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1.
In this study,cloud base height(CBH) and cloud top height(CTH) observed by the Ka-band(33.44 GHz) cloud radar at the Boseong National Center for Intensive Observation of Severe Weather during fall 2013(September-November) were verified and corrected.For comparative verification,CBH and CTH were obtained using a ceilometer(CL51) and the Communication,Ocean and Meteorological Satellite(COMS).During rainfall,the CBH and CTH observed by the cloud radar were lower than observed by the ceilometer and COMS because of signal attenuation due to raindrops,and this difference increased with rainfall intensity.During dry periods,however,the CBH and CTH observed by the cloud radar,ceilometer,and COMS were similar.Thin and low-density clouds were observed more effectively by the cloud radar compared with the ceilometer and COMS.In cases of rainfall or missing cloud radar data,the ceilometer and COMS data were proven effective in correcting or compensating the cloud radar data.These corrected cloud data were used to classify cloud types,which revealed that low clouds occurred most frequently. 相似文献
2.
Flow resistance, ventilation, and pollutant removal for idealized two-dimensional (2D) street canyons of different building-height to street-width (aspect) ratios $AR$ are examined using the friction factor $f$ , air exchange rate (ACH), and pollutant exchange rate (PCH), respectively, calculated by large-eddy simulation (LES). The flows are basically classified into three characteristic regimes, namely isolated roughness, wake interference, and skimming flow, as functions of the aspect ratios. The LES results are validated by various experimental and numerical datasets available in the literature. The friction factor increases with decreasing aspect ratio and reaches a peak at $AR = 0.1$ in the isolated roughness regime and decreases thereafter. As with the friction factor, the ACH increases with decreasing aspect ratio in the wake interference and skimming flow regimes, signifying the improved aged air removal for a wider street canyon. The PCH exhibits a behaviour different from its ACH counterpart in the range of aspect ratios tested. Pollutants are most effectively removed from the street canyon with $AR = 0.5$ . However, a minimum of PCH is found nearby at $AR = 0.3$ , at which the pollutant removal is sharply weakened. Besides, the ACH and PCH are partitioned into the mean and turbulent components to compare their relative contributions. In line with our earlier Reynolds-averaged Navier–Stokes calculations (Liu et al., Atmos Environ 45:4763–4769, 2011), the current LES shows that the turbulent components contribute more to both ACH and PCH, consistently demonstrating the importance of atmospheric turbulence in the ventilation and pollutant removal for urban areas. 相似文献
3.
Hyunjung Kim Hyun Mee Kim Jinwoong Kim Chun-Ho Cho 《Asia-Pacific Journal of Atmospheric Sciences》2018,54(1):1-17
In this study, CarbonTracker, an inverse modeling system based on the ensemble Kalman filter, was used to evaluate the effects of data assimilation parameters (assimilation window length and ensemble size) on the estimation of surface CO2 fluxes in Asia. Several experiments with different parameters were conducted, and the results were verified using CO2 concentration observations. The assimilation window lengths tested were 3, 5, 7, and 10 weeks, and the ensemble sizes were 100, 150, and 300. Therefore, a total of 12 experiments using combinations of these parameters were conducted. The experimental period was from January 2006 to December 2009. Differences between the optimized surface CO2 fluxes of the experiments were largest in the Eurasian Boreal (EB) area, followed by Eurasian Temperate (ET) and Tropical Asia (TA), and were larger in boreal summer than in boreal winter. The effect of ensemble size on the optimized biosphere flux is larger than the effect of the assimilation window length in Asia, but the importance of them varies in specific regions in Asia. The optimized biosphere flux was more sensitive to the assimilation window length in EB, whereas it was sensitive to the ensemble size as well as the assimilation window length in ET. The larger the ensemble size and the shorter the assimilation window length, the larger the uncertainty (i.e., spread of ensemble) of optimized surface CO2 fluxes. The 10-week assimilation window and 300 ensemble size were the optimal configuration for CarbonTracker in the Asian region based on several verifications using CO2 concentration measurements. 相似文献
4.
A large-eddy simulation (LES) with a one-equation subgrid-scale (SGS) model was developed to investigate the flow field and
pollutant dispersion inside street canyons of high aspect ratio (AR). A 1/7th power-law wall model was implemented near rigid
walls to mitigate the demanding near-wall resolution requirements in LES. This LES model had been extensively validated against
experimental results for street canyons of AR = 1 and 2 before it was applied to the cases of AR = 3 and 5. A ground-level
passive pollutant line source, located in the middle of the street, was used to simulate vehicular emissions. Three and five
vertically aligned primary recirculations were developed in the street canyons of AR 3 and 5, respectively. The ground-level
mean wind speed was less than 0.5% of the free stream value, which makes it difficult for the pollutant to be transported
upward for removal. High pollutant concentration and variance were found near the buildings where the air flow is upwards.
It was found that the velocity fluctuation, pollutant concentration and variance were all closely related to the interactions
between the primary recirculations and/or the free surface layer. Several quantities, which are non-linear functions of AR,
were introduced to quantify the air quality in street canyons of different configurations. 相似文献
5.
Kwang-Yul Kim Seoyeon Lee Maeng-Ki Kim Chun-Ho Cho 《Asia-Pacific Journal of Atmospheric Sciences》2014,50(4):519-529
Cold surge statistics have been analyzed from the 52-year (1961–2012) winter temperature data archived at Korea Meteorological Administration. Despite a significant winter warming in South Korea, there is no statistically affirmable sign of the occurrence frequency, duration and total days of cold surges to have changed in the record; there is little abatement in the frequency, duration, and total days of cold surges. Thermal advection anomalies were also derived from the NCEP/NCAR and ERA interim reanalysis datasets. Cold surges defined in terms of thermal advection anomalies do not exhibit any statistically significant change either. The increased mean and the decreased variance of thermal advection both indicate that cold advection from north has, in fact, decreased gradually in South Korea. It appears that cold surges are statistically rare enough to be affected by increased mean and decreased variance of thermal advection. Polar warming is often interpreted as weakening of jet stream and increasing southward flux of cold air. Analysis of thermal advection, on the other hand, does not show increased advection of cold air from north in South Korea. 相似文献
6.
A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the
flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity
aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow
in the streamwise direction x. The flow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence
above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind
and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum
wind velocity standard deviations σ
i
in the streamwise (σ
u
), spanwise (σ
v
) and vertical (σ
w
) directions are located near the roof-level windward corners. Moreover, a second σ
w
peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σ
i
by the local friction velocity u
*, it is found that σ
u
/u
* ≈ 1.8, σ
v
/u
* ≈ 1.3 and σ
w
/u
* ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux
u′′w′′ shows that, while the inward and outward interactions are small, the sweeps and ejections dominate the momentum transport
over the street canyons. In the x direction, the two-point correlations of velocity R
v,x
and R
w,x
drop to zero at a separation larger than h but R
u,x
(= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient Ω
T
of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons. 相似文献
7.
The Gaussian model of plume dispersion is commonly used for pollutant concentration estimates. However, its major parameters, dispersion coefficients, barely account for terrain configuration and surface roughness. Large-scale roughness elements (e.g. buildings in urban areas) can substantially modify the ground features together with the pollutant transport in the atmospheric boundary layer over urban roughness (also known as the urban boundary layer, UBL). This study is thus conceived to investigate how urban roughness affects the flow structure and vertical dispersion coefficient in the UBL. Large-eddy simulation (LES) is carried out to examine the plume dispersion from a ground-level pollutant (area) source over idealized street canyons for cross flows in neutral stratification. A range of building-height-to-street-width (aspect) ratios, covering the regimes of skimming flow, wake interference, and isolated roughness, is employed to control the surface roughness. Apart from the widely used aerodynamic resistance or roughness function, the friction factor is another suitable parameter that measures the drag imposed by urban roughness quantitatively. Previous results from laboratory experiments and mathematical modelling also support the aforementioned approach for both two- and three-dimensional roughness elements. Comparing the UBL plume behaviour, the LES results show that the pollutant dispersion strongly depends on the friction factor. Empirical studies reveal that the vertical dispersion coefficient increases with increasing friction factor in the skimming flow regime (lower resistance) but is more uniform in the regimes of wake interference and isolated roughness (higher resistance). Hence, it is proposed that the friction factor and flow regimes could be adopted concurrently for pollutant concentration estimate in the UBL over urban street canyons of different roughness. 相似文献
8.
Kwang-Yul Kim Ji-Won Kim Maeng-Ki Kim Chun-Ho Cho 《Asia-Pacific Journal of Atmospheric Sciences》2013,49(5):675-685
Daily winter temperatures in Korea have been analyzed via CSEOF analysis. Then, each PC time series was detrended and was fitted to an AR (autoregressive) model. Based on the identified AR model, an artificial time series of arbitrary length can be generated by using an arbitrary white-noise time series. In this way, one hundred new sets of PC time series were generated over the period of 1973–2058. Then, the trend for each PC time series was added back to the artificial PC time series extending the trend until 2058. Ultimately, artificial daily winter temperatures in Korea have been constructed by using the artificial PC time series and the original loading vectors derived from the observational data. The 100 new data sets have been investigated in order to understand the winter temperature variability 50 years into the future. Regression analysis in CSEOF space shows that temperature increase in Korea is associated with increased 850-hPa air temperature over most of the Asian domain (97°-153°E × 22°-73°N) and increased 850-hPa geopotential height in the southern part of the domain. As a result, southerly and southeasterly wind anomalies develop carrying positive temperature anomalies northward and northwestward. Both the 200-hPa air temperature and geopotential height changes indicate that there will be fairly significant northward shift of the jet stream in future. The standard deviation of the 200-hPa potential vorticity increases implying that shortwave trough and henceforth baroclinic instability will increase in future. Finally, GEV (Generalized Extreme Value) distribution and GPD (Generalized Pareto distribution) distribution have been compared between the observational records and the future records of the same length. The extreme value distributions based on the synthetic datasets show that warm extreme events will be more extreme in future and cold extreme events, on the other hand, will be less extreme. This study provides an estimate of future temperatures based on the observational data and serves as an independent baseline solution for comparisons with numerical model solutions. 相似文献
9.
Sanghun Lee Deg-Hyo Bae Chun-Ho Cho 《Asia-Pacific Journal of Atmospheric Sciences》2013,49(5):619-624
We used daily precipitation data from a global high-resolution climate scenario to analyze the features of future precipitation including extreme and heavy rainfall. The scenario shows that the model reproduces the daily precipitation over South Korea well. The projections show an increase in annual precipitation of approximately 18% in the late 21st century, with the highest increase (38%) occurring in winter. The number of days with daily precipitation of less than 5 mm decreases, but that of daily precipitation of more than 5 mm increases slightly in the latter part of the 21st century. The peak of precipitation days shifted from July to August. The number of days with relatively small amounts of precipitation (10 and 30 mm d?1) increases most substantially in the winter season, but that for large amounts of precipitation (50, 80, 100, and 130 mm d?1) increases most in the summer season. Events with heavy precipitation rates of 100 and 130 mm d?1 are expected to occur in the winter season in the late 21st century, although no such events occurred during the winter season in the reference period. 相似文献
10.
Variation of soil moisture during active and weak phases of summer monsoon JJAS (June, July, August, and September) is very important for sustenance of the crop and subsequent crop yield. As in situ observations of soil moisture are few or not available, researchers use data derived from remote sensing satellites or global reanalysis. This study documents the intercomparison of soil moisture from remotely sensed and reanalyses during dry spells within monsoon seasons in central India and central Myanmar. Soil moisture data from the European Space Agency (ESA)—Climate Change Initiative (CCI) has been treated as observed data and was compared against soil moisture data from the ECMWF reanalysis-Interim (ERA-I) and the climate forecast system reanalysis (CFSR) for the period of 2002–2011. The ESA soil moisture correlates rather well with observed gridded rainfall. The ESA data indicates that soil moisture increases over India from west to east and from north to south during monsoon season. The ERA-I overestimates the soil moisture over India, while the CFSR soil moisture agrees well with the remotely sensed observation (ESA). Over Myanmar, both the reanalysis overestimate soil moisture values and the ERA-I soil moisture does not show much variability from year to year. Day-to-day variations of soil moisture in central India and central Myanmar during weak monsoon conditions indicate that, because of the rainfall deficiency, the observed (ESA) and the CFSR soil moisture values are reduced up to 0.1 m3/m3 compared to climatological values of more than 0.35 m3/m3. This reduction is not seen in the ERA-I data. Therefore, soil moisture from the CFSR is closer to the ESA observed soil moisture than that from the ERA-I during weak phases of monsoon in the study region.
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