Characterising the vertical structure of buildings in cities for use in atmospheric models

Urban schemes for numerical weather prediction (NWP) often assume an infinite street canyon with constant height and width, impacting turbulent and radiative fluxes. We develop parameterisations for urban morphology profiles, with five complexity levels, using data from six cities at 2 km × 2 km resolution. Comparisons of parameterised building plan area to these ‘true’ data show that 90% of building fraction profiles have bias errors (BE) at any height of <0.03. An effective building diameter (D) is used to characterise the proportionality between building plan area and building normalised perimeter length. The six-city mean D is 21 m. Relations for D have normalised BE (nBE) < 16%, increasing to 26% when total wall area is assumed to be unknown. Impacts from using these new morphology relations are tested with SPARTACUS-Urban radiative transfer simulations. The effective shortwave albedo has a nBE 2–10% (cf. ‘true’). Within-canyon absorption have larger nBEs, suggesting the bulk albedo hides within-canopy errors. Overall, nBE increase as less morphology data are provided, notably when omitting total wall area. We conclude that urban vertical variability using the proposed relations are acceptable for NWP, requiring only: surface building plan area, mean building height, and effective building diameter.