Abstract:Based on the WRF(Weather Research Forecast) model and its own Nudging assimilation system,combined with the observation data(after quality control) of 2 588 automatic stations in the Three Gorges area in January 2014,this paper carries out the assimilation test of automatic station and establishes the high resolution temperature field of 3 km in the Three Gorges area.Comprehensive comparative analyses are carried out on the monthly mean temperature field and the hourly temperature change among the results of the assimilation test of automatic station,the non-assimilation test and the assimilation test of sparse field with NCEP sparse observation stations.Results show that compared with the non-assimilation test,the average deviation of temperature field is reduced to ±0.5℃ in most areas for the assimilation test of automatic station.Absolute deviation of the hourly temperature is reduced to less than 1℃ in plains,hills and mountains.Correlation coefficient of the hourly temperature is above 0.9,the temperature deviation ranges of the three types of terrain are reduced by more than 1.14℃,and the root mean square errors are reduced to more than 0.55℃.Relative standard deviations are close to 1 in plains and hills,and relative standard deviation is reduced to 1.11 in mountains in the Taylor diagram for the assimilation test of automatic station.The assimilation test of automatic station is superior to the assimilation test of sparse field.The assimilation test of automatic station has a good effect on the establishing of temperature field of the Three Gorges area in January 2014,which provides an effective reference for the establishment of high resolution temperature field in the area.

Abstract:Based on the European Centre for Medium-Range Weather Forecasts(ECMWF),National Centers for Environmental Prediction(NCEP) and the China Meteorological Administration(CMA) ensemble forecasts of 2 m surface air temperature taken from TIGGE data archive,the inconsistency of the surface air temperature has been investigated.The results show that the time mean inconsistency index of each model for surface air temperature is quite different.The time mean inconsistency index of ECMWF forecast system is the smallest,followed by NCEP,and CMA is the largest.In addition,the inconsistency indices of the control forecast in the NCEP forecast system,the control forecast and the ensemble mean forecast in the ECMWF forecast system increase with the forecast lead time.In the ensemble forecast system,the ensemble mean forecast is superior to the control forecast,especially for longer term forecast.But in the CMA forecast system,the inconsistency indices of the control forecast and the ensemble mean forecast are substantially stable at a higher level.The further study indicated that the time mean inconsistency index of the surface air temperature forecast in the ECMWF ensemble forecasting system varies with areas and seasons.The forecast inconsistency index during winter(summer) is relatively larger(smaller) among four seasons in a year.Furthermore,the occurrence frequency of the forecast flip doesn't vary with forecast lead time significantly,while that of the forecast flip-flop and forecast flip-flop-flip increases with the forecast lead time considerably.

Abstract:Low temperature event is one of many meteorological disasters,which always causes much loss in agriculture,transportation,electricity and so on.The analysis of extreme low temperature can improve our understanding.As a powerful complement to observation data,the reanalysis data play an important role in related researches.Hence,the quality of reanalysis data is related to the accuracy of research results.Therefore,it is necessary to explore the performance of reanalysis data on low temperature filed.Based on the daily minimum temperature data of 192 gauge stations in China and the reanalysis minimum temperature data of NCEP/NCAR,NCEP/DOE,JRA-55,ERA-Interim from 1979 to 2013,and the homogenization temperature data from 1979 to 2004,this study calculates low temperature threshold,and explores the features and differences between reanalysis data and observation data on climatology,interannual and interdecadal variations,and long-term trend.The following conclusions are obtained by calculating correlation coefficient and linear trend,and by using EOF analysis and other methods.Results show that:(1) Compared with the observations,the thresholds of homogenization data are lower in Northeast China,western Nei Mongol,Guangdong and Guangxi while higher in the east side of the Tibetan Plateau,northern Xinjiang and the middle and lower reaches of the Yellow River,but the linear trends are the opposite.(2) The thresholds of reanalysis data are lower in the south of China while higher in the northeast.The correlation coefficients display that the credibility is better in the east of China than that in the west,which may be related to the terrain and the number of gauge stations.(3) The reanalysis data can capture the features of cooling trend over the central and western Nei Mongol and warming trend over the Tibetan Plateau,but there are differences in values and ranges,and the data underestimate the significant warming trend in North China Plain based on the observations.(4) The thresholds derived from the four reanalysis data can depict the observed characteristics of nationwide consistent and reverse between the northeast and other regions.The features of interannual variation can be inferred from the four reanalysis data,but only JRA-55 and ERA-Interim can reappear the interdecadal variation of the low temperature thresholds.(5) Counting the number of low temperature days by threshold value,the results of reanalysis data are generally less than those of the observation data.From the results,we can conclude that the applicability of reanalysis data is better in eastern China,and the effect of ERA and JRA is better than that of NCEP products.

Abstract:Influenced by many climatic factors,the precipitation anomaly over the Maritime Continent (MC) shows strong regional features.Based on the NCEP/NCAR,GPCP and GODAS monthly mean reanalysis data from the period of 1979 to 2013,in this study we investigated the regional characteristics of precipitation in the Indian Ocean side of the MC,as well as the relationships between different precipitation patterns with variations of the East Asian monsoon and tropical circulation,using regression and other statistical methods.The results show that one oceanic area to the west of Java Island (Box-A),and another in the equatorial Indian Ocean to the west of Sumatra (Box-B),are two key regions where the precipitation varies inter-annually with larger variance in the west of the MC.It is found that there are significant differences in the rainfall variation in Box-A from that in Box-B,although the rainfall variations between these two key regions are significantly correlated.When the summer precipitation is abnormally high in these two areas,the water vapor from the equatorial Indian Ocean,equatorial Pacific,South China Sea and Bay of Bengal is strong.However,precipitation in Box-A is closely related to the IOD mode,while that in Box-B has a closer relationship with monsoon activities.When the precipitation anomalies in Box-A are positive,the warm sea water in the mixed layer converges to Box-A to sustain the higher than normal SST in this area,thereby facilitating an anomalous cyclonic circulation over Box-A and its southwestern side to the Box-A in the lower troposphere.This warmer SSTA,along with the cyclonic circulation,are responsible for the anomalous upward motion and greater precipitation there.When the precipitation is abnormally high over Box-B,the anomalous circulation appears similar to that in Box-A,albeit slightly weaker and more westward.The warmer SST in Box-B is mainly related to the upwelling anomalies in the ocean.All of these results are helpful for our better understanding of the mechanisms of variations of both regional climate in the MC and the Asian summer monsoon anomalies.

Abstract:In this study,the atmosphere and oceanic features in association with large-scale sea surface temperature(SST) anomalies over the North Pacific in winter are examined in terms of their dominant roles in local air-sea interactions,by using NCEP/NCAR Reanalysis and Extended Reconstructed Sea Surface Temperature from the period of 1948-2014.The statistic results show that a greater number of cases of atmospheric forcing to the ocean are identified,thus indicating that atmospheric forcing to the underlying ocean plays a dominant role in local air-sea interaction over the North Pacific in winter.However,some cases for oceanic forcing to the overlying atmosphere still exist,which is indicative of oceanic feedback.In the cases for atmospheric forcing to the ocean,a dipole pattern of geopotential height anomalies(an anomaly anticyclone in the northeast,while an anomaly cyclone in the southeast),together with southeasterly and deep warm anomalies,lie only over the positive SST anomalies,which is indicative of equivalent barotropic atmospheric structures,while the opposite is true for these large-scale SST anomalies.In the cases for oceanic forcing to the atmosphere,a triple pattern of geopotential height anomalies(an anomaly anticyclone in the northwest,an anomaly cyclone in the southwest,and an anomaly cyclone in the east),along with surface easterly anomalies,are located over the positive SST anomalies.However,a pair of geopotential height anomalies(an anomaly anticyclone in the south,and an anomaly cyclone in the east),along with surface westerly wind anomalies,appear over the negative SST anomalies.Overall,the geopotential height anomalies still have the same equivalent barotropic structure features as the cases of atmospheric forcing to the ocean,yet their values are much smaller and are mainly limited to the lower troposphere.In addition,the ocean temperature anomalies propagate downward when the atmospheric forcing dominates,while they are almost stationary when the oceanic forcing plays an active role.

Abstract:Based on the Hadley Center monthly sea surface temperature data,the NCEP/NCAR monthly sea level pressure data,sea surface wind data and the vertical velocity data at 500 hPa,the characteristics of Southern Indian Ocean Dipole(SIOD) variation and its relationship with ENSO are studied.The results showed that the SIOD is characterized by occurrence of a cold(warm) SST anomaly in the northeast Indian Ocean and a warm(cold) SST anomaly in the southwest,which are seasonal phase-locked.It develops from October to December,reaches a climax in the following January to March,and decays from April to June.The formation mechanism of SIOD is affected by the large-scale atmospheric circulation.The location and intensity variations of Mascarene high and Australian High(Low) contribute to the sea surface wind anomalies,which results in SST anomalies and then leads to the SIOD.There is a close relationship between the sea surface temperature in the austral subtropical Indian Ocean and that in the equatorial central eastern Pacific Ocean.The SIOD event is significantly negatively correlated with ENSO which occur in the previous winter.Most of the positive SIOD events occur after La Niña events,whereas most of the negative SIOD events occur after El Niño events.In some cases,SIOD appears after neither of the two events.The anomalous vertical motion and anomalous tropical zonal wind,which are induced by ENSO,can affect the intensity and location of sea surface pressure and sea surface wind fields in the austral subtropical Indian Ocean,and can further influence the evolution of the eastern and western poles of SIOD,respectively.Furthermore,SIOD can also occur in isolation,and the occurrence of the negative SIOD events is usually one month earlier than that of the positive SIOD events.The anomalous sea surface temperature cannot develop continuously without the effect of ENSO,which leads to a short life span of the SIOD.

Abstract:Based on the Hadley sea ice concentration data and NCEP/NCAR reanalysis data,this paper analyzes interdecadal variations of ice-melting amount of sea ice in Arctic region and its interannual relationship with atmospheric variables.Results show that:Arctic sea ice has a significant interdecadal variation and strong regional characteristics.Average and variance of ice-melting amount of East Siberian Sea and Beaufort Sea increase,while average and variance of ice-melting amount in ocean east Greenland decrease.Empirical Orthogonal Function (EOF) analysis is made on ice-melting amount of Arctic sea ice during the three different climate periods (1960-1990,1970-2000 and 1980-2010).The first two EOF modes both have significant interdecadal variations during the three different climate periods.The increase of ice-melting amount of East Siberian Sea is correlated to interdecadal variation of the first EOF mode,while the second EOF mode causes interdecadal melting of sea ice of Beaufort Sea significantly.Furthermore,the corresponding general atmospheric circulation also shows significant interdecadal variations,their interannual relationships with AO/NAO also have interdecadal transition.The second EOF mode of ice-melting amount has a closer interannual relationship with AO,and their correlation coefficient is just 0.186 from 1960 to 1990,but it increases to 0.367 from 1980 to 2010.There is an interdecadal enhancement in the interannual relationship between ice-melting amount of the whole Arctic Ocean and AO,especially in the interannual relationship between ice-melting amount of East Siberia region and AO,and their correlation coefficient is more than 0.4 from 1980 to 2010.The correlation coefficient between ice-melting amount of Beaufort Sea and AO varies greatly,and it reaches up to-0.488 from 1960 to 1990,but it decreases to 0.161 after 1980.However,AO has no significant interdecadal variation.It is Beaufort high that mainly causes the interdecadal variation of interannual relationship between ice-melting amount of sea ice in Arctic region and atmospheric variables.The interdecadal weakening of Beaufort high leads to the decrease of transportation of sea ice from Pole region to East Siberia Sea and Beaufort Sea,resulting in the reduction of sea ice in the two regions,which causes the interdecadal transition of interannual relationship between AO and Arctic sea ice.

Abstract:A squall line is formed in Shandong province,which is influenced by the high trough and surface convergence line in the afternoon 18 August 2012.Based on the automatic weather station data and Jinan Doppler Weather Radar data,this paper studies the merging characteristics of mergence between cell and cell,bow echo and cell during the development of the squall line.Results indicate that:(1) Merging process experiences the primary phase,the intermediate phase and the complete combined phase.The cloud bridge emerges at the middle level of the storm.The echo intensity first decreases and then strengthens during the merging period.(2) The results of the mergence are that the upstream echo decreases and merges with the downstream echo,and provides the abundant water vapor for the downstream echo.The convergence between downdraft outflow and warm moisture air increases the ascending movement of the downstream echo,so the downstream echo develops continuously and replaces the old echo.That is,the mergence belongs to the feeder type.The low level small scale vortexes are strengthened and their base heights reduce because of the mergence,so the meso-scale cyclone or uncorrelated shear is produced.(3) The intensity of two cells weakens during the intermediate merging phase of cell with cell,and strengthens during the complete combined phase.The low vortexes have the characteristics of small scale and strong intensity,the vertical stretching thickness and maximum shear value of the meso-cyclone or uncorrelated shear are larger,and their base heights are lower.The tornado storm expresses characteristics such as long duration,large influenced area and higher intensity,and can cause more serious disasters.(4) The cell maintains development during the entire merging period between the bow echo and cell,while the bow echo decreases in the period of complete combined phase.The low vortexes have larger scale and weaker intensity.(5) The base height,vertical thickness and maximum shear value of the meso-cyclone or uncorrelated shear show high relativity to the intensity of tornado.During the merging process of cells,when the base height or the maximum shear height of the meso-scale cyclone or uncorrelated shear reduces,or the maximum shear value increases abruptly,the tornado would be produced.If the mergence is located in the apex of the bow echo,the tornado would be produced.

Abstract:Based on the observed precipitation data from 394 stations over Qinling and surrounding areas,the ERA-Interim reanalysis data from the European Centre for Medium-range Weather Forecasts(ECMWF),the Empirical Orthogonal Function(EOF),wavelet and regression analysis methods,the major modes of summer precipitation in Qinling and surrounding areas and their associated atmospheric circulation anomalies are analyzed.Results show that:(1)The first four EOF modes account for 73% of the total variance and the principal component has significant interannual variability with 2-4 years period.EOF1 reflects the difference climate change between south and north Qinling.EOF2 reflects the reverse phase oscillation on both sides of the loess plateau.EOF3 is characterized by opposite distribution between southwest and northeast Qinling.EOF4 reflects Guanzhong plain topography precipitation contribution.The EOF3 and EOF4 also have 4-8 years interannual variations.(2)Regression analysis showed that when EOF1 in positive phase,200 hPa Jet is weak,at the same time,midlatitude trough and West Pacific subtropical high are weak,anomalous water vapor transport from East China Sea favors more precipitation in northern Qinling.EOF2 and EOF3 have significant Mongolia low and Northeast cold vortex circulation characteristics respectively.500 hPa circulation anomaly of EOF4 was not significant.(3)A summer monsoon index in Qinling is defined,the monsoon index of the regression field is significantly related to the time coefficient of the precipitation spatial mode,more(less) precipitation in Northern Qinling,less(more) rainfall in the South,reflecting the strong(weak) monsoon year conversion.Otherwise,in different strong(weak) monsoon years,the atmosphere circulation in Qinling has big difference,which can trigger a variety of precipitation mode and phase oscillations.

Abstract:In this paper,using precipitation data for China,NCEP/NCAR reanalysis data and SST data provided from NOAA during the period of 1961-2006,the interdecadal and interanual characteristics and causes of precipitation in the Huanghuai area were studied.On the interdecadal scale,when the Pacific Decadal Oscillation(PDO) was in the warm phase,the Southern Oscillation was weaker and the precipitation in the Huanghuai area was heavier.However,when the PDO was in the cold phase,the Southern Oscillation was stronger and the precipitation in the Huanghuai area was lighter.On the interannual scale,when the SST was higher in the north and lower in the south of the India Ocean,the west pacific subtropical high was stronger and the double Blocking High appeared near the Ural mountains and Okhotsk at 500 hPa.The westerly jet was found to the south of the average position at 200 hPa.The anticyclone appeared in the Huanghuai area.The shear of northeast and southwest wind appeared in Huanghuai on an 850 hPa wind.The upward current was on the Huanghuai area.The circulation led to greater precipitation in the Huanghuai area.It was found that the prediction model built on the India Ocean SST had a more accurate prediction ability on the precipitation in the Huanghuai area.

Abstract:The change of low cloud(cloud base height below 450 m) has great effect on the take-off and landing of the aircraft.Based on the low cloud cover data provided by Shanghai Hongqiao Airport(SHA),ERA-Interim reanalysis data and sea surface temperature(SST) data provided by NOAA during 1986-2015,this paper investigates the interannual variation of the low cloud days at SHA in June and its possible physical mechanism.Results show that the low cloud days are the largest at SHA in June,and the low cloud days in June have significant interannual variation.The main reasons affecting interannual variation of the low cloud days at SHA in June are that:When the subtropical westerly jet moves northward,SHA is located on the right side of jet entrance area,where the abnormal updrafts are good for the occurrence of low cloud above SHA;When the southeast monsoon strengthens,the water vapors from the low latitudes and the air flows from the north converge in the middle and lower reaches of Yangtze River,which also increases the low cloud days at SHA.In addition,ENSO plays an important role in the interannual variation of the low cloud days at SHA in June.When the SSTs in the tropical central Pacific are cooler,the local convective activity will be suppressed,which leads to an anomalous anticyclone at lower troposphere in the low latitude region of the northwestern Pacific.The anomalous anticyclone is enhanced continually by the local wind-evaporation-SST positive feedback process,and can persist from preceding winter to June.Hence,the vapor from the ocean can be transported by the easterly wind in the south of the anomalous anticyclone to the middle and lower reaches of Yangtze River,which can lead to more precipitation in the region and be beneficial to more low cloud days at SHA.

Abstract:Atmospheric aerosol possesses the characteristics of indirect radiative forcing,i.e.part of the particles in the aerosol are activated as CCN to form cloud droplets,which are then able to change the radiation characteristics of the cloud and affect the micro-physics in the cloud.The influence of CCN on cloud and precipitation is very complex.In order to study this process,we designed two sets of experiments.The WDM6 two-parameter microphysical scheme is incorporated into the mesoscale WRF (V3.7) to simulate a stratiform precipitation over the East China region from 12:00 July 26,2014 to 6:00 July 28,2014.By changing the number of initial cloud condensation nuclei (CCN) in the model and parameterization scheme,sensitivity experiments were used to analyze the simulation results.The first experiment used double bi-directional nesting to perform the simulation.The initial concentration of CCN is based on the default of 10⁸ kg^-1 in the WRF model,which is roughly equivalent to 100 cm^-3,representing the CCN condition in a very clean area and performing the control test.The other representative values of CCN are 10⁹ kg^-1,4×10⁹ kg^-1 and 10¹⁰ kg^-1,respectively corresponding to approximately 1 000 cm^-3,4 000 cm^-3 and 10 000 cm^-3,representing the CCN concentrations in the cleaner,contaminated and heavily contaminated areas.The results show that the influence of CCN concentration on precipitation is complex and nonlinear.With the increase of the CCN concentration,the precipitation increases.The mixing ratio of cloud water and graupel always shows a rising trend;however,the mixing ratio of rainwater shows the tendency of first increasing,then decreasing,then increasing again.The mixing ratio of ice crystal,by contrast,first decreases,then increases,then decreases again.In addition,the mixing ratio of snow crystals first increases,then decreases.In addition to the above control and sensitivity experiments,another experiment was set up to further analyze the influence of the CCN parameterization scheme on the precipitation process by changing the parameterization scheme of CCN activation.The two-parameter scheme is N_ccn=C·S^k,where the parameters C and K are respectively set to 1 596 and 0.46;and the three-parameter scheme is N_ccn=N₀ (1-exp(-BS^k)),where N₀,B and K are respectively set to 1 510,4 and 1.1.All of the parameters are obtained by using the observation data from Huangshan for the time range of June 30 to July 28,2014,by Qing Miao.The results show that there are differences between the two simulated precipitation areas.Compared with the two-and three-parameter schemes,the simulated rainfall area is slightly different,and the precipitations of both are higher than that of the real-time precipitation,and the precipitation simulated by the three-parameter scheme is more realistic.After the precipitation has occurred,the concentration of CCN in the three-parameter scheme is always higher than that in the two-parameter scheme,and the value of CCN concentration undergoes little change.The results of the two-parameter scheme show that the warm-cloud precipitation is strengthened,and the cold-cloud precipitation is slightly weakened.The results of the three-parameter scheme show that the warm-cloud precipitation is weaker,while the cold-cloud precipitation is stronger.

Abstract:In this study,fields of wind and temperature from the ERA-Interim reanalysis for the period of 1980-2000 are used to calculate the Eliassen-Palm flux and its divergence of planetary waves in the Northern Hemisphere winter.Next,the characteristics of decadal-scale variability throughout the layer of stratospheric temperature and wind and the relationships to changes in planetary wave activity during December and January-February are analyzed.While the high-latitude middle and lower stratosphere show a warming tendency in December,the same region cools during January-February.The zonal-mean zonal winds in the mid-to-high latitude middle and upper stratosphere decelerate evidently in December,but show a significant accelerated tendency during January-February with increasing latitude and altitude in the high-latitude middle and upper stratosphere.The planetary waves mainly propagate upward along the low-latitude and polar wave guides in the Northern Hemisphere winter.However,in December the propagation along the low-latitude wave guide decreases,whereas that along the polar wave guide and throughout the entire layer stratosphere increases evidently.On the contrary,during January-February the propagation along the low-latitude wave guide strengthens significantly,whereas that along the polar wave guide and throughout the stratosphere weakens.Therefore,during December and January-February for the period of 1980-2000,the respective situations of the decadal-scale changes of wave-mean flow interactions in the Arctic stratosphere tend to be opposite,and it is necessary to discuss the decadal variability during different winter months on a case-by-case basis.

Abstract:In this study,we used the ECMWF ERA-Interim reanalysis datasets and NOAA/NCDC station datasets to construct a new coldness intensity(CI) index based on the amplitude of daily anomalously cold temperatures in winter,which was different from the sub-seasonal temperature variance.Next we analyzed the abnormal changes of atmospheric circulation,shortwave and longwave radiation and temperature advection,so as to reveal the change characteristics of winter cold events,as well as the causes of recent increasing winter cold events in the mid-latitudes of Eurasia during the period of 1998-2012.The results showed the following:1) Cold winters and events were frequently found in the mid-latitudes of Eurasia over the past 16 years,especially in northwestern Europe,Siberia and East Asia.The linear trend of winter coldness intensity was consistently clearly enhanced in these areas,but the sub-seasonal variance of winter temperature did not exhibit a clear intensified linear trend.The consistency of coldness intensity and cold events showed that the coldness intensity may indicate the recent intensification of cold events.2) Since 1998,an intensification and northwest expansion of the Siberian High associated with enhancing northerly wind and weakening westerly wind in the mid-latitudes of Eurasia was observed,which led to negative winter temperature anomalies and winter extreme cold events increase over the mid-latitudes.In addition,recent frequent and persistent cold surges facilitated the significant decreasing of winter temperature over the mid-latitudes of Eurasia,coupled with continuously strengthening of winter coldness intensity.3) We further decomposited the temperature advection,and observed that the temperature advection was mainly dominated by cold advection in the mid-latitudes of Eurasia during the period of 1998-2012,and this feature had been particularly significant since 2005.Cold zonal advection mainly dominated in Europe and East Asia,while meridional temperature advection mainly cooled Siberia,and both zonal and meridional advections were critical components.Aside from the meridional wind and temperature gradient components cooling effects,the zonal wind and temperature gradient components also played a particularly significant role in cold anomalies in the mid-latitudes of Eurasia.The meridional temperature gradient was one aspect that affected the temperature drop in the mid-latitudes of Eurasia,and its value was smaller than the other contributions.Abnormal zonal and meridional temperature gradients and zonal wind drove the cooling effect over Europe,among which the zonal wind value was the strongest.The changes of zonal and meridional winds played a key role cooling Siberia,while the abnormal zonal temperature gradient mainly cooled down East Asia,which may have been related to the temperature differences enhanced between sea and land there.In general,the dominant temperature advection components varied among different regions.Compared to the cooling effects of temperature advections throughout the entire Eurasian continent,shortwave and longwave radiations only cooled the local areas where autumn and winter snow cover increased.