Urban Effect on Sea-Breeze-Initiated Rainfall: A Case Study for Seoul Metropolitan Area

We examined the sea-breeze-initiated rainfall in the Seoul Metropolitan area (SMA) on 6 July 2017 using the weather research and forecasting (WRF) model. The model captures the arrival of the sea breeze front (SBF), the development of afternoon rainfall in the SMA, and the location of the sea-breeze-initiated maximum rainfall in the northeastern SMA reasonably well but overestimates the subsequent rainfall. We conducted sensitivity tests to better understand the urban effect on the sea-breeze-initiated rainfall event. Through factor separation analysis, we first examined the explicit role of sea and urban effect on sea-breeze-initiated rainfall. The results show that the interaction of sea and urban effects cause rainfall in the northwest and northeast of the SMA, indicating that both urban heat island circulation (UHIC) and sea breeze play an important role in the study case’s rainfall. We further examined the relative role of urban roughness and anthropogenic heat on the sea-breeze-initiated rainfall through factor separation analysis. Both anthropogenic heat and urban roughness play a role in increasing precipitation in the northeastern area of the SMA, with a larger contribution of anthropogenic heat than urban roughness. The relationship between low-level convergence at the SBF and urban factors is discussed.

Distributed LES-based drag parameterisation for heterogeneous urban neighbourhoods

<p><span>The form and density of buildings modify the air flow and momentum exchange within cities, and therefore strongly affect local wind, temperature, humidity and pollution. Numerical weather prediction (NWP) models currently do not account for heterogeneity in their surface layer parameterisation. Regional models represent buildings, if at all, based on quantities such as plan and frontal area indices, and parameterise their impact at the lowest grid level, even though buildings can protrude a significant height into the atmospheric boundary layer.</span></p><p><span>To investigate how to parameterise urban roughness in NWP, we analysed high-resolution building-resolving large eddy simulations (LES) with the uDALES model over a range of heterogeneous urban neighbourhoods. The simulation setups have a similar building density and frontal aspect ratio, but vary in complexity with different building heights, plan areas and street geometries. Using the LES data we developed a parameterisation model that describes the vertical distribution of building drag inside a heterogeneous urban canopy. The drag force exerted on the atmosphere represents the momentum loss in the urban canopy due to buildings and can be incorporated as additional stress term in the momentum equations. The parameterisation represents the spatial heterogeneity effects in a one-dimensional vertical function, and links the building drag force to the heterogeneity of building layouts. A characterisation of the vertical and horizontal heterogeneity of built-up neighbourhoods is used as model input.</span></p>

Parametrizing Horizontally-Averaged Wind and Temperature Profiles in the Urban Roughness Sublayer

Tower-based measurements from within and above the urban canopy in two cities are used to evaluate several existing approaches that parametrize the vertical profiles of wind speed and temperature within the urban roughness sublayer (RSL). It is shown that current use of Monin–Obukhov similarity theory (MOST) in numerical weather prediction models can be improved upon by using RSL corrections when modelling the vertical profiles of wind speed and friction velocity in the urban RSL using MOST. Using anisotropic building morphological information improves the agreement between observed and parametrized profiles of wind speed and momentum fluxes for selected methods. The largest improvement is found when using dynamically-varying aerodynamic roughness length and displacement height. Adding a RSL correction to MOST, however, does not improve the parametrization of the vertical profiles of temperature and heat fluxes. This is expected since sources and sinks of heat are assumed uniformly distributed through a simple flux boundary condition in all RSL formulations, yet are highly patchy and anisotropic in a real urban context. Our results can be used to inform the choice of surface-layer representations for air quality, dispersion, and numerical weather prediction applications in the urban environment.

NUMERICAL MODELING OF TSUNAMI INUNDATION USING SUBGRID SCALE URBAN ROUGHNESS PARAMETERIZATION

The importance of accurate numerical modeling of tsunami inundation in an urban area has clearly realized due to the devastating damage from 2011 Tohoku Earthquake Tsunami. Although, numerical inundation simulations using high resolution topography data (O(1m)), the medium resolution tsunami inundation model (O(10m)-O(100m)) needs and useful for tsunami hazard assessment. This study develops and validates a numerical model of tsunami inundation using upscaled urban roughness parameterization: Drag Force Model (DFM) which deals with the effect of structures as drag force acting on flow based on physical modeling. The validation of the DFM reveals that the DFM can express the effect of the flow direction and inundation ratio.