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Parameterization of net radiation in urban and suburban environments Doerksen, Geoff N.

Abstract

Radiation budgets are currently understudied in urban environments. It is especially difficult, almost impossible, to find an existing urban radiation site where the radiation budget is continually being monitored. This creates the need to model the radiative components in cities: such information is used for several applied purposes (solar energy, building and urban design) and as input to meteorological pre-processors used to calculate urban heat, mass and momentum fluxes, atmospheric stability and mixed layer depth, that in turn drive climate and air quality models. Recently field measurements of the component surface radiation budget fluxes have been gathered at several urban sites in different climates and with different surface structure and cover. Data collected in Basel, Lodz, Marseille, Miami, and Vancouver are used here to devise an urban radiation scheme that uses measurements of solar radiation and routine weather observations to estimate net radiation. The simplest approach to obtain net radiation (Q*) is to use linear regression relations between net radiation (Q*) and incoming solar radiation (K↓) derived from data measured at the above urban, suburban and rural radiation sites. Multiple regression incorporating a cloud parameter shows a marked improvement over such simple linear regression at the study sites. The major limitation of these regression methods is that they are strictly daytime Q* schemes and cannot generate estimates during the night. An alternative is to parameterize each surface radiation budget component separately. Here this involved both tests of existing models and schemes and development of a new urban L↑ scheme. Several incoming longwave all-sky radiation schemes were tested at the study sites where the Maykut and Church all-sky L↓ scheme provides lower error and a smaller bias than Crawford and Duchon all-sky L↓, when both are combined with the Dilley and O'Brien clear-sky L↓ parameterization. To estimate outgoing longwave radiation a correction term (CT), to account for the difference between the surface and air temperature, was evaluated at a densely-built urban site in Basel. A new urban outgoing longwave radiation scheme (termed LUST) was created based on the strong correlation found between the difference between the surface and air temperature (T[sub s] – T₃) and solar radiation. The scheme, also evaluated at the densely-built urban site in Basel, uses measurements of air temperature and solar radiation to estimate outgoing longwave radiation. The LUST scheme generally performed better than the CT models at most sites with a reduction of RMSE by as much as 30% at the urban and suburban sites. Finally, two net radiation parameterization schemes, both including LUST, were tested. The Q* scheme where L↓ was modelled with a cloud parameter was found to give the best results at two suburban sites, while the scheme that modelled L↓ with cloud observations performed best at two heavily-built urban sites, an urban parking lot site and a rural grassland site, and error was similar at a third central city site. It is recommended that if cloud observations are available that latter Q* scheme be used, however if they are not then use of the other Q* scheme is acceptable.

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