TY - JOUR
T1 - Revisiting the relation between momentum and scalar roughness lengths of urban surfaces
AU - Li, Qi
AU - Bou-Zeid, Elie
AU - Grimmond, Sue
AU - Zilitinkevich, Sergej
AU - Katul, Gabriel
N1 - Funding Information:
information National Center for Atmospheric Research (NCAR), Cornell University, the NOAA-Princeton Cooperative Institute for Climate Science (CU00029), Princeton's Andlinger Center for Energy and the Environment, and the U.S. National Science Foundation (NSF) Sustainability Research Network Additioal support was provided by NCAR's Computational and Information Systems Laboratory the Academy of Finland,1444758;1664091;P36861020 and UPRI0007;314 798/799;NSF AGS-1644382 and IOS-1754893Support was provided by start-up funds from Cornell University, the small allocation grant for new faculty by the National Center for Atmospheric Research (NCAR), NOAA-Princeton Cooperative Institute for Climate Science (CU00029), Princeton's Andlinger Center for Energy and the Environment, and the U.S. National Science Foundation (NSF) Sustainability Research Network (Cooperative Agreement 1444758 and grant 1664091). High-performance computing support from Cheyenne (doi:10.5065/D6RX99HX) provided by NCAR's Computational and Information Systems Laboratory (NSF projects P36861020 and UPRI0007) is acknowledged. SZ acknowledges support from the Academy of Finland (grant 314 798/799), and GK acknowledges partial support from NSF-AGS-1644382 and NSF-IOS-1754893. The Metropolis Project is acknowledged for supporting the collaborative visit of GK to Princeton University. We thank valuable comments from Wilfried Brutsaert and three anonymous reviewers.
Funding Information:
Support was provided by start‐up funds from Cornell University, the small allocation grant for new faculty by the National Center for Atmospheric Research (NCAR), NOAA‐Princeton Cooperative Institute for Climate Science (CU00029), Princeton's Andlinger Center for Energy and the Environment, and the U.S. National Science Foundation (NSF) Sustainability Research Network (Cooperative Agreement 1444758 and grant 1664091). High‐performance computing support from Cheyenne (doi:10.5065/D6RX99HX) provided by NCAR's Computational and Information Systems Laboratory (NSF projects P36861020 and UPRI0007) is acknowledged. SZ acknowledges support from the Academy of Finland (grant 314 798/799), and GK acknowledges partial support from NSF‐AGS‐1644382 and NSF‐IOS‐1754893. The Metropolis Project is acknowledged for supporting the collaborative visit of GK to Princeton University. We thank valuable comments from Wilfried Brutsaert and three anonymous reviewers.
Funding Information:
National Center for Atmospheric Research (NCAR), Cornell University, the NOAA‐Princeton Cooperative Institute for Climate Science (CU00029), Princeton's Andlinger Center for Energy and the Environment, and the U.S. National Science Foundation (NSF) Sustainability Research Network Additioal support was provided by NCAR's Computational and Information Systems Laboratory the Academy of Finland,1444758;1664091;P36861020 and UPRI0007;314 798/799;NSF AGS‐1644382 and IOS‐1754893 Funding information
Publisher Copyright:
© 2020 Royal Meteorological Society
PY - 2020/10/1
Y1 - 2020/10/1
N2 - Large-Eddy Simulations (LESs) of neutral flow over regular arrays of cuboids are conducted to explore connections between momentum (z0m) and scalar (z0s) roughness lengths in urban environments, and how they are influenced by surface geometry. As LES resolves the obstacles but not the micro-scale boundary layers attached to them, the aforementioned roughness lengths are analyzed at two distinct spatial scales. At the micro-scale (roughness of individual facets, e.g., roofs), it is assumed that both momentum and scalar transfer are governed by accepted arguments for smooth walls that form the basis for the LES wall-model. At the macro-scale, the roughness lengths are representative of the aggregate effects of momentum and scalar transfer over the resolved roughness elements of the whole surface, and hence they are directly computed from the LES. The results indicate that morphologically based parametrizations for macro-scale z0m are adequate overall. The relation between the momentum and scalar macro-roughness values, as conventionally represented by (Formula presented.) and assumed to scale with (Formula presented.) (where Re∗ is a roughness Reynolds number), is then interpreted using surface renewal theory (SRT). SRT predicts n = 1/4 when only Kolmogorov-scale eddies dominate the scalar exchange, whereas n = 1/2 is predicted when large eddies limit the renewal dynamics. The latter is found to better capture the LES results. However, both scaling relations indicate that z0s decreases when z0m increases for typical urban geometries and scales. This is opposite to how their relation is usually modelled for urban canopies (i.e., z0s/z0m is a fixed value smaller than unity).
AB - Large-Eddy Simulations (LESs) of neutral flow over regular arrays of cuboids are conducted to explore connections between momentum (z0m) and scalar (z0s) roughness lengths in urban environments, and how they are influenced by surface geometry. As LES resolves the obstacles but not the micro-scale boundary layers attached to them, the aforementioned roughness lengths are analyzed at two distinct spatial scales. At the micro-scale (roughness of individual facets, e.g., roofs), it is assumed that both momentum and scalar transfer are governed by accepted arguments for smooth walls that form the basis for the LES wall-model. At the macro-scale, the roughness lengths are representative of the aggregate effects of momentum and scalar transfer over the resolved roughness elements of the whole surface, and hence they are directly computed from the LES. The results indicate that morphologically based parametrizations for macro-scale z0m are adequate overall. The relation between the momentum and scalar macro-roughness values, as conventionally represented by (Formula presented.) and assumed to scale with (Formula presented.) (where Re∗ is a roughness Reynolds number), is then interpreted using surface renewal theory (SRT). SRT predicts n = 1/4 when only Kolmogorov-scale eddies dominate the scalar exchange, whereas n = 1/2 is predicted when large eddies limit the renewal dynamics. The latter is found to better capture the LES results. However, both scaling relations indicate that z0s decreases when z0m increases for typical urban geometries and scales. This is opposite to how their relation is usually modelled for urban canopies (i.e., z0s/z0m is a fixed value smaller than unity).
KW - large-eddy simulation
KW - scalar roughness length
KW - surface renewal theory
KW - urban canopy
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U2 - 10.1002/qj.3839
DO - 10.1002/qj.3839
M3 - Article
AN - SCOPUS:85089293580
SN - 0035-9009
VL - 146
SP - 3144
EP - 3164
JO - Quarterly Journal of the Royal Meteorological Society
JF - Quarterly Journal of the Royal Meteorological Society
IS - 732
ER -