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2010 Vol. 27, No. 3

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CCSC (climate change study in China)
Influences of Freshwater from Major Rivers on Global Ocean Circulation and Temperatures in the MIT Ocean General Circulation Model
Boyin HUANG, Vikram M. MEHTA
2010, 27(3): 455-468. doi: 10.1007/s00376-009-9022-6
Responses of global ocean circulation and temperature to freshwater runoff from major rivers were studied by blocking regional runoff in the global ocean general circulation model (OGCM) developed at the Massachusetts Institute of Technology. Runoff into the tropical Atlantic, the western North Pacific, and the Bay of Bengal and northern Arabian Sea were selectively blocked. The blocking of river runoff first resulted in a salinity increase near the river mouths (2 practical salinity units). The saltier and, therefore, denser water was then transported to higher latitudes in the North Atlantic, North Pacific, and southern Indian Ocean by the mean currents. The subsequent density contrasts between northern and southern hemispheric oceans resulted in changes in major ocean currents. These anomalous ocean currents lead to significant temperature changes (1oC--2oC) by the resulting anomalous heat transports. The current and temperature anomalies created by the blocked river runoff propagated from one ocean basin to others via coastal and equatorial Kelvin waves. This study suggests that river runoff may be playing an important role in oceanic salinity, temperature, and circulations; and that partially or fully blocking major rivers to divert freshwater for societal purposes might significantly change ocean salinity, circulations, temperature, and atmospheric climate. Further studies are necessary to assess the role of river runoff in the coupled atmosphere-ocean system.
A Comparison of Polar Vortex Response to Pacific and Indian Ocean Warming
Shuanglin Li
2010, 27(3): 469-482. doi: 10.1007/s00376-009-9116-1
During recent decades, the tropical Indo-Pacific Ocean has become increasingly warmer. Meanwhile, both the northern and southern hemispheric polar vortices (NPV and SPV) have exhibited a deepening trend in boreal winter. Although previous studies have revealed that the tropical Indian Ocean warming (IOW) favors an intensifying NPV and a weakening SPV, how the tropical Pacific Ocean warming (POW) influences the NPV and SPV remains unclear. In this study, a comparative analysis has been conducted through ensemble atmospheric general circulation model (AGCM) experiments. The results show that, for the Northern Hemisphere, the two warmings exerted opposite impacts in boreal winter, in that the IOW intensified the NPV while the POW weakened the NPV. For the Southern Hemisphere, both the IOW and POW warmed the southern polar atmosphere and weakened the SPV. A diagnostic analysis based on the vorticity budget revealed that such an interhemispheric difference in influences from the IOW and POW in boreal winter was associated with different roles of transient eddy momentum flux convergence between the hemispheres. Furthermore, this difference may have been linked to different strengths of stationary wave activity between the hemispheres in boreal winter.
Mean Kinematic Characteristics of Synoptic Easterly Disturbances over the Atlantic
2010, 27(3): 483-499. doi: 10.1007/s00376-009-9092-5
This study investigates the mean kinematic characteristics of the tropical Atlantic easterly disturbances in January--March (JFM), April--June (AMJ), July--September (JAS) and October--December (OND) from 1968--1998. For each season, the preferential tracks of these disturbances in the 3--10-day band periods were computed and spatialized, as well as their associated wavelength, velocity and main period, which lies between 3--5 days and between 6--9 days depending on the track and the season. Two main tracks are highlighted over the Atlantic Ocean. During OND and JFM these two tracks are well separated and located in each hemisphere around 15oS and 12.5oN. From AMJ to JAS these tracks migrate northward; in JAS, they merge into one over the northern tropical Atlantic along 17.5oN.The associated wavelength fields exhibit a meridional gradient, with large wavelengths (greater than 4000 km) around the equator, between 5oN and 5oS, and smaller wavelengths outside of this latitude band (between 2500--3500 km). The phase speed is also found to exhibit poleward decreasing values from 12--6 m s-1. Over the north Atlantic track, 6--9-day disturbances were found to occur from January to May and approximately from October to December. From June to September, the 3--5-day waves dominate the synoptic activity. Over the south Atlantic track, between May and August the synoptic variability is mainly explained by the 3--5-day disturbances but from January to April and from September to December both 3--5-day waves and 6--9-day waves can occur.
An Assessment of the Quality of Surface Sensible Heat Flux Derived from Reanalysis Data through Comparison with Station Observations in Northwest China
ZHOU Lian-Tong, HUANG Ronghui
2010, 27(3): 500-512. doi: 10.1007/s00376-009-9081-8
The present study compares seasonal and interdecadal variations in surface sensible heat flux over Northwest China between station observations and ERA-40 and NCEP-NCAR reanalysis data for the period 1960--2000. While the seasonal variation in sensible heat flux is found to be consistent between station observations and the two reanalysis datasets, both land-air temperatures difference and surface wind speed show remarkable systematic differences. The sensible heat flux displays obvious interdecadal variability that is season-dependent. In the ERA-40 data, the sensible heat flux in spring, fall, and winter shows interdecadal variations that are similar to observations. In the NCEP-NCAR reanalysis data, sensible heat flux variations are inconsistent with and sometimes even opposite to observations. While surface wind speeds from the NCEP-NCAR reanalysis data show interdecadal changes consistent with station observations, variations in land-air temperature difference differ greatly from the observed dataset. In terms of land-air temperature difference and surface wind speed, almost no consistency with observations can be identified in the ERA-40 data, apart from the land-air temperature difference in fall and winter. These inconsistencies pose a major obstacle to the application in climate studies of surface sensible heat flux derived from reanalysis data.
Effects of Street-Bottom and Building-Roof Heating on Flow in Three-Dimensional Street Canyons
Jae-Jin KIM, Jong-Jin BAIK
2010, 27(3): 513-527. doi: 10.1007/s00376-009-9095-2
Using a computational fluid dynamics (CFD) model, the effects of street-bottom and building-roof heating on flow in three-dimensional street canyons are investigated. The building and street-canyon aspect ratios are one. In the presence of street-bottom heating, as the street-bottom heating intensity increases, the mean kinetic energy increases in the spanwise street canyon formed by the upwind and downwind buildings but decreases in the lower region of the streamwise street canyon. The increase in momentum due to buoyancy force intensifies mechanically induced flow in the spanwise street canyon. The vorticity in the spanwise street canyon strengthens. The temperature increase is not large because relatively cold above-roof-level air comes into the spanwise street canyon. In the presence of both street-bottom and building-roof heating, the mean kinetic energy rather decreases in the spanwise street canyon. This is caused by the decrease in horizontal flow speed at the roof level, which results in the weakening of the mean flow circulation in the spanwise street canyon. It is found that the vorticity in the spanwise street canyon weakens. The temperature increase is relatively large compared with that in the street-bottom heating case, because relatively warm above-roof-level air comes into the spanwise street canyon.
The Role of Land--sea Distribution and Orography in the Asian Monsoon. Part II: Orography
XU Zhongfeng, QIAN Yongfu, FU Congbin
2010, 27(3): 528-542. doi: 10.1007/s00376-009-9045-z
The role of various mountains in the Asian monsoon system is investigated by AGCM simulations with different mountains. The comparison of the simulation with Asian mountains (MAsia run) with the simulation without mountains (NM run) reveals that the presence of the Asian mountains results in a stronger South Asian summer monsoon (SASM), characterized by enhanced lower-tropospheric westerly winds, upper-tropospheric easterly winds, and stronger water vapor convergence. In East Asia, the southerly winds and water vapor convergence are significantly strengthened in association with the intensified zonal pressure gradient between the East Asian continent and the Pacific Ocean. Both the dynamical and thermodynamic forcing of the Tibetan Plateau play important role in strengthening the Asian summer monsoon. In winter, the presence of Asian mountains significantly strengthens the continental high, which leads to a stronger Asian winter monsoon. The presence of African--Arabian mountains helps to intensify the exchange of mass between the Southern Hemisphere and Northern Hemisphere by strengthening the cross equatorial flows in the lower and upper troposphere over East Africa. Asian mountains also play a crucial role in the seasonal evolution of Asian monsoons. In comparison with the NM run, the earlier onset and later withdrawal of lower-tropospheric westerly winds can be found over South Asia in the MAsia run, indicating a longer SASM period. The African--Arabian mountains also moderately contribute to the seasonal variation of the South Asian monsoon. In East Asia, the clear south-to-north march of the southerly winds and subtropical rainfall starts to occur in early summer when the effects of Asian mountains are considered.
What Causes the Springtime Tropospheric Ozone Maximum over Northeast Asia?
Jae H. KIM, Hyunjin LEE
2010, 27(3): 543-551. doi: 10.1007/s00376-009-9098-z
Scientists have long debated the relative importance of tropospheric photochemical production versus stratospheric influx as causes of the springtime tropospheric ozone maximum over northern mid-latitudes. This paper investigates whether or not stratospheric intrusion and photochemistry play a significant role in the springtime ozone maximum over Northeast Asia, where ozone measurements are sparse. We examine how tropospheric ozone seasonalities over Naha (26oN, 128oE), Kagoshima (31oN, 131oE), and Pohang (36oN, 129oE), which are located on the same meridional line, are related to the timing and location of the jet stream. The ozone seasonality shows a gradual increase from January to the maximum ozone month, which corresponds to April at Naha, May at Kagoshima, and June at Pohang. In order to examine the occurrence of stratospheric intrusion, we analyze a correlation between jet stream activity and tropospheric ozone seasonality. From these analyses, we did not find any favorable evidence supporting the hypothesis that the springtime enhancement may result from stratospheric intrusion. According to trajectory analysis for vertical and horizontal origins of the airmass, a gradual increasing tendency in ozone amounts from January until the onset of monsoon was similar to the increasing ozone formation tendency from winter to spring over mainland China, which has been observed during the build-up of tropospheric ozone over Central Europe in the winter--spring transition period due to photochemistry. Overall, the analyses suggest that photochemistry is the most important contributor to observed ozone seasonality over Northeast Asia.
Effects of Vertical Wind Shear on Intensity and Rainfall Asymmetries of Strong Tropical Storm Bilis (2006)
YU Jinhua, TAN Zhemin, Yuqing WANG
2010, 27(3): 552-561. doi: 10.1007/s00376-009-9030-6
The effects of environmental vertical wind shear (VWS) on the intensity and rainfall asymmetries in Tropical Storm (TS) Bilis (2006) have been analyzed based on TRMM/TMI-estimated surface rainfall data, QuikSCAT wind fields, 850- and 200-hPa winds of the NCEP-NCAR reanalysis, precipitation data at 5-min intervals from automatic weather stations over mainland China, and the best track data of TS Bilis (2006). The results show that the simultaneous and 6-hour-lagged correlation coefficients between VWS and storm intensity (the minimum central sea level pressure) are 0.59145 and 0.57438 (P-1 and thus suppressed the intensification of Bilis (2006). Distribution of precipitation in Bilis (2006) was highly asymmetric. The azimuthally-averaged rainfall rate in the partial eyewall, however, was smaller than that in a major outer rainband. As the storm intensified, the major rainband showed an unusual outward propagation. The VWS had a great impact on the asymmetric distribution of precipitation. Consistent with previous modeling studies, heavy rainfall generally occurred downshear to downshear-left of the VWS vector both near and outside the eyewall, showing a strong wavenumber-one asymmetry, which was amplified as the VWS increased.
Aerosol Optical Properties Affected by a Strong Dust Storm over Central and Northern China
XIN Jinyuan, DU Wupeng, WANG Yuesi, GAO Qingxian, Zhanqing LI, WANG Mingxing
2010, 27(3): 562-574. doi: 10.1007/s00376-009-9023-5
Aerosol observational data at 8 ground-based observation sites in the Chinese Sun Hazemeter Network (CSHNET) were analyzed to characterize the optical properties of aerosol particles during the strong dust storm of 16--21 April 2005. The observational aerosol optical depth (AOD) increased significantly during this dust storm at sites in Beijing city (86%), Beijing forest (84%), Xianghe (13%), Shapotou (27%), Shenyang (47%), Shanghai (23%), and Jiaozhou Bay (24%). The API (air pollution index) in Beijing and Tianjin also had a similar rise during the dust storm, while the Angstrom exponent (alpha) declined evidently at sites in Beijing city (21%), Beijing forest (39%), Xianghe (19%), Ordos (77%), Shapotou (50%), Shanghai (12%), and Jiaozhou Bay (21%), respectively. Furthermore, The observational AOD and alpha demonstrated contrary trends during all storm stages (pre-dust storm, dust storm, and post-dust storm), with the AOD indicating an obvious ``Valley--Peak--Valley' pattern of variation, while alpha demonstrated a ``Peak--Valley-- Peak' pattern. In addition, the dust module in a regional climate model (RegCM3) simulated the dust storm occurrence and track accurately and RegCM3 was able to basically simulate the trends in AOD. The simulation results for the North China stations were the best, and the simulation for dust-source stations was on the high side, while the simulation was on the low side for coastal sites.
Influence of the Asian-Pacific Oscillation on Spring Precipitation over Central Eastern China
ZHOU Botao, ZHAO Ping
2010, 27(3): 575-582. doi: 10.1007/s00376-009-9058-7
The linkage between the Asian-Pacific oscillation (APO) and the precipitation over central eastern China in spring is preliminarily addressed by use of the observed data. Results show that they correlate very well, with the positive (negative) phase of APO tending to increase (decrease) the precipitation over central eastern China. Such a relationship can be explained by the atmospheric circulation changes over Asia and the North Pacific in association with the anomalous APO. A positive phase of APO, characterized by a positive anomaly over Asia and a negative anomaly over the North Pacific in the upper-tropospheric temperature, corresponds to decreased low-level geopotential height (H) and increased high-level H over Asia, and these effects are concurrent with increased low-level H and decreased high-level H over the North Pacific. Meanwhile, an anticyclonic circulation anomaly in the upper troposphere and a cyclonic circulation anomaly in the lower troposphere are introduced in East Asia, and the low-level southerly wind is strengthened over central eastern China. These changes provide advantageous conditions for enhanced precipitation over central eastern China. The situation is reversed in the negative phase of APO, leading to reduced precipitation in this region.
A Regional Air--Sea Coupled Model and Its Application over East Asia in the Summer of 2000
FANG Yongjie, ZHANG Yaocun, TANG Jianping, REN Xuejuan
2010, 27(3): 583-593. doi: 10.1007/s00376-009-8203-7
A regional air--sea coupled model, comprising the Regional Integrated Environment Model System (RIEMS) and the Princeton Ocean Model (POM) was developed to simulate summer climate features over East Asia in 2000. The sensitivity of the models behavior to the coupling time interval (CTI), the causes of the sea surface temperature (SST) biases, and the role of air--sea interaction in the simulation of precipitation over China are investigated. Results show that the coupled model can basically produce the spatial pattern of SST, precipitation, and surface air temperature (SAT) with five different CTIs respectively. Also, using a CTI of 3, 6 or 12 hours tended to produce more successful simulations than if using 1 and 24 hours. Further analysis indicates that both a higher and lower coupling frequency result in larger model biases in air--sea heat flux exchanges, which might be responsible for the sensitivity of the coupled models behavior to the CTI. Sensitivity experiments indicate that SST biases between the coupled and uncoupled POM occurring over the China coastal waters were due to the mismatch of the surface heat fluxes produced by the RIEMS with those required by the POM. In the coupled run, the air--sea feedbacks reduced the biases in surface heat fluxes, compared with the uncoupled RIEMS, consequently resulted in changes in thermal contrast over land and sea and led to a precipitation increase over South China and a decrease over North China. These results agree well observations in the summer of 2000.
The Adaptive Wavelet Collocation Method and Its Application in Front Simulation
HUANG Wenyu, WU Rongsheng, FANG Juan
2010, 27(3): 594-604. doi: 10.1007/s00376-009-8189-1
The adaptive wavelet collocation method (AWCM) is a variable grid technology for solving partial differential equations (PDEs) with high singularities. Based on interpolating wavelets, the AWCM adapts the grid so that a higher resolution is automatically attributed to domain regions with high singularities. Accuracy problems with the AWCM have been reported in the literature, and in this paper problems of efficiency with the AWCM are discussed in detail through a simple one-dimensional (1D) nonlinear advection equation whose analytic solution is easily obtained. A simple and efficient implementation of the AWCM is investigated. Through studying the maximum errors at the moment of frontogenesis of the 1D nonlinear advection equation with different initial values and a comparison with the finite difference method (FDM) on a uniform grid, the AWCM shows good potential for modeling the front efficiently. The AWCM is also applied to a two-dimensional (2D) unbalanced frontogenesis model in its first attempt at numerical simulation of a meteorological front. Some important characteristics about the model are revealed by the new scheme.
Analysis on the Interaction between Turbulence and Secondary Circulation of the Surface Layer at Jinta Oasis in Summer
HAN Bo, LU Shihua, AO Yinhuan
2010, 27(3): 605-620. doi: 10.1007/s00376-009-9015-5
The kinetic energy variations of mean flow and turbulence at three levels in the surface layer were calculated by using eddy covariance data from observations at Jinta oasis in 2005 summer. It is found that when the mean horizontal flow was stronger, the turbulent kinetic energy was increased at all levels, as well as the downward mean wind at the middle level. Since the mean vertical flow on the top and bottom were both negligible at that time, there was a secondary circulation with convergence in the upper half and divergence in the lower half of the column. After consideration of energy conversion, it was found that the interaction between turbulence and the secondary circulation caused the intensification of each other. The interaction reflected positive feedback between turbulence and the vertical shear of the mean flow. Turbulent sensible and latent heat flux anomaly were also analyzed. The results show that in both daytime and at night, when the surface layer turbulence was intensified as a result of strengthened mean flow, the sensible heat flux was decreased while the latent heat flux was increased. Both anomalous fluxes contributed to the cold island effect and the moisture island effect of the oasis.
Using the OSPM Model on Pollutant Dispersion in an Urban Street Canyon
2010, 27(3): 621-628. doi: 10.1007/s00376-009-9064-9
An observational campaign was conducted in the street canyon of Zhujiang Road in Nanjing city in 2007. Hourly mean concentrations of P10 were measured at street and roof levels. The Operational Street Pollution Model (OSPM) street canyon dispersion model was used to calculate the street concentrations and the results were compared with the measurements. The results show that there is good agreement between measured and predicted concentrations. The correlation coefficient R2 values (R2 is a measure of the correlation of the predicted and measured time series of concentrations) are 0.5319, 0.8044, and 0.6630 for the scatter plots of PM10 corresponding to light wind speed conditions, higher wind speed conditions, and all wind speed conditions, respectively. PM10 concentrations tend to be smaller for the higher wind speed cases and decrease rapidly with increasing wind speed. The presentations of measured and modelled concentration dependence on wind direction show fairly good agreement. PM10 concentrations measured on the windward side are relatively smaller, compared with the corresponding results for the leeward side. This study demonstrates that it is possible to use the OSPM to model PM10 dispersion rules for an urban street canyon.
An Inter-hemispheric Teleconnection and a Possible Mechanism for Its Formation
LIU Ge, JI Liren, SUN Shuqing, ZHANG Qingyun
2010, 27(3): 629-638. doi: 10.1007/s00376-009-8172-x
Through observational analyses, an inter-hemispheric teleconnection is identified between the subtropical region of the South China Sea to the western Pacific near the Philippines (WP) and the region to the east of Australia (AE). The teleconnection is significantly correlated with sea surface temperature anomalies (SSTAs) in key sea areas (including the Indian Ocean, the South China Sea, and the area to the east of Australia). Based on the IAP T42L9 model, numerical experiments are performed to explore a possible mechanism for the formation of the teleconnection. The results show that the positive SST anomalies in the key sea areas may jointly contribute to the occurrence and maintenance of the positive geopotential height anomalies over both the WP and AE and be a critical factor in the teleconnection formation. The large-scale SST anomaly in the Indian Ocean, involving the tropics and subtropics of both hemispheres, may lead to concurrent atmospheric responses over both the WP and AE, while the effect of the local SST anomaly to the east of Australia seems to reinforce and maintain the positive height anomaly over the AE.
Fog Research in China: An Overview
NIU Shengjie, LU Chunsong, YU Huaying, ZHAO Lijuan, LU Jingjing
2010, 27(3): 639-662. doi: 10.1007/s00376-009-8174-8
Fog can adversely affect human activity directly and indirectly, resulting in large losses both in terms of the local economy and lives. Much effort has been devoted to studies of fog across many areas of China, and in that context this paper aims to summarize climatic characteristics and review fog field experiments and their major results relating to fog mechanisms, physical properties and chemical characteristics. Progress in the application of remote sensing techniques and numerical simulation in fog research are also discussed. In particular, the effects of urbanization and industrialization on fog are highlighted. To end, perspectives on future fog research are outlined. The goal of this review paper is to introduce fog research in China to the global academic community and thus promote international collaboration on fog research. This is important because most papers on fog in China are published in Chinese, which are unreadable for the vast majority of non-Chinese researchers.
Size Distributions and Elemental Compositions of Particulate Matter on Clear, Hazy and Foggy days in Beijing, China
YANG Yongjie, WANG Yuesi, HUANG Weiwei, HU Bo, WEN Tianxue, ZHAO Yanan
2010, 27(3): 663-675. doi: 10.1007/s00376-009-8197-1
Total suspended particulates (TSP) samples were collected using low pressure impactors (Andersen Series 20-800, USA) on typical clear, hazy and foggy days in Beijing in order to investigate the characteristics of size distributions and elemental compositions of particulate matter (PM) in different weather conditions. The concentrations of sixteen elements, including Na, Mg, Al, K, Ca, Mn, Fe, Ni, Cu, Zn, As, Se, Cd, Ba, Tl and Pb were detected using inductively coupled plasma mass spectrometry (ICP-MS). The results showed that Ca, Al, Fe, Mg and Ba on foggy days were 2.0--2.6 times higher than on clear days, and 2.3--2.9 times higher than on hazy days. Concentrations of Cu, Zn, As, Se and Pb on foggy days were 163.5, 1186.7, 65.9, 32.0 and 708.2 ng m-3, respectively, in fine particles, and 68.1, 289.5, 19.8, 1.6 and 103.8 ng m-3, respectively, in coarse particles. This was 1.0--8.4 times higher and 1.4--7.4 times higher than on clear and hazy days, respectively. It is then shown that Mg, Al, Fe, Ca and Ba were mainly associated with coarse particles, peaking at 4.7--5.8 μm; that Cd, Se, Zn, As, Tl and Pb were most dominant in fine particles, peaking at 0.43--1.1 μm; and that Na, K, Ni, Cu and Mn had a multi-mode distribution, with peaks at 0.43--1.1 μm and 4.7--5.8 μm. The enrichment factors indicated that coal combustion along with vehicle and industry emissions may be the main sources of pollution elements.
The Development of a Nonhydrostatic Global Spectral Model
2010, 27(3): 676-684. doi: 10.1007/s00376-009-9080-9
With the development of numerical weather prediction technology, the traditional global hydrostatic models used in many countries of the world for operational weather forecasting and numerical simulations of general circulation have become more and more unfit for high-impact weather prediction. To address this, it is important to invest in the development of global nonhydrostatic models. Few existing nonhydrostatic global models use consistently the grid finite difference scheme for the primitive equations of dynamical cores, which can subsequently degrade the accuracy of the calculations. A new nonhydrostatic global spectral model, which utilizes the Eulerian spectral method, is developed here from NCAR Community Atmosphere Model 3.0 (CAM3.0). Using Janjic9s hydrostatic/nonhydrostatic method, a global nonhydrostatic spectral method for the primitive equations has been formulated and developed. In order to retain the integrity of the nonhydrostatic equations, the atmospheric curvature correction and eccentricity correction are considered. In this paper, the Held-Suarez idealized test and an idealized baroclinic wave test are first carried out, which shows that the nonhydrostatic global spectral model has similar climate states to the results of many other global models for long-term idealized integration, as well as better simulation ability for short-term idealized integration. Then, a real case experiment is conducted using the new dynamical core with the full physical parameterizations of subgrid-scale physical processes. The 10-day numerical integration indicates a decrease in systematic error and a better simulation of zonal wind, temperature, and 500-hPa height.
Finescale Spiral Rainbands Modeled in a High-Resolution Simulation of Typhoon Rananim (2004)
LI Qingqing, DUAN Yihong, YU Hui, FU Gang
2010, 27(3): 685-704. doi: 10.1007/s00376-009-9127-y
Finescale spiral rainbands associated with Typhoon Rananim (2004) with the band length ranging from 10 to nearly 100 km and band width varying from 5 to 15 km are simulated using the Fifth-Generation NCAR/Penn State Mesoscale Model (MM5). The finescale rainbands have two types: one intersecting the eyewall and causing damaging wind streaks, and the other distributed azimuthally along the inner edge of the eyewall with a relatively short lifetime. The formation of the high-velocity wind streaks results from the interaction of the azimuthal flow with the banded vertical vorticity structure triggered by tilting of the horizontal vorticity. The vertical advection of azimuthal momentum also leads to acceleration of tangential flow at a relatively high altitude. The evolution and structures of the bands are also examined in this study. Further investigation suggests that the boundary inflection points are related tightly to the development of the finescale rainbands, consistent with previous findings using simple symmetric models. In particular, the presence of the level of inflow reversal in the boundary layer is a crucial factor controlling the formation of these bands. The near-surface wavy peaks of vertical vorticity always follow the inflection points in radial flow. The mesoscale vortices and associated convective updrafts in the eyewall are considered to strengthen the activity of finescale bands, and the updrafts can trigger the formation of the bands as they reside in the environment with inflow reversal in the boundary layer.
Temporal Variability in Fine Carbonaceous Aerosol over Two Years in Two Megacities: Beijing and Toronto
YANG Fumo, Jeffrey BROOK, HE Kebin, DUAN Fengkui, MA Yongliang
2010, 27(3): 705-714. doi: 10.1007/s00376-009-9103-6
Time-series of weekly total carbon (TC) concentrations of fine aerosol particles (PM2.5 in Beijing and Toronto were compared to investigate their respective levels and temporal patterns over two years from August 2001 through July 2003. In addition to this comparison, differences in the factors contributing to the observed concentrations and their temporal variations are discussed. Based upon past knowledge about the two megacities with highly contrasting air pollutant levels, it is not surprising that the average TC concentration in Beijing (31.5 μg C m-3) was greater than that in Toronto by a factor of 8.3. Despite their large concentration differences, in both cities TC comprised a similarly large component of PM2.5. TC concentrations exhibited very different seasonal patterns between the two cities. In Beijing, TC experienced higher levels and greater weekly fluctuations in winter whereas in Toronto this behavior was seen in summer. As a result, the greatest gap in TC concentrations between Beijing and Toronto (by a factor of 12.7) occurred in winter, while the smallest gap (a factor of 4.6) was in summer. In Beijing, seasonal variations in the emissions probably played a greater role than meteorology in influencing the TC seasonality, while in Toronto during the warm months more than 80% of the hourly winds were recorded from the south, along with many potential anthropogenic sources for the days with high TC concentrations. This comparison of the differences provides insight into the major factors affecting carbonaceous aerosol in each city.