Abstract:Based on the ECMWF(European Centre for Medium-Range Weather Forecasts) globe ensemble Prediction System(EPS),the forecasting performance on Eurasia blocking circulation in January 2008 when the freezing rain and snow storm occurred in South China were assessed and discussed.The results showed that the observed blocking frequencies during January 2008 were extremely high and the blocking frequencies forecasted by ECMWF decreased with the lead time increasing.The circulation forecasting verification showed that the performance of ensemble forecast was better than that of deterministic forecast when the lead time was longer than 6 days.So the advantage of ensemble forecast mainly lied in the mid-range forecast.The blocking predictability analysis indicated that the predictability during the decay stage was lower than that during the onset stage.In addition,the blocking occurrence probability can be obtained based on the ensemble forecast,and such information could be offered to the forecasters in advance.

Abstract:Based on the ensemble mean outcomes from forecasts of the surface temperature 2 m over the ground in the Northern Hemisphere,which were provided by ECMWF,JMA,NCEP and UKMO in the TIGGE(THORPEX Interactive Grand Global Ensemble) data archive,a statistical downscaling forecast was studied by using the interpolation,linear regression in conjunction with multimodel superensemble(SUP) and bias-removed ensemble mean(BREM).The results showed that the statistical downscaling technique significantly improved the forecast skill of four single models during three months in the summer of 2007.The SUP and BREM methods further reduced the errors of the single model downscaling forecasts.The improvement percentage of the 1-7 d forecast error of the downscaling forecast with BREM and SUP forecast schemes of the best single model ECMWF was over 20%.In addition,the forecast skill of the statistical downscaling with SUP forecast was superior to that with BREM forecast and the forecast skiu by using the bilinear interpolation method was better than that by using the other three interpolation methods in both two schemes.

Abstract:Based on the GRAPES_Meso regional ensemble forecast system,three ensemble perturbation schemes are designed and compared,namely,multi-initial,multi-initial-physics and multi-initial-physics-boundary.Consecutive experiments for a period of one month in July 2008,during which heavy rainfall processes occurred in Jianghuai,are conducted in this paper.The results are as follows:All ensemble schemes can outperform the control forecast,the improvement for small rain forecast is obvious whereas the improvement for heavy rain forecast is slight;the multi-initial scheme can only produce limited ensemble spread;the introduction of physical parameterization perturbation and lateral boundary perturbation could significantly amplify ensemble spread and improve the ensemble forecast of each quantity.After all,the scheme of multi-initial-physics-boundary is the best.The results indicate that model physics and lateral boundary are important factors for GRAPES_Meso regional ensemble forecast uncertainty.

Abstract:Wilcoxon Signed Rank Test,a non-parametric test,is introduced and applied to the evaluation of tropical cyclone(TC) intensity forecast.The distribution of forecasting error is a significant non-normal one,so those common parametric tests can not be used and this non-parametric test is proposed.As a result,the prediction method of TC intensity can be evaluated by using Wilcoxon Signed Rank Test,which is helpful not only to better understand the forecast method of TC intensity,but also to improve the prediction accuracy of TC intensity.

Abstract:The total sky imager(TSI-440) is designed to automatically monitor cloud conditions at daytime with higher spatial resolutions and is able to provide more accurate cloud cover data.The paper introduced the fundamental mechanism and data format of the TSI in detail and then analyzed the imaging characteristics under different weather conditions and the cloud cover computational errors based on the routine surface observation data from TSI-440 at Wuxi from May to October 2008 by using statistical method.The results show that the imaging characteristic is closely related with the visibility and the red blue ratio increases as the visibility reduces.After analysis of the images,it is also found that the TSI-440 makes certain errors in computing cloud cover when processing cloudy images under overcast or cloudy conditions.In view of this situation,the paper sets new thresholds(sunny points at 0.62;cloudy points at 0.66) to simulate the sky images and count the cloud points.The recalculated results are much more reliable than the raw results from the TSI.Setting new thresholds could reduce the cloud cover errors effectively,especially on cloudy days.

Abstract:Based on the homogeneous daily surface air temperature dataset in 2013 from the National Meteorological Information Center,the linear trends of mean temperature and extreme temperature indices and their urban biases at Shenyang station before and after homogenization were compared and evaluated.The result shows that homogeneity of temperature series exerts weak effect on the daily maximum temperature and the relevant extreme temperature indices trend estimation.Meanwhile,it exerts obvious effect on the daily minimum temperature and the relevant extreme temperature indices trend estimation.The result also indicates that the urban bias of annual mean temperature increases after homogenization,especially that of annual mean minimum temperature.However,the urban bias of extreme temperature indices relevant to cold events reduces while that relevant to warm events increases after homogenization.Therefore,homogenization can correct the non-uniformity of surface temperature observation data effectively such as relocation of weather stations but it also brings new urban bias to the observation records to a certain extent.

Abstract:Using the NCEP/NCAR reanalysis data and the OLR(outgoing longwave radiation) data from NOAA during 1980-2010,an index is defined to describe the longitudinal position of the western Pacific subtropical high based on 500 hPa potential height variation in the key areas.With the index,an investigation is carried out closely with the East Asian summer monsoon system,and the circulation and convective activity differences are revealed in the monsoon area as the subtropical high position is abnormal in June and July.Results show that the abnormal eastern-western position of the subtropical high reflects the Asian summer monsoon intensity,that is,when the subtropical high position is west(east),the South China Sea's summer monsoon is weak(strong),and the subtropics summer monsoon is strong(weak).There exists some interaction between the advance and retreat of the subtropical high and members of the East Asia monsoon system.In the western subtropical high years,the South Asia high is strong with eastern position,the monsoon trough is weaker,the long-wave trough develops deeper,the Mascarene high and the Australia high is weaker,and the cross-equatorial flow is also weaker.The situation is contrary in the eastern subtropical high years.

Abstract:Based on 753 stations' daily rainfall data from China Meteorological Administration,NCEP/NCAR daily reanalysis data from 1978-2007 and monthly extended reconstructed sea surface temperature(ERSST) data supplied by NOAA,the regional Meiyu periods during 30 years(1978-2007) over the Yangtze-Huaihe river valley(YHRV) are defined from a regional integration viewpoint.The characteristic of spatial inhomogeneous distribution of Meiyu rainfall is discussed based on EOF(empirical orthogonal function)analysis.In particular,the relationship between spatial inhomogeneous distribution of Meiyu rainfall and previous SST(sea surface temperature) is studied.The results show that in-phase change across the whole YHRV region is closely linked to the previous winter SST anomaly near the Sea of Okhotsk in North Pacific.When the SST is higher(lower) than normal,the winter monsoon tends to be abnormally weak(strong) and the next Meiyu is flood(drought).In addition,when the SST from South China Sea(SCS) to east of Taiwan and Philippines is lower(higher) than normal in May,the following Meiyu rainfall is also to increase(decrease).The south-north out-of-phase rainfall pattern over the YHRV is intimately associated with the previous fall and winter SST in middle Indian Ocean.When the middle Indian Ocean SST is abnormally high from last October to January,the wind shear at 850 hPa between the Yangtze and the Huaihe River is apt to come into being,which is favorable for "south flood and north drought" pattern in the YHRV region,and vice versa.The east-west out-of-phase pattern is related to the SST in the eastern and central tropical Pacific in previous autumn and winter.ENSO events may exert an effect on the east-west location of western Pacific subtropical high,which leads to the general circulation anomaly and the east-west out-of-phase pattern of Meiyu further.When the eastern and central tropical Pacific SST is abnormally high(low),which corresponds to warm(cold) ENSO events,the location of western Pacific subtropical high is more westward(eastward) than normal,which leads to "east dry and west wet"("east wet and west dry") pattern over the YHRV.

Abstract:Based on the daily precipitation data of 753 stations during 1961-2011 provided by National Meteorological Information Center of China and the NCEP/NCAR 1961-2011 daily wind and specific humidity data,this paper investigates low-frequency characteristic of the multi-year mean summer precipitation in South China and its relationships with low-frequency water vapor transport.Results show that summer precipitation in South China has the tendency of increasing,and is obviously more after 1992(1993-2011,period Ⅱ) than before(1961-1992,periodⅠ),especially in most Guangxi,northern Guangdong and the boundary beteen Fujian and Jiangxi.Summer precipitation in South China shows significant 10-20 d low-frequency oscillation,which is more significant in the period Ⅱ than in the periodⅠ.In periodⅠ,the anticyclonic water circulation in northwest Pacific and a pair of cyclonic and anticyclonic water circulations in east of the Indo-China Peninsula and south of South China Sea are the main systems affecting 10-20 d summer rainfall,with water vapor mainly from the bay of Bengal,South China Sea and west Pacific,and cold air from Caspian Sea and east of Lake Baikal.In period Ⅱ,a pair of cyclonic and anticyclonic water circulations in northwest Pacific and in east of Indonesia are the main systems influencing the low frequency rainfall,with water vapor from South China Sea and west Pacific and cold air from east of Lake Baikal.

Abstract:Based on the conventional,AWS,NCEP/NCAR(1°×1°,interval of 6 h) reanalysis and WRF hourly data,the falling area of a heavy rain process in Shandong Province from 30 June to 2 July 2010 was analyzed.Results show that the warm sector and cold sector heavy rain are two features of the rainfall process.The warm sector heavy rain is stronger,wider,more concentrated than the cold sector,locating on the right of the 925 hPa meridional shear line or in the southeastern quadrant of low vortex,the rear of warm temperature ridge,and the southerly region in front of surface low pressure.The cold sector heavy rain is weak and scattered,locating on the left of 925 hPa meridional shear line,in front of cold temperature trough,and in the northerly region behind surface low pressure.The warm and cold sector heavy rain both lie in the area where the atmospheric precipitable water(APW) greater than 70 kg/m²,and on the left to LLJ(low level jet) top.LLJ begins with the heavy rain simultaneously or one hour ahead,and precipitation intensity maximum period appears one to three hours after 850 hPa wind speed jumped.When only the cold sector heavy rain,the cold air is weak with lower height,and the rainstorm lies behind the cold front and in front of the θ_se frontal zone,at the top of θ_se warm ridge and the center of strong upward motion.When the cold and warm sector heavy rain coexist,the intensity of warm and cold air are stronger than that of only cold sector,and the rainstorm locates in wind speed convergence region,a latitude distance south of the strong upward motion center.When only the warm sector heavy rain,cold air is stronger with higher altitude,and the rainstorm is situated in one latitude distance ahead of cold front,the intersection of θ_sewarm ridge and ground,and the center of the strong upward motion.The difference value in the north-south span between the lower frontal area which slants northward with height and the middle frontal area which slants southward with height,directly affect the strength of upward motion and the distribution of heavy rain.

Abstract:Classification studies were conducted in the paper on the temporal and spatial variations as well as the effect of water vapor on the tropical cyclone(TC) frequency over the South China Sea(SCS) and Northwest Pacific Ocean(WNP) in different periods during 1961-2010.The major results are summarized as follows:The whole year can be divided into an active period(June-November) and a calm period(December-May) for TC.The first eigenvectors during both periods show a uniform enhancement or reduction of the TC frequency over the SCS and WNP,while the second eigenvector in the active period shows that the TC frequency over the WNP is in opposite phase with that over the SCS,and the second eigenvector in the calm period shows that the TC frequency to the west of 130°E is in opposite phase with that between 130°E and 150°E.Correlation and composite analysis indicates that the divergence of water vapor flux shows a significant negative correlation with the TC frequency over the WNP in both periods and over the eastern SCS and the vicinity of Philippines in the calm period,but they show only a weak negative correlation over the central and northern SCS in the active period.Therefore,the water vapor conditions may lead to higher TC frequency over the northern(eastern) SCS and the central and eastern(western) parts of the WNP during the active(calm) period.

Abstract:The WRF (weather research and forecasting model) is used to simulate the typhoon Fengshen in 2008.PV-ω inversion diagnostic is applied to quantitatively diagnose the simulated data of typhoon Fengshen at 1000 UTC on 20 June.In this study,the contributions of latent heating,friction and dry dynamical processes to the forced secondary circulations(FSCs) of a typhoon vortex are analyzed respectively.The results show that the latent heating FSC accounts most for the forced secondary circulations.The friction FSC is mostly constricted in the boundary layer(PBL) and dry dynamical processes mainly influence the central area of the typhoon.It is shown that the inverted quasi-balanced flow with friction and latent heating can depict 70% of the secondary circulations.In contrast,the dry dynamical forcing,which is determined by vertical shear and system-relative flow,can account for as much as 40% of vertical motion in typhoon core area.The FSC reduces the typhoon vertical motion in upshear direction and intensifies the vertical motion in downshear direction,thus the asymmetry of typhoon comes into being.It is found that a mesoscale convective system in the typhoon outer core region develops on the right side of down shear direction.An ideal quasi-balanced typhoon superposed by the mesoscale convective system is constructed to examine the influence of various vertical shear and system relative flow.The result shows that the shear FSC is almost linearly related to the magnitude of environmental vertical shear,while increasing the magnitude of the system-relative flow may possibly weaken the FSC.The disturbance aloft on the mesoscale convective system,which is caused by the shear FSC,may be the reason for the persistence of the mesoscale convective system.

Abstract:Based on daily rainfall data and NCEP/NCAR 1°×1° FNL data during the Jianghuai Meiyu period in the summer of 2011,the main influential system and its circulation characteristics of the Meiyu were analyzed.It was found the features of the blocking situation in the 2011 Meiyu period and those during other typical Meiyu times were different:High-latitude blocking had mobility and the main cold air in the 2011 Meiyu period was from the polar vortex splitting southward caused by development of the middle-high latitude circulation.Moreover,the cutting trough caused by the weak resistance situation in middle and high latitude provided the dynamic condition for the Meiyu.It was also found the development of the Meiyu rainstorm was associated with the oscillation of the Northwest Pacific subtropical high ridge.In addition,the influence mechanism of Meiyu front system,the jets and the sub-circulation triggered by them as well as the dry cold air's intrusion on the rainstorm were discussed.

Abstract:Using the climatological mean pentad precipitation data in eastern China,annual variations of monsoon precipitation within the four subareas,i.e. South China,the middle-lower reaches of Yangtze River,Huaihe Basin and North China,are studied.Especially the stages and regional features of summer monsoon precipitation are discussed in detail.By harmonic analysis method,climatological variation of precipitation within each subarea is decomposed into annual cycle mode,seasonal mode,intra-seasonal oscillation mode and intra-monthly oscillation mode.Results show that the impact of interactions among the four modes on the stages and regional features of precipitation are significant.The annual cycle mode is the main mode influencing rainy season,and the seasonal mode and CISO(climatological intraseasonal oscillation) mode are important to determine the flood season.Based on the four climate modes,a new division method of rainy season and flood season in eastern China is put forward.It may be more objective and reasonable as compared to former division methods,though the method is simple.