Cooling Effects of Urban and Peri-Urban Wetlands: Remote Sensing

The evapotranspiration (ET) of urban hedges has been assumed to be an important component of the urban water budget and energy balance for years. However, because it is difficult to quantify the ET rate of urban hedges through conventional evapotranspiration methods, the ET rate, characteristics, and the cooling effects of urban hedges remain unclear. This study aims to measure the ET rate and quantify the cooling effects of urban hedges using the ‘three-temperature model + infrared remote sensing (3T + IR)’, a fetch-free and high-spatiotemporal-resolution method. An herb hedge and a shrub hedge were used as field experimental sites in Shenzhen, a subtropical megacity. After verification, the ‘3T + IR’ technique was proven to be a reasonable method for measuring the ET of urban hedges. The results are as follows. (1) The ET rate of urban hedges was very high. The daily average rates of the herb and shrub hedges were 0.38 mm·h−1 and 0.33 mm·h−1, respectively, on the hot summer day. (2) Urban hedges had a strong ability to reduce the air temperature. The two hedges could consume 68.44% and 60.81% of the net radiation through latent heat of ET on the summer day, while their cooling rates on air temperature were 1.29 °C min−1 m−2 and 1.13 °C min−1 m−2, respectively. (3) Hedges could also significantly cool the urban underlying surface. On the summer day, the surface temperatures of the two hedges were 19 °C lower than that of the asphalt pavement. (4) Urban hedges had markedly higher ET rates (0.19 mm·h−1 in the summer day) and cooling abilities (0.66 °C min−1 m−2 for air and 9.14 °C for underlying surface, respectively) than the lawn used for comparison. To the best of our knowledge, this is the first research to quantitatively measure the ET rate of urban hedges, and our findings provide new insight in understanding the process of ET in urban hedges. This work may also aid in understanding the ET of urban vegetation.

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Spatial distribution of aerosol microphysical and optical properties and direct radiative effect from the China Aerosol Remote Sensing Network

10.5194/acp-2019-405 ◽

2019 ◽

Cited By ~ 1

Author(s):

Huizheng Che ◽

Xiangao Xia ◽

Hujia Zhao ◽

Oleg Dubovik ◽

Brent N. Holben ◽

...

Keyword(s):

Remote Sensing ◽

Optical Properties ◽

Urban Areas ◽

Radiative Effect ◽

Remote Sites ◽

Fine Mode ◽

Cooling Effects ◽

Aerosol Remote Sensing ◽

Rural Sites ◽

Urban Sites

Abstract. Long-term observations of aerosol microphysical and optical properties obtained through ground-based remote sensing at 50 China Aerosol Remote Sensing Network (CARSNET) sites were used to characterize the aerosol climatology for representative remote, rural, and urban areas over China to assess effects on climate. The annual mean effective radii for total particles (Refft) decreased from north to south and from rural to urban sites, and high total particle volumes were found at the urban sites. The aerosol optical depth at 440 nm (AOD440 nm) increased from remote/rural sites (0.12) to urban sites (0.79), and the extinction Ångström exponent (EAE440–870 nm) increased from 0.71 at the arid/semi-arid sites to 1.15 at the urban sites, presumably due to anthropogenic emissions. Single scattering albedos (SSA440 nm) ranged from 0.88 to 0.92 indicating slightly to strongly absorbing aerosols. Absorption AOD440 nm's were 0.01 at the remote sites versus 0.07 at the urban sites. The average direct aerosol radiative effect (DARE) at the bottom of atmosphere increased from the sites in the remote (−24.40 W/m2) to the urban area (−103.28 W/m2) indicating increased cooling at the latter. The DARE for the top of the atmosphere increased from −4.79 W/m2 at the remote sites to −30.05 W/m2 at the urban sites, indicating overall cooling effects for the earth-atmosphere system. A classification method based on SSA440 nm, fine mode fraction (FMF), and EAE440–870 nm showed that coarse mode particles (mainly dust) were dominant at the rural sites near the northwestern deserts, while light-absorbing, fine-mode particles were important at most urban sites. This study will be useful for understanding aerosol climate effects and regional environmental pollution, and the results will provide useful information for satellite validation and the improvement of climate modelings.

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