Jiachuan Yang

Enhancing Hydrologic Modelling in the Coupled Weather Research and Forecasting-Urban Modelling System

Urbanization modifies surface energy and water budgets, and has significant impacts on local and regional hydroclimate. In recent decades, a number of urban canopy models have been developed and implemented into the Weather Research and Forecasting (WRF) model to capture urban land-surface processes. Most of these models are inadequate due to the lack of realistic representation of urban hydrological processes. Here, we implement physically-based parametrizations of urban hydrological processes into the single layer urban canopy model in the WRF model. The new single-layer urban canopy model features the integration of, (1) anthropogenic latent heat, (2) urban irrigation, (3) evaporation from paved surfaces, and (4) the urban oasis effect. The new WRF–urban modelling system is evaluated against field measurements for four different cities; results show that the model performance is substantially improved as compared to the current schemes, especially for latent heat flux. In particular, to evaluate the performance of green roofs as an urban heat island mitigation strategy, we integrate in the urban canopy model a multilayer green roof system, enabled by the physical urban hydrological schemes. Simulations show that green roofs are capable of reducing surface temperature and sensible heat flux as well as enhancing building energy efficiency.

Assessing the Impact of Enhanced Hydrological Processes on Urban Hydrometeorology with Application to Two Cities in Contrasting Climates

AbstractTo enhance the capability of models in better characterizing the urban water cycle, physical parameterizations of urban hydrological processes have been implemented into the single-layer urban canopy model in the widely used Weather Research and Forecasting (WRF) Model. While the new model has been evaluated offline against field measurements at various cities, its performance in online settings via coupling to atmospheric dynamics requires further examination. In this study, the impact of urban hydrological processes on regional hydrometeorology of the fully integrated WRF–urban modeling system for two major cities in the United States, namely, Phoenix and Houston, is assessed. Results show that including hydrological processes improves prediction of the 2-m dewpoint temperature, an indicative measure of coupled thermal and hydrological processes. The implementation of green roof systems as an urban mitigation strategy is then tested at the annual scale. The reduction of environmental temperature...

Land surface energy partitioning revisited: A novel approach based on single depth soil measurement

The partitioning of solar energy into sensible, latent, and ground heat fluxes over the land surface is responsible for changes of state variables in the soil-atmosphere system. Recent research enables the reconstruction of the land surface temperature and ground heat flux using Greens function approach, as well as the estimate of the distribution of available energy into latent and sensible heat fluxes based on linear stability analysis. Combining the Greens function approach and linear stability analysis, we propose a new physically based numerical procedure to estimate the land surface energy partitioning in this paper. The new method is capable of predicting all surface energy budgets using a single depth soil measurement; the model reliability is evaluated with comparisons to flux tower measurements. The results of this study deepen our insight into the implicit link between surface energy partition and subsurface soil dynamics and how the link can be employed to related research areas.

Relative efficiency of surface energy partitioning over different land covers.

Aims: In this paper, we aim to assess different parameterization schemes for quantifying the surface energy portioning process, in particular, the latent and sensible heat fluxes, and their applicability to various surface cover types. Study Design: This study intercompares theoretical models that predict the relative efficiency of the latent heat (evapotranspiration) with respect to the sensible heat flux. Model predictions are compared with field measureme nts over surface covers with different physical characteristics and soil water availability. Place and Duration of Study:This study was carried out at the Arizona State University, Tempe, AZ, between August 2012 and December 2012. Methodology: Three theoretical models for prediction of the relative efficiency of the latent heat were investigated, based on the lumped heat transfer (Priestley), the linear stability analysis (LSA) and the maximum entropy principle (MEP), respectively .Model predictions werecompared against field measurements over three different land cover types, viz. water, grassland and suburban surfaces. An explicit moisture availability parameter�≤is incorporated in the MEP model, to facilitate direct comparison against the LSA and field measurements. Standard post-processing and quality control were applied

Empirical modeling and spatio-temporal patterns of urban evapotranspiration for the Phoenix metropolitan area, Arizona

In this study, an empirical model for predicting urban evapotranspiration (ET) is examined for the Phoenix metropolitan area that is in a subtropical desert climate using in situ ET measurements from a local flux tower and remotely sensed moderate-resolution imaging spectroradiometer land products. Annual ET maps of Phoenix are then created for the period from 2001 to 2015 using the empirical model developed. A time-series trend analysis is finally performed using predicted ET maps to discover the spatio-temporal patterns of ET changes during the study period. Results suggest that blue-sky albedo and land surface temperature are two statistically significant variables explanatory to model urban ET for Phoenix. Areas that have experienced significant increases of ET are highly spatially clustered, and are mainly found on the outskirts of the city, while areas of decreasing ET are generally associated with highly developed areas, such as downtown Phoenix.

A Backward-Lagrangian-Stochastic Footprint Model for the Urban Environment

Built terrains, with their complexity in morphology, high heterogeneity, and anthropogenic impact, impose substantial challenges in Earth-system modelling. In particular, estimation of the source areas and footprints of atmospheric measurements in cities requires realistic representation of the landscape characteristics and flow physics in urban areas, but has hitherto been heavily reliant on large-eddy simulations. In this study, we developed physical parametrization schemes for estimating urban footprints based on the backward-Lagrangian-stochastic algorithm, with the built environment represented by street canyons. The vertical profile of mean streamwise velocity is parametrized for the urban canopy and boundary layer. Flux footprints estimated by the proposed model show reasonable agreement with analytical predictions over flat surfaces without roughness elements, and with experimental observations over sparse plant canopies. Furthermore, comparisons of canyon flow and turbulence profiles and the subsequent footprints were made between the proposed model and large-eddy simulation data. The results suggest that the parametrized canyon wind and turbulence statistics, based on the simple similarity theory used, need to be further improved to yield more realistic urban footprint modelling.

Should Cities Embrace Their Heat Islands as Shields from Extreme Cold

AbstractThe higher temperature in cities relative to their rural surroundings, known as the urban heat island (UHI), is one of the most well documented and severe anthropogenic modifications of the...

Evaluating the Effectiveness of Mitigation Options on Heat Stress for Sydney, Australia

AbstractGlobal warming, in combination with the urban heat island effect, is increasing the temperature in cities. These changes increase the risk of heat stress for millions of city dwellers. Give...