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ABSTRACT:To bridge the gaps between traditional mesoscale modelling and microscale modelling, the National Center for Atmospheric Research, in collaboration with other agencies and research groups, has developed an integrated urban modelling system coupled to the weather research and forecasting (WRF) model as a community tool to address urban environmental issues. The core of this WRF/urban modelling system consists of the following: (1) three methods with different degrees of freedom to parameterize urban surface processes, ranging from a simple bulk parameterization to a sophisticated multi-layer urban canopy model with an indoor-outdoor exchange sub-model that directly interacts with the atmospheric boundary layer, (2) coupling to fine-scale computational fluid dynamic Reynolds-averaged Navier-Stokes and Large-Eddy simulation models for transport and dispersion (T&D) applications, (3) procedures to incorporate highresolution urban land use, building morphology, and anthropogenic heating data using the National Urban Database and Access Portal Tool (NUDAPT), and (4) an urbanized high-resolution land data assimilation system. This paper provides an overview of this modelling system; addresses the daunting challenges of initializing the coupled WRF/urban model and of specifying the potentially vast number of parameters required to execute the WRF/urban model; explores the model sensitivity to these urban parameters; and evaluates the ability of WRF/urban to capture urban heat islands, complex boundary-layer structures aloft, and urban plume T&D for several major metropolitan regions. Recent applications of this modelling system illustrate its promising utility, as a regional climate-modelling tool, to investigate impacts of future urbanization on regional meteorological conditions and on air quality under future climate change scenarios.

We incorporated a single-layer urban canopy model into a simple two-dimensional atmospheric model in order to describe the fundamental impact of the urban canopy model on an idealized urban heat island simulation. We found that the heat island circulation developed less strongly than when using the atmospheric model with the standard slab urban model. Additionally, the coupling with urban canopy model (i) delays the phase of surface air temperature, (ii) reduces the diurnal range of the temperature, and (iii) produces a nocturnal heat island, which results from the difference in atmospheric stability between city and its surroundings. The features from the atmospheric model coupled with the canopy model agree well with those from observation, although the atmospheric model with the slab model does not. The simulated nocturnal heat island is caused by the larger heat storage of the canopy model which releases sensible heat after sunset.

For an increasing number of applications, mesoscale modelling systems now aim to better represent urban areas. The complexity of processes resolved by urban parametrization schemes varies with the application. The concept of fitness-forpurpose is therefore critical for both the choice of parametrizations and the way in which the scheme should be evaluated. A systematic and objective model response analysis procedure (Multiobjective Shuffled Complex Evolution Metropolis (MOSCEM) algorithm) is used to assess the fitness of the single-layer urban canopy parametrization implemented in the Weather Research and Forecasting (WRF) model. The scheme is evaluated regarding its ability to simulate observed surface energy fluxes and the sensitivity to input parameters. Recent amendments are described, focussing on features which improve its applicability to numerical weather prediction, such as a reduced and physically more meaningful list of input parameters. The study shows a high sensitivity of the scheme to parameters characterizing roof properties in contrast to a low response to road-related ones. Problems in partitioning of energy between turbulent sensible and latent heat fluxes are also emphasized. Some initial guidelines to prioritize efforts to obtain urban land-cover class characteristics in WRF are provided.

A single-layer urban canopy model is incorporated into a simple two-dimensional atmospheric model in order to examine the individual impacts of anthropogenic heating, a large heat capacity, and a small sky-view factor on mesoscale heat island formation. It is confirmed that a nocturnal heat island on a clear, calm summer day results from the difference in atmospheric stability between a city and its surroundings. The difference is caused by anthropogenic heating and the following two effects of urban canyon structure: (i) a larger heat capacity due to the walls and (ii) a smaller sky-view factor. Sensitivity experiments show that the anthropogenic heating increases the surface air temperature though the day. (This factor strongly affects the nocturnal temperature, and the maximum increase of 0.67°C occurs at 0500 LST.) The larger heat capacity due to the walls decreases the daytime temperature and increases the nocturnal temperature. (The maximum increase of 0.39°C occurs at 0600 LST.) The smaller sky-view factor increases the temperature though the day, particularly during the first several hours after sunset. (The maximum increase of 0.52°C occurs at midnight.) In urban areas, this factor results in uniform cooling that occurs at a constant rate. The impact of the canyon structure is shown to be as significant as anthropogenic heating.

[1] High-resolution simulations from the Advanced Research Weather Research and Forecasting (ARW-WRF) model, coupled to an urban canopy model (UCM), are used to investigate impacts of soil moisture, sea surface temperature (SST), and city of Houston itself on the development of a stagnant wind event in the Houston-Galveston (HG) area on 30 August 2000. Surface and wind profiler observations are used to evaluate the performance of WRF-UCM. The model captures the observed nocturnal urban-heat-island intensity, diurnal rotation of surface winds, and the timing and vertical extent of sea breeze and its reversal in the boundary layer remarkably well. Using hourly SST slightly improves the WRF simulation of offshore wind and temperature. Model sensitivity tests demonstrate a delicate balance between the strength of sea breeze and prevailing offshore weak flow in determining the duration of the afternoon-evening stagnation in HG. When the morning offshore flow is weak (3-5 m s −1 ), variations (1°-3°C) in surface temperature caused by environmental conditions substantially modify the wind fields over HG. The existence of the city itself seems to favor stagnation. Extremely dry soils increase daytime surface temperature by about 2°C, produced more vigorous boundary layer and faster moving sea breeze, favoring stagnation during late afternoon. The simulation with dry soils produces a 3 h shorter duration stagnation in the afternoon and 4 h longer duration in the evening, which may lead to more severe nighttime air pollution. Hourly variations of SST in shallow water in the Galveston Bay substantially affect the low-level wind speed in HG.Citation: Chen, F., S. Miao, M. Tewari, J.-W. Bao, and H. Kusaka (2011), A numerical study of interactions between surface forcing and sea breeze circulations and their effects on stagnation in the greater Houston area,

This study presents the projected urban climate for the 2070s' August in the three largest urban areas, Tokyo, Osaka, and Nagoya in Japan. To accurately evaluate the urban climate, the simulations use the Weather Research and Forecast (WRF) model with 3-km grid increment coupled to an urban canopy model (UCM). To project future urban climate, the simulations apply dynamical downscaling to three GCMs (MIROC3.2-medres, MRI-CGCM2.3.2a, CSIRO-Mk3.0) and use the ensemble average for results. The results provide estimates of the heat stress to future residents of Tokyo, Osaka, and Nagoya.The WRF-UCM model reproduces the observed spatial distribution of the surface air temperature in the 2000s' August, giving an all-domain mean bias of À1.2 C and RSME of 2.7 C. For Tokyo, Nagoya, and Osaka, these biases are À0.6, À0.1, and À0.4 C. Moreover, the diurnal temperature variations at these urban stations are well reproduced. The projected monthly average August temperatures in the 2070s are about 2.3 C higher than those in the 2000s at the three urban areas and comparable to those in the record-breaking hot summer of 2010. (Predictions by individual ensemble members di¤er by 0.8-1.2 C.) As a result, urban areas will experience uncomfortable sleeping nights nearly every day in August, with roughly as many heat-induced sleeping-discomfort nights as those in 2010. Moreover, application of the wet-bulb globe temperature (WBGT) shows that people in Tokyo will be warned not to strenuously exercise outdoors for 62% of the daytime hours in the 2070s, a sharp increase from the 30% of the 2000s. (Predictions by individual ensemble members range from 54-67%). Osaka and Nagoya will have even more restrictions on outdoor exercise. Finally, the urban heat island intensity is 1.5 C in Tokyo of the 2070s, comparable to the background climate warming of 2.3 C.

The changes of a sea breeze and a daytime heat island due to land-use alteration during an 85 year period have been numerically simulated. The domain of interest is the Kanto Plain (15000 km2), including the Tokyo metropolitan area. This urban area is located in the southern part of the plain and consists of many cities in Tokyo and its suburbs. The horizontal scale of the area is about 40 km and has increased by a factor of four during the 85 year period. The simulations were conducted under a summer synoptic condition with weak gradient wind and almost clear sky. The model is based on the threedimensional anelastic equations, taking into account the hydrostatic assumption. First, it was confirmed that the simulated wind field and temperature distribution with using the land-use data for 1985, agreed with observed data. The simulations were then conducted using the land-use data for 1950 and 1900. From comparison among the three simulations, the following two major conclusions were obtained: (1) Land-use alteration modified the wind system over the Kanto Plain. In particular, the simulated sea breeze front in 1985 was more clearly defined around the northern end of the Tokyo metropolitan area. The time required for the sea breezes to reach inland areas increased by two hours. (2) The warming due to land-use alteration is found over the Tokyo metropolitan area and the northwestern part of the Kanto Plain. In particular, the area of the most prominent warming is found in the northern end of the Tokyo metropolitan area. Intensity of daytime heat island in the area were estimated as 3-4C and 2-3C during the 85 year period, and latest 35 years respectively. The above warming is confirmed to result from the enhanced sensible heat flux and the change of interaction between the boundary layer heating and sea breeze front.