Abstract:Since 2000, the main Meiyu front rainband moved northward to the Huaihe River basin. The poor or no Meiyu occurred in the Yangtze River very often, due to the anomaly of the atmospheric circulation. However, in poor Meiyu season, a heavy rainfall still occurred in Wuhan area during 0000 UTC 29 June-0000 UTC 30 June 2009. The mentioned above case is diagnosed by using NCEP data, operational observation from China Meteorological Administration, four times per day SCHeREX sounding, automatic weather stations data, radar data, FY-2C Black Body Temperature (TBB), and CMORPH data. It is noticed that the Baikal trough as cold air source favorable the heavy rainfall occurrence has been replaced by the ridge. However, the other favorable factors still benefit the precipitation. They have been noticed that: 1) Existance of col pressure field in East Asia provided the important condition for formation of mesoscale disturbance to the eastern part of the Tibetan Plateau. 2) Eastward shifting trough from the Tibetan Plateau intensified the vertical motion in the middle reaches of the Yangtze River and a meso low formed between Chongqing City and Hubei Province which was very helpful for heavy rainfall in Wuhan. 3) Westward extending of the West Pacific Subtropical High forced the low level jet to intensify in Jiangxi and Hunan provinces and Low Level Jet (LLJ) transported very rich moisture to the Wuhan area. 4) Overlaying of the cold and dry in upper troposphere on the warm and moist in low troposphere produced the significant potential instability. High Convective Available Potential Energy (CAPE) value was released and the convection was initiated due to the triggering of the mesoscale disturbance. 5) The small meso-b scale convective systems relating directly with the heavy rainfall in Wuhan have been revealed.

Abstract:The distribution of annual precipitation, which is affected by mountainous terrain, decreases from south to north in Shaanxi Province. Annual precipitation is 400-600 mm in northern Shaanxi and approximately 500-700 mm in Guanzhong. In the south, however, annual precipitation is 700-900 mm. In the Micang, Daba, and Qinling mountains of southern Shaanxi, as well as in the western mountains areas, annual precipitation reaches 900-1250 mm. In addition, this precipitation exhibits significant seasonal variation. Precipitation is significantly higher in summer than in other seasons, accounting for 39% to 64% of the entire annual precipitation, and is concentrated in northern Shaanxi. In early summer before late June to early July, heavy rainfall gathers in southern Shaanxi. Due to the sudden increase in local precipitation, rainfall prediction is difficult. The case of a heavy rainfall event occurring in southern Shaanxi and Guangdong during 2-3 Jun 2006 is examined. Data was acquired through a variety sources including observation analysis, numerical simulation, and diagnosis analysis.The effects of a short-wave trough at the bottom of a cold eddy at 500 hPa, a shear line at 700 hPa, and a surface easterly are examined. The main factor triggering this storm was a sudden increase in the southeastward jet at 925 hPa and 850 hPa in the low troposphere. The process of this storm was simulated by the Weather Research and Forecasting (WRF) model. The results are essentially consistent with the actual precipitation values. The heavy rain area, the center of maximum precipitation, and vertical velocity are consistent with the observations. Moreover, present value potential vorticity analysis indicates that a very strong cold dry air mass propagating from the top of troposphere in this heavy rainfall process promoted the formation of severe precipitation. The change in vertical velocity and the general potential temperature and distribution of the abnormal convective vorticity vector (CVV), which were all obtained through simulation, accurately reproduce the area and changes in heavy rainfall. These three quantities demonstrate accuracy in forecasting the storm. Therefore, the CCV could be applied to forecast rainfall in Shaanxi Province.

Abstract:Utilizing GPS sounding data collected during the Surface Heat Budget of the Arctic Ocean campaign over the period of one year (SHEBA, October 1997-October 1998), the authors characterized the boundary layer inversion and low-level jet over the Arctic Ocean. The results showed that 96% of the soundings (1115 UTC and 2315 UTC) had near-surface inversion layers, where 22% were surface-based and 70% were between 250 m and 850 m. During the winter months, the frequency of the surface-based inversion, the inversion depth, and the temperature change in the inversion layer were stronger than that during the summer months. During the year, the low-level jets occurred with a frequency of 41% and their average height was 520 m. The jets appeared mostly around 150 m, and 70% were below 600 m. The mean speed of the jets was 10.6 m·s^-1 and the predominant wind direction was east and northeast; 75% of the wind speeds recorded ranged between 4 m·s^-1 and 13 m·s^-1. The analysis of the turning angle distribution between the jet core and the ground wind indicated that inertial oscillation was the main cause of the low-level jet in the Arctic region.

Abstract:The authors analyzed the tendency of the annual and summer average surface temperatures in the Northern Hemisphere for a period covering nearly 60 years and the variation of the convective precipitation occurred in summer (June-August). The possible causes of the variation were also researched in this article by investigating the impact factors of the convective precipitation. The analysis was based on the monthly data, daily and monthly relative humidity, temperature data of NCEP/NCAR, and the daily precipitation records of 60 selected stations in the mid-lower reaches of the Yangtze River using trend analysis and mutation testing method. The results show that for nearly 60 years the annual and summer average surface temperatures in the Northern Hemisphere have been rising with an abrupt temperatures leap in nearly 1998 so that this year can be regarded as an temperature discontinuity. With the increasing effect of global warming, both the frequency and intensity of the convective precipitation events that occurred in summer were enhanced in the reaches of the Yangtze River. In years when the temperature was higher than average, the water vapor content in the upper and middle troposphere showed a continuously decreasing trend and in the lower troposphere showed an increasing trend in July and August. Apart from a few individual years, the heat content at the 700 hPa, 850 hPa, and 1000 hPa levels had an obvious positive trend. The atmospheric instability also showed a clear tendency of strengthening. These factors have a strong correlation with the increasing frequency and intensity of the convective precipitation. It was concluded that changes in water vapor content and enhancement of heat content and atmospheric instability caused by global warming may have an important influence on the increase of convective precipitation events.

Abstract:Classical methods, such as the harmonic wave superimposing method and the linear filtering method, cannot simulate the universal fractal feature of wind fluctuations. The authors proposed a simple method using the stochastic Weierstrass-Mandelbrot function to simulate wind fluctuations with the fractal feature. The fractal dimension, which is an important parameter that describes the fractal feature of wind fluctuations, can be related to the exponents of the inertial-range spectrum. Simulation results show that the method can effectively simulate the probability distributions and variations of wind fluctuations in the medium-high frequency range.

Abstract:Using the NCEP reanalysis data and observed rainfall data in China from 1951 to 2008, the interdecadal variation in Southern Hemisphere (SH) circulation and the influence of Antarctic Oscillation (AAO) on summer rainfall in eastern China under the various decadal backgrounds was analyzed. The results indicate that interdecadal variation of SH circulation occurred during the late 1970s. While the subtropical highs in the southeastern Pacific and the southern Atlantic were weakened, the Mascarene high, also known as the Indian Ocean subtropical high, was intensified. In addition, the circumpolar low-pressure belt was deepened. The AAO with a negative phase was changed into a positive phase. The influence of AAO on summer rainfall in China was also changed. A stronger AAO in the boreal spring before 1976 related to more rainfall in the southern Yangtze River and in North China and less rainfall in the Yangtze-Huaihe River valley. Conversely, rainfall amounts were higher from south China to the Yangtze-Huaihe River valley and lower in northern and northeastern China after 1976. Therefore, the influence of AAO on summer rainfall in China is related to the decadal background. After 1976, the influence of AAO tended to intensify and extend more northward. When the effects of sea surface temperature are minimal, AAO can be used as an important tool for summer rainfall prediction in China.

Abstract:The differences of air quality between the northern and southern parts of Beijing were studied by using observation data of air quality and meteorological factors, as well as synoptic maps of North China. Analyzing the weather patterns and meteorological factors during air pollution episodes, the authors conduct a preliminary discussion on the formation mechanism of this phenomenon. The results show that in 2006 there were 47 days with this phenomenon. It appeared in autumn and winter, mostly between midnight and the following morning. Most of time, the air quality in south Beijing was worse than that in the northern part of the city. The authors found that the phenomenon was related to the meso-α-scale weather system. The path and speed of dry cold air are considered to be the main factors. When strong convergence appeared in the southwestern part of Beijing, the dispersion condition varied between the south and the north, causing heavy air pollution in the southwest.

Abstract:The variations in the trends for the start dates of the seasons in the Tibetan Plateau over 47 years (1961-2007) were analyzed using the daily temperature data of 1961-2007 provided by the National Meteorological Information Center, China Meteorological Administration. The results show that spring and summer are starting earlier while autumn and winter are starting later; the changes in autumn and summer were not as significant as those of spring and winter. This trend is most obvious in the 1990s and the beginning of the 21st century as the effect of global warming increases. This trend has different seasonal spatial distributions in the Tibetan Plateau; spring and winter have the same spatial distribution—larger changes in the central region than in the surrounding area summer and autumn are similar but different than spring and winter. Before the 1990s, the start date of spring showed little change, but the trend of earlier start date became obvious from the 1990s onward; the trend of earlier start date of summer is more stable than that of spring but the interannual variability is greater. The trend of the earlier start date of autumn is also more stable than that of spring with greater interannual variability; from the 1990s, the start date of autumn shows a more obvious delay trend than that of winter, while in the early phase of 21st century the delay trend is strong in winter.

Abstract:The frequency and intensity of temperature extremes in Beijing was studied by using the standardized weather station data from 1979 to 2008. The differences between rural and urban areas and their relationship to the urban heat island intensity are discussed. The analysis results show that the decrease in the minimum temperature events over the urban area [-5.94 d (10 a)^-1] was slightly higher than that over rural areas [-5.28 d (10 a)^-1] over the 30-a period, while the difference of maximum temperature changes between the urban [4.33 d (10 a)^-1] and rural [4.42 d (10 a)^-1] areas were much smaller. The authors also found that the frequencies of extreme events between the urban and rural areas were very small. The investigation of the intensity revealed similar trends. Although the temperature extremes over the urban area were clearly stronger than over the rural areas, in the urban area the decreasing trend of minimum temperature extremes was slightly greater [0.042 °C (10 a)^-1] than the increasing trend of the maximum temperature extremes, while in the rural area the deceasing trend of the minimum temperatures was slightly less [0.052 °C (10 a)^-1] than the increasing trend of the maximum temperatures. The study on the differences between the urban and rural areas suggested that the intensity of the temperature extremes was stronger over the urban area than that over the rural area. The difference in temperature extremes ship between the urban and rural areas was generally a positive value (except for the negative difference in maximum high temperature in 1982 and 1885). On the other hand, statistical analysis showed no correlationship between the intensity of the urban heat island and the rural-urban difference of extreme temperature events, indicating that the impact of the urban heat island on the urban and rural areas is mainly coincident for a super-city like Beijing.

Abstract:The first snowfall over North China in winter 2009 was investigated through numerical modeling and diagnostic analysis with a focus on the dynamic and thermodynamic effect of the wind field in the lower troposphere and the characteristics of atmospheric stratification. The results show that this snowfall was induced by the combined effect of the trough in the middle troposphere, the shear line in the lower troposphere, and the inverted trough on the ground. With the southeastward movement of the influencing systems, the snow area moved southward. The easterly reflux that had important dynamic effects on the snowfall was mainly located at and below 850 hPa. On the one hand, the easterly reflux and the westerly flux formed a wind convergence line, which favored upward motion. On the other hand, the effect of the topography combined with the easterly reflux triggered a southerly flow that prompted the transport of warm airflow into the snow area with a result of frontogenesis when the warm airflow met the cold airflow. The easterly reflux had thermodynamic effects in addition to the dynamic effects, for it played an important role in the convergence of water vapor over the snow area. There was a pronounced temperature drop in the lower troposphere during the snowfall. The temperature in the boundary layer and at 850 hPa fell below 0 ℃ and to -5 ℃, respectively. This result suggests that to improve the forecasting of the nature of winter precipitation, close attention should be paid to the simulated characteristics of temperature stratification in the lower troposphere.

Abstract:Using the Madden-Julian oscillation (MJO) index provided by Climate Prediction Center, NOAA, and the tropical cyclone (TC) best track dataset over the western North Pacific (WNP) provided by Shanghai Institute of Typhoon of CMA, the quantified statistics of the modulation of MJO on tropical cyclone activity are performed. The results show that the genesis, intensity, landfall, and path of TCs are all modulated by MJO significantly. When the upper-air convergent center locates in 120°E-160°E (MJO phases 3-5), there are less TCs form and the genesis position locates more north in the WNP; While the upper-air convergent center locates in 10°W-70°E(MJO phases 8-10), more TCs form and the genesis position locates more southward. When the upper-air convergent center locates at 70°E, i.e., MJO phase 10, the intensity of TCs is significantly modulated; during phases 1-4 and 10, the vigorous TCs occur in the east of the Philippines of WNP, indicating the dominant northwestward and northward paths, possibly making landfall on the East and North China; while during the phases 5-8, the TCs occur frequently in the vicinity of the Philippines to the South China Sea, suggesting the dominant westward tracks and possible TC landfall on the south China. These statistical facts could be useful in the prediction of TC intraseasonal activity.

Abstract:Using climatological methods, the forecast performance of the ECMWF model over the Northeast Hemisphere is discussed through analytic comparison between the ECMWF daily analysis and its seven-day forecast products. The results show that: 1) the time series verification indicates that the ECMWF’s daily forecast exhibits a seasonal trend, that is the forecast performance for summer was worse than those for the other seasons and the forecast accuracy decreased to its minimum in July. 2) The prediction ability differed among the various meteorological fields. In general, the analysis showed good correlation with the seven-day forecast 850-hPa temperature and 500-hPa geopotential height while the correlation with the 850-hPa relative humidity was poor. The forecast bias increased with extended prediction time. 3) The seven-day forecast temperature was higher than the ECMWF analysis over the land and lower over the tropical area; the forecast performance over the nearby area of the equator was weaker than over the higher latitude region, showing similar behavior to some degree in other meteorological prediction fields. 4) There was a remarkable OKJ wave chain in the 500-hPa geopotential height or a 500-hPa wind difference between the seven-day forecast and the analysis. 5) The zonal wind forecast was better than the meridional wind forecast field, and showed a high correlation with the analysis near 30°N. 6) Overall, the ECMWF model has better forecasting capability at the high level fields than at the lower level ones, with the exception of the 700-hPa wind fields.

Abstract:In this paper, a β channel linear two-layer barotropic quasi-equilibrium ocean model forced by time-varying wind, that is wind anomalies, is established, and its analytical solutions are obtained. It should be noted that the following discussions pertain to the baroclinic mode because the barotropic mode is inconsistent with actual ocean conditions. In the western boundary, the upper corresponding flow field to time-varying wind is expressed as a couple of vortices with cyclonic and anti-cyclonic curvatures. In the upper ocean, obvious western and eastern flow can be forced on the β channel centerline at t=0 and t=T/2, where T is the varying period of wind field, because west and east wind are strongest at that time. Moreover, an obvious countercurrent appears at the south-north lateral boundary of the β channel. Although the forcing of wind disappears near t=T/4 and t=3T/4, oceanic flow anomalies remain due to inertia. In addition, the abnormal flow is strong and changes slowly at t=0 and t=T/2 but becomes weak, acute, and reverses its direction at t=T/4 and t = 3T/4. The corresponding ocean mode also shows that the strength of the lower flow is similar to that of the upper flow; however, the flow is in an opposite direction, which denotes the baroclinicity of the ocean. In analytical solutions, the fluctuation frequency of the ocean flow is the same as its forcing wind field with the exception of a fixed phase difference. Although the nature of the flow anomalies is a quasi-equilibrium vortex wave because only the vortex wave is included in the model, it will transform to a Rossby wave when the forcing wind stress and Rayleigh friction is small. Estimation of the flow amplitude shows that the corresponding flow anomalies become stronger with a higher wind stress, a wider half-width of β channel, a smaller dissipation and a slower changing frequency of wind stress. In addition, the low-frequency wind stress can initiate stronger flow anomalies than those for high-frequency wind stress under the same conditions. The spatial pattern of the corresponding ocean mode is similar to that of the flow anomalies over southeast Honshu Island in Japan. The analytical solutions obtained in this paper can reflect the upper flow anomalies forced by actual wind in the mid-latitude North Pacific to some extent; therefore, these results are helpful for demonstrating the corresponding mechanism of the upper flow to time-varying wind fields.

Abstract:Proven theories and observations in specific regions have demonstrated that average climates often exhibit significant nonlinear relationships with extreme climates. Two different types of climate variables that show symmetric and asymmetric probability distributions correspond to nonlinear changes from average to extreme climates. In this paper, the data from several representative stations and relative climatic variable fields are used to prove that theoretical estimated probabilities for extreme climates are more consistent with actual values. Examples are used to demonstrate cases in which the various probability distributions are observed.