Plateau Meteorology

Current Issue

28 April 2009, Volume 28 Issue 2   
  • Numerical Simulation of Local and Non-Local Effects of Vegetation Change and Urbanization in a Semi-Arid Region in China
  • CHEN Lei;TIAN Wen-shou;WANG Chan
  • 2009 Vol. 28 (2): 233-245. 
  • Abstract ( ) PDF (7410KB) ( )
  • Using a mesoscale model, the local and non\|local responses of the atmosphere to changes in vegetation cover and surface types in a semi\|arid region are analysed.The results show that the changes in vegetation cover have a complex effect on the surface temperature with a significant diurnal cycle. Contrast to a strong and persistent warming caused by urbanization of the area,decreasesin vegetation cover only give rise to a weak and intermitted warming depending on the partition of the surface net available energy between sensible heat flux, latent heat flux and soil heat flux.The local temperature anomalies caused by the vegetation and surface type change can propagate out of the source region by background winds with details of the propagation depending on wind fields and underlying topography. Under suitable wind conditions, those temperature signals can pass around a mountain to form a persistent warming region lee of the mountain. A decrease in vegetation cover tends to force a 1.1 km deep secondary circulation. Decreases in vegetation cover also lead to decreases in column total water vapour over the area for one hand, and force secondary circulations due to temperature contrast on the other. The forced secondary circulations tend to transport inward the water vapour outside of the area to compensate the decrease of water vapour input from the surface. The integrated effect of a decrease in vegetation cover is to decrease column total water vapour over the area. Outside of the area, the colume total water vapour tends to increase upwind of the area and decrease downwind of the area.
  • Investigation of Spatial Representativeness for Surface Flux Measurementswith Eddy Covariance System and Large Aperture Scintillometer
  • GONG Li-juan;LIU Shao-min;SHUANG Xi;CAI Xu-hui;XU Zi-wei
  • 2009 Vol. 28 (2): 246-257. 
  • Abstract ( ) PDF (4972KB) ( )
  • Basedon the analysis of sensitivity of analytical footprint models of an eddy covariancesystem (EC)and a Large Aperture Scintillometer(LAS), as well as the observation of EC and LAS at Miyun flux station of Beijing (from August 2006 to December 2007), the flux spatial representativeness of EC and LAS were preliminarily analyzed. The results are as follows: (1)The source areas of EC and LAS are sensitive to wind direction, Monin-Obukhov length, aerodynamic roughness length and measurement height. (2)The source areas of the eddy covariance system distribute in the southwestand south area during the daytime, while in the northeast and north area during the nighttime. The source areas of the LAS are in the direction from southeast to northwest. The source areas of the eddy covariancesystem are differentin each month, but all locatein an area of 1000 m (S-N) by 850 m (W-E) area.The source areas of the LAS are in the direction from southeast to northwestin each month, about 2060 m long and 620m wide. (3)The main typesof land use to determine the measured fluxes of the eddy covariancesystem are orchard(67%) and farmland(19%). The flux contributionof orchardis larger in summer and autumn, but smaller in winter and spring. And it is contraryfor farmland. The primary land use types of flux contribution for LAS are orchard(49%), farmland(28%) and residential area(11%). The seasonal variationof flux contribution of orchardand farmlandis the same as the eddy covariancesystem, but it is not obvious.
  • Characteristics of the Inversion and the Water VaporTransport during a Duration Fog Event
  • WU Bin-gui -;ZHANG Hong-sheng;WANG Jing;ZHANG Chang-chun;YU li-li;LIU Bin-xian;XIE Yi-yang
  • 2009 Vol. 28 (2): 258-267. 
  • Abstract ( ) PDF (10356KB) ( )
  • Based on the station observation, boundary layer observation and NCEP reanalysis data, the characteristics of water vapor and temperature inversion are discussed during the durative fog, which occurred in the North Chinaon February 13~14, 2006. The results show that the upper and low\|layer system setting of a warm high ridge at middle level and the transformed surface cold high pressure over the east of China, which are advantageous for fog formation to form temperature inversion and the feeble wind adjacent to the surface. With the deep inversion temperature, the drizzle which occurs in the water convergence directly leads to the fog when its vapor condenses again in the colder layer. The southwestern low jet stream is served to the formation and duration of the fog, which mainly contributes to the inversion existenceand water vapour transport. The fog thickness is thinner and thinner when the the low level is controled by the northwestern dry and cold advection and becomes dry; meantime, the stable stratification is distroyed due to the increased temperature in the surface layer contributed by the shortwave radiation, which lead to increased mix between the up and down layers and finally achievethe dissipation of fog. It is also showed that the fog will be accompanied with the drizzle when the water vapor converges, otherwise, the fog without the drizzle company.
  • Numerical Simulation of Mass Forcingon a Rainstorm in North China
  • 2009 Vol. 28 (2): 278-292. 
  • Abstract ( ) PDF (10738KB) ( )
  • In rainstorm system, mass of wet air can be decreased by heavy precipitation, and this decrease introduces a force on the development of rainstormand this force is called “mass forcing” in this article. First, the tendency equation of perturbation pressure for ARPS numerical model, which includes mass forcing, is deduced by introducing a modified continuity equation. Secondly, with this model, a cold-front rainstorm in North China is simulated and diagnosed. Result shows: (1)Mass forcing can influence the amount of precipitation. For this rainstorm process, the maximum mass forcing shows up in the height of 5750 m where both the water vapor condensation and pressure declination are strong, also it produces both upward and downward movement of the air flow. Furthermore, the downward movement suppresses the rising movement of lower inflow, which decreases the amount of precipitation and downgrades the rainstorm. (2)The distribution of mass forcing is coincide with that of cloud water content. The change of mass force agrees with that of precipitation rate, especially in the height of 5750 m. The terminal falling speed of hydrometeor places the biggest influence on the mass forcing. (3)The change of local pressure field caused by change of wet air mass field can result in adjustment of dynamic field, such as convergent, divergent and vertical movement. Reciprocally, the change of dynamic field can also affect the pressure field. So this interacting process will yield visible effect on the intensity of rainstorm. (4)The dominating terms which lead to different local perturbation pressuress are the difference of convergent term and that of vertical term, while that mass forcing has an indirect effect on it. The leading term which controls the distribution of local water vapor and water material is the difference of microphysical term, followed by the terminal falling speed of hydrometeor.
  • The Effects of Tibetan Plateau and Mountains overSoutheastern China on the Formation of the Meiyu Rain
  • 2009 Vol. 28 (2): 299-305. 
  • Abstract ( ) PDF (5545KB) ( )
  • The Meiyu Rain (MR) to the south of the mid\|and lower\|reaches of Yangtze River over Southeastern China (SEC) is one of the important components of the East Asia Subtropical Monsoon system. It is the result of the process of the marching northward of warm and wet southwesterly with season warming. Climate mean data analyses and numerical model sensitivity experiments are used to study the effect of Tibetan Plateau and the mountains over southeastern China on the formation of the Meiyu. Data analyses shows, at lower troposphere, SEC is located at the downstream of a southwesterly velocity centerand there is mechanically forced rise of the southwesterly flow by the mountains over SEC. As a result, there is strong southwesterly jet wind velocity and moisture convergences over SEC. That is the immediate climatic cause of MR formation. Sensitivity numerical experiments shows, without the Tibetan Plateau (TP), the southwesterly jet over SEC become stronger, its vorticity increases, and MR increases by 2 mm/d. In other words, with TP standing, its Sensible Heat Air Pump (SHAP) makes the circulation′s vorticity increasing over its surrounding areas but decreasing over SEC by suppressing the rising over there and thus makes MR decrease. On the other hand, without the mountains over SEC, the MR decreases by 2 mm/d and the rain belt shifts north to the Huaihe River reach. On the contrary, with the mountains lifted by 300 m, MR increases and the rain belt shifts south. These indicatingthat the mountains over SEC can force the southwesterly rising and magnify MR. So the mountains over SEC also have important effect on the formation of MR. Thus it can be seen, MR is originally produced by the typical east Asia summer monsoon, which is caused by sea-land thermal contrast. MR is also affected heavily by the nearby huge plateau and mountains through their thermal and mechanical forcing. These can help us to understand the originof the summer monsoon and its rains over East China.
  • A Case Study of Plateau Vortex Moving East Ward with HeavyRainfalls Based on Helicity and Non\|Geostrophic Wet Q-Vector
  • 2009 Vol. 28 (2): 319-326. 
  • Abstract ( ) PDF (4933KB) ( )
  • Using NCEP global 1°×1° final\|analysis data and the routine observation data， with the theories of helicity and non\|geostrophic wet Q\|vector, a case of Plateau vortex moving east which caused heavy precipitation was analyzed by both synoptic analysis and dynamic methods. The results show that 500 hPa horizontal distribution of z\|helicity can give good direction to the movement of vortex center and distribution of rainfall, the heavy precipitation is located in the great grads ofpositivehelicity. z-helicity can reflect the dynamic character of weather when the rainstorm occurred, and divergence of negative vorticity on higher levelmatched convergence of positive vorticityon lower leve1 is a dynamic mechanism to trigger heavy precipitation. Relative spiral of more comprehensively reflect the rainfall districts and rainfall distribution center, and the next 6 h after the precipitation to the districts and have a better prediction of heavy precipitation occurred in a relatively central spiral of plus and minus centres connect the maximum gradient value of the positive side.Low level convergence of non-geostrophic wet Q-vector as a powerful tool to diagnose precipitation qualitatively has good relation to rain falls, convergence center are correlation with precipitation intensity; cross-section of wet Q-vector divergence provided good reference information for the distribution and movement of precipitation.
  • Comprehensive Analyses on Rainstorm Caused by MCCin Guangxi and Guizhou
  • 2009 Vol. 28 (2): 335-351. 
  • Abstract ( ) PDF (4242KB) ( )
  • Using the synoptic data such as satellite image, Doppler radar and upper wind, the floodingrainstorm in Guangxi and Guizhou on June 8～9, 2007 caused by MCC is analyzed. The main conclusions are as follows: MCC is the direct influence system to rainstorm; the disconnected recession to western of low-level jet provides enough transmission of vapor to generate and develop MCC. The middle and lower of troposphere inclining to western with heightforms the high-energy pipe, which maintains the convective instability in generationarea of MCC. Cold air brought by northeast air on bottom of North China High moving southward along east of Tibetan Plateau generates frontogenesis, which benefits to form MCC. For the generating,maintaining and disappearingof MCC, we can see from the radial velocity map of CAPPI(1.5 km): The southwest low-level jet forms first, then the cyclonicor radial convergenceon left appears; combing with moving eastward and turning to southward of severe convective cloud zone similar to squall line, large-scale of radial and severe divergence area is formed. Weakening and vanishing of low-level jet predicts weakening and dissipation of MCC.
  • A Numerical Case Study on the Evolution of Hail Cloudand the Three-Dimensional Structure of Supercell
  • 2009 Vol. 28 (2): 352-365. 
  • Abstract ( ) PDF (1961KB) ( )
  • Asevere hailstorm occurred in Beijing on 31 May 2005 are simulated using a high resolutionmesoscalenumerical model WRF(Weather Research and Forecast). Theresults show that the main characteristic of case are successfully simulated. it is well reproduced from synoptic scale system toconvectivecell, the simulated trough on the bottom of cold vortex, the worm moisture tongue,the topographic flow convergence line, the accumulated precipitation,the evolution of the hail cloudand the convective cell structure are consistent with observation. The supercell plays a key role in hailstorm case. So the research on the formation, structure and evolution of supercell are very important. The simulated a long-lived hailstorm has a supercell and the structure of the supercell has features common to supercellular conception model, such as bounded weak echo, spliting, right moving and one giant pair of slantwise updraft and downdraft. We put emphasison the evolution of this supercellular hailstorm cloud and the three-dimensional reflectivity and flow structure of the supercell. The intent is to understand the three-dimensional structureof supercell in our country′s hail case, and the intent is to learn what structuralfactors of this supercell case allow hailstorm to be long-lived.
  • Case Simulation on the Interaction between SevereConvective (Hail) Cloud and Environment
  • 2009 Vol. 28 (2): 366-373. 
  • Abstract ( ) PDF (1298KB) ( )
  • On the base of successfully simulated hailstorm case which happened on 31 May 2005, the dependence of convective storm on the environmental factors including CAPE(the convective available potential energy) and shear(vertical wind shear) and the interaction between convective storm and local environment were studed. The result shows that the shear is very importantto convective storm type for a given amount of CAPE. The long-lived supercell storm need balance between CAPE and shear. The relationship between CAPE and shear has been expressed in term of Richardson number (Ri= CAPE/shear). In the simulated “531”case, when CAPE is more than 1000 J·kg-1 small shear bring short-lived ordinary convective cell while large shear often produce long-lived supercell storm and Richardson number is more than 10 in the former and between 1~5 in the latter. The convective storm effect local surrounding distinctly. The squall-line of ordinary cell tend to increase surrounding shear in front of it while supercell storm would dissipate more and more shear on the process of development and the area influenced spread to hundreds of kilometers. The blocking of the large-scale updraft on the horizontal airflow and the mixing of evere convective activities may be reasonable interpretation for phenomenaabove. Corresponding to the decrease of CAPE, the local shear also diminishes in the regime of evere convection. As a result, the Richarson number holds between 1~5, which would benefitfor the maintenance of supercell.
  • Impact of Planetary Wave on the Stratospheric Residual Circulation
  • 2009 Vol. 28 (2): 402-410. 
  • Abstract ( ) PDF (963KB) ( )
  • Using the global ECMWF/ECR-40 reanalysisdataand calculating the residual velocities, the distribution characteristic of the residual circulation are analyzed. And the planetary wave amplitudes of waves 1 and 2 were also calculated by the geopotential height data. Then the changes of the residual circulation wereanalyzed in different activity intensities of planetary wave, as well as the change after wave 1 transformed into wave 2 or wave 2 transformed into wave 1. The results showedthat there wasa nice coherence between the amplitudes of the wave 1 or the one of wave 2 and the residual velocities, the coherence between wave 2 and the residual velocities was better than that between wave 1 and the residual velocities. In addition, the phase of the wave amplitude was unanimous with that of *, but earlier about two months than that of *. The residual circulation became more intense while the wave became more intense. After the planetary waves transformedfrom waves 1 to 2, the residual circulation became stronger under 50 hPa in the stratosphere, but became weaker above 30 hPa. At the same time, after the planetary wave transformed from waves 2 into 1, the residual circulation gotweaker under 30 hPa, but stronger above 20 hPa in the stratosphere.
  • Effects of Black Carbon Aerosol in SouthAsia on Asian Summer Monsoon
  • 2009 Vol. 28 (2): 419-424. 
  • Abstract ( ) PDF (985KB) ( )
  • The effects of black carbon aerosol (BC) in the South Asia on Asian summer monsoon have been investigated by using the Community Atmosphere ModelVersion 3 (CAM3)developed by the National Center for Atmospheric Research(NCAR). The results show that BC heats the lower tropospherebecause of strongly absorbing the solar radiationin the South Asia during thelate spring. This may lead to an advance of the rainy periods of the Bay of Bengaland its coast and the outbreak of the Indian summer monsoon. In summer, the heated air forms a steady heating layer at upper air through ascending along the southern slope of the Tibetan Plateau, which gives birth to a local motion of deep convection. Thus, a meridional circulation thatascends in the north and subsides in the south appears between the South Asia continent and the Indian Ocean, which enhance the northwardmotion and intensifies the Indian summer monsoon. However, the BC of the South Asia weakens the East Asian summer monsoon, induces western Pacific Subtropical Highto extendnorthwestward, and causes the Meiyu belt of China to shift northeastward through impacting on the surface pressure, vertical motion, precipitation, wind field of 850 hPa and so on.
  • Correlation Analysis and Statistical Prediction of AbnormalCold/Warm in the Southwestern Region of China Spring
  • 2009 Vol. 28 (2): 425-432. 
  • Abstract ( ) PDF (756KB) ( )
  • Using the spring (April~May) air temperature data of Southwest China from 1951 to 2006, 500 hPa geopotential heights of the Northern Hemisphere, and the SST data of the north Pacific ocean, the relation between the climatic change of spring atmospheric temperature in the southwest region of China, 500 hPa geopotential heights of the Northern Hemisphere, the sea surface temperature (SST) in the tropical region, and the Southern Oscillation were analyzed, through methods of composited analysis, relation analysis and association table. It is found that the spring air temperature in the southwestern China has obvious decadal climate variation characteristics. In years with extraordinarily cold or warm springs, 500 hPa geopotential height departures in the Northern Hemisphere and the SST departure in the tropical region are quite different. The intensity change of subtropical high of the Northern Hemisphere in the former period of January to March and the tropical ocean temperature in the former period of August to February, especially the SST change in the third area of NINO in the former period of December to January, are well predictive to the spring atmospheric temperature in the southwestern China. They are close positive relation. The intensity change of the South Oscillation in the former period of August to September is also meaningful for it. They have close negative correlation.
  • Effect of Urbanization and Observation Environment Changeon Wind Speed Trend in Hebei Province, China
  • 2009 Vol. 28 (2): 433-439. 
  • Abstract ( ) PDF (528KB) ( )
  • Applying a data set of annual mean surface wind speedfrom 143 stations for 1975\_2004 period and census data Hebei Province in 1990 and 2000, the effects of urbanization and observational environment change on long\|term wind speed trends inHebei Province are analyzed, Based on the records of population growth, the station relocation, change of instrumental height,microscopic environmental variationnear observational stations, the stations are classified to 4 groups, and a comparative analysisis made to identifythe differenteffects of the human factors. Results show significant decreasing trends ofannual mean surface wind speed for most of the stations in the region. The urbanization and changes in observational environment have significantly influenced the trends of annual mean wind speed. The urbanization of cities or towns near the stations is one of important factors, which leads to the obvious slowdown of wind speed for approximately 25% stations. The micro environmental variation immediatelyaround stations even has a larger effecton the decrease of wind speed, which generally surpassesthe regional baseline wind speed change.It is noted that, although the regional baseline wind speed has decreased, the urbanization and observational environment change have made a larger contribution to the observed slowdown of surface wind speed in the study region. We also find that some in\|homogeneities of wind speed data have been caused by the the observation environmental changes.
  • Correlation Analyses on Total Precipitable Water andSurface Dew Point Temperature over Hefei
  • 2009 Vol. 28 (2): 452-457. 
  • Abstract ( ) PDF (485KB) ( )
  • Based on four\|year\|observation using ground\|based dual\|channel microwave radiometer WVR-1100 and temperature/humidity sensors at Hefei station, the empirical relation between surface dew point tdand total precipitable water Won thebasis of mean daily values have been statistically analyzed. The calculated results show that the correlation coefficient r between surface tdand the natural logarithm of Wis higher than 0.90 and the value of empirical coefficients aand b are -0.521 and 0.0901℃-1, respectively, mirroring particularity of theclimate condition of Hefei region. The monthly correlation coefficients range from 0.120 to 0.813 and seasonal values from 0.740 to 0.865, with the highest value in fall and lowest in summer. The degree of correlation is excellent in fall and winter months, but weaker in late spring and summer months, especially for July and August. The monthly variations of aare gentlyduring spring, fall and winter months, but acutelyduring the summer months. It′s annual variation is nearly reverseto that of monthly correlation coefficient. Proposed that it is not advisableto calculate W using surfacetdbased on empirical calculating model on monthly scale.

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