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Simulations of five November months (2010–2014) using the urban version of the numerical weather prediction system of the Meteorological Service Singapore (uSINGV) are used to analyse the urban effect on convective precipitation over Singapore and Johor Bahru (Malaysia). The model is able to closely predict locations where rainfall peaks occur, but rainfall totals are overestimated compared to radar data. The temporal variability of rainfall in the region shows that urban areas increase the frequency and severity of rainfall events and that such impact increases with the rainfall intensity. Results show that low-level moisture advection is enhanced in this coastal conurbation as a result of the strengthening of wind convergence. The latter is likely caused by increasing sea-breeze strength due to lower surface pressure over the urban area, and higher urban surface roughness, respectively. As a consequence, more precipitable water is available in the region, enhancing convection and increasing the probability of heavy rainfall over the centre and north of Singapore island and Johor Bahru. Stronger convection further increases moisture advection from the vicinity. By studying the temporal variability and the spatial distribution of rainfall events, the present study provides new insights on the urban impact on heavy rainfall in tropical areas. The conclusions are only valid for the November inter-monsoon period, when local forcing, rather than large-scale influences, dominates rainfall generation. National Environment Agency - Singapore, R-109-000-338 236-490

To date, existing urban land surface models (ULSMs) have been mostly evaluated and optimised for mid- and high-latitude cities. For the first time, we provide a comparative evaluation of four ULSMs for a tropical residential neighbourhood in Singapore using directly measured energy balance components. The simulations are performed offline, for an 11-month period, using the bulk scheme TERRA_URB and three models of intermediate complexity (CLM, SURFEX and SUEWS). In addition, information from three different parameter lists are used to quantify the impact (interaction) of (between) external parameter settings and model formulations on the modelled urban energy balance components. Encouragingly, overall results indicate good model performance for most energy balance components and align well with previous findings for mid-latitude regions, suggesting the transferability of these models to (sub)tropical regions. Similar to results from mid-latitude regions, the outgoing longwave radiation and latent heat flux remain the most problematic fluxes. In addition, the various combinations of models and different parameter values suggest that error statistics tend to be dominated more by the choice of the latter than the choice of model. Finally, our intercomparison framework enabled the attribution of common deficiencies in the different model approaches found previously in mid-latitude regions: the simple representation of water intercepted by impervious surfaces leading to a positive bias in the latent heat flux directly after a precipitation event; and the positive bias in modelled outgoing longwave radiation that is partly due to neglecting the radiative interactions of water vapor between the surface and the tower sensor. These findings suggest that future developments in urban climate research should continue the integration of more physically-based processes in urban canopy models, ensure the consistency between the observed and modelled atmospheric properties and focus on the correct representation of urban morphology, water storage and thermal and radiative characteristics.

SUMMARY A comprehensive study of atmospheric turbulence over an urban surface has been carried out. In this Part I of the study the spectral characteristics are analysed. Results involving all the important atmospheric variables, measured at two heights (z’/zg = 21 and 37) under unstable conditions, are presented, normalized by the respective variances/covariances, and within the Monin-Obukhov similarity framework. Although the shapes and location of the peaks of the urban spectra/cospectra are in good agreement with homogeneous surface layer data, the analysis of the normalized dissipation rates and spectral correlation coefficients reveals differences which can be attributed to the rough surface. For example, the dissipation of turbulent kinetic energy is relatively small, perhaps owing to an increase in the transport and export of locally produced turbulent energy; there is a very efficient transfer of momentum which it is suggested may be a result of wake production associated with bluff bodies, and it is found that the transfer of heat and moisture are dissimilar.

A comprehensive study of atmospheric turbulence over an urban surface has been carried out. Part II presents the normalized standard deviations of all important atmospheric variables from unstable conditions Measured at z'/z0=37 and analysed within the Monin-Obukhov similarity framework. They differ from the rural reference data in the following ways. (1)The normalized standard deviation of vertical velocity is systematically smaller. (2)The efficiency of urban transfer of heat and especially momentum are larger. (3)The humidity statistics cannot be described within the similarity framework. It is suggested that the roughness sublayer effects and large-scale space and time inhomogeneities of the motions in the suburban atmosphere are responsible for the observed dissimilarities.