Vol.17, NO.3, 2006

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Diagnostic Analysis of a Heavy Rain Event in North China Caused by the Development of Yellow River Cyclone
Liang Feng, Tao Shiyan, Zhang Xiaoling
2006, 17(3): 257-265.
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Yellow River Cyclone is one of the important weather systems that could produce heavy rain in North China. A heavy rain event occurred from 9 to 11 June 1991 in North China and caused by the development of a Yellow River Cyclone is studied in detail through a combined analysis of NCEP/NCAR reanalysis dataset, the temperature of black body (TBB) data of Geostationary Meteorological Satellite (GMS) and radiosonde data. It is found that the occurrence and development of Yellow River Cyclone are results of intense development of atmospheric baroclinicity not usual in summer season. The joint effects of strong divergence and positive advection of vorticity at high level trigger the Yellow River Cyclone. The warm advection at low-level in the troposphere facilitates the Cyclone's development and leads it to move. The heavy rain region is under the right side of the exit of the high level jet. In the south of the region, from 18:00 (UTC) June 9, low level jet (LLJ) with wind speed greater than 14 m·s-1 occurs at 850 hPa and transports water vapor to it. Heavy rain occurs at early stage of the Yellow River Cyclone and mainly comes from the shield clouds of the warm section of the cyclone. There are two transportation paths of water vapor in this heavy rain event: one is from southwest, the other is from southeast. And the water vapor flux from southwest is larger. The west extension and intensify of the west Pacific subtropical high help the transportation of water vapor to North China along the LLJ. The total precipitable water vapor increases obviously in North China before the occurrence of heavy rain. It is also found that the contours of relative humidity and potential temperature are dense and decrease with height over heavy rain region. The atmosphere is of intensive convective instability. From the cross sections of vorticity and vertical velocity, it is found that an obvious positive vorticity is transferred from high level vortex to the Yellow River Cyclone. Strong convergence of water vapor flux and ascending motion are found in the Cyclone, and the ascending velocity ahead of the Cyclone is larger than the subsidence velocity behind the Cyclone. Therefore, the vertical circulation is asymmetry, which improves the rapid growing of raindrops. As it is found in other studies of Yellow River Cyclone, there are meso-β-scale convective systems developping and moving along the southwest low level jet in this case. These meso-scale convective systems (MCS) in the frontal clouds cause the inhomogeneous of the precipitation and produces intensive precipitation centers. And the release of latent heat caused by precipitation has positive feed back to the development of the Cyclone.
Semiannual Oscillation of the Extratropical Circulation in the Southern Hemisphere and Its Numerical Simulation
Gao Hui, Xue Feng
2006, 17(3): 266-272.
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The semiannual oscillation (SAO) of the atmospheric circulations in the extratropic regions is one of the unique characteristics in the Southern Hemisphere. Based on the NCEP/NCAR reanalysis datasets of the sea level pressure (SLP) and the geopotential height from January 1979 to December 2000 the oscillation is studied. Results show that the SAO is most active in the extratropic regions in the lower troposphere but the tropics in the middle-higher level. The extratropical SAO of SLP is found to be most significant in 40°S and 65°S, i.e., the two branches of the Antarctic oscillation (AAO) or named as the Southern Hemisphere annular mode (SAM), where each variance percentage of the half-year period exceeds 70%. That is to say that the anti-phase relationship for the annual cycle of SLP between the two latitudes is primarily caused by their SAO components. But the SAO is not remarkable between these two latitudes, especially in 55°S, i.e., the node position of the standing wave of the AAO. In 55°S, the variance percentage of SAO is less than 20% while that of the annual cycle is greater than 70%. Results also indicate that the phases at the latitudes of 40°S and 65°S are inconsistent, with maximam variance percentages occurring in March and September while minimums in June and December respectively in 40°S, but maximums in January and July and minimums in April and October respectively in 65°S. In other words, the phase of the SAO in the near-polar latitude has a one-month lag than in the middle counterpart.At present, most of the general circulation models (GCMs) are poor in simulating the characteristics of the extratropical SAO in the Southern Hemisphere. So the SAO is used widely as a criterion to verify a GCM's simulation skill. Based on the above results, a nine-level atmospheric general circulation model developed in the Institute of Atmospheric Physics (IAP 9L AGCM) is employed to simulate the SAO. In general, the model is successful in simulating the SAO of SLP along 65°S, with its intensity some what weaker than the observation. On the other hand, the IAP 9L AGCM fails to simulate the SAO along 40°S.
Mean Features of Continuing Heavy Rain Process in Sichuan Basin During 2—6 September 2004
Chen Zhongming, Min Wenbin, Gao Wenliang, He Guangbi
2006, 17(3): 273-280.
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The persistent precipitation which lasts a long period of time, concentrates on an area and induces the most serious disasters is the essential concern of the researchers and weather reporters. Previous researches have mainly paid close attention to the mechanism of the formation and development of heavy rain, but not attached much importance to the analysis of mean features of heavy rain process.A continuing heavy rain process occurs in Sichuan Basin during 2—6 September 2004. The composite analysis is used in order to analyze mean features of this heavy rain process. The composite technique of meteorological data is as the follow: , where F (p, k) represents mean value of meteorological factor at k-station on p-isobaric surface; i represents time level of observation; p represents depth of isobaric surface; k is the ordinal number of meteorological station. Diagnostic analysis on time averaging values F (p, k) of meteorological factors from 20:00 on 3 September to 08:00 5 September 2004 is made. The results show that: ① "Sanda" typhoon moves westward, causing the mainland high cell stable and meanwhile the cold trough moving southeastward along the northeast side of the Tibetan Plateau blocking on the north of Sichuan Basin. The cold trough forms and dissipates repeatedly over this area, creating a favorable environment for the persistent torrential rain. ② The divergence intensity is higher than the vorticity in lower troposphere. Atmospheric motion is characterized by convergence of air current. The relationship between them is quite the contrary in upper troposphere. ③ The vertical coupling of meso-scale convergence belt in low layer with divergence belt in high layer constructs a meso-scale system with long lifespan and deep upward current. ④ The sources of watervapour over heavy rain area are the South China Sea and the Bay of Bengal. The moisture flux transferring to heavy rain area from the South China Sea is more than the Bay of Bengal, but there is not significant difference between them. To analyze the intensity of the moisture transfer from different sources contributes to the reveal of the moisture source of continuing heavy rain process. ⑤ The coupling between meso-scale convergence belt in lower troposphere and meso-scale divergence belt in upper troposphere constructs a deep and long life meso-scale system. It directly contributes to the maintenance of heavy rain. ⑥ Atmospheric motion is strongly unbalanced during heavy rain. Force faction of the unbalance leads to the maintenance of the strong convergence in low layer and is also the main dynamic factor leading to the continuing heavy rain.
Hailstorms Characteristics in Doppler Radar Radial Velocity Fields
Wang Ling, Zheng Guoguang, Kang Yuxia, Fang Wen, Bian Sufen, Xu Huanbin
2006, 17(3): 281-287.
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In severe hailstorms, hailstones always falls accompanied with some other weather disasters such as thunder, strong wind and rainstorm. Generally speaking, different kinds of strong convective weather bring damage and even social influences to different extents. Beijing is one of the areas that hail occurs frequently and suffers from previous damage every year. With the widely deployment of Doppler radars all over the country in recent years, more studies on the local convective weather systems including hails are gradually put into practice. It is well-known that the hail formation relates closely to various atmospheric physical conditions. The formation of those large hailstones to a great extent relies on the changes of wind structure which always has a storm scale. Among the 32 hailstorms occurred in 2001 and 2002 of Beijing, four kinds of weather phenomena are categorized according to their surface observation including hailfall together with wind and rain, hailfall with wind only, hailfall with rain only, and the others. The radar image characteristics and the Doppler radar radial velocity fields from these 32 hailstorms observed in Beijing are discussed in detail. Statistical analysis shows that the characteristics of "gale region" and "mesocyclone" can always be identified from the radar echo images. The "gale region" is an area which displays a maximum Doppler radar radial velocity along the wind direction in radar PPI (V) echoes. The radial velocity is more than 20 m/s. On the front edge of the wind, the velocity grads is so great that the wind direction changes abruptly. The "gale region" is a downwards airflow related to cold air, often stimulates squall line and falls into the category of β mesoscale synoptic system. Observed from radar PPI (V) echoes, the "mesocyclone" is a cyclonic convergence or rotating area in the Doppler radar radial velocity fields. It is easy to be identified from continuous animated radar echoes. Different air flows converge together and form such a special cyclone. With a small scale and short lifetime, the "mesocyclone" belongs to γ mesoscale synoptic system where weather disaster bursts out very easily. In terms of the statistical analysis, the "gale region" observed in the Doppler radar radial velocity fields usually appears with hailfall together with strong wind, while the "mesocyclone" generally indicates the occurrence of heavy rain besides the hailfall. Though the physical pro-cess of hail formation is very complicated and also there are many other scientific hypothesis on the mechanism of hail formation, the characteristics of "gale region" and "mesocyclone" observed in the Doppler radar radial velocity fields indicate the dynamic structure of the atmospheric stream field is in favor of the occurrence of those previous mentioned convective phenomena. And the study can contribute to the forecast of weather phenomena probably occurring in the detected hailstorm. Combined with other observations, it can improve the ability of the nowcasting for the severe convective disastrous weather too.
Large-scale Environmental Characteristics of Heavy Rainfall Before and After Tropical Cyclone Landing in South China
Wang Jiechun, Jiang Jixi
2006, 17(3): 288-295.
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In order to develop some operational forecasting methods in the period of 24 hours for precipitation before and after tropical cyclones landing in South China, the analysis of the relationship between the environmental factors and the rain field is made, using NCEP/NCAR reanalysis data, satellite cloud images and 38 daily precipitation data of 7 tropical cyclones. The regions with daily rainfall amount over 10 mm are selected, especially those areas exceeded 50 mm rainfall caused by TC which are divided into three types: zonal, meridional and NE—SW orientated kinds. The detail analysis on the main meteorological factors related to the heavy rainfall is done by means of diagnosis and statistical methods. The relationship between streamline field and rainfall distribution, the effect of different subtropical high intensity and position, the function of the jet and wind shear at low-level on rainfall zones and the function of the all moisture flux convergence and monsoon cloud surge and so on are investigated. The result indicates that before and after tropical cyclones landing in South China, the rainfall distribution and intensity in 24 hours relate closely to meteorological factors such as streamline field at upper level, the subtropical high at middle level, the low-level jet, the low-level wind shear, the moisture flux convergence, the monsoon cloud surge and so on. On the other hand, the different allocation of these factors leads to different types of rainfall region and different precipitation intensity. The main characteristics of the zonal model can be described as the follows: the subtropical high's position at middle level is more westward, low-level wind shear connects to the center of tropical cyclone, and an easterly jet or southeasterly jet is around tropical cyclone; southwest monsoon cloud surge is northeastward; a divergent center on the west or the south to the tropical cyclone at the upper level streamline field. And the main pointsof meridional model are like these: subtropical high position at middle level is more eastward and its western edge is with a square figure; a wind sheer line is in the direction of N—S or NE—SW through the tropical cyclone center, and a northwestward or northward low-level jet is at eastern side; southwest monsoon cloud surge moves to tropical cyclone; a middle-level trough in westerlies or a subtropical jet is with an N—S axis next to the tropical cyclone at western side. As to the NE—SW orientated type, the subtropical high position is more eastward and its intensity is weaker; a trough in the westerlies at middle level moves towards tropical cyclone, the low-level wind shear line through the center of tropical cyclone is in the direction of NE—SW, a northeastward low-level jet is on the southeastern side. The conceptual models according to the above relationship between the large-scale environmental characteristics and rain field are summarized. Also, some physical causes of the heavy rainfall formation in each are explained. The result may provide a referable method for the very short-range rainfall prediction before and after tropical cyclone's landfall.
Relationship Between the Development of Inertia-gravity Wave and the Movement of Rain-band During Meiyu and Storm Rainfall Period
Chang Yue, He Jinhai, Ding Zhiying, Yang Shuai
2006, 17(3): 296-302.
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Storm rainfall in Meiyu season in 1991 causes great damages to mid-lower reaches of the Yangtze River, and many researchers have done a lot of studies on it. But there are only a few researches on the relationship between the movement of Meiyu rain-band and inertia-gravity wave. NCEP/NCAR reanalysis data and MM5 nonhydrostatic model are used to simulate the precipitation procedure during the period of 00:00 (UTC), July 5, 1991 to 12:00, July 6, 1991, and the result is diagnosed to reveal the inner relationship between the propagation of inertia-gravity wave and the movement of rain-band in this process.The analysis on the meso-scale perturbation field of the storm rainfall in the Meiyu season indicates that the development and propagation of inertia-gravity wave is closely related to the development and propagation of rain-band, and the propagation form of upper inertia-gravity wave is different from the lower one. At the early stage of the rainfall, positive and negative vorticity band lies from east to west in the upper and lower layer of mid latitudes. The positive and negative vorticity in the southern part of rainfall area distributes alternately from northeast to southwest. Typical inertia-gravity character and southward-propagation tendency is also revealed in divergence diagram. The upper inertia-gravity wave propagates southwards later than the lower one, and vorticity and divergence bands distribute like the shape of "8". North-east vorticity and divergence bands propagate southwards, but south-west ones propagate northwards. In the mid-period of precipitation, upper inertia-gravity wave shows the tendency of counter clock-wise rotation around the lower vortex center, at this time, rainfall begins to increase and moves eastwards. Inertia-gravity wave has the character of distributing in the out-of-phase form. The lower layer does not shows the rotation around vortex feature similar to the upper layer. The allocation of upper and lower layers provides beneficial dynamical condition to the long-term maintaining of this precipitation.The main resaosn of the propagating inertia-gravity wave in the upper and the non-propagating inertia-gravity wave in the lower layer is that there are more convective stability and symmetric instability areas in the upper layer, and there are more convective instability areas in the lower layer. And the reason of the easy development of inertia-gravity when it propagates southwards in lower layer but the hard development vice versa is that the southern part of rain area is convective relative stability area, but the northern part is convective instability area. These reasons are revealed in the aspect of equivalent potential vorticity in the study. The convective instability in the lower layer triggers precipitation and inertia-gravity wave. The non-propagating inertia-gravity is firstly produced, then, the convective instability and symmetric instability in the upper layer motivate the production of propagating inertia-gravity. These should be the cause of the production of inertia-gravity rotating around vortex in the upper layer. The inertia-gravity wave propagating northwards in the upper layer is beneficial to the rain band's and vortex's movement northwards. The inertia-gravity wave propagating southwards in the upper and lower layer is beneficial to the formation of many rain-bands.
The Abnormal Activity of the Westerlies System and Its Impacts on 2003 Summer Heavy Rainfall over Huaihe Basins
Li Feng, Lin Jian, He Lifu
2006, 17(3): 303-309.
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The blocking activity and its impacts on heavy rainfall over Huaihe Basins during the period of Meiyu-season in 2003 is studied based on NCEP/NCAR daily reanalysis data and observation. The results show that the blocking activities are very remarkable and the evolution of blocking can be divided into three phases. When the blocking establishes and during its maintenance, it leads westerly front to move southwards as well as cold air consequently bursting out to south. As a result, cold and warm air mass and energy exchange frequently in midla titude zone. Especially, the blocking in the west part of Baikal is getting more active, intensifies and moves southwards, thus, energy front and subtropical front maintain stably over the north of Huaihe Basins after 20th of June. Atmospheric momentum and heat diffuse to the south is led by the blocking, and atmospheric negative disturb momentum transfers to mean kinetic energy over the Huaihe Basins, so westerly jet is strengthened, which is favorable for the subtropical high's maintenance and persisting rainfall over this region.Statistical results indicate that the intensity and location of the blocking high are related closely to the low eddy occurred in front of it. The movement of the blocking system in zonal and meridional directions are in agreement with the latter, their correlation coefficients are 0.60 and 0.65 respectively based on the daily data. Their intensity variations are also in close relation, the blocking's strengthen and weaken are corresponding to low pressure eddy's deepening and weakening. The low eddy intensity variation and its horizontal movement have an impact on the position of the subtropical high ridge, the low eddy's strengthening/weakening leads to the subtropical high's southwards/northwards movement, similarly, the low eddy's movement to the west/east agrees with subtropical high's north/south shift.Additionally, results show that the blocking activity can trigger Rossby wave fluctuation in longitude direction so that to affect the subtropical high by changing westerly basic flow and helix configuration of long wave. Especially, during the beginning and the end of Meiyu, long wave pattern evolves gradually to northwest-southeast feature as the guidance in Asian mid and high latitudes. At the same time, basic flow changes remarkably companying the configuration of long wave, which leads the Rossby wave spreading to high-latitudes, atmosphere with high geopotential in lower and middle latitudes transmits to north companying the Rossby wave, thereby, subtropical high undergoes seasonal jump to north and rainfall belt shifts to north. On the contrary, rain belt moves several times southwardly during the period of Meiyu season, which are mainly because long wave pattern in mid-high latitudes evolves gradually to northeast-southwest festure or the basic flow changes reversely, and the Rossby wave spreads southwards, the atmosphere with low geopotential in mid-high latitudes transmits to south, forcing the subtropical high to recede southwards.
Numerical Study on the Effects of Persistent Cold Sea Surface Temperature on the Northwest Pacific Ocean in Atmosphere Circulation During the Early Summer in East Asia
Yuan Jianshuang, Zheng Qinglin
2006, 17(3): 310-315.
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Ocean as a large heat resource, its persistent sea surface temperature anomaly (SSTA) during the winter half year stirs upper air heating anomaly over it, contributing to atmospheric circulation variation in East Asia region in early summer. By employing the improved NCAR/CCM3 climate model, and designing two experiments which use different sea surface temperature anomaly (SSTA) over northwest Pacific ocean (6.66°—51.11°N, from East Asia western coast to about meridian) , using real monthly SSTA in Exp-A and climate mean SSTA in Exp-B, study is undertaken of the effects of cold SSTA on circulation transformation in the early summer in East Asia in 1992. This model considers well the influence of Tibetan Plateau topography, and the surface roughness length is improved in the context of new TIPEX data, thereby improvs the long term prediction and simulation ability over East Asia region. The investigations indicate that persistent cold SSTA from winter to early summer in 1992 over northwest Pacific ocean is significantly unfavorable for south branch westerly jet's northward seasonal shifting and Tibetan Plateau temperature gradient backspin from winter to summer, exhibiting the temperature decrease of 0—1 ℃ in the north side and increase of 1—2 ℃ in the south side of Tibetan Plateau at early summer, maintaining a west wind center of maximum speed of 5 m/s, 2 m/s on 200 hPa and 500 hPa respectively, resulting in the south branch westerly jet's persistence and enhancement. The cold SSTA in the northwest Pacific ocean reduces the heat release into the atmosphere, inducing the low trough's obuious deepening and the average geopotential height in east Asia dropping by 4.66 dagpm, therefore the cold air in the rear of the cold front invades the lower latitude area so the south branch of the front is southing, and is unfavorable for the west Pacific subtropical high's westwards move and strengthening. It also makes surface mean temperature at early summer decrease by 1—2 ℃ in central China or south China, especially decreasing by 2—5 ℃ at northeast China. Thus cold SSTA in northwest Pacific ocean is an important factor for colder summer in northeast China. The experiment results suggest that those phenomena occurring in early summer in 1992, such as the early or late arrival of the Meiyu in the Yangtze-Huaihe River Basins and scarce rainfalls in the rainy season, are in great relation to the cold SSTA in the close vicinity of East Asia land.The result shows that anomaly heating by SSTA in winter half year over northwest Pacific ocean plays an important role in the circulation variation in early summer. Persistent cold SSTA over northwest Pacific ocean can trigger changes of the temperature over Tibetan Plateau, and then of the westerly jet and long wave trough along Asias east coast, also of the sub-tropical high over western Pacific. Those changes are the main background of the planetary scale flow pattern transforming in North Hemisphere from winter to summer. And then, by influencing surface temperature, it affects the circulation transform in East Asia.
Evaluation of Quantitative Precipitation Forecasting of Multiple NWP Models in Summer of 2004
Wang Yu
2006, 17(3): 316-324.
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With the fast development of numerical weather prediction (NWP) technology, the quantitative precipitation forecast of NWP models has become a major basis of everyday operation weather forecast in most countries all over the world at the moment. The precipitation forecasts of five NWP models, including T213L31, HLAFS 0.25, Huabei-meso-MM5, a Germen global model and Japanese global model which are widely applied in National Meteorological Center and other weather prediction offices in China, are verified and assessed synthetically by the spatial distribution of seasonal mean daily rain rate, the time series of regional mean rainfall rate and statistic verification method. The result of the verification shows that there is certain spatial forecast capability for the quantitative precipitation forecasting of short term (for forecast durations of 24/36~48/60 h at 24-hour intervals) by NWP models at present, but there are some biases or errors for position of heavy rainfall center or rain belt. With the forecasting valid time increasing, the errors of rain belt increase. Most models often overestimate the rainfall in the western part of China or underestimate the precipitation in the eastern part of China. Through the verification of the time series of regional mean daily precipitation, it is found that there is more obvious forecasting capability of NWP model for the developing trends of heavy rainfall processes in some regions in the eastern part of China than ordinary processes, and the forecast of the middle and lower reaches of the Yangtze is the best. However, there are some errors for the quantity of forecast precipitation against the observation rainfall. For example, MM5 model often overestimates the rainfall in South China while HLAFS model usually overestimates the precipitation in Southwest China. According to the results of accumulative verification, there are not much differences for Ts of light rain and moderate rain among five models. The global models' Ts are higher than those of regional models, and the forecast by Japanese model is the best among the five models forecast for the two classes. But for heavy rain or more severe rain, the performance of domestic models is super to foreign models. There are unique advantages of MM5 and T213L31 model for different valid time or rainfall classification, but for quantity and area of forecast rainfall are greater than those of observation.
Data Warehouse and Its Potential in Weather Forecast
Tan Xiaoguang
2006, 17(3): 325-332.
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An important problem of current forecaster's forecasting platform is that although the system provides lot of data (over 2GB, several thousand weather fields data one day) forecasters only use a few of them (less than 1%) in operational forecast. And how to enable the system to have a flexible data management ability for forecasters to efficiently use historical data is another important issue. Data warehouse is a good solution to these problems. The data warehouse is a subject-oriented, integrated, time-variant and non-volatile collection of data in support of management's decision making process. The data in the warehouse are processed ones called "analytic data" correspond to original operational data:"Subjects" are defined as objects to be analyzed in weather forecast, e.g. those concepts in forecaster's experiences. "Analytic data" are referred to as the real values corresponding to the "subjects" transformed from original operational data according to the definitions of subjects (for weather forecast, transformation based on the weather system is of the most importance). By creating a subject system from the concept set of forecaster's knowledge, defining the data transformation to change operational data into analytic data for each subject in the subject system, running the transformation program on operational data every day to get real-time analytic data and save them in a database, a data warehouse is built. Data warehouse is a data set of "analytic data". In this way, from concept to subject to analytic data, the data used in analyses directly match the concepts in forecaster's mind, and make data analysis more quickly and use more data in operational forecast.There are two important analysis tools in data warehouse. Data Mining (DM) is an exploring tool. The relationships among the subjects (i.e., concepts from forecasters) are automatically explored from the analytic data set in DM system. The resulting relationship is saved in the knowledge base of data warehouse, and reinforce forecaster's knowledge. Mining of association rules is noteworthiness because sometimes it is more reasonable than linear regression analysis. On-line analysis process (OLAP) is another analysis tool, an interactive validating tool. Forecasters use it to view data, validating relationships (including forecaster's guess and results from DM) and then make forecast decisions. The kernel technology is multi-dimensional analysis. Especially for weather forecast, "Compare analysis", "Multi-analysis" and "Analog analysis" based on multi-dimensional analysis are also used in OLAP. OLAP will be the main workbench for forecasters in data warehouse.In data warehouse, metadata is used. Data management and maintenance become easier and flexible, historical data and heterogeneous data such as special observation data, even Internet data, will be easier to use by applications. The bottleneck of traditional knowledge base system is knowledge acquirement. In data warehouse, forecasters put their concepts into subject system of data warehouse firstly, then get relationships between concepts from DM (or manually input some certain relationships) and validate them by OLAP. The knowledge of forecasters will be systematically used in forecast process, and the bottleneck problem will be moderated.
Radar Echoes Characteristics of the Sudden Convective Rainstorm over Beijing Area on July 10, 2004
Chen Mingxuan, Yu Xiaoding, Tan Xiaoguang, Gao Feng
2006, 17(3): 333-345.
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An intense thunderstorm attacks Beijing and brings sudden severe rainstorm over local Beijing urban area in the afternoon of July 10, 2004. An analysis of the paroxysmal rainstorm characteristics is made using weather observation data, radar echo data of CINRAD/SA S-band at Tanggu of Tianjin and a thunderstorm identification, tracking and analysis algorithm. The analysis illuminates the clear characteristics of the strong convective rainstorm which is of small extent, high precipitation, paroxysmal initiation and development, and slow motion. The evolution and motion properties indicate that the convection has direct and close relation to the large scale warm southerly airflow with abundant water vapor characteristics. The convective cloud clusters are gestated and given birth in warm and wet southwestern air stream. The cloud clusters rapidly intensify when they arrive in Beijing area, then gradually a super cell storm comes into being with meso-β scale over Beijing urban zone and result in a heavy rainfall. The detailed analysis results show the thunderstorm cells come from two different directions into the city area, southwest and southeast respectively, produce the strong convective rainfall. The southwest thunderstorms gradually move and develop northeastward near the urban zone, and merge and reinforce with newly inspired storm cells, bring a heavy rainfall in Shijingshan, Mentougou and Haidian districts of Beijing. The two small cells born at southeast of Beijing develop rapidly and gradually move northwestward into urban area. They merge into a storm when reaching urban zone, then the storm quickly strengthens. However, the storm has very low velocity and hovers and maintains near 2 hours just over Beijing urban zone. The storm pours a great deal precipitation into urban zone in only 2 hours. But the precipitation is not evenly distributed over urban zone. Statistical analyses and storms tracking on the evolution characteristics of the convection are performed by using Tianjin radar data and automatic rain gauge data of Beijing area. During the course of the heavy rain, the convection has an evolution characteristic from intensifying to weakening, then intensifying again. There is an evidently out-of-phase between the top of the thunderstorms and the height of maximal reflectivity. The height of maximal reflectivity is less than or close to the height of 0 ℃ isotherm, and the heights of centroid and reflectivity-weighted centroid for the whole storm are also less than the height of 0 ℃ isotherm on the whole. Therefore, the results confirm the convection can only give birth to liquid state rainfall without any hail. That is consistent with the fact that there is not any surface hail log. Another analysis, based on relative humidity from soundings at Beijing Observatory and radar data comparison between Tianjin S-band and Beijing C-band, indicats that these strong radar echoes of Tianjin radar at about 20:00 Beijing local time are resulted from anomalous propagation (AP), so they are inveracious superrefraction echoes.
Influence of Moist Schemes in MM5 on the Uncertainties of "03·7" Heavy Rainfall Numerical Simulation
Wang Hongli, Zhang Man, Liu Ming
2006, 17(3): 346-353.
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The weather forecast produced by numerical weather prediction models has been an important means in the operational forecast, however there exists intrinsic uncertainty. This uncertainty is caused by both the ones in the initial conditions and the ones in the numerical model itself. In the process of the numerical prediction, it is important to consider these errors since the nature of the atmospheric dynamics is to amplify errors originating from either of them. At present, with the application of the massive non-conventional data, such as satellite and radar data in the assimilation system, it is possible to produce the relatively more accurate initial fields. Thus, the uncertainty of numerical model may be more important to forecast especially in the various moist physical process schemes. In recent years, with the ever-improving performance of numerical prediction models and ever-increasing computational resources, the explicit schemes are used to forecast the strong precipitation process in the high resolution mesoscale models gradually with hybrid approach. However, what is the role of explicit schemes and what extension they lead to the Meiyu front heavy rainfall forecast uncertainty? All these issues are worth being further studied. In terms of non-hydrostatic MM5, the impact of explicit moist schemes on the uncertainties in the numerical simulation of the "03·7" Nanjing heavy rainfall is investigated. According to the simulated 24-hour accumulated rain spatial pattern and Ts skill of the variance experiments, the uncertainty depends on the explicit and implicit schemes' relative coordination and sensitivity when the hybrid approach is used to describe the cloud and precipitation process. The implicit schemes determine the spatial pattern of the rain bands, while the explicit schemes adjust the local rain pattern and density. The extent to which the rain band adjusted is related to the convective parameterization. The difference in the rates of resolve scale rainfall to total rainfall in different experiments indicates the uncertainty due to explicit schemes. At the same time, in the center of the simulated strong precipitation region, for the convective parameterization of Grell and KF2, the resolve resolution precipitation of the explicit schemes accounts for the 1/3—2/3 of the total rainfall; for the BM, the unresolved rain dominates the total accumulated rainfall. It indicates that, the explicit schemes play an important role in the precipitation simulation for Grell and KF2 with the hybrid approach, on the other hand, it is inhibited for the BM schemes. Thus, the uncertainties of explicit schemes should be considered when implicit scheme Grell or KF2 is used to simulate the heavy rainfall.
The Climatic Background and Medium-range Circulation Features of Continuous Torrential Rain from April to June in Guangdong
Xie Jiongguang, Ji Zhongping, Gu Dejun, Liang Jian, Zeng Cong, Xiong Yali, Lin Zhenguo
2006, 17(3): 354-362.
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Abstract:
To better understand the cause of medium-range continuous torrential rain and forecast its inducing flood from April to June in Guangdong, the climatic background and medium-range circulation characteristics of continuous torrential rain happens in the target season during 1961—2001 in the Guangdong Province are analyzed using statistic and composite analysis. It shows that the continuous torrential rain has the highest probability (47%) in June and lowest probability (13%) in April. The wavelet transform and maximum entropy spectral demonstrate that the continuous torrential rain in Guangdong exhibits periodic oscillation of about 2—5 and 10 years, and the number of the continuous torrential rain will increase in the coming years. The medium-range circulation characteristics and evolving circulation features of high-and low-latitudes prone to Guangdong continuous torrential rain are analyzed with the pentad mean 500 hPa geopotential height. The two indexes of mediumrange circulation characteristics favorable for Guangdong continuous torrential rain are summarized. Their common feature is that the very stable blocking high in the west and blocking high in the east happen in the midhigh latitudes. The superposition between the east blocking high in the mid-high latitude and the positive anomalies east of 120°E in the low-latitudes make the pattern of east high and west low very steady. At 850 hPa, the significant composite pentad southwest wind belt in 10°—25°N, 80°—115°E provides water vapor for continuous torrential rain, from which the water vapor of the Bay of Bengal is continually transported to South China. The positive vertical velocity center is located over Guangdong Province in composite pentad vertical velocity field at 850 hPa. All these conditions are in favor of continuous torrential rain.Based on the above results and pentad mean 500 hPa geopotential height from daily prediction products of the European Center for Medium-Range Weather Forecasts (ECMWF), medium-range forecasting of continuous torrential rain in the Guangdong Province is made by dynamic-similarity diagnostic weather analysis and interpretation of Ki, Si, depression of the dew point in 850 hPa and other physical factors from numerical simulation according to forecasting experiments for precipitation. The accuracy is 12/17 (71%) for 17 forecasted continuous torrential rains process from April through June during 2003—2005. This demonstrates that the method has good reference for operational forecasting of continuous torrential rain.
Variational Data Assimilation of Satellite Radiance to a Mesoscale Rainstorm Simulation
Huang Bing, Liu Jianwen, Zhong Zhong, Bai Jie
2006, 17(3): 363-369.
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Abstract:
With the development of the numerical weather forecasting technology, the models are becoming more and more mature, such that the quality of forecast depends more on the initial conditions. But the conventional observation is about 200 km away to each other, it can not describe the initial condition of atmosphere well to meet the need of numerical model. To improve these conditions with ATOVS satellite radiance data some researches use T213 products as the background based on GRAPES 3D-Var and MM5 models. The GRAPES (Global and Regional Assimilation and PrEdiction System) is a new generation hydrostatic/non-hydrostatic, multi-scale numerical model of China developed by Research Center for Numerical Meteorological Prediction of CAMS (Chinese Academy of Meteorological Sciences), CMA. GRAPES 3D-Var is the assimilating part of the system, it can assimilate many kinds of observations if the observation operator and observation error matrix is provided. MM5 model is the mesoscale prediction system developed by NCAR/PSU, it can be used for real-time studies, simulating or predicting the weather system such as mesoscale convective systems, fronts, land-sea breezes, mountain-valley circulations, and urban heat islands. A heavy rainstorm occurs in the middle-lower reaches of Yangze River and in the east of the southwest China from 22 to 24 June 2004. It brings serious disasters to the people in and out Hunan Province because of mud slide and flood caused by the rainstorm. So it is very urgent to pay attention to the research of such rainstorms as to mitigate the damage. The ATOVS detector of NOAA satellite consists of HIRS, AMSU-A, AMSU-B channels, of which the radiance of AMSU channels can penetrate the cloud to reach the satellite, so the AMSU radiance contain much information in the cloud which is very important to the initial condition of numerical model. In order to identify the effect of numerical weather prediction by assimilating satellite radiance data, two experiments are carried out by use of the GRAPES 3D-Var assimilation system and the MM5 numerical prediction model to simulate the storm, the NOATOVS experiment only assimilates the conventional sounding data, the ATOVS experiment simultaneously assimilates the conventional sounding data and the ATOVS radiance data. The assimilation results show that directly assimilating ATOVS radiance data can improve the temperature, humidity and wind fields within the troposphere. The temperature increment in the lower troposphere in the mid-lower reaches of Yangtze River and Korea peninsula is positive, the humidity increment in the sea area is positive. The assimilations are applied as the initial field of the MM5 model for prediction, which then undergo analysis for comparison. The comparisons between both the ATOVS and the NOATOVS experiments show that ATOVS experiment can not only simulate the weather situation well, but also describe the intensity and the location of the rainstorm better than the NOATOVS experiment. All these analyses imply that the simulations of mesoscale convective system and heavy local rainstorm may be improved to some extent through assimilating satellite radiance data directly, but further case studies are needed to verify it.
Analyses on a Tornado Event in Front of a Typhoon
He Caifen, Yao Xiuping, Hu Chunlei, Quan Caifeng
2006, 17(3): 370-375.
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Abstract:
By employing the basic and derived data sets from the products of the new generation Doppler weather radar and the conventional weather observation data sets, a tornado event in front of a typhoon happened in Gaoqiao town, Zhejiang Province on 25 August, 2004 is synthetically analyzed. The results show that the intensity of this tornado belongs to the F1 tornado intensity category. And its generation and development has a close relationship with the strong wind shear in front of the typhoon. The vertical humidity change (greater in the lower level and less in the upper level), the significant wind vertical shear and the topographic impact play an important role in the occurrence of the local tornado, though the vorticity and the divergence patterns may not be very essential for the deep convective development. The analysis of the Doppler weather radar output products also indicates that the storm has a life span lasting for more than 3 hours, with its echo top extends the height of less than 6 km. And the tornado locates on the edge of the strongest echo gradient region near the V-shaped weak echo region which means that the strong updraft contributes most to the tornado. It can be discovered that quite a few radar products can help show the occurrence of the typhoon, such as the significant hook echo pattern with 60 dBz reflectivity intensity, microcyclone characteristic in the products of storm-relative mean radial velocity map in all four volume scans, with a maximum composite shear approaching to 189×10-3 s-1 and so on. But the warning time is only twelve minutes in advance of the occurrence of the tornado and the weak microcyclone has a several-kilometer scale in the vertical direction. Moreover the analysis of the 3-dimentional characteristic of the tornado reveals that the tornado is neither a typical supercell tornado nor a non-supercell one, it is a mixed tornado of the above two kinds. The tornado may develop while a vortex meets with the cyclonic region resulting from the wind vertical shear and the ascending area coming from the convergence effect and rotates acceleratedly. The tornado develops in the edge of the local vortex source in front of the downdraft while the rotation and updraft enhance owing to the great wind vertical shear and the effects of the buoyancy and the helicity. Therefore, the analysis is condutive to the tornado forecast in the future.
The Variation of Snow-cover and Snow Disaster in Qinghai During 1961—2004
Shi Xinghe, Li Fengxia, Zhaxi Cairang, Guo Anhong, Da Chengrong, Tang Hongyu
2006, 17(3): 376-382.
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Abstract:
Using the grid data of 500 hPa height field which is provided by the Natoinal Climate Center and the ground meteorological observation data of seven meteorological stations over the eastern Haixi and around Qinghai Lake (36°~38°N, 96°~101°E) during 1961—2004, the snow disaster's annual table and ground snow sequence is collected, the circulation characteristic of the snow disaster occurred in snowy year and none-snowy year over the eastern Haixi and around Qinghai Lake is analyzed, and based on the climatic diagnosis and the mathematical statistics method the interannual variation characteristic and the generating mechanism as well as the cause of snow disaster are studied. The result shows that the probability of the occurrence of the regional snow disaster over the eastern Haixi and around Qinghai Lake is 15.9% (7/44), while the probability of the occurrence of the local snow disaster is only 9.1% (4/44). Among the eleven snow disasters the probability of snow disaster is 91% (10/11) in later winter to spring (from January to April), and the probability is 9% (1/11) in early winter (from mid October to December). The slowly increasing of the accumulated snow-cover amount over the eastern Haixi and around Qinghai Lake in winter in the 44 years is prone to snow-cover and low temperature damage. The primary influencing system of precipitation over this region is the plateau trough, Mongolia trough and plateau low cyclone (at the same time accompanying convergence line and shearing line), i.e., when the 500 hPa polar vortex center of the north hemisphere from winter to spring is leaning against west/east hemisphere, or the height anomaly of Tibetan Plateau and eastern coast region forms into anomaly distribution with "low in west but high in east (or high in east but low in east)", there has more (only less) snow over the eastern Haixi and around Qing hai Lake. The variation trend of the snow-cover amount in winter and spring in the future 10 years is corresponded with the variation in 1990s, and it also keeps increasing. The snow disasters in later winter to spring over the eastern Haixi and around Qinghai Lake have relationships not only with the local weather and changing climate, but also with the local landform.
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