Does the restoration of an inner-city stream in Seoul affect local thermal environment?

Summary Changes in local thermal environment associated with the restoration of an inner-city stream in Seoul, Korea, are investigated using observational data. The stream, called the Cheonggye stream, which had been hidden and covered with cement/asphalt for 46 years, runs 5.8 km eastward through a central region of Seoul. Intensive observations were made in the stream area for a number of summertime periods before, during, and after the stream restoration to detect the effects of the stream on local environment and to quantify them. It is estimated that after the stream restoration the near-surface temperature averaged over the stream area dropped by 0.4 °C, with the largest local temperature drop being 0.9 °C. However, it cannot be stated that this 0.4 °C temperature drop is due entirely to the stream effect only, because synoptic-scale and local-scale weather conditions during the two periods were inevitably not identical. The stream effect on air temperature is also evident in the temperature distribution along a street traversing the stream. In the daytime after the stream restoration, the sensible heat flux was greatly reduced and the ratio of sensible heat flux to net radiative flux dramatically decreased. These first-time results of the restored-stream effects on urban thermal environment could contribute to the scientific basis of urban planning which aims to make a large city comfortable to live in and nature- and environment-friendly. Authors’ addresses: Yeon-Hee Kim, Il-Soo Park, Hae-Jung Koo, and Jae-Cheol Nam, Meteorological Research Institute, Korea Meteorological Administration, Seoul 156-720, Korea; Sang-Boom Ryoo, Suwon Meteorological Office, Korea Meteorological Administration, Suwon 441-856, Korea; Jong-Jin Baik, School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea     

Indicators of Climate Warming in Minnesota: Lake ICE Covers and Snowmelt Runoff

Records of hydrologic parameters, especially those parameters that are directly linked to air temperature, were analyzed to find indicators of recent climate warming in Minnesota, USA. Minnesota is projected to be vulnerable to climate change because of its location in the northern temperate zone of the globe. Ice-out and ice-in dates on lakes, spring (snowmelt) runoff timing, spring discharge values in streams, and stream water temperatures recorded up to the year 2002 were selected for study. The analysis was conducted by inspection of 10-year moving averages, linear regression on complete and on partial records, and by ranking and sorting of events. Moving averages were used for illustrative purposes only. All statistics were computed on annual data. All parameters examined show trends, and sometimes quite variable trends, over different periods of the record. With the exception of spring stream flow rates the trends of all parameters examined point toward a warming climate in Minnesota over the last two or three decades. Although hidden among strong variability from year to year, ice-out dates on 73 lakes have been shifting to an earlier date at a rate of −0.13 days/year from 1965 to 2002, while ice-in dates on 34 lakes have been delayed by 0.75 days/year from 1979 to 2002. From 1990 to 2002 the rates of change increased to −0.25 days/year for ice-out and 1.44 days/year for ice-in. Trend analyses also show that spring runoff at 21 stream gaging sites examined occurs earlier. From 1964 to 2002 the first spring runoff (due to snowmelt) has occurred −0.30 days/year earlier and the first spring peak runoff −0.23 days/year earlier. The stream water temperature records from 15 sites in the Minneapolis/St Paul metropolitan area shows warming by 0.11^∘C/year, on the average, from 1977 to 2002. Urban development may have had a strong influence. The analysis of spring stream flow rates was inconclusive, probably because runoff is linked as much to precipitation and land use as to air temperature. Ranking and sorting of annual data shows that a disproportionately large number of early lake ice-out dates has occurred after 1985, but also between 1940 and 1950; similarly late lake ice-in has occurred more frequently since about 1990. Ranking and sorting of first spring runoff dates also gave evidence of earlier occurrences, i.e. climate warming in late winter. A relationship of changes in hydrologic parameters with trends in air temperature records was demonstrated. Ice-out dates were shown to correlate most strongly with average March air temperatures shifting by −2.0 days for a 1^°C increase in March air temperature. Spring runoff dates also show a relationship with March air temperatures; spring runoff dates shift at a rate of −2.5 days/1^°C minimum March air temperature change. Water temperatures at seven river sites in the Minneapolis/St Paul metropolitan area show an average rise of 0.46^°C in river temperature/1^°C mean annual air temperature change, but this rate of change probably includes effects of urban development. In conclusion, records of five hydrologic parameters that are closely linked to air temperature show a trend that suggests recent climate warming in Minnesota, and especially from 1990 to 2002. The recent rates of change calculated from the records are very noteworthy, but must not be used to project future parameter values, since trends cannot continue indefinitely, and trend reversals can be seen in some of the long-term records.     

长沙城市化进程对局地气候的影响

利用1961—2010年湖南省长沙及周边郊县共5个气象站的逐日气温、降水和相对湿度观测数据,在对各气象要素资料序列进行均一性检验和订正的基础上,以郊县站作为背景场,系统地分析了长沙城市化不同阶段对城市气候的影响。结果表明:受城市化的影响,近50年长沙市区地面气温及其上升速率远高于郊县的,城市热岛效应明显,热岛效应对市区年平均气温的增温贡献率达30%,且主要出现在20世纪90年代之后,四季中城市化贡献率的最大值出现在夏季,秋季和春季的次之,冬季的最小。50年来长沙市区和郊县年降水量差值序列呈显著增加趋势,长沙城市雨岛效应较为显著,且主要发生在春季和夏季,城市化带来的雨岛效应加速了长沙降水结构两极分化,使城市内涝发生的概率进一步增大。50年来长沙市区和郊县年平均相对湿度及其差值的变化趋势均不显著,就全年平均而言,长沙的城市干岛效应并不明显,但近20年来干岛效应显著增强;分季节来看,近50年干岛效应主要发生在夏季。     

Thermal Energy Balance Analysis of the Tokyo Metropolitan Area Using a Mesoscale Meteorological Model Incorporating an Urban Canopy Model

The summer climate around the Tokyo metropolitan area has been analysed on an urban scale, and the regional characteristics of the thermal energy balance of a bayside business district in the centre of Tokyo (Otemachi) have been compared with an inland residential district (Nerima), using a mesoscale meteorological model incorporating an urban canopy model. From the results of the analysis, the mechanism of diurnal change in air temperature and absolute humidity in these areas is quantitatively demonstrated, with a focus on the thermal energy balance. Moreover, effective countermeasures against urban heat-islands are considered from the viewpoint of each region’s thermal energy balance characteristics. In addition to thermal energy outflux by turbulent diffusion, advection by sea-breezes from Tokyo Bay discharges sensible heat in Otemachi. This mitigates temperature increases during the day. On the other hand, because all sea-breezes must first cross the centre of Tokyo, it has less of a cooling effect in Nerima. As a result, the air temperature during the day in Nerima is higher than that in Otemachi.     

A numerical study of the influence of urban expansion on monthly climate in dry autumn over the Pearl River Delta, China

Summary In this study, we employed a regional model to simulate the impact of urban expansion on monthly climate in Pearl River Delta (PRD) region. Two experiments were performed by prescribing two different land covers in the PRD region. One land cover represents vegetation in the 1970s which is derived from the United States Geological Survey (USGS) data with 24-category (hereafter referred to as NU). The other land cover represents the current urban condition which is derived from remote sensing data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) in 2004 (hereafter referred to as HU). Using the two land cover datasets, monthly climate of October 2004 was simulated, which was a very dry season in the PRD region. The results obtained from the numerical simulation show a distinct difference in simulated shelter-level temperature, humidity, surface fluxes and the height of planetary boundary layer (PBL) with two different land cover data sets being specified. The maximum difference in simulated monthly mean temperature over urban areas was 0.9 °C. A large temperature difference was found in urbanized area in Guangzhou, Dongguan, Zhongshan and Shenzhen. The monthly mean relative humidity in urban areas decreased by 1.4% as a result of urban expansion (from 59.2% in NU to 57.8% in HU). The maximum decrease in mixing ratio was 0.4 g/kg in Guangzhou and Dongguan, whereas the maximum decrease in relative humidity was 2.4%. There was an increase of sensible heat flux in developed lands and the maximum increase was 90 W m⁻². In contrast, latent hear flux in urban area decreased and the maximum decrease was 300 W m⁻². In addition, the increase in mean height of PBL ranged from 20 to 80 m (HU compared with NU), and the maximum change of the height was 180 m over urban area in city of Guangzhou.     

Relative effects of climate change and wildfires on stream temperatures: a simulation modeling approach in a Rocky Mountain watershed

Freshwater ecosystems are warming globally from the direct effects of climate change on air temperature and hydrology and the indirect effects on near-stream vegetation. In fire-prone landscapes, vegetative change may be especially rapid and cause significant local stream temperature increases but the importance of these increases relative to broader changes associated with air temperature and hydrology are not well understood. We linked a spatially explicit landscape fire and vegetation model (FireBGCv2) to an empirical regression equation that predicted daily stream temperatures to explore how climate change and its impacts on fire might affect stream thermal conditions across a partially forested, mountainous landscape in the western U.S. We used the model to understand the roles that wildfire and management actions such as fuel reduction and fire suppression could play in mitigating stream thermal responses to climate change. Results indicate that air temperature increases associated with future climates could account for a much larger proportion of stream temperature increases (as much as 90 % at a basin scale) than wildfire. Similarly, land management scenarios that limited wildfire prevalence had negligible effects on future stream temperature increases. These patterns emerged at broader spatial scales because wildfires typically affected only a subset of a stream’s network. However, at finer spatial and temporal scales stream temperatures were sensitive to wildfire. Although wildfires will continue to cause local, short-term effects on stream temperatures, managers of aquatic systems may need to find other solutions to cope with the larger impact from climate change on future stream warming that involves adapting to the increases while developing broad strategies for riparian vegetation restoration.     

地球上的人工气候岛和大气污染源——天气与城市水问题

文章探讨了城市对气温等气象要素的影响规律，指出了城市气候是异于周围农村气候的人工气候岛，例如城市热岛（气温），雨岛（降水量），干岛（相对湿度）等。特殊的城市条件会给城市造成灾害，例如高温灾害，城市洪灾，城市风灾，酸雨，酸雾，光化学烟雾等，城市还有它它自己特有的气象现象，如城市热岛环流（乡村风），城市是地球上的主要大气污染源，在这个人工污染岛中居住，对居民的健康是不利的。     

北京城市热岛效应的流动观测和初步研究

采用车载气象观测仪器结合GPS定位、连续数据采集系统的流动观测方法研究了北京城区内不同城市地表覆盖物对城市局地小气候的影响和气象要素分布,发现城市下垫面分布与城市的温度和湿度有密切关系,一般在城市建筑物密集、水体和植被少的区域具有较高的气温和较低的相对湿度,在建筑物稀疏和植被、水体较多的区域则相反。观测结果表明,流动观测方法是对常规气象观测资料的有益补充,也是城市气候观测中的一种有效方法。     

Climate change effects on tropical night days in Seoul, Korea

In Seoul (37.57°N, 126.97°E), South Korea (located at mid-latitudes), the frequency of tropical night (TN) days, which have been defined as days with a minimum temperature greater than 25°C, have shown an increase due to the effects of temperature and water vapor. It was found that TN days accounted for almost 10.2% (July) and 22.1% (August) of the total number of days in respective months during the last decade in Seoul, and these figures may be increasing with climatic change. The daytime and nighttime sky on TN days can contain water vapor when the monsoonal southwesterly flow prevails. This strong wind may induce moisture advection from the warm ocean, and consequently, there is much larger specific humidity over the city during TN days in comparison to non-TN days. The effect of climatic change on the specific humidity is related to an increase in the number of TN days, which has shown an upward trend of 13-day/100-year and is significantly modulated by both water vapor and air temperature during July and August. Moreover, the relative role of water vapor in increasing the frequency of TN days has become much more significant after the 1960s in comparison to that of air temperature, which may be attributed to urbanization in Seoul since the 1960s.     

武汉城市区域水陆风环流的形成与转化特征研究

大气细颗粒物PM_2.5污染引起的雾霾天气既与本地污染物排放密切有关,也受局地特殊的风场影响.本文以武汉城市区域为研究对象,分别研究了长江沿岸的江陆风环流、东湖沿岸湖陆风环流的形成与转化特征,发现江风、湖风开始时间均为07：00-08：00,一年中最大风速均能达到2 m/s左右,而春夏季江风的持续时间高于秋冬季,夏季湖风的持续时间高于春季.同时发现区域附近温度和相对湿度之间有明显的相关性,湿度变化趋势大体上跟温度变化趋势相反,且温度对相对湿度的影响存在一定的滞后性,延迟时间大约为1 h.     

Objective method for classifying air masses: an application to the analysis of Buenos Aires’ (Argentina) urban heat island intensity

Summary ?During recent years, numerous studies have examined the Buenos Aires urban climate, but the relationship between large-scale weather conditions and the Buenos Aires urban heat island (UHI) intensity has not been studied. The goal of this paper is to apply an objective synoptic climatological method to identify homogeneous air masses or weather types affecting Buenos Aires during winter, and to relate the results to the UHI intensity. A K-means clustering method was used to define six different air masses considering the 03:00, 09:00, 15:00 and 21:00 LT surface observations of dry bulb temperature, dew point, cloud cover, atmospheric pressure and wind direction and velocity at Ezeiza, the most rural meteorological station of the Buenos Aires metropolitan area (Fig. 1). Results show that the mean UHI intensity is at its maximum (2.8 °C) a few hours before sunrise when conditions are dominated by cold air masses associated with cold-core anticyclones, weak winds and low cloud cover. Inverse heat islands are found during the afternoon for all air masses indicating that surface processes are not dominant at that time. The relatively infrequent and warmest air mass is the only one that presents a mean negative urban-rural temperature difference (−0.1 °C) during the afternoon with the smallest diurnal cycle of the UHI intensity probably due to the prevailing high humidity and cloudy sky conditions. The paper provides an insight into the Buenos Aires urban–rural temperature difference under a variety of winter weather types and results could be useful to improve local daily temperature forecasts for the metropolitan area of Buenos Aires on the basis of the routine forecasts of weather types. Received October 24, 2001; revised June 12, 2002; accepted October 10, 2002     

营口和鞍山城市气候变化对比分析及原因探讨

利用1951—2005年辽宁省沿海轻工业城市营口和重工业城市鞍山的气温、降水、云量、相对湿度及平均风速等气候资料,采用对比分析方法,分析了近55 a来2城市的各类气候变化特征及产生的原因。结果表明:近55 a来营口和鞍山平均气温呈递增趋势,降水、总云量、平均风速和相对湿度均呈递减趋势,只有低云量变化趋势不同,其中营口呈递增趋势,鞍山呈递减趋势,鞍山平均气温递增趋势及总云量、平均风速和相对湿度递减趋势均强于营口同类气候要素变化;营口和鞍山城市平均气温、相对湿度、低云量和风力气候要素特征趋势变率较大,降水和总云量趋势变率相对较小。     

Stream temperature sensitivity to climate warming in California’s Sierra Nevada: impacts to coldwater habitat

Water temperature influences the distribution, abundance, and health of aquatic organisms in stream ecosystems, so understanding the impacts of climate warming on stream temperature will help guide management and restoration. This study assesses climate warming impacts on stream temperatures in California’s west-slope Sierra Nevada watersheds, and explores stream temperature modeling at the mesoscale. We used natural flow hydrology to isolate climate induced changes from those of water operations and land use changes. A 21 year time series of weekly streamflow estimates from WEAP21, a spatially explicit rainfall-runoff model were passed to RTEMP, an equilibrium temperature model, to estimate stream temperatures. Air temperature was uniformly increased by 2°C, 4°C, and 6°C as a sensitivity analysis to bracket the range of likely outcomes for stream temperatures. Other meteorological conditions, including precipitation, were unchanged from historical values. Raising air temperature affects precipitation partitioning into snowpack, runoff, and snowmelt in WEAP21, which change runoff volume and timing as well as stream temperatures. Overall, stream temperatures increased by an average of 1.6°C for each 2°C rise in air temperature, and increased most during spring and at middle elevations. Viable coldwater habitat shifted to higher elevations and will likely be reduced in California. Thermal heterogeneity existed within and between basins, with the high elevations of the southern Sierra Nevada and the Feather River watershed most resilient to climate warming. The regional equilibrium temperature modeling approach used here is well suited for climate change analysis because it incorporates mechanistic heat exchange, is not overly data or computationally intensive, and can highlight which watersheds are less vulnerable to climate warming. Understanding potential changes to stream temperatures from climate warming will affect how fish and wildlife are managed, and should be incorporated into modeling studies, restoration assessments, and licensing operations of hydropower facilities to best estimate future conditions and achieve desired outcomes.     

Simulation of urban climate with high-resolution WRF model: A case study in Nanjing, China

In this study, urban climate in Nanjing of eastern China is simulated using 1-km resolution Weather Research and Forecasting (WRF) model coupled with a single-layer Urban Canopy Model. Based on the 10-summer simulation results from 2000 to 2009 we find that the WRF model is capable of capturing the high-resolution features of urban climate over Nanjing area. Although WRF underestimates the total precipitation amount, the model performs well in simulating the surface air temperature, relative humidity, and precipitation frequency and inter-annual variability. We find that extremely hot events occur most frequently in urban area, with daily maximum (minimum) temperature exceeding 36°C (28°C) in around 40% (32%) of days. Urban Heat Island (UHI) effect at surface is more evident during nighttime than daytime, with 20% of cases the UHI intensity above 2.5°C at night. However, The UHI affects the vertical structure of Planet Boundary Layer (PBL) more deeply during daytime than nighttime. Net gain for latent heat and net radiation is larger over urban than rural surface during daytime. Correspondingly, net loss of sensible heat and ground heat are larger over urban surface resulting from warmer urban skin. Because of different diurnal characteristics of urban-rural differences in the latent heat, ground heat and other energy fluxes, the near surface UHI intensity exhibits a very complex diurnal feature. UHI effect is stronger in days with less cloud or lower wind speed. Model results reveal a larger precipitation frequency over urban area, mainly contributed by the light rain events (< 10 mm d^?1). Consistent with satellite dataset, around 10?C20% more precipitation occurs in urban than rural area at afternoon induced by more unstable urban PBL, which induces a strong vertical atmospheric mixing and upward moisture transport. A significant enhancement of precipitation is found in the downwind region of urban in our simulations in the afternoon.     

Future heat vulnerability in California, Part II: projecting future heat-related mortality

Through the 21^st century, a significant increase in heat events is likely across California (USA). Beyond any climate change, the state will become more vulnerable through demographic changes resulting in a rapidly aging population. To assess these impacts, future heat-related mortality estimates are derived for nine metropolitan areas in the state for the remainder of the century. Heat-related mortality is first assessed by initially determining historical weather-type mortality relationships for each metropolitan area. These are then projected into the future based on predicted weather types created in Part I. Estimates account for several levels of uncertainty: for each metropolitan area, mortality values are produced for five different climate model-scenarios, three different population projections (along with a constant-population model), and with and without partial acclimatization. Major urban centers could have a greater than tenfold increase in short-term increases in heat-related mortality in the over 65 age group by the 2090s.     

北京秋季一次降雪前污染天气的激光雷达观测研究

以2009年11月5~8日北京地区发生的一次特殊天气形势下的重污染天气过程为例,研究分析本次污染特点和大气边界层结构特征以及此天气过程的大气温度和相对湿度结构特点。激光雷达是探测大气边界层及气溶胶的一个高效工具,利用ALS300激光雷达系统测量信号,应用Fernald方法反演大气消光系数,根据反演的气溶胶消光系数的最大突变,即最大递减率的高度来确定大气边界层的高度。利用其观测的退偏比分析大气污染物特性。利用微波辐射计数据,确定大气温度和湿度时空特征。研究结果表明:在本次污染天气下,大气具有很强的逆温结构,逆温最大可达近1 K(100 m)-1,500 m以上的大气相对湿度很低,在这种天气特征下的大气边界层高度在400 m左右,非常稳定。污染结束降雪开始前,大气逆温结构消失,大气湿度大幅度增加,接近饱和。根据lidar(light detection and ranging)退偏比的分析,本次污染天气是一次典型的烟尘类颗粒物的污染,污染具有区域性特点。PM2.5(空气动力学当量直径小于等于2.5μm的颗粒物)与AOT(Aerosol Optical Thickness)之间有明显的线性关系,相关系数达到0.72。该lidar系统能够反演出秋季降雪前本次污染天气背景下北京城区上空的大气污染特性和大气边界层高度。     

Quantifying the influence of land-use and surface characteristics on spatial variability in the urban heat island

The urban thermal environment varies not only from its rural surroundings but also within the urban area due to intra-urban differences in land-use and surface characteristics. Understanding the causes of this intra-urban variability is a first step in improving urban planning and development. Toward this end, a method for quantifying causes of spatial variability in the urban heat island has been developed. This paper presents the method as applied to a specific test case of Portland, Oregon. Vehicle temperature traverses were used to determine spatial differences in summertime ~2 m air temperature across the metropolitan area in the afternoon. A tree-structured regression model was used to quantify the land-use and surface characteristics that have the greatest influence on daytime UHI intensity. The most important urban characteristic separating warmer from cooler regions of the Portland metropolitan area was canopy cover. Roadway area density was also an important determinant of local UHI magnitudes. Specifically, the air above major arterial roads was found to be warmer on weekdays than weekends, possibly due to increased anthropogenic activity from the vehicle sector on weekdays. In general, warmer regions of the city were associated with industrial and commercial land-use. The downtown core, whilst warmer than the rural surroundings, was not the warmest part of the Portland metropolitan area. This is thought to be due in large part to local shading effects in the urban canyons.     

重庆市主城区雾霾天气过程的污染特征分析

利用重庆主城区沙坪坝站2002~2014年逐日气象观测资料和2009~2011年10月至翌年3月逐时气象观测及污染监测资料,对雾霾天气过程中SO₂、NO₂和PM10污染物浓度变化及与气温、风和降水等地面气象要素的关系进行分析。结果表明:2009~2011年重庆主城区共出现Ⅰ级至Ⅳ级不同强度的雾霾天气过程共27次,雾霾天气过程中SO₂浓度变化为单峰型,峰值出现在中午,白天浓度大于夜间;NO₂浓度变化为双峰型,主峰值出现在晚上20时,次峰值出现在中午12时;PM10的日变化幅度较SO₂和NO₂变幅小,呈双峰双谷型;污染物浓度与气温、相对湿度的相关性比较好,静风条件有利于污染物积累,降水对污染物有较明显的清除作用。      

Urban-rural humidity and temperature differences in the Beijing area

In this paper, urban-rural temperature, relative humidity and vapour pressure differences at fixed hours in Beijing area were analyzed and compared. The data from one station with different observation surroundings and one rural station (0200, 0800, 1400 and 2000 hours local standard time (LST)) for the period 1971–2003 were used. The effect of urban on local temperature, relative humidity and vapour pressure were discussed. Results showed that the urban environment caused great increase in local temperature at 0200, 0800 and 2000 hours and small increase at 1400 hours. Also, the urban effect on humidity suggested that the urban environment can decrease relative humidity and vapour pressure at all fixed hours, while the urban-rural relative humidity differences are great at 2000 and 0200 hours and the urban-rural vapour pressure differences are great at 1400 and 2000 hours. The annual average temperature, relative humidity and vapour pressure difference between urban and rural are about 1.76°C, 6.3% and 1.5475 hpa during the period from 1981 to 1997, respectively. Results indicated that the urban effect on local temperature and humidity is significant in Beijing area.     

Projected global climate change impact on water temperatures in five north central U.S. streams

The effect of projected global climate change due to a doubling of atmospheric CO₂ on water temperatures in five streams in Minnesota was estimated using a deterministic heat transport model. The model calculates heat exchange between the atmosphere and the water and is driven by climate parameters and stream hydrologic parameters. The model is most sensitive to air temperature and solar radiation. The model was calibrated against detailed measurements to account for seasonally variable shading and wind sheltering. Using climate projections from the GISS, GFDL and OSU GCMs as input; stream temperature simulations predict a warming of freely flowing river reaches by 2.4 °C to 4.7 °C when atmospheric CO₂ doubles. In small shaded streams water temperatures are predicted to rise by an additional 6 °C in summer if trees along stream banks should be lost due to climate change or other human activities (e.g. logging). These projected water temperature changes have significant consequences for survival and growth of fishes. Simulation with the complete heat budget equations were also used to examine simplified water temperature/air temperature correlations.