Plateau Meteorology

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28 August 2009, Volume 28 Issue 4   
  • Anomaly Features of Temperature over Qinghai-Xizang Plateauin Summer and Pacific SSTA in Winter and Its Correlation Analysis
  • 2009 Vol. 28 (4): 711-722. 
  • Abstract ( ) PDF (9419KB) ( )
  • With EOF, REOF and SVD, the spatial anomalyfeatures of both the North Pacific SSTA and the summer temperature over Qinghai-Xizang Plateau and their correlations are studied by using the air temperature data at 59 observational stations in summer for the period 1971-2004and the North Pacific sea surface temperature of 286 grid points in the areas of 10°S~50°N, 120°E~80°W in winter from 1970 to2003.The characteristics of the North Pacific SSTA and the temperature anomalies over Qinghai-Xizang Plateau in summer among the decades have also been analyzed. The results show that there are four main spatial patterns of the North Pacific SSTA in winter. The first pattern is a seesaw between the Equatorial Mid-East Pacific and the Northwest Pacific; the second pattern is a variation in the opposite phasesbetween the eastern and the westernregions of the Subtropical Pacific; the third pattern is a seesaw between the southern and the northernparts of the North Pacific; the fourth pattern is consistently higher(lower) than normal. Among the above patterns, the first one is the the main feature of the spatial pattern of North Pacific SSTA in winter. The sea surface temperature of the North Pacific in winter can be divided into six sub-regions: The Equatorial Mid-East Pacific, the area of California Current, the Kuroshio district, the Oyashio district, the Alaska Current and Mid Pacific. There are three main spatial patterns of the temperature over the Qinghai-Xizang Plateau in summer. The first pattern is consistently higher(lower) than normal; the second pattern is a seesaw between southern and northern regions; the third pattern is an opposite trend of variation between the border and the hinterland. The temperature over the Qinghai-Xizang Plateau in summer can be divided into four sub-regions: the Northeast Plateau, the southeast part of Tibet, the Middle of the Plateau, and the southern border of Tibet. Clear teleconnections really exist between the SSTA in east equatorial Pacific in winter and the next summer temperature on Qinghai-Xizang Plateau. When the SSTA is high(low), that is to say, when El Nio (La Nia) appears in winter， the temperature over Qinghai-Xizang Plateau is low (high) in the next summer.
  • Analysis of Turbulent Characteristics in the Surface Layerin Litang Region on the East Edge of Tibetan Plateau
  • 2009 Vol. 28 (4): 745-753. 
  • Abstract ( ) PDF (4711KB) ( )
  • A comprehensive field experiment of atmospheric boundary layer over grassland surface is introduced. The experiment has been held in Litang region on the east edge ofTibetan Plateau by IPM, CMA (Institute of Plateau Meteorology, China Meteorological Administration) since November 2005. Some micrometeorological characteristics of windspeed,wind direction, atmospheric stability, and turbulent intensity, variance similarity, surfacefluxesin the near-surface layer are analyzed and compared based on the observational data acquired by using the eddy correlation method in January and July 2006 (representing winterandsummer,respectively). The main results are as follows: Atmospheric stability mostly varies from -0.5 to 0.5 in January, from -0.25 to 0.25 in July. Turbulence is very active when wind speed is less than 2 m·s-1 and decreases rapidly as wind speed increasing. The normalized variance of three-dimensional winds and z/L satisfies the similarity law under both unstable and stable stratification, their universal functions also can be fitted according to the “law of 1/3 fractional power”. The diurnal variations of surfacefluxes are evident, and sensible heat fluxis leading, latent heat fluxis very weak in January while reversely in July.
  • The Observational Analysis of Atmospheric Boundary LayerStructure in East Part and Downstream of Tibetan Plateau
  • 2009 Vol. 28 (4): 754-762. 
  • Abstract ( ) PDF (5524KB) ( )
  • The atmospheric boundary layer (ABL) structure in east part and downstreamof Tibetan Plateau(TP) was investigated utilizing the intensifying radiosonde data in December 2007. The results indicate that there is a surface-based temperature inversion layer (SBTIL) with an average heights of 500 m at nighttime, and a mixed layer (ML) with the heights of 2000 m in east part of TP in winter. The water vapor and wind speed were well mixed in the ML, and there is a deeply humidity inversion layer (HIL) when the ML was matured. The diurnal range is smalland the SBTIL trap water vapor in surface layer (SL), and the water vapor may be saturated before sunrise in Sichuan Basin where the downstream of TP. At daytime, the ML will mature in the midday and its height is only 300 m at the average level. There is a stronger temperatureinversion layer at lower troposphere(TILLT) over Sichuan Basin, which was produced by the effect of elevated heating cased by TP and the west wind in winter. The variations of the TILLT arise from the diurnal variations of air temperatureat ABL in east part of TP and local westerly. The vertical distribution of atmospheric momentum, heat and water vapor were strongly changed by the TILLT. The change of radiation caused by the stratus depended on the TILLT is the main reason why there is a small diurnal range and lower ML height.
  • Analysis on Heterogeneous Underlying Surface Roughness Lengthin Semi-Arid Mountains Area of Loess Plateau, China
  • 2009 Vol. 28 (4): 763-768. 
  • Abstract ( ) PDF (1671KB) ( )
  • Using the observational data from semi-arid climate and environment observatory of Lanzhou University meteorological observation tower which contained wind speed, wind direction, temperature, pressure, humidity data from April 17, 2007 to April 16, 2008. The surface roughness length at the observation station in the neutral atmosphere condition was calculated with classic wind profile and wind speed standard deviation methods, meanwhile, the data analysis obtained the spatial and temporal features of the surface roughness length in Loess Plateau geographical environment. The results show that the impact of seasonal variation on the roughness length can rate up to 0.159 m and the impact of heterogeneous underlying surface on the roughness length can rate up to 0.155 m. The surface roughness length near the observation station is: 0.017 in spring, 0.662 m in summer, 0.065 in autumnm and 0.018 m in winter. The roughness of northwest direction at station reaches 0.17 m in spring, 0.22 m in summer, 0.34 m in autumn and 0.05 m in winter. The roughness of southeast direction at observation station reaches 0.11 m in spring, 0.17 m in summer, 0.19 m in autumn and 0.05 m in winter. The best selection of data for roughness calculation is wind speed 6±1.5 m·s-1 and the wind direction changes within 30° in this station.
  • The Circulation Indices of Polar Vortex on 10 hPa inNorthern Hemisphere and Preliminary Analysis
  • 2009 Vol. 28 (4): 777-785. 
  • Abstract ( ) PDF (5038KB) ( )
  • A set of circulation indices are definedand calculated to characterize monthly mean polar vortex at 10 hPa geopotential heights in the Northern Hemisphere, including area-(S)， intensity-(P) and centre position-(λc, φc)-indices by use of 10 hPa 1948-2007 NCEP/NCAR reanalyzed value monthly height data. These indices seriesare used to investigate the seasonal variation and interannual anomaly of polar vortex, together with the relation to global warming, ozone anomaly and AO. The results are as follows: (1) there is anticyclonic(cyclonic) from June to August(from September to March). The change of autumn circulation pattern is slower than spring. (2) P can be replaced by Sin analysis due to the interannual synchronal variations of the intensity and area for polar vortex. The interannual(interdecadal) variation of Panomaly is significantly in January(July). (3) The anomalies of system centre position in January are more evident than in July. (4) The variationsof mean temperature at mid-stratosphere in the vicinity of pole zone in January are different from in July, but they are synchronal withcorresponding P and not significantly correlative with the trend of global warming. However the relationship betweenPand total ozone in Julyare obviously. (5) There is so notable correlation between Pand AO that P can represent AO.
  • Analysis of a Gust Front Process in EarlySummer of Nanjing Region
  • 2009 Vol. 28 (4): 836-845. 
  • Abstract ( ) PDF (12659KB) ( )
  • Gust front is often produced by mesoscale convective system and produce low altitude wind shear that can be hazardous to aircraft operations. A gust front, produced by a severe mesoscale convective system happened in Nanjing on 29 June 2006, was examined . The gust front structure was analyzed with single Doppler radar data and surface mesonet data collected in Nanjing region. When gust front occurred, some physical features were obviously observed by surface mesonet, including wind direction change, wind speed increase, pressure intensifying, temperature descending, and then the rainfall. These were compatible with classical gust front theories. Refraction was figured out by using pressure, relative humidity and temperature. This parameter zoomed when gust front arrived, and dropped slowly when gust front moved away. Because rearward of convective system after the leading edge was very moist, refraction kept high for a time. The reflectivity and echo height of gust front were stronger than those of both sides resulted from radar observation data. The distance between gust front and leading edge extended from 5 to 10 km. Wind field simulated from radar observation radial velocity showed faster wind at upper level than at low level, and the center of large velocity area at upper level was anterior. Rear flow could been seen from the simulated wind field at low level. The feature of gust front showed in radar observation was obviously intensity composed shear line. Using this feature, an auto detection method was designed, which could availably detect gust front. Combinatorial shear line indicated the development and track of gust front.
  • Impacts of Climatic Changes in Tarim River Basinin Past 45 Years on Water Resources
  • 2009 Vol. 28 (4): 854-860. 
  • Abstract ( ) PDF (2323KB) ( )
  • Effects of climatic changes (precipitation and temperature changes) on water resource in the Tarim River basin were analyzed based on the daily precipitation and temperature dataset at 40 meteorological stations and annual streamflow data at 7 hydrological stations in 1961-2005 and 1957-2005 respectively. Trends within hydro-meteorological series were analyzed by using the nonparametric Mann-Kendall trend test (M-K). The results indicated that: (1) temperature in the Tarim River basin is increasing, especially in autumn; (2) precipitation in the Tarim River basin is increasing in the past years. However, only the precipitation in the Tianshan area is significant increasing trend. M-K test results of annual streamflow seires indicated that annual streamflow of the headwater is not significant trends, except for the Akesu River. The streamflowof Akesu River is in significant increasing trend. Due to the fact that the sources of the river streamflow in theTarim River basin hearily depends on the precipitation and melted glacial water, therefore, increasing streamflow is the result of both the precipitation and melting ice increasing. These results are of practical and scientific merits in basin-scale water resource management under the global warming scenario and regional response of climatic changes to global warming in the Tarim River basin.
  • Characteristics of Outgoing Longwave Radiation Related to TypicalFlood and Drought Years over the East of Southwest China in Summer
  • 2009 Vol. 28 (4): 861-869. 
  • Abstract ( ) PDF (7600KB) ( )
  • The abnormal OLR features of the typical flood and drought years in summer are analyzed based on the global daily and monthly OLR grid data from 1979 to 2006 and the daily precipitation data at 20 stations in the east of Southwest China from 1959 to 2006. The rainfall indexes show that the summer precipitation in 2006(1998) was the least(most) one since 1959.The OLR distributions represent obvious difference between the extreme drought years and flood ones. The OLR values were lower(higher) than normal over the area from the eastern Tibetan Plateau to Changjiang-Huaihe valleys, and also so over the south of Bengalbay and the east Indian ocean near equator areas, but higher(lower) over the Philippinesand its surrounding region in flood(drought) years. The convection activities over the east Indian ocean near equator areas in extremely severe flood years were stronger than those of drought ones, but oppositeover the Philippinesand its surrounding region. The area from the eastern Tibetan Plateau to Changjiang-Huaihe valleys including the east of Southwest China prevail ascending flow in summer of flood years. The main movement features of tropicalITCZ in flood years were westward whereaseastward in the early summer of drought years. The northward features were obvious of convection over the south of Bengalbay and the east Indian ocean near equator areas in flood yearsand it arrived to the north of 30°N after the middle dekad of June. In the area the convection presented south-north oscillation and break sporadicallyand it arrived to 30°N rarely. The key area OLR represents low frequency period about 40 d and quasi-bi-weekly period about 12～15 d duringsome days. The OLR 40 d low frequency convection made stronger(weaker) and resulted in more(less) rainfall in summer of flood(drought) years throughmeridional and zonal propagation.
  • Analysis of Changes in Different-Class Precipitationover Eastern China under Global Warming
  • 2009 Vol. 28 (4): 889-896. 
  • Abstract ( ) PDF (2568KB) ( )
  • Daily precipitation data simulated byhigh-resolution version of MIROC（MIROC_Hires， Model for Interdisciplinary Research On Climate) is used to analyze changes in different-classprecipitation characteristics over eastern China under global warming.The results show that the MIROCmodel can well reproducenot only the spatial distribution but also the seasonal changes of precipitationover eastern Chinain 1971-2000. Since in the whole eastern China, the precipitation intensity differs much from south to north, characteristics of precipitation in South China, Yangtze-Huaihe Basin, North China and Northeast China are analyzed, respectively. Comparing with the quantityand frequency of different-class precipitation between the period of 1971-2000 and 2071-2100, there are siginficant changesin the four regions. In South China, increase in frequencies of non-precipitationand heavy (≥24 mm·d-1) rainfall days is significant, up to 13.8 d·a-1 and 4.2 d·a-1, respectively, and those of relatively weak (1~12 mm·d-1) rainfall days decrease. In Yantze-Huaihe Basin, the mean annual precipitation averaged over the region increases while frequencies of heavy rainfall days increase by 3.6 d·a-1 and those of relatively weak rainfall days decrease. In North China, the increase of mean annual precipitation is the most significant in the four regions, up to 30.5%, while frequencies of non-precipitation increase. In Northeast China, though the rainfall intensity in present days is not so much as in the other three regions, the frequencies of rainfall days increase either. As the rainfall intensity increases in the four regions, events of the extreme rainfall days are supposed to increase.
  • Analysis on Impact of Terrain on Extremely Rainstormin Southern Shanxi under the Background of Block High
  • 2009 Vol. 28 (4): 897-905. 
  • Abstract ( ) PDF (3663KB) ( )
  • Take a rainstorm occurred in Yuanqu county, Shanxi from 29 to 30 July 2007 as a case, based on the observed data, automatic meteorological data, and GMS infrared satellite image, the development characteristics of terrain vertical velocity, shear vortex, divergence, and energy were analyzed. The 500 hPa heights and temperature before rainstorm were combined and analyzed, combining experimental results by numerical model, all shows that: (1) The forming and long-term maintaining of block high and cutting low at 500 hPa before rainstorm made cold air pile up over Shanxi, and so strong cold gushing was formed. Meanwhile, the strengthening and maintaining of West Pacific subtropical high offered energy and water vapor source for the rainstorm. Under background of the above special weather, regional rainstorm and heavy rain in large scope in Shanxi were occurred, and the center of heavy rainstorm located on the windward side of the mountain. (2) In the shear cloud band, there were a series of mesoscale rainstorm cloud cluster formed which arranged in a band along Taihang mountain. The wind convergence at lower levels by special terrain of Yuanqu made water vapor draw into cloud and caused rainstorm cloud cluster forming and developing continually. The rainstorm in Yuanqu county was caused directlyby the γ-mesoscale rainstorm cloud cluster in the shear cloud band. (3) The air flow field nearYuanqu county was changed by its special terrain. It can not only help the mesoscale disturbanceform and develop under the advantageous weather condition, but also make terrain vertical velocity strengthenand maintain. It can also make water vapor draw into cloud and so prolong the durian time of rainstorm cloud above Yuanqu county. It can make coagulated water drop efficiently by physical process of cloud and so increase rainfall. The shrink of trumpet terrain was important, too. (4) The forming and maintaining of K-H instabilitywas an important reason of causing vortex on the shear line maintain, strengthen, and causing rainstorm. While the special terrain in Yuanqu county made K-H in stability strengthen and maintain. (5) There was a saddle structure of θse at the vertical profile and a dustpan of high energy area above the rainstorm area. The energy front sloped towards the north and convective instability developed deep. The action between strongly releasing of high instability and low cloud on the side of facing wind of the mountain each other made rainfall increase and it was an important reason of rainfall near Yuanqu larger than other areas.
  • Numerical Simulation of Mesoscale System of a RainStorm in Warm Section of Southern China
  • 2009 Vol. 28 (4): 906-914. 
  • Abstract ( ) PDF (8272KB) ( )
  • In order to understand the rainstorm process in warm section over southern China, the rainstorm occurring on May 9, 2005 is analyzed using the synopticobservation data and satellite image firstly. Then the numerical simulation of the rainfall is conducted by using MM5V3.6 model. Based on the outputs of successful simulation, the structure and cause of the mesoscale rainstorm system are studied. Results show that the divergencegenerated on the right side of the upper level jet entrance region overlapping the convergencegenerated on the left side of lower level jet exit region in southern part of South China leads to the heavy rain happeningin the warm section. The mesoscale convective clouds are direct affecting system of the rainstorm. The southerly flow vertically ascends strongly over the rainstorm system and the ascending draft forms a meridional verticalcounter-circulation with the downdraft in upper troposphereon the southern side of the system in cross-section along longitude. The velocity convergenceof lower level flow plays a crucial role in formation and development of the heavy rain system of warm sector. It is indicated that the higher rotational helicity might be one important mechanism for the rainstormand can be used to judge the development and movement of the rainstorm system.
  • Structure of Meiyu Front and Its Sensitivity Experiment ofRelationship between Front and Vortex Disturbance
  • 2009 Vol. 28 (4): 924-934. 
  • Abstract ( ) PDF (8851KB) ( )
  • Using the meso-scale model MM5 the relationship between the development of south-west vortex and Meiyu front in the middle and lower reaches of Yangtze River in June 1999 are studied. The intensity transform of front or not, and space-time relationship between frontogenesis, frontolysis and vortex evolution were studied through sensitivity experiments.The results show that the low vortex was sensitive to intensity transform of middle level front. If intensity of middle level front was changed, vortex vorticity at every level would change in phase. Middle level system had great effect on development of cyclone along Meiyu front. As for vortex evolution, middle level front was more important than boundary layer front. Perpendicular sub-circulation would develop if intensity of middle level front changed. The development of sub-circulation lead to divergence changing in low level, and finally it would effect to the intensity of vortex. Only increasing the intensity of boundary layer front could not result in low vortex developing. If we changed the intensity of middle level front in the initial time, the intensity of middle level front would change out of phase after integrating some hours. Sensitive experiments proved that the development of middle level front would influence the developmentof vortex after 6 hours or so. The development of south-west vortex and Meiyu front was not at the same time. The frontogensisi appeared about 6 hours earlier than the development of vortex. The place that intensity of front and low vortex changed was almost at the same longitude. The place that front intensity changed was at the north of that vortex intensity changed. So there was a good relationship between Meiyu front and low vortex along it.
  • Analyses on Observational Structure Characteristics and NumericalSimulation of Cold-Air Convective Cloud and Wind of a TypicalCold-Vortex Snowstorm Process in Shandong Peninsula
  • 2009 Vol. 28 (4): 935-944. 
  • Abstract ( ) PDF (9250KB) ( )
  • The analysis of observational structure characteristics of cold-air convective cloud and wind of heavy snow eventsof the typical cold vortexoccurred in Shandong Peninsula on4 December 2005 is undertaken using the satellite VIS image data， Doppler radar data and observational station data.Meanwhile， the numerical simulation is performed to be tested with the real observational data.The results show when outbreaks of winter cold air occur from the Eurasian Continent， snow cloud develop over the Bohai sea. The FY-2C VIS satellite image shows the cloud streaks which the alignment of SW to NE is almost perpendicular to the northwesterly wind in the lower troposphere. Doppler radar data show that the reflectivity of heavy snow at 0.5 elevation of the snowband extended from Bohai sea toward to Shandong Peninsula is about 30 to 40 dBz. In the velocity field, the anticlockwise movement of wind with height presented cold air current and a windconvergencein the low-level isstudied by analyzing the Doppler radar data also. Velocity cross-section indicatesthe relationship betweenthe wind velocity andsnowstorms. The numerical simulated results display that there isa low-level jet at 1.5 km heightswhen heavy snow events occur in Shandong Peninsula. The time-height cross-section of horizontal wind above Weihai shows that there is warmair current from 1000 to 700 hPa.The changes of numerical simulated vertical velocity and divergence are in better agreement with observational data. The updrafts and the convergence in the lower layer arethe dynamic mechanism of the snowstorms by analyzing the time-height cross-section of vertical velocityand horizontal divergence.

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