Search

Rapid and uncontrolled urbanization in tropical Africa is increasingly leading to unprecedented socio-economical and environmental challenges in cities, particularly urban heat and climate change. The latter calls for a better representation of tropical African cities' properties relevant for urban climate studies. Here, we demonstrate the possibility of collecting urban canopy parameters during a field campaign in the boreal summer months of 2018 for deriving a Local Climate Zone (LCZ) map and for improving the physical representation of climate-relevant urban morphological, thermal and radiative characteristics. The comparison of the resulting field-derived LCZ map with an existing map obtained from the World Urban Data and Access Portal Tool framework shows large differences. In particular, our map results in more vegetated open low-rise classes. In addition, site-specific fieldwork-derived urban characteristics are compared against the LCZ universal parameters. The latter shows that our fieldwork adds important information to the universal parameters by more specifically considering the presence of corrugated metal in the city of Kampala. This material is a typical roofing material found in densely built environments and informal settlements. It leads to lower thermal emissivity but higher thermal conductivity and capacity of buildings. To illustrate the importance of site-specific urban parameters, the newly derived site-specific urban characteristics are used as input fields to an urban parametrization scheme embedded in the regional climate model COSMO-CLM. This implementations decreases the surface temperature bias from 5.34 to 3.97 K. Based on our results, we recommend future research on tropical African cities to focus on a detailed representation of cities, with particular attention to impervious surface fraction and building materials. [ABSTRACT FROM AUTHOR]

This study aims at assessing and understanding the impact of recent urbanization on the (surface) urban heat island ((S)UHI) under clear‐sky conditions in a tropical African city using different sources of remotely sensed data sets together with an urban climate model (UCM). The observed SUHI during clear sky conditions is found to be about 4°C on average over the capital city of Kampala, Uganda. The UCM, consisting of TERRA_URB embedded in COSMO‐CLM, represents the SUHI well during night but overestimates it by about 3°C in the mean during day. Moreover, a systematic warm land surface temperature bias of about 4°C is identified by night. Improved urban input parameters—derived from Local Climate Zones following the World Urban Database and Access Portal Tool (WUDAPT) framework—lead to a more realistic representation of spatial land surface temperatures patterns. In addition, this parameterization of the UCM can properly represent atmospheric variables such as air temperature, specific and relative humidity, as observed by the automated weather stations. A model sensitivity study furthermore demonstrates that the stronger urban heat island induced by the recent urbanization of Kampala over the past 15 years strongly interacts with the lake–land breeze circulation. Stronger daytime convection over the hotter city leads to areas of convergence that amplify the afternoon lake breeze in the Southern parts of the metropolis. Overall, this study demonstrates that the city of Kampala has a tangible effect on the regional climate that needs to be considered when studying present and future climate impacts. [ABSTRACT FROM AUTHOR]

With more than 1.5 million inhabitants, the capital city of Kampala, Uganda, is one of the most populated areas in Sub-Saharan Africa. Moreover, with a growth rate of more than 4 percent, it is considered as one of the fastest growing cities on the planet. As current and future impacts of this urbanization on the regional climate is still unknown, some recent studies developed frameworks in order to apply urban climate models over this city. Brousse et al. (2019) uses the simple urban canopy scheme TERRA URB embedded in the COSMO-CLM model. Although this scheme has already proven its validity in several cases (Wouters et al., 2016; Demuzere et al., 2017), obtaining input parameters for the urban canopy model valid for Kampala is difficult. In fact, as urban climate models requires thermal and geometrical canopy input variables, Local Climate Zones are often considered as powerful tools for parameterization of the model following the WUDAPT framework (Ching et al., 2018). Yet the values extracted out of the ranges proposed by Stewart and Oke that were applied until now, are mostly derived out of Western cities. Therefore in order to improve the parametrization of the urban climate model and to validate the applicability of the LCZ scheme over East African cities, this study sets up a methodology for obtaining a novel range of parameters per LCZ class within the city of Kampala.In situ data were obtained during a field exercise in the summer months of 2018. A representative sample of measuring points was selected along transects throughout the city of Kampala. This selection required a point coverage that is proportional to the spatial occurrence of each LCZ class. This sampling procedure permitted the acquisition of about 1300 measurement points. Measured variables consist of quantitative (road width, distance between houses, heights of buildings) and qualitatively estimated (vegetation fraction, road-wall-roof material) variables needed (in)directly by the urban parametrization scheme. Combining these measurements with the LCZ map created by Brousse et al. (2018) allow for the definition of a more trustworthy representation of the city of Kampala. The new ranges obtained from the field campaign are thoroughly examined for each of the abovementioned parameters and compared with the defaults, focusing on physical explanations for statistically significant differences between LCZ classes. First, based on deviations from default settings, the potential effect of the tropical urban structure on the local climate is estimated. In a next step, the results of this study will serve as input for the urban parametrization scheme. Online coupled with a high resolution climate model, the effects of the newly defined urban presence on the regional climate in larger Kampala will be explored. [ABSTRACT FROM AUTHOR]