Abstract
The spatiotemporal characteristics of air temperature and humidity mediated by urban bluespace are investigated using a combination of dense network of climatological observations in a medium-sized US city, computational fluid dynamics, and analytical modeling approaches. Both numerical simulation and observational results show that the rate of change of hourly averaged air temperature and humidity at 3.5 m over urban areas peaks 2 h after sunset, while it decreases with time monotonically over greenspace, indicating different impacts due to presence of urban lakes. The apparent temperature decreases with distance to lakes in urban area due to higher near-shore humidity. This highlights that urban lakes located near city center can deteriorate the nighttime cooling effects due to elevated humidity. Finally, two analytical models are presented to explain the connection between the surface and air temperature as well as the spatial variation of air temperature and humidity adjacent to the urban lakes. These simplified models with parameters being inferred from the network of measurements have reasonably good performance compared to the observations. Compared to other sophisticated numerical simulations, these analytical models offer an alternative means that is easily accessible for evaluating the efficacy of bluespace on urban nocturnal cooling.
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Availability of data and material
The data that support the findings of this study are available from the corresponding author upon reasonable request. The original datasets are publicly available from https://lter.limnology.wisc.edu/content/wsc-temperature-and-relative-humidity-data-150-locations-and-around-madison-wisconsin-2012.
Code availability
The Fluent code is available to all users.
Abbreviations
- \(AT\) :
-
Apparent temperature
- \({AT}_{3.5}\) :
-
Apparent temperature at 3.5 m above the surface
- \({C}_{s}\) :
-
Roughness constant
- \(c\) :
-
Integral constant
- \({c}_{p}\) :
-
Specific heat capacity of air at constant pressure
- \(e\) :
-
Water vapor pressure
- \(G\) :
-
Ground heat flux
- \(H\) :
-
Sensible heat flux
- \({K}_{s}\) :
-
Roughness height
- \(k\) :
-
Parameter related to air cooling rate
- \(LE\) :
-
Latent heat flux
- \({L}_{net}\) :
-
Net long wave radiation
- \({L}_{v}\) :
-
Latent heat of vaporization of water
- \(Q\) :
-
Available energy
- \({Q}_{l}\) :
-
Available energy of lake surface
- \({Q}_{u}\) :
-
Available energy of urban surface
- \(q\) :
-
Mass fraction of water vapor
- \({q}_{l}\) :
-
Humidity above lakes
- \({q}_{ls}\) :
-
Humidity of lake surface
- \({q}_{ref}\) :
-
Reference humidity
- \({q}_{us}\) :
-
Humidity of urban surface
- \({R}_{n}\) :
-
Net all wave radiation
- \(RH\) :
-
Relative humidity
- \(T\) :
-
Ambient temperature
- \({T}_{i}\) :
-
Initial surface temperature
- \({T}_{l}\) :
-
Air temperature above lakes
- \({T}_{ls}\) :
-
Temperature of lake surface
- \({T}_{ref}\) :
-
Reference temperature
- \({T}_{s}\) :
-
Surface temperature
- \({T}_{s\_average}\) :
-
Nighttime averaged surface temperature
- \({T}_{us}\) :
-
Temperature of urban surface
- \({T}_{3.5}\) :
-
Air temperature at 3.5 m above surface
- \(t\) :
-
Time after sunset
- \({t}_{sunrise/sunset}\) :
-
Sunrise/sunset time
- \(u\) :
-
Velocity
- \({u}_{x}\) :
-
Streamwise velocity
- \({u}_{y}\) :
-
Vertical velocity
- \({u}_{10}\) :
-
Velocity at 10 m above surface
- \(\alpha\) :
-
Derivative of the saturated specific humidity with respect to temperature at \(T={T}_{ls}\)
- \({\beta }_{l}\) :
-
Ratio of actual lake surface specific humidity to the saturated value
- \({\beta }_{u}\) :
-
Ratio of actual urban surface specific humidity to the saturated value
- \(\kappa\) :
-
Molecular thermal diffusivity
- \(\lambda\) :
-
Molecular thermal conductivity
- \(\Delta t\) :
-
Time step
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Acknowledgements
We thank Dr. Ying Sun for generously providing the server for computation. The data used in this study include NASA ECOSTRESS LST products (ECO2LSTE.001) from NASA that are downloaded via https://lpdaacsvc.cr.usgs.gov/appeears/ and the dense urban observations from NSF-funded Long-Term Ecological Research of North Temperate Lakes site that are accessible from https://lter.limnology.wisc.edu/node/56132.
Funding
This work is funded by NASA’s Interdisciplinary Research in Earth Science (IDS) program (80NSSC20K1263). QL acknolwedges support from the National Science Foundation (NSF-AGS (2028644), NSF-CBET (2028842)).
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YC performed the simulation and analysis; LH processed the data; Everyone contributed to writing the manuscript; QL, YC conceptualized this work.
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Cui, Y., Hu, L., Wang, Z. et al. Urban nocturnal cooling mediated by bluespace. Theor Appl Climatol 146, 277–292 (2021). https://doi.org/10.1007/s00704-021-03727-5
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DOI: https://doi.org/10.1007/s00704-021-03727-5