Decrease In Wind Speed Research Articles

A possible recovery of the near-surface wind speed in Eastern China during winter after 2000 and the potential causes

A lasting decrease in the near-surface wind speed (SWS) in China has been revealed, but a following short-term strengthening in the SWS was rarely noted. In this paper, the daily mean SWS observed datasets from 328 measurement stations in Eastern China during the period 1981–2011 were used to investigate the facts and causes of the observed short-term strengthening in winter SWS in recent decades. The major results are summarized as follows: the SWS showed a significant decrease in the last 30 years, but a short-term strengthening in SWS was observed during the winter since 2000 in Eastern China. The SWS in Eastern China showed a significant decrease of − 0.11 m s−1 decade−1 from 1981 to 1999, followed by a weak increase of 0.0008 m s−1 decade−1 from 2000 to 2011. The short-term strengthening in the SWS since 2000 was mainly induced by the changes of the pressure-gradient force (PGF), which could be attributed to the changes of the sea-level pressure (SLP) in the region (51°–69.75° N, 51.75°–111.75° E). Furthermore, the changes of the PGF during the two periods of 1981–1999 and 2000–2011 were consistent with those of the SLP in the region (51°–69.75° N, 51.75°–111.75° E). The correlation coefficient between PGF and SLP was 0.32 and 0.66 during the period 1981–1999 and 2000–2011, respectively. Therefore, the effects of the changes in SLP over the region (51°–69.75° N, 51.75°–111.75° E) on changes of SWS in the Eastern China should be significant.

An urban-based climatology of winter precipitation in the northeast United States

Varying forms of precipitation during winter weather events cause disruptions to commercial operations and transportation networks, particularly in densely populated regions. Developing a better understanding of the characteristics surrounding these events may lead to better prediction and subsequent mitigation. This study constructs a 21-year cold season climatology of precipitation type over highly urbanized areas of the northeastern United States. By using quality-controlled station reports, a specific focus is placed on the influence of urbanization in precipitation processes. In events involving multiple precipitation types, the ambient atmospheric profile is very close to the freezing point in lower levels. The influence of the boundary layer urban heat island may play a role in increasing melting of hydrometeors. Statistically significant findings from linear regression modeling show that proximity to urban centers, as derived from mean road density, plays a role in the surface observation of mixed precipitation events. 21% of any mixed precipitation observation may be attributed to its distance from a high density urban area. Decreases in mean surface wind speed and direction during mixed precipitation events increase the likelihood of an intact boundary layer urban heat island and melting of hydrometeors when compared to stronger wind speeds during snowfall events.

Spatio-temporal variation of potential evapotranspiration and climatic drivers in the Jing-Jin-Ji region, North China

Potential evapotranspiration (ET0) is a key component of the water cycle and a main factor to water resources assessment. Analysis of variation in ET0 is significant in understanding the climate change and agricultural water management; It is affected by multiple climatic factors, including air temperature, relative humidity, wind speed, and sunshine duration. To understand the variations of ET0 in the Jing-Jin-Ji region (Beijing Municipality, Tianjin Municipality, and Hebei Province), values of ET0 and other climatic parameters at 22 national meteorological stations from 1961 to 2015 were analyzed. Cramer’s test showed a shift point in the ET0 data at approximately 1991. Annual ET0 decreased from 1961 through 1991 at a rate of −20.95 mm per decade, but annual ET0 increased at a rate of 7.40 mm per decade from 1992 to 2015. Partial correlation analysis and multiple linear regression methods were used to determine the dominant climatic driving factors for ET0, and to reveal the causes of change in ET0. From 1961 to 1991, the decrease of wind speed and sunshine duration offset the effect of increased air temperature, leading to reduced ET0. However, from 1992 to 2015, average air temperature was the most dominant factor contributing to the increase in ET0. Increased of air temperature and wind speed, and decreased relative humidity caused an increase in ET0. This change in ET0 could impact hydrological cycle and agriculture irrigation management in the Jing-Jin-Ji region.

Vertical Distribution and Daily Flight Periodicity of Ambrosia Beetles (Coleoptera: Curculionidae) in Florida Avocado Orchards Affected by Laurel Wilt.

Ambrosia beetles have emerged as significant pests of avocado ((Persea americana Mill. [Laurales: Lauraceae])) due to their association with pathogenic fungal symbionts, most notably Raffaelea lauricola T.C. Harr., Fraedrich & Aghayeva (Ophiostomatales: Ophiostomataceae), the causal agent of the laurel wilt (LW) disease. We evaluated the interaction of ambrosia beetles with host avocado trees by documenting their flight height and daily flight periodicity in Florida orchards with LW. Flight height was assessed passively in three avocado orchards by using ladder-like arrays of unbaited sticky traps arranged at three levels (low: 0-2 m; middle: 2-4 m; high: 4-6 m). In total, 1,306 individuals of 12 Scolytinae species were intercepted, but six accounted for ~95% of the captures: Xyleborus volvulus (Fabricius) (Coleoptera: Curculionidae), Xyleborinus saxesenii Ratzeburg (Coleoptera: Curculionidae), Euplatypus parallelus (Fabricius) (Coleoptera: Curculionidae), Xyleborus bispinatus Eichhoff (Coleoptera: Curculionidae), Xyleborus affinis Eichhoff (Coleoptera: Curculionidae), and Hypothenemus sp. (Coleoptera: Curculionidae). The primary vector of R. lauricola, Xyleborus glabratus Eichhoff (Coleoptera: Curculionidae), was not detected. Females of X. volvulus showed a preference for flight at low levels and X. bispinatus for the low and middle levels; however, captures of all other species were comparable at all heights. At a fourth orchard, a baiting method was used to document flight periodicity. Females of X. saxesenii and Hypothenemus sp. were observed in flight 2-2.5 h prior to sunset; X. bispinatus, X. volvulus, and X. affinis initiated flight at ~1 h before sunset and Xylosandrus crassiusculus (Motschulsky) (Coleoptera: Curculionidae) at 30 min prior to sunset. Results suggest that ambrosia beetles in South Florida fly near sunset (when light intensity and wind speed decrease) at much greater heights than previously assumed and have species-specific patterns in host-seeking flight.

Decadal changes of reference crop evapotranspiration attribution: Spatial and temporal variability over China 1960–2011

Atmospheric evaporative demand can be used as a measure of the hydrological cycle and the global energy balance. Its long-term variation and the role of driving climatic factors have received increasingly attention in climate change studies. FAO-Penman-Monteith reference crop evapotranspiration rates were estimated for 644 meteorological stations over China for the period 1960–2011 to analyze spatial and temporal attribution variability. Attribution of climatic variables to reference crop evapotranspiration rates was not stable over the study period. While for all of China the contribution of sunshine duration remained relatively stable, the importance of relative humidity increased considerably during the last two decades, particularly in winter. Spatially distributed attribution analysis shows that the position of the center of maximum contribution of sunshine duration has shifted from Southeast to Northeast China while in West China the contribution of wind speed has decreased dramatically. In contrast relative humidity has become an important factor in most parts of China. Changes in the Asian Monsoon circulation may be responsible for altered patterns of cloudiness and a general decrease of wind speeds over China. The continuously low importance of temperature confirms that global warming does not necessarily lead to rising atmospheric evaporative demand.

Influence of climate variables on FAO Penman–Monteith reference evapotranspiration in the Upper Chenab Canal command area of Pakistan

Examining the changing trends of climatic variables, along with their respective influences identified by conventional sensitivity analysis, could play a decisive role in distinguishing the prominent variables controlling evapotranspiration variations over time. The trends of climatic variables, their influences, and subsequent contributions to FAO Penman–Monteith reference evapotranspiration (ETo) were systematically analysed for the command area of the Upper Chenab Canal, Pakistan, from 1980 to 2012. The nonparametric Mann–Kendall test was employed for trend detection, and ETo sensitivity to climatic variables was quantified via sensitivity coefficients. A significant overall increase in maximum temperature (Tmax) and minimum temperature (Tmin), a significant decrease in sunshine hours (n) and net solar radiation (Rn), and a non-significant decrease in relative humidity (RH) and wind speed (u2) resulted in a slight decrease in ETo. ETo was highly sensitive to Rn, followed by Tmax, n, Tmin, RH, and u2. The highly influential Rn and the comparatively less influential n, Tmin, and u2 appeared to be the active factors controlling the ETo fluctuations. Rn and n contributed to downward trends in ETo in the study area, whereas u2 and Tmin contributed to upward trends. The contributions of Tmax, despite being the second-most influential variable, to ETo variations were not prominent. RH had negligible impact on ETo, as it was least influential and showed non-significant variations.

Temporal variation of the wind environment and its possible causes in the Mu Us Dunefield of Northern China, 1960–2014

Research on the wind environment variation improves our understanding of the process of climate change. This study examines temporal variation of the near-surface wind environment and investigates its possible causes in the Mu Us Dunefield of Northern China from 1960 to 2014, through analyzing the meteorological data from seven stations and the land use and land cover (LUCC) change data with 100 m resolution. The wind speed had a widespread significant decrease with an average trend of − 0.111 m s−1 decade−1, although the rate of decrease differed seasonally. This negative trend was also found in the winds that were above a 5 m s−1 threshold, as well as the percentage of their days, which influenced the wind speed change more strongly. Overall, 88.69% of the annual decrease resulted from decreases in the maximum wind speed, and the percentage even reached 100% in autumn and winter. We further found that the drift potential decreased at decadal time scales, mainly focusing on three prevailing wind groups: the northerly, westerly, and southerly winds. This revealed the weakened East Asian monsoon and westerly circulation in the lower atmosphere. Against the context of climate warming, the decline of wind speeds in spring was closely related to the greenhouse gas, while the winter decline was closely associated with the aerosol or atmospheric dust. Moreover, the LUCC change showed the decreased areas of sand land and the increased areas of vegetation-covered land, which increased the ground surface roughness and was another reason for the weakened wind environment.

Effects of the Reynolds number and the tip losses on the optimal aspect ratio of straight-bladed Vertical Axis Wind Turbines

Aspect Ratio (AR) is one of the main design parameters of straight-bladed vertical axis turbines. This paper will examine whether a high AR, with long blades and low tip losses, or a low AR, with a higher diameter and higher losses, is more suitable to achieve the maximum power output given a fixed cross-sectional area. Traditional Double-Multiple Stream-Tube (DMST) approaches are limited by a lack of tip loss formulations specifically conceived for vertical axis turbines. Therefore, a CFD-3D investigation covering a power range from micro-generation to MW has been done. Results show that both Reynolds number and tip losses strongly influence the aerodynamic performance of the rotor. More advantages seem to be achieved by limiting tip losses rather than increasing chord-based Reynolds number (Rec), addressing towards high AR at least for medium and large-size turbines. However, as turbine size and wind speed decrease, this difference narrows considerably. For micro turbines, tip losses are balanced by the effects of Rec, thus a variation of AR does not imply a variation of CP. For all the cases that have been analysed, turbine size and therefore Rec does not appreciably affect the normalized CP distribution along the blade, which only depends on AR.

Quantifying Climatic Impact on Reference Evapotranspiration Trends in the Huai River Basin of Eastern China

Reference evapotranspiration (ETref) is an important study object for hydrological cycle processes in the context of drought-flood risks of the Huai River Basin (HRB). In this study, the FAO-56 Penman–Monteith (PM) model was employed to calculate seasonal and annual ETref based on 137 meteorological station data points in HRB from 1961 to 2014. The Mann–Kendall (MK) trend analysis was adopted together with Theil–Sen’s estimator to detect tendencies of ETref and climate factors. Furthermore, a developed differential equation method based on the FAO-56 PM model was applied to quantify the sensitivities of ETref to meteorological factors and their contributions to ETref trends. The results showed that the ETref demonstrated a strong spatially heterogeneity in the whole HRB at each time scale. ETref showed a significant decreasing trend in the upper-middle HRB and Yi-Shu-Si River Basin, especially at the annual time scale, in growing season and summer, while a generally increasing trend in ETref was detected in the lower HRB, and the significance only showed in spring. These phenomena could be reasonably explained by a significantly increasing mean temperature (TA), a significantly decreasing wind speed (WS), solar radiation (SR), and a slightly decreasing relative humidity (RH). The most sensitive factor to ETref was RH in most sub-regions and most time scales, except in the growing season and summer. Based on the developed differential equation method, the dominant factor of the decreasing ETref was WS in the annual time scale, spring, autumn, and winter in most sub-regions, except the lower HRB, which then shifted to SR in the growing season and summer. However, in the lower HRB, the significantly decreasing RH was the most dominant factor, especially in the annual time scale, growing season, and spring, which might be responsible for the slightly increasing ETref there.

Spatial and temporal evolution of natural and anthropogenic dust events over northern China

Mineral dust interacts with radiation and cloud microphysics in East Asia can affect local and regional climate. In this study, we found that the occurrences of dust storms, blowing dust, and floating dust over northern China has decreased 76.7%, 68.5%, and 64.5% considerably since the beginning of this century. Based on a multi-dimensional ensemble empirical mode decomposition (MEEMD) method, a steady decrease in zonal maximum wind speed (up to −0.95 m/s) in the Northern Hemisphere was largely responsible for this recent decline in dust event occurrences. Then, a new detection technique that combines multi-satellite datasets with surface observations of dust events is developed to estimate the contribution of anthropogenic dust column burden from disturbed soils to the observed total dust. It is found that the percentage of the anthropogenic dust column burdens to total mineral dust is up to 76.8% by human activities during 2007–2014 in eastern China, but only less than 9.2% near desert source regions in northwestern China. However, we note that the anthropogenic effects on the dust loading for both regions are non-negligible.

On the Attribution of Changing Reference Evapotranspiration in a Coastal Area of China

Reference evapotranspiration (ET0) is a crucial parameter for hydrological modeling, land-atmospheric interaction investigations and agricultural irrigation management. This study investigated changes in ET0 and attributed those changes to climate variations in a coastal area (Zhejiang province) of China by a numerical experiment method. The results indicated that annual ET0 increased significantly (p < 0.05) at a rate of 1.58 mm·year−1 from 1973 to 2013, which was mainly caused by an obvious increase in ET0 in spring. Air temperature and water vapor pressure deficits increased significantly (p < 0.05) at rates of 0.04 °C·year−1 and 0.005 kPa·year−1, respectively, at an annual time scale during the study period, while wind speed and solar radiation decreased significantly (p < 0.05) at rates of −0.01 m/s·year−1 and −3.94 MJ·m−2·day−1·year−1, respectively. The contributions of changes in air temperature, wind speed, water vapor pressure deficits and solar radiation to increases in ET0 were 0.39, −0.56, 2.62 and −0.61 mm·year−1, respectively. The decrease in wind speed and solar radiation negatively affected the increase in ET0, which was offset by the positive effects of the air temperature and water vapor pressure deficits increase, where the water vapor pressure deficits was the dominant factor in increasing ET0 in the coastal area. Moreover, the impact of topography on ET0 was further discussed. ET0 changes at plain stations were approximately 5.4 times those at hill stations, which may be due to the impact of a large water body and the augment of surface roughness from intense human activities in the well-developed plain area. The results are helpful for investigating spatial and temporal changes in the evaporative demand for well-developed regions under energy-limited conditions.

The Trend and Influencing Factors of Potential Evapotranspiration in the Yellow River

Based on the Penman-Monteith formula recommended by the World Food and Agriculture Organization (FAO) and the Mann-Kendall trend test method, the variation trend of potential evapotranspiration in the Yellow River Basin from 1952 to 2014 is analyzed. The results showed that the potential evapotranspiration of 43.3% of the 90 meteorological stations in the Yellow River Basin showed a significant upward trend. 30% of the stations showed a significant downward trend, and 26.7% of the stations had no obvious trend of change. In all the secondary areas of water resources, the stations above Longyangxia are basically marked upward. The Longyangxia to the northern part of Lanzhou, the Longmen to the east of Sanmenxia and the Sanmenxia to Huayuankou are all significant descending sites. The change trend of the sites below the Huayuankou is not obvious. In other partition three kinds of sites are distributed. Through the correlation analysis, it is found that the increase of temperature has a great influence on the stations where the potential evapotranspiration is significantly increased, and the decrease of wind speed is the main reason for the significant decrease of potential evapotranspiration in some stations.

Role of Satellite Observation and Geospatial Technologies for the Maintenance and Mitigation of Risks to Critical Energy Infrastructure of the Caspian Sea

Abstract The main goal of this research was to study oil pollution, determine oil spill frequencies and approximate oil leak sources around the Oil Rocks Settlement, the Chilov and Pirallahi Islands in the Caspian Sea using 136 time-series ENVISAT radar images acquired during 2006-2010. Oil slicks were mainly detected from radar images acquired under the wind speed range of 2.5 - 12.4 m/s. The following oil spill frequencies were observed around the Oil Rocks Settlement, the Chilov and Pirallahi Islands: 2-10 (3471.04 sq. km.), 11-20 (971.66 sq. km.), 21-50 (692.44 sq. km.), 51-128 (191.38 sq. km.). The most critical oil leak sources with the frequency range of 41-128 were observed at the Oil Rocks Settlement. The exponential regression analysis between wind speeds and oil slick areas detected from 136 multi-temporal ENVISAT images revealed the regression coefficient equal to 63%. The regression model showed that larger oil spill areas were observed with decreasing wind speeds.

Modeling the Dynamics of the Atmospheric Boundary Layer Over the Antarctic Plateau With a General Circulation Model

AbstractObservations evidence extremely stable boundary layers (SBL) over the Antarctic Plateau and sharp regime transitions between weakly and very stable conditions. Representing such features is a challenge for climate models. This study assesses the modeling of the dynamics of the boundary layer over the Antarctic Plateau in the LMDZ general circulation model. It uses 1 year simulations with a stretched‐grid over Dome C. The model is nudged with reanalyses outside of the Dome C region such as simulations can be directly compared to in situ observations. We underline the critical role of the downward longwave radiation for modeling the surface temperature. LMDZ reasonably represents the near‐surface seasonal profiles of wind and temperature but strong temperature inversions are degraded by enhanced turbulent mixing formulations. Unlike ERA‐Interim reanalyses, LMDZ reproduces two SBL regimes and the regime transition, with a sudden increase in the near‐surface inversion with decreasing wind speed. The sharpness of the transition depends on the stability function used for calculating the surface drag coefficient. Moreover, using a refined vertical grid leads to a better reversed “S‐shaped” relationship between the inversion and the wind. Sudden warming events associated to synoptic advections of warm and moist air are also well reproduced. Near‐surface supersaturation with respect to ice is not allowed in LMDZ but the impact on the SBL structure is moderate. Finally, climate simulations with the free model show that the recommended configuration leads to stronger inversions and winds over the ice‐sheet. However, the near‐surface wind remains underestimated over the slopes of East‐Antarctica.

Variation characteristics and influencing factors of free water evaporation in recent 58 years in Tai’an area

The mutation test and trend analysis of free water evaporation in Tai’an from 1956 to 2013 were carried out with Pettitt detection method, Mann-Kendall method and linear tendency estimation method. The results indicate that: (1) the free water evaporation in Tai’an was distributed unevenly within the year, and the maximum appeared in June, accounting for 14.5% of the whole year; (2) in the last 58 years, the free water evaporation in Tai’an changed significantly in 1985. The evaporation decreased obviously before the change, then fluctuated around the mean value, and gradually became stable; (3) by means of correlation analysis and partial differential decomposition, it was found that free water evaporation in Tai’an before 1985 was positively correlated with air temperature, solar radiation and wind speed, and negatively correlated with actual water vapor pressure. The significant decrease in solar radiation and wind speed are the main reasons for the significant decline in annual evaporation; (4) after 1985, the air temperature played a dominant role, and the rise of air temperature offset the negative influence of decline of wind speed and solar radiation on the free water evaporation.

기상 관측소 주변 토지 피복 변화와 기후 자료간의 관련성 분석

This study analyzes relationship between land cover change and local climate data of 46 weather stations in South Korea from the 1980s to 2000s. The used area shows proportional relationships to mean daily min. temperature and mean temperature, and a reverse relationship is found between the used area and relative humidity. However, the forest indicates reverse relationships to mean daily min. temperature and mean temperature. The agricultural land causes to increase in relative humidity and decrease in mean wind speed, while the water increases mean daily min. temperature, daily min. temperature and mean temperature as well as mean wind speed. The urbanization type (used area + barren) shows high correlations with temperature and humidity. The suburbanization type (agricultural land + forest + grass + wetland) has high correlations with temperature and wind speed. High correlations are also found between the waterfront type (wetland + water), and temperature and wind speed. It can be concluded that change in land cover around weather station obviously influences on climate data of the weather station and it is also expected that reliability and homogeneity of climate data from a weather station can be enhanced by this study.

The impact of urban canopy meteorological forcing on summer photochemistry

The regional climate model RegCM4.4, including the surface model CLM4.5, was offline coupled to the chemistry transport model CAMx version 6.30 in order to investigate the impact of the urban canopy induced meteorological changes on the longterm summer photochemistry over central Europe for the 2001–2005 period. First, the urban canopy impact on the meteorological conditions was calculated performing a reference experiment without urban landsurface considered and an experiment with urban surfaces modeled with the urban parameterization within the CLM4.5 model. In accordance with expectations, strong increases of urban surface temperatures (up to 2–3 K), decreases of wind speed (up to −1 ms−1) and increases of vertical turbulent diffusion coefficient (up to 60–70 m2s-1) were found. For the impact on chemistry, these three components were considered. Additionally, we accounted for the effect of temperature enhanced biogenic emission increase. Several experiments were performed by adding these effects one-by-one to the total impact: i.e., first, only the urban temperature impact was considered driving the chemistry model; secondly, the wind impact was added and so on.We found that the impact on biogenic emission account for minor changes in the concentrations of ozone (O3), oxides of nitrogen NOx = NO + NO2 and nitric acid (HNO3). On the other hand, the dominating component acting is the increased vertical mixing, resulting in up to 5 ppbv increase of urban ozone concentrations while causing −2 to −3 ppbv decreases and around 1 ppbv increases of NOx and HNO3 surface concentrations, respectively. The temperature impact alone results in reduction of ozone, increase in NO, decrease in NO2 and increases of HNO3. The wind impact leads, over urban areas, to ozone decreases, increases of NOx and a slight increase in HNO3. The overall impact is similar to the impact of increased vertical mixing alone. The Process Analysis (PA) technique implemented in CAMx was adopted to investigate the causes of the modeled impacts in more details. It showed that the main process contributing to the temperature impact on ozone is a dry-deposition enhancement, while the dominating process controlling the wind impact on ozone over cities is the advection reduction. In case of the impact of enhanced turbulence, PA suggests that ozone increases are, again as assumed, the result of increased downward vertical mixing supported by reduced chemical loss. Comparing the model concentrations with measurements over urban areas, a slight improvement of the model performance was achieved during afternoon hours if urban canopy forcing on chemistry via meteorology was accounted for. The study demonstrates that disregarding the urban canopy induced meteorological effects in air-quality oriented modeling studies can lead to erroneous results in the calculated species concentrations. However, it also shows that the individual components are not equally important: urban canopy induced turbulence effects dominate while the wind-speed and temperature related ones are of considerably smaller magnitude.

Dynamic analysis of pan evaporation variations in the Huai River Basin, a climate transition zone in eastern China

Pan evaporation (Epan), which we examine in this study to better understand atmospheric evaporation demand, represents a pivotal indicator of the terrestrial ecosystem and hydrological cycle, particularly in the Huai River Basin (HRB) in eastern China, where high potential risks of drought and flooding are commonly observed. In this study, we examine the spatiotemporal trend patterns of climatic factors and Epan by using the Mann-Kendall test and the Theil-Sen estimator based on a daily meteorological dataset from 89 weather stations during 1965–2013 in the HRB. Furthermore, the PenPan model is employed to estimate Epan at a monthly time scale, and a differential equation method is applied to quantify contributions from four meteorological variables to Epan trends. The results show that Epan significantly decreased (P<0.001) at an average rate of −8.119mm·a−2 at annual time scale in the whole HRB, with approximately 90% of stations occupied. Meanwhile, the generally higher Epan values were detected in the northern HRB. The values of the aerodynamic components in the PenPan model were much greater than those of the radiative components, which were responsible for the variations in the Epan trend. The significantly decreasing wind speed (u2) was the most dominant factor that controlled the decreasing Epan trend at each time scale, followed by the notable decreasing net radiation (Rn) at the annual time scale also in growing season and summer. However, the second dominant factor shifted to the mean temperature (Ta) during the spring and winter and the vapor pressure deficit (vpd) during the autumn. These phenomena demonstrated a positive link between the significance of climate variables and their control over the Epan trend.

Evaluation of near-surface wind speed simulations over the Tibetan Plateau from three dynamical downscalings based on WRF model

The Tibetan Plateau (TP) has experienced significant climate changes in recent decades. Wind stilling is one of the most evident changes based on observations. This study assesses near-surface wind speed simulations from three WRF dynamical downscalings over the TP. The assessment is conducted by comparing with observations at 83 stations over the TP and ERA-Interim in 1980–2005. Results show that the three dynamical downscalings and ERA-Interim all overestimated the annual and seasonal wind speeds over the TP, especially in winter. Overestimation of the U component is the main contributor to the overestimation of near-surface wind speeds. The difference between grid and station elevations partly contributes to the larger overestimation, particularly in 3000–4000 m. Impacts of the land surface scheme on the simulations of wind speed are also noticed. Observations show significant decreasing wind speed trend and the variability is larger at higher elevations above 4000 m, which are not represented in the ERA-Interim. Affected by forcing, the dynamical downscalings do not capture the observed significantly decreasing trend, but they outperform the forcing in winter. The better performance in winter but summer is mainly because the dynamical downscalings are unable to reproduce the observed direction of the U component in summer. The ability of dynamical downscalings to simulate the near-surface wind speed may be improved by choosing forcing with better linear trends, enhancing the momentum transports in the planetary boundary parameterization scheme, and considering the unresolved topographic features sufficiently.

Effects of the tree distribution and species on outdoor environment conditions in a hot summer and cold winter zone: A case study in Wuhan residential quarters

Residential district is the important space for people living and outdoor activities. Vegetation is proved to be effectively regulate microclimate. Living in Wuhan, residents have to suffer heat stress in summer and strong cold wind in winter simultaneously. It is necessary to dictate vegetation types and layout in residential area to get comfortable environment both in hot summer and cold winter. This study examined the vegetation influences of residential wind environment in hot and cold seasons by using the ENVI-met model V4. Field measurement validated the performance of ENVI-met model. The simulation was based on multi-story buildings representing the current primary form of residential area in Wuhan. 3 scenarios with three tree arrangements and 8 vegetation species were simulated. Height-to-distance ratio of trees (as “Aspect ratio of trees”, ART) was used to describe the tree distribution. Results showed that the impact of vegetation on both heat environment and ventilation depended on tree arrangement, LAI, crown width and tree height. The comparison of 3 tree distributions revealed that trees with an ART < 2 should be a priority to mitigate hot environments due to the large effects on PET reduction in summer. Evergreen species with an ART < 2 also effectively decreased wind speed in winter as well as blocked direct sunlight, resulting in negative effects on PET. Tall trees with a large LAI and canopy diameter should be a priority to improve the comfort of outdoor environments.