Vol.28, NO.3, 2017

Display Method:
Interdecadal Variation of Haze Days over China with Atmospheric Causes in Recent 50 Years
Pan Wei, Zuo Zhiyan, Xiao Dong, Zhang Renhe
2017, 28(3): 257-269. DOI: 10.11898/1001-7313.20170301
Characteristics of interdecadal variations of haze days over China and plausible meteorological causes during 1961 to 2013 are analyzed, using observations from 745 meteorological stations in China. Results show that most haze weather occurs over the east part of China from South China to North China where haze days exhibit an increasing trend especially in economically developed areas, such as North China, the Huanghuai and Jianghuai Plains. Haze days are fewer in large parts of Northeastern China and Western China, and haze days show decreasing tendency in these regions. Generally, haze days are more frequent in autumn and winter than those in spring and summer. Also, autumn and winter are seasons that variation of haze days is most significant. Haze days occur more frequently in January and December than other months. The first mode of EOF reflects a monotonically increasing trend in the east part of China. The second mode shows that the region from South China to the Huaihe River and the region from the Huaihe River to North China present opposite variation tendency of phase. Focusing on regions from South China to the Huaihe River and from the Huaihe River to North China, haze days show an increasing trend year by year over the region from South China to the Huaihe River in autumn and winter before 2000, and then the trend is smoother. In the region from the Huaihe River to North China, however, haze days change gently in recent 30 years in autumn and winter. The variation of haze days relate to surface wind speed and relative humidity. Haze days show a significant negative relationship with surface wind speed before 1990s. Haze days may be influenced by variations of east wind in the region from South China to the Huaihe River in autumn, while northeast wind in the winter. For region from the Huaihe River to North China, haze days is concerned with south wind in autumn, while haze days have nothing to do with wind in winter. The relativity between haze days and surface wind speed weakens, but haze days have significant relationship with relative humidity after 1990s. Haze days experience an increasing tendency because of the reduced surface wind before 1990s and the decrease of relative humidity after 1990s over the region from South China to the Huaihe River in autumn and winter. In comparison, haze days show moderate variations in the region from the Huaihe River to North China, which are probably related to the moderate variability in surface speed and relative humidity in autumn and winter.
Use of Representative Pixels of Motion for Wind Vector Height Assignment of Semi-transparent Clouds
Zhang Xiaohu, Zhang Qisong, Xu Jianmin
2017, 28(3): 270-282. DOI: 10.11898/1001-7313.20170302
Height assignment of semi-transparent clouds needs two variables:Radiation from semitransparent clouds themselves and radiation from the background underneath semitransparent clouds. Generally, the former radiation is calculated using algorithms based on all pixels in tracer boxes. A new algorithm, however, is proposed considering only pixels with great contribution to correlation coefficients during wind vector tracing stage, and only allow these pixels to join the calculation. Scatter diagrams are made with infrared brightness temperature against their contribution to the feature tracking correlation coefficients. Scatter diagrams show a "C" shape pattern and can be used to recognize representative pixels of motion. Pixels with relative less contribution to correlation coefficients are at the left side of the "C" shape with infrared brightness temperature in the middle range. The other pixels with relative more contributions are naturally separated into two branches with warmer and colder infrared brightness temperature respectively. When tracer boxes are partly covered by clouds, the warm branch pixel brightness temperature is with typical ground feature at both the scope and the variability. And when tracer boxes are totally covered by clouds, warm branch pixels are in the slot of clouds with typical cloud features among cloud cells. Thus, no matter tracer boxes are partly covered or totally covered by clouds, cold branch pixels with greater contribution to correlation coefficients are better at representing the cloud motion and can be named as pixels with contribution to the motion. The algorithm using different pixel's contribution to the feature tracking improves height assignment quality by effectively removing rapid changing pixels in tracer boxes when estimating the cloud height. Using representative pixels of motion at the estimation of radiation from semi-transparent clouds themselves, FY-2 wind vector height assignment quality is improved.
Estimation of Background Radiation Underneath Clouds for Wind Vector Height Assignment of Semi-transparent Clouds
Zhang Xiaohu, Zhang Qisong, Xu Jianmin
2017, 28(3): 283-291. DOI: 10.11898/1001-7313.20170303
Height assignment of semi-transparent clouds needs two variables:Radiation from the semi-transparent clouds themselves and radiation from the background underneath semi-transparent clouds. The background radiation is not directly observed by the satellite, but can be estimated with infrared/water vapor scatter diagrams of satellite measurements around tracer image boxes. According to the cloud amount and type observed, tracer image boxes can be divided into 4 types:With different levels of clouds inside, near the cloud edge, with only high or middle-high clouds inside and with only middle-low clouds or ground in tracer box. By studying characteristics of infrared/water vapor scatter diagrams of satellite measurements in 8 typical cases of 4 types which are variety, different estimation methods are discussed and a new estimation method for only high or middle-high clouds in tracer box is proposed. When there are cloud-free pixels in tracer boxes, infrared maximum brightness temperature may represent the background radiation for infrared channel; on the other hands, observation pixels with the highest infrared but relative lower water vapor brightness temperatures are used to estimate the background radiation for water vapor channel. Observation pixels with high infrared, but relative lower water vapor brightness temperatures are located in the clear sky region near the cloud edge, where atmospheric stratification conditions are relatively wetter than the region far away from clouds. The clear sky region near the cloud edge relatively better represent the condition underneath clouds better. Due to absorption and retransition processes, the water vapor brightness temperatures with wetter atmospheric stratification conditions in the clear sky region near the cloud edge are relative lower. When there are no cloud-free pixels in the tracer image box, the searching area should be expanded in the east-west direction to find cloud free pixels, until the background radiation can be estimated.Based on the new estimation method, FY-2 wind vector height assignment algorithm is improved, and effectiveness is analyzed by comparing with ECMWF reanalysis data, showing that FY-2 wind vector quality including biases and standard deviations is improved obviously.
Spatiotemporal Variations of Snowfall Days over the Tibetan Plateau from 1981 to 2010
Chu Duo, Luosang Quzhen, Yang Zhigang, Yang Yong
2017, 28(3): 292-305. DOI: 10.11898/1001-7313.20170304
Snow is a vital component of alpine regions due to its large seasonal variations and distinctive physical properties, it greatly affects climate, hydrology, and ecology at regional and global scales. The Tibetan Plateau (TP), also called "the roof of the world", is the highest land and typical alpine region on the earth with over 4000 m above sea level. Snow on the TP is a vital fresh water source in the Himalayan region and downstream areas, and also a significant indicator of climate change from regional to global scales. Therefore, under global climate warming, the variation of snowfall days over the TP, which is defined as the number of days with snowfall (including sleet) weather at a station from 2000 BT to 2000 BT, is concerned by international scientific community and local people.Spatial and temporal variations of snowfall days over the TP are analyzed using snow observations from 94 meteorological stations for the period from 1981 to 2010, and the following results are obtained. Snowfall days over the TP are generally characterized by more snowfall days in the central and eastern alpine region, southern slope of the Himalayan range and the Qilian mountains in the northeast, and less snowfall days in valleys in south and the Qaidam Basin in the north. The number of averaged annual snowfall days for the TP is 60 days, of which 45% occurs in spring, 28% in winter, 22% in autumn and 5% in summer, respectively. There is a very significant decreasing trend in mean annual snowfall days from 1981 to 2010 over the TP with a rate of 10.5 days per decade. The abrupt climate change of snowfall days from more to less occurs around 1997 and in the later 10 years, annual snowfall days decrease obviously especially in 2010, which is an anomalous snowless year. Intra-annual distribution of snowfall days over the TP is characterized by double-peak type. The first peak occurs during transition period of atmospheric circulation from winter to summer and the second peak takes place in transition period of atmospheric circulations from summer to winter with less peak value. During the transition time of atmospheric circulation, low value weather system along with upward air movement and temperature and humidity conditions in the upper air is favorable for snowfall over the TP. A very significant correlation exists between decrease in snowfall days and increase in temperature from 1981 to 2010 over the TP. Under global warming and more rapid temperature increasing on the TP, the rainfall previously in snow form converts into liquid rainfall, which may significantly contribute to the decrease of snowfall days over the TP in the recent 30 years.
A Comparison of Optimal-score-based Correction Algorithms of Model Precipitation Prediction
Wu Qishu, Han Mei, Liu Ming, Chen Fajing
2017, 28(3): 306-317. DOI: 10.11898/1001-7313.20170305
Based on data from national meteorological stations, one year quasi-symmetrical mixed running training period (QSRTP), and precipitation prediction from CMA (T639), ECMWF, NCEP, JMA, both optimal threat score (OTS) method and optimal equitable threat score (OETS) method are designed to conduct a comparison experiment on correction algorithms for model precipitation with frequency matching (FM) method. Through classification correction, three methods are used merely to calibrate model precipitation amount with the predicted rain-belt location and shape kept unchanged. The OTS method figures out correction coefficients of different precipitation classes by optimizing threat score (TS) of corrected precipitation within training period. OETS is similar to OTS but achieved by optimizing ETS. Correction experiments are conducted twice a day with forecast time at 0000 UTC and 1200 UTC, respectively. To consider seasonal background, 20 days before the forecast day and 20 days after the same day in the previous year are adopted to constitute training period. For each national meteorological station, there are 80 samples in total. The correction experiment shows that for either precipitation products of ECMWF, JMA, NCEP, CMA, or their ensemble mean, both OTS and OETS show much better performance than FM in 24 h accumulated precipitation classification calibration with different lead time according to traditional verification methods like TS and ETS. In particular, OTS is the best and can improve precipitation prediction in all lead times. After correction, both OTS and OETS incline to forecast larger precipitation area than observation for most classes but less precipitation amounts. Compared to FM, both methods tend to produce a little higher false alarm rates in middle and low classes, which is much less than the reduced missing rate, thereby leading to a higher threat score. In terms of ECMWF correction, OTS and OETS have a relatively stable Bias score of 1.1, although there are much fewer samples in high class. By contrast, FM produces an unstable Bias score, especially in maximum class with score over 2.2, indicating an excessively high missing rate. As for stable equitable error in probability space (SEEPS), OTS has superiorities over all lead times, all single models and multi-model mean. Furthermore, TS of corrected ECMWF precipitation using OTS method in 2015 are also better than subjective forecast from all aspects, with national averaged threat score of 1 d rainstorm forecast reaching 0.194.
Rapid Intensification and Associated Large-scale Circulation of Super Typhoon Rammasun in 2014
Cheng Zhengquan, Lin Liangxun, Yang Guojie, Sha Tianyang
2017, 28(3): 318-326. DOI: 10.11898/1001-7313.20170306
Most offshore tropical cyclones (TCs) would be weaken because of energy decay due to land surface friction, while a small part of them intensify instead. There is an average of 0.8 TCs per year which intensifies abruptly in offshore waters of South China. Statistical, diagnostic and numerical work reveal that sea surface temperature (SST), TC inner structure and characteristics of atmospheric circulation are the most important factors. And for the latter, strength or strengthening of a low-level jet linking with TC and weak environmental wind vertical shear are significant, and widely applied in operation to judge the TC intensity change trend. The mechanism of TC abrupt intensification in offshore waters is still not fully revealed, and therefore it is still very difficult to give an accurate forecast for these phenomena in the route operation. Wrong judge and forecast for TC abrupt intensification in offshore waters would lead to underestimation and insufficient defense of TC disasters. Super typhoon Rammasun is intensifying continuously for over 24 h before it makes landfall at Wenchang of Hainan at 0730 UTC 18 July 2014, with 40 m·s-1 of maximal central wind speed, 960 hPa of minimal central sea level pressure at central part of South China Sea at 0000 UTC 17 Jul 2014, 72 m·s-1 888 hPa at 0600 BT 18 Jul 2014. And Rammasun becomes the strongest landfalling TC on national record.Based on CMA TC track data, satellite and radar images, NCEP reanalysis data, NOAA high-resolution blended analysis of daily optimum interpolation SST and synoptic analysis and kinetic energy budget, study is carried out on the characteristic of abrupt intensity change of Rammasun and its cause.Synoptic analysis show that the continuous abrupt intensification of Rammasun is concerned with the favorable atmospheric background circulation, the anomalous warm SST in central and northern part of South China Sea during the middle ten days of July 2014, intensification of the southwestern low level jet and cross-equatorial flow linking with Rammasun, and the maintenance of upper level outflow especially the unstable atmosphere in the downstream region. Its moving into the unstable circumstance is advantageous to the convective activities and the efficiency of convective condensation latent heat release within the cyclone circulation, and leads to the maintenance or intensification of the TC.Kinetic energy budget output reveals, main kinetic energy source at low level is from the wind crossing through the isobar, and this is connected with vertical circulation of ascending in the central area and descending in the outer region forced by convective condensation latent head. To consider in view of convective activities and condensation latent heat release is helpful for understanding of TC intensity change and forecast in operation.
The Wind Measuring Performance of WINDCUBE V2 Pulse Laser Wind Profiler Under Different Weather Conditions
Chen Wenchao, Song Lili, Wang Zhichun, Liu Aijun
2017, 28(3): 327-339. DOI: 10.11898/1001-7313.20170307
The wind data observed by the gradient wind observation system set on 100 m high tower in Donghai Island meteorology station located in Zhanjiang, Guangdong Province are compared with the wind data measured by WINDCUBE V2 pulse laser wind profiler installed at the same site from 21 May to 16 August in 2012. Based on time synchronization adjustment, validity test and sample selection, a comparative analysis is conducted using several wind parameters observed at heights of 50 m, 65 m, 70 m, 100 m by cup anemometer, ultrasonic anemometer and WINDCUBE V2 under weak wind, strong wind, sunny and rainy days.Results show that fitting degrees of horizontal wind parameters are quite good. Fitting degrees of wind speed and wind direction measured by WINDCUBE V2 and cup anemometer are up to more than 0.99. The fitting degree of 3 s gust wind speed is over 0.96. Fitting degrees of 3 s gust wind speed, turbulence intensity and wind speed standard deviation are over 0.96, 0.67 and 0.79, respectively. The WINDCUBE V2 performs a little better under strong wind and no rainfall condition when measuring the above wind parameters. Fitting degrees of wind speed and wind direction of strong wind samples are a litter larger than that of weak wind samples. More than 90% strong wind speed relative deviation vary between-4.5% and 2.5% while more than 90% wind speed relative deviation vary between-11.5% and 5.5% under weak wind condition. Overall, the precipitation has no obvious influence on the measurement of wind speed, wind direction, turbulence intensity and gust wind speed by WINDCUBE V2 when the 10 min rainfall intensity is less than 15 mm while measurements of wind speed standard deviation are a bit influenced by precipitation. Fitting degrees of wind speed and wind direction of samples without rain are a litter larger than that on rainy days, but both are over 0.99. The WINDCUBE V2 performs weaker at the vertical airflow velocity measurement under the increasing of horizontal wind speed and the precipitation. The fitting degree of vertical airflow velocity is 0.872 when the horizontal wind speed is between 0-1.5 m·s-1. But the fitting degree of vertical airflow velocity decreases to 0.25 when the horizontal wind speed increases to 0-6 m·s-1. The vertical airflow velocity measured by WINDCUBE V2 decreases more significantly with the stronger rainfall. When 1 min rainfall intensity is over 0.5 mm, the vertical airflow velocity measured by WINDCUBE V2 is found to be 4 m·s-1 less than the measurement of ultrasonic anemometer. These results can provide some technical references for the reliability test method of laser wind profiler measured data.
Diagnosis of Thermal and Dynamic Mechanisms of Two Rainstorm Processes in Northern Shaanxi
Zhao Qiang, Wang Nan, Li Pingyun, Qu Liwei
2017, 28(3): 340-356. DOI: 10.11898/1001-7313.20170308
Based on the conventional meteorological observations and 6 h 1°×1° NCEP FNL analysis data, two heavy rain processes occurred on 21-22 July 2013 and 8-9 July 2014 in northern Shaanxi are diagnosed with synoptic method and dynamic diagnosis method. It shows that both processes can be attributed to the intersection of the warm moist air flow along the edge of subtropical high at 500 hPa and the cold air brought from plateau troughs. Low-level jet plays an important role, as it provides adequate water vapor and water vapor convergence lifts on the left side of the shear line. The vertical secondary circulation produced by the coupling of upper-and low-level jet is an important triggering factor. Heavy rainfall in "0721" process occurs mainly at Yan'an where strong coupling of upper-level jet and low-level jet is located. There is strong convective instability in the atmosphere in the initial stage of precipitation during both rainstorm processes. Convergence lifting from the low-level shear line triggers convection energy, resulting in strong precipitation. The latent heat of condensation released by the precipitation extends downward to the middle atmosphere, and leads to thermal discontinuity of the middle and lower atmosphere. Atmospheric wet baroclinicity and frontogenesis significantly enhances, causing the uplift of whole layer saturated atmosphere and strong precipitation, finally producing heavy rainfall process. Because the convective precipitation is stronger and latent heat released is greater in "0721" rainstorm process, the feedback of the low-level jet and the middle atmosphere frontogenesis is stronger, and therefore the precipitation is heavier. The vertical component of generalized convective vorticity vector describes the enhancing of vertical wind shear very well, and describes the frontogenesis which is increased by condensation latent heat that released by water vapor phase transition in the middle and lower layers very well. Therefore, the changing trend of generalized convective vorticity vector can reflect the development and decrease of precipitation. The large value center and high gradient area on the south side of the vertically integrated moist thermodynamic advection parameters is consistent with rainstorm fall area, and it appears about 6 hours before the precipitation, indicating it can be used to effectively forecast regional precipitation.
Environmental Conditions of Three Squall Lines in the North Part of Zhejiang Province
Chen Shuqin, Zhang Lina, Yu Xiaoding, Xu Zheyong, Liu Han
2017, 28(3): 357-368. DOI: 10.11898/1001-7313.20170309
Based on radar data and intensive surface observations, combined with JMA reanalysis data, a typical case with three consecutive squall lines is analyzed, which occurred in the north part of Zhejiang Province on 2 July 2008. Corresponding atmospheric conditions are investigated in detail. Special emphasis is given on the relationship between convection current and underlying surface state such as temperature, humidity and convergence of wind, especially the impact of land-sea boundary on rebirth and strengthening of convection current. Besides, the forming processes of the third typical bow echo, including development, attenuation, effluent, inflow, rebirth and dissemination of convection cell are studied in horizontal and vertical direction. After that, favorable conditions for all evolutionary stages and interrelation of three squall lines are summarized.It shows that there are specific places where newly-born convections and convection reinforcements are likely to be found, such as high temperature region, high humidity region, frontal surface, convergence line and coastline. In general, it's favorable for convection when the ground temperature is more than 32℃, the dew point temperature is greater than 23℃, the ground temperature gradient is greater than 0.1℃/km, or the ground level wind shear is greater than 5 m/s. Severe convective systems also react on underlying surface, and then convective systems are influenced as well, severe thunderstorm causes strong divergent outflow and cool pool is formed on ground layer. Cold air in the front of thunderstorm diverges outwards and causes gust front, which lifts the pre-frontal warm and moist. New convective cells develop close to the gust front, so that convective systems can diffuse forward. The question about convective systems' change crossing coastlines is complex. If they move to the sea by day, the temperature of underlying surface will descend and system's intensity would be weaken easily, and the situation will become opposite by night. In addition, the convergence of wind and intensity of convective systems enhance over the sea on account of small frictional forces and strong wind speed. Severe thunderstorm generates and strengthens at coastlines frequently, and particularly at the junction between the gust front and the coastline due to convergence caused by wind discontinuity around coastlines. Finally, the convection weather concept model before the trough is summarized:The area is within the scope of subtropical high before the trough at 500 hPa. There is strong southwest jet and warm wet tongue at low level, which forms unstable stratification. There is a big wind belt at 500 hPa, which forms a larger vertical wind shear from 0 to 6 km level, adding to the potential instability, and a strong convection system is triggered if a cold front comes.
The Simulation and Evaluation of Soil Moisture Based on CLDAS
Han Shuai, Shi Chunxiang, Jiang Lipeng, Zhang Tao, Liang Xiao, Jiang Zhiwei, Xu Bin, Li Xianfeng, Zhu Zhi, Lin Hongjin
2017, 28(3): 369-378. DOI: 10.11898/1001-7313.20170310
The national weather service modernization is the core and key to the modernization of the national weather, which is an important symbol to enhance China meteorological technology level and professional ability. China Meteorological Administration publishes the national meteorological modernization objectives and evaluation plan (2014-2020), which clearly proposes the development of multi-source data fusion data set, and the land surface data fusion is one of the most important parts. Using the technique of multi-source data fusion, China Meteorological Administration Land Data Assimilation System (CLDAS) integrates observation of ground, satellite and numerical model to obtain the high-quality temperature, pressure, humidity, wind speed, the grid point data of precipitation and radiation and other factors, and then to drive land surface model to simulate different depths of soil temperature and moisture.CLM3.5 land surface model is used to simulate land surface soil moisture of different depths, and then results are assessed using 3 ground datasets. The first is the automatic soil moisture observation of CMA in 2013, which is checked strictly by quality control process, the second is CTP-SMTMN data, and the last is GLDAS soil moisture and ERA-Interim Reanalysis. A comprehensive assessment for soil moisture is conducted and it shows that the correlation coefficient reaches a high level in most provinces, which can better reflect the objective change of soil moisture and has a strong guiding role. In statistical analysis of time series by selecting the representative station, surface soil moisture changing rates are higher than deeper layers, because the interaction between surface soil and the atmospheric boundary layer feedback is more sensitive, and the water heat exchange are more frequent. On the Tibetan Plateau, using Taylor diagrams comparison, it's found simulation results in the appraisal process of different indices are better than the other two kinds of foreign soil moisture data. In summary, the correlation coefficient is up to 0.8, and deviation is about-0.04 mm3·mm-3 to 0.04 mm3·mm-3 and root mean square error is lower than 0.04-0.05 mm3·mm-3 from stations of provinces average and all over the country.
Correction of TRMM Monthly Precipitation Data from 1998 to 2013 in Xinjiang
Lu Xinyu, Wei Ming, Wang Xiuqin
2017, 28(3): 379-384. DOI: 10.11898/1001-7313.20170311
Using monthly TRMM precipitation data and precipitation observations from 105 national basic weather stations in Xinjiang region from 1998 to 2013, a stepwise regression model and a back-propagation (BP) neural network are established to correct TRMM precipitation. Results show that models added with geographical factors can increase the accuracy of TRMM precipitation effectively. Corrected by two models, overall correlation coefficients are 0.75 and 0.80, and relative errors are 4.88% and 3.19%. On the monthly scale, the relative error of TRMM monthly precipitation ranges from-5.68% to 54.44%, from-4.26% to 32.57% after stepwise regression and from 5.33% to 24.48% after neural network, respectively. In addition, results show that qualities of satellite precipitation products are improved in different degrees from ST, with 0.01-0.49 for stepwise regression model and 0.03-0.70 for neural network, respectively. Compared with TRMM data before correction, the stepwise regression model and BP neural network model can accurately and quantitatively reproduce the actual distribution of precipitation, and provide a more practical method for the area lack of precipitation data.