fine-scale prediction test with a low-resolution "forecast field" and fine-scale "analysis field" for daily average temperature,using the self-adapting Kalman Filter-type decaying average statistical downscaling technique,was designed,without the effect of forecasting error in numerical forecasting production.The result of the downscaled prediction was compared with the interpolation method and analyzed for its possibility of application in China.The decaying average technique in this paper filtered the observational data in order and determined the change of the dynamic system constantly.Then,systematic bias (called the "downscaling vector",DV) was estimated.Finally,the prediction outcome was then corrected by the bias.This was a kind of self-adapting bias-estimated method,similar to the Kalman filter and a statistical post-processing method.The DV,defined as the difference between the "forecast field" and "analysis field" at the same time,presents the statistical relationship and systematic bias between the forecast and analysis.The DV that is weight-averaged between the last DV and the forecast error at the same time is updated by the decaying average algorithm.Thus,we can extract error information between the forecast and the observational data to estimate the forecast bias.The result show that: (1)The 1-3 d forecast accuracy rate of the self-adapting Kalman filter-type decaying average statistical downscaling technique was 70%-80%,which is basically satisfactory for professional application.The RMSE of the 1-3 d forecasts was between 1.4 ℃ to 1.7 ℃,and the average value in China was 1.5 ℃.The error increased from Southeast China (RMSE of 1.4 ℃) to Northwest China (RMSE of 1.8 ℃).The forecasting ability decreased with the increase of the forecast limitation.The forecast effect was best in summer and worst in winter. (2)The IDS interpolation method was able to provide the best estimated field,but the decaying average statistical downscaling technique was better than any interpolation method.The critical DV was structured reasonably,able to estimate the interpolation bias of the estimated field well.The RMSE decreased to 50% on average (approximately 1.4 ℃),and the forecast accuracy rate increased by 20%-30% in the 1-3 d forecasts.In particular,the large gap in forecasting ability between West China and East China was reduced,and the accuracy rate increased more than 30% in West China,where there was a large error.In conclusion,the forecasting ability of the method was verified by producing a low-resolution "forecast field" and fine-scale "analysis field" without the influence of forecasting error in the numerical forecasting product.Therefore,the decaying average statistical downscaling technique is feasible for operational fine-scale surface temperature forecasting in China.In future work,we intend to combine numerical forecasting products and reanalysis temperature data to generate a realistic prediction test for demonstrating how the Kalman filter-type self-adapting decaying average downscaling technique performs.