Urbanization has stronger impacts than regional climate change on wind stilling: a lesson from South Korea

Wind stilling has been observed in many regions across the Northern Hemisphere; however, the related mechanisms are not well understood. Analyses of the wind speed variations in South Korea during 1993–2015 in this study reveal that the annual-mean surface wind speeds at rural stations have increased by up to 0.41 m s ^−1 decade ^−1 , while those at urban stations have decreased by up to −0.63 m s ^−1 decade ^−1 . The local wind speed variations are found to be negatively correlated with the population density at the corresponding observation sites. Gustiness analyses show the increase in local surface roughness due to urbanization can explain the observed negative wind speed trends at urban stations as the urbanization effect overwhelms the positive wind speed trend due to climate change. The observed negative wind speed trend in urban areas are not found in the regional climate model simulations in the Coordinated Regional Climate Downscaling Experiment—East Asia (CORDEX-EA) as these models do not take into account the impact of urbanization on wind variations during the period. This study suggests that urbanization can play an important role in the recent wind stilling in rapidly developing regions such as South Korea. Our results suggest that future climate projections in CORDEX-EA may overestimate wind speeds in urban areas, and that future regional climate projections need to consider the effects of urbanization for a more accurate projection of wind speeds

Comprehensive assessment of vertical variations in urban atmospheric CO2 concentrations by using tall tower measurement and an atmospheric transport model

© 2022 Elsevier B.V.In this study, we performed a comprehensive assessment of the vertical CO2 concentration in the urban atmosphere using measurements at two different heights (113 m and 420 m) in Seoul, South Korea. The difference in CO2 concentration between the two altitudes (△CO2 = CO2 at 113 m minus CO2 at 420 m) showed a significant diurnal variation, with the highest at 07:00 (19.9 ppm) and the lowest at 16:00 (3.9 ppm). When the planetary boundary layer (PBL) rose above the two sites (daytime), the CO2 concentrations at the two altitudes were highly correlated (r = 0.87) with low △CO2. In contrast, when the PBL was located between the two sites (night time), the correlation coefficient of the CO2 concentration between the two altitudes decreased by 0.55 with a high △CO2. To explain the cause of this variation in △CO2 according to PBL, we performed Weather Research and Forecasting-stochastic time-inverted Lagrangian transport (WRF-STILT) simulations. Simulations showed that CO2 measurements at two different heights were influenced by the same nearby urban areas during the daytime. However, the site above the PBL only measured the CO2 of air transported from the outside downtown area during the night time. Consequently, the observed night time △CO2 is explained by the difference in air mass between the two measurements owing to PBL variations. The night time △CO2 further implicates the local attribution of observed CO2 below the PBL by removing the effect from the remote area. Because of this unique night time characteristic of △CO2, we evaluated the changes in CO2 concentration in Seoul during the COVID-19 period. Compared to the pre-COVID-19 period, △CO2 clearly decreased from 26.5 ppm to 6.2 ppm with the implementation of social distancing, thus confirming the decreasing local influence of CO2 concentrations. Our findings highlight the potential of atmospheric CO2 monitoring at high altitudes as an observation-based method to assess the effectiveness of local carbon management.N

Spatiotemporal variations in urban CO2 flux with land-use types in Seoul

Background Cities are a major source of atmospheric CO2; however, understanding the surface CO2 exchange processes that determine the net CO2 flux emitted from each city is challenging owing to the high heterogeneity of urban land use. Therefore, this study investigates the spatiotemporal variations of urban CO2 flux over the Seoul Capital Area, South Korea from 2017 to 2018, using CO2 flux measurements at nine sites with different urban land-use types (baseline, residential, old town residential, commercial, and vegetation areas). Results Annual CO2 flux significantly varied from 1.09 kg C m(- 2) year(- 1) at the baseline site to 16.28 kg C m(- 2) year(- 1) at the old town residential site in the Seoul Capital Area. Monthly CO2 flux variations were closely correlated with the vegetation activity (r = - 0.61) at all sites; however, its correlation with building energy usage differed for each land-use type (r = 0.72 at residential sites and r = 0.34 at commercial sites). Diurnal CO2 flux variations were mostly correlated with traffic volume at all sites (r = 0.8); however, its correlation with the floating population was the opposite at residential (r = - 0.44) and commercial (r = 0.80) sites. Additionally, the hourly CO2 flux was highly related to temperature. At the vegetation site, as the temperature exceeded 24 celcius, the sensitivity of CO2 absorption to temperature increased 7.44-fold than that at the previous temperature. Conversely, the CO2 flux of non-vegetation sites increased when the temperature was less than or exceeded the 18 celcius baseline, being three-times more sensitive to cold temperatures than hot ones. On average, non-vegetation urban sites emitted 0.45 g C m(- 2) h(- 1) of CO2 throughout the year, regardless of the temperature. Conclusions Our results demonstrated that most urban areas acted as CO2 emission sources in all time zones; however, the CO2 flux characteristics varied extensively based on urban land-use types, even within cities. Therefore, multiple observations from various land-use types are essential for identifying the comprehensive CO2 cycle of each city to develop effective urban CO2 reduction policies.N

Nonlinear response of vegetation green-up to local temperature variations in temperate and boreal forests in the Northern Hemisphere

The annual cycle of vegetation growth may be altered in response to climate changes affecting ecosystem dynamics. However, our understanding of vegetation seasonality is mostly limited to the mechanisms and attributes of phenological events, such as spring emergence and fall senescence. Here we have investigated the seasonal evolution of vegetation growth from winter dormancy to summer maturity of four forest types in the Northern Hemisphere (NH) temperate and boreal forests for 1982–2011. The present study assesses large-scale variations in the vegetation green-up rate (VGrate) and its connection to temperature variability using remotely sensed normalized difference vegetation index (NDVI) and surface air temperature. The average of the VGrate of the analysis period increases with latitude, which indicates that the canopy develops more rapidly from dormancy to maturity for vegetation in higher-latitude or colder climate zones. VGrate and precedent temperature also show a positive correlation (r) over temperate and boreal forests (67% of the forest area in the NH), indicating that increased temperatures lead to faster canopy development within the same climate zone or latitude band. Responsiveness of VGrate to temperature variability shows that despite the same magnitude of local temperature variability during extremely cold and warm years, the magnitude of VGrate acceleration in warm years (0.07 (15-day)! 1) is larger than the VGrate deceleration in cold years (!0.03 (15-day)!1), suggesting that the response of VGrate to temperature variability is nonlinear. Among the four forest types examined in this study, the nonlinear responses are most clearly observed in deciduous broadleaf forests indicating that forest composition may regulate the large-scale response of canopy development to temperature variability. Overall, our results suggest that anomalous seasonal warming will significantly affect canopy developments over wide deciduous forest areas

Short-term reduction of regional enhancement of atmospheric CO2 in China during the first COVID-19 pandemic period

Recent studies have reported a 9% decrease in global carbon emissions during the COVID-19 lockdown period; however, its impact on the variation of atmospheric CO2 level remains under question. Using atmospheric CO2 observed at Anmyeondo station (AMY) in South Korea, downstream of China, this study examines whether the decrease in China's emissions due to COVID-19 can be detected from the enhancement of CO2 mole fraction (Delta CO2) relative to the background value. The Weather Research and Forecasting-Stochastic Time-Inverted Lagrangian Transport model was applied to determine when the observed mole fractions at AMY were affected by air parcels from China. Atmospheric observations at AMY showed up to a -20% (-1.92 ppm) decrease in Delta CO2 between February and March 2020 compared to the same period in 2018 and 2019, particularly with a -34% (-3.61 ppm) decrease in March. Delta CO, which was analyzed to explore the short-term effect of emission reductions, had a decrease of -43% (-80.66 ppb) during the lockdown in China. Particularly in East China, where emissions are more concentrated than in Northeast China, Delta CO2 and Delta CO decreased by -44% and -65%, respectively. The Delta CO/Delta CO2 ratio (24.8 ppb ppm(-1)), which is the indicator of emission characteristics, did not show a significant difference before and after the COVID-19 lockdown period (alpha = 0.05), suggesting that this decrease in Delta CO2 and Delta CO was associated with emission reductions rather than changes in emission sources or combustion efficiency in China. Reduced carbon emissions due to limited human activity resulted in a decrease in the short-term regional enhancement to the observed atmospheric CO2.N

Short-term reduction of regional enhancement of atmospheric CO2in China during the first COVID-19 pandemic period

International audienceRecent studies have reported a 9% decrease in global carbon emissions during the COVID-19 lockdown period; however, its impact on the variation of atmospheric CO2 level remains under question. Using atmospheric CO2 observed at Anmyeondo station (AMY) in South Korea, downstream of China, this study examines whether the decrease in China's emissions due to COVID-19 can be detected from the enhancement of CO2 mole fraction (ΔCO2) relative to the background value. The Weather Research and Forecasting-Stochastic Time-Inverted Lagrangian Transport model was applied to determine when the observed mole fractions at AMY were affected by air parcels from China. Atmospheric observations at AMY showed up to a -20% (-1.92 ppm) decrease in ΔCO2 between February and March 2020 compared to the same period in 2018 and 2019, particularly with a -34% (-3.61 ppm) decrease in March. ΔCO, which was analyzed to explore the short-term effect of emission reductions, had a decrease of -43% (-80.66 ppb) during the lockdown in China. Particularly in East China, where emissions are more concentrated than in Northeast China, ΔCO2 and ΔCO decreased by -44% and -65%, respectively. The ΔCO/ΔCO2 ratio (24.8 ppb ppm-1), which is the indicator of emission characteristics, did not show a significant difference before and after the COVID-19 lockdown period (α = 0.05), suggesting that this decrease in ΔCO2 and ΔCO was associated with emission reductions rather than changes in emission sources or combustion efficiency in China. Reduced carbon emissions due to limited human activity resulted in a decrease in the short-term regional enhancement to the observed atmospheric CO2

Enhanced regional terrestrial carbon uptake over Korea revealed by atmospheric CO 2 measurements from 1999 to 2017

International audienceUnderstanding changes in terrestrial carbon balance is important to improve our knowledge of the regional carbon cycle and climate change. However, evaluating regional changes in the terrestrial carbon balance is challenging due to the lack of surface flux measurements. This study reveals that the terrestrial carbon uptake over the Republic of Korea has been enhanced from 1999 to 2017 by analyzing long‐term atmospheric CO2 concentration measurements at the Anmyeondo Station (36.53°N, 126.32°E) located in the western coast. The influence of terrestrial carbon flux on atmospheric CO2 concentrations (ΔCO2) is estimated from the difference of CO2 concentrations that were influenced by the land sector (through easterly winds) and the Yellow Sea sector (through westerly winds). We find a significant trend in ΔCO2 of −4.75 ppm per decade (p < .05) during the vegetation growing season (May through October), suggesting that the regional terrestrial carbon uptake has increased relative to the surrounding ocean areas. Combined analysis with satellite measured normalized difference vegetation index and gross primary production shows that the enhanced carbon uptake is associated with significant nationwide increases in vegetation and its production. Process‐based terrestrial model and inverse model simulations estimate that regional terrestrial carbon uptake increases by up to 18.9 and 8.0 Tg C for the study period, accounting for 13.4% and 5.7% of the average annual domestic carbon emissions, respectively. Atmospheric chemical transport model simulations indicate that the enhanced terrestrial carbon sink is the primary reason for the observed ΔCO2 trend rather than anthropogenic emissions and atmospheric circulation changes. Our results highlight the fact that atmospheric CO2 measurements could open up the possibility of detecting regional changes in the terrestrial carbon cycle even where anthropogenic emissions are not negligible