Urban Microclimate and Outdoor Thermal Comfort of Public Spaces in Warm-Humid Cities: A Comparative Bibliometric Mapping of the Literature

Intensified human population encourages urbanization changing the morphology and metabolism of urban environments, thus altering the local climate and outdoor thermal comfort (OTC) in public spaces. OTC is an increasingly urgent area of research for tropical climates. This study explores the literature from the Scopus database on urban microclimate and OTC in public spaces and contrasts the studies in warm-humid cities through a bibliometric mapping of literature. The adapted methodology includes; Bibliometric Search, Scientometric Analysis, and Content analysis using VOSviewer software to identify the evolution paths, gaps, and the most recent movement of OTC assessments in urban public spaces. Results reveal five evolution paths related to all climatic regions; 1) materials and cooling strategies, 2) simulation modeling and urban planning, 3) design parameters affecting thermal perception, 4) cooling effects of green infrastructure, and 5) thermal adaptation in urban design. Although urban morphology and vegetation have been received the highest attention respectively, only a few for blue infrastructure related to warm-humid cities. This review identified five research gaps; the impact of blue infrastructure on OTC, strategies to overcome the effect of reflective materials, vegetation configurations in street canyons with wind flow, OTC improvements in asymmetrical street canyons, and how local climate zone (LCZ) classification approach could be used for OTC assessments. Past empirical studies have revealed that urban vegetation, using micro-meteorological simulation modelling to examine the impact of urban design interventions. Finally, comprehensive content analysis, bibliographic coupling based on documents, co-occurrence of all-keywords, are suggested for future bibliometric reviews. Finally, further research on recommended areas would assist decision-makers in planning and design to en-hance livability by improving microclimate and OTC in urban spaces.

two types in the categorization as "built types" and "natural landscapes without artificial erections". Because of climate change and ambient temperature increases throughout the world, biometeorological assessments and outdoor thermal comfort (OTC) in urban spaces have received increasingly urgent attention (Mayer et al., 2008;Holst & Mayer, 2011;Taleghani et al., 2015;Morakinyo et al., 2018;Venhari et al., 2019;Zaki et al., 2020;Manteghi et al., 2020;Wai et al., 2021). Currently, modeling techniques which have been improving over many years are introduced to analyze the biometeorology of the urban environments considering the local climate. However, majority of the current OTC assessment methods provide results only for specific meteorological conditions instead of long-term analysis (Acero et al., 2020). However, meteorological conditions cannot fully account for changes in subjective thermal comfort though it is a significant factor in OTC assessment (Nikolopoulou & Steemers, 2003;Han et al., 2007).
Even though tropical urban environments are more vulnerable to heat stress and thermal discomfort, research in assessing human biometeorology in tropical climates is scarce. However, there have been several studies on urban design and landscape design parameters such as morphology, geometry, density, greenery, configurations with warm-humid climates (Johansson & Emmanuel, 2006;Sharmin et al., 2015;Chow et al., 2016;Johansson et al., 2018;Yahia et al., 2018). It must be noted that a proper understanding of the microclimatic differences influenced by urban morphology and landscape design elements provides impor-cally. Mathematical analysis of published documents extracted from a database can be quantitatively and statistically investigated on the basic properties, publication patterns, and use of documents of a research domain with bibliometric analysis (Agarwal et al., 2016;Mora et al., 2017). A large volume of bibliographical data and information analysis, visualization and graphical representation techniques, and literature mapping can become conveniently accessible with the aid of software programs due to the fast advancement in data mining (Meng et al., 2020). Although there is an increasing number of empirical studies that have analyzed urban microclimate, outdoor thermal comfort and its' impacts, investigations in tropical warm cities have received less attention. Therefore, it is important to understand the evolution of the literature, key authors, research gaps, methodologies, and future directions of this research domain.
VOSviewer software has been developed in the Netherlands by Van Eck and Waltman of Leiden University. This software can build networks of journals, researches based on co-citations, keyword analysis according to co-occurrences, and visualize the outcome in bibliometric networks (Waltman, Eck, & Noyons, 2010;Eck & Waltman, 2019). VOSviewer bibliometric mapping has been applied in various fields of research such as local climate classification (Xue et al., 2020), smart and regenerative urban growth (Peponi & Morgado, 2020), mental health and well-being (Hernández-Torrano et al., 2020), fear of crime in urban envi-  (Kawshalya et al., 2020), construction safety research (Jin et al., 2019), urban street space and residents' health (Meng et al., 2020), and in international mobile library research (Zhao & Shen, 2018). Therefore, Scientometric Analysis using VOSviewer software was adapted for the network analysis algorithms to comprehend the connectivity of the existing knowledge in the domain of urban microclimate and thermal comfort of public spaces in warm-humid cities. The objectives of the study are as follows: 1) To identify influential authors, journals, countries, of the research domain of urban microclimate and thermal comfort related to all climatic regions.
2) To categorize the keywords and identify trends through an author keyword and co-occurrence analysis.
3) To compare the research attention received in warm-humid cities and provide the evolution paths, gaps, and latest movement of research advancement, through author keywords and document analysis.

Methodology
The research methodology has three main phases; Bibliometric Search, Scientometric Analysis, and Content analysis the approach of science mapping literature (Agarwal et al., 2016;Mora et al., 2017) was adapted to understand the research domain of urban microclimate and OTC in public spaces and compare the specific research attention received in warm-humid climate and the future directions. Figure 1 is a diagrammatical representation of the methodology.

Bibliometric Search and Data Acquisition-Phase I
Bibliometric search was conducted based on Scopus database to explore the initial list of papers (csv format). The bibliometric data for the scientometric analysis is limited to only the one (Scopus) database, because of the inability to access several databases in a single analysis. Mongeon and Paul-Hus (2016) have mentioned that the Scopus database includes the largest number of exclusive journals in all fields and a lot more journals than Web of Science. Further, the Scopus database includes most of the articles indexed in Web of Science in both fields of Natural Science and Engineering, and Social Sciences. The retrieval of data was performed from the Scopus database in two searches, for the first search, the keywords used were "microclimate", "thermal comfort" and "urban public spaces". Thus, the following search procedure was conducted in the Scopus database. The retrieval of data was performed on 14 June 2021, and 390 bibliographic items with all information were exported. The inclusion of the publication comprises the year 1983 to 2021.
2nd search-("thermal comfort" AND "microclimate") AND ("warm-humid" OR "hot-humid" OR "tropical" OR "tropics") AND ("urban space" OR "park" OR "open space" OR "streets" OR "public space") The second search was conducted adding terms "warm-humid" OR "hot-humid" OR "tropical" OR "tropics" to the first search. The retrieval of data was performed on the same day and 57 bibliographic items with all information were exported. The inclusion of the publication comprises the year 1999 to 2021. Both searches were undertaken with digital filtering process. The manual screening was done only for the second search for further analysis. Article screening was carried out with inclusion and exclusion criteria. The digital screening was conducted with the initial inclusion of subject area to environmental science, engineering, social sciences, earth and planetary science, energy, decision science, materials science, multidisciplinary, and completion stage to final, the document type to articles, and language to English. Then, from the first and second search, 256 and 37 articles respectively were explored. In the manual screening, unrelated articles were excluded referring to the article title and the abstract of the paper in warm-humid cities of public spaces in tropical cities. Three (3) articles were removed in this process due to the irrelevance of the studies (semi-outdoor spaces, artificial neural network models, and ecosystem services).

Scientometric Analysis-Phase II
Scientometric analysis was conducted for acquired data from the bibliometric search and filtering process (256 articles from the first search and 34 from the second) using the version 1.6.15 of VOSviewer software, which quantifies and illustrates the relationships among scientific literature (Waltman, Van Eck, & Noyons, 2010;Eck & Waltman, 2020;Xue et al., 2020). It manifests the distribution of the publications, network of authors, journals, co-occurrence of key-  (Waltman et al., 2010). Temporal distribution, influential authors, contribution countries, author keywords were descriptively analyzed. The illustrations show the relationships among the selected articles creating a network using nodes and connections. The nodes visualize the importance of an item whereas the connections indicate the closeness among them (Eck & Waltman, 2020;Tanudjaja & Kow, 2018). In VOSviewer, the importance of an item is indicated by weight and score attributes.
In visualization maps, an item with higher weight is displayed more prominently (with the size of the circles and labels) when compared with items of lower weights. Thus, the items with higher much weightages are highlighted with larger labels and circles.

Content Analysis-Phase III
A comprehensive quantitative analysis was conducted for the finalized (34) papers to identify the most influential papers on urban microclimate and thermal comfort in warm-humid cities. The document analysis was done using VOSviewer and ranked based on document weight and normal citation scores. Finally, a content analysis was conducted manually to determine the most recent movement, research gaps, and future trends in research advancement of urban microclimate OTC of public spaces in tropical cities. Although the scientometric analysis was limited to one (Scopus) database, the findings from content analysis were discussed referring to the most relevant literature from other scientific databases to validate the identified research gaps and trends.

Performed Analysis
This section shows the results of the scientometric Analysis and content analysis. This includes the illustrations of the evolution of the publications, leading journals, contributing counties, influential authors, and research topics by generating and clustering the citation network and Co-occurrence network for both 256 articles (analysis A) and 34 articles (analysis B) separately. Then, the selected publications were further reviewed manually to identify the extent of research attention received by the studies in warm-humid cities. Analysis A-Studies of urban microclimate and OTC in urban public spaces irrespective of the climatic region with the search results of first bibliometric search.
Analysis B-Studies of urban microclimate and thermal comfort in urban public spaces in warm-humid climatic region with the search results of second bibliometric search.

Leading Journals
Main contributing journals in the research domain of urban microclimate and OTC in public space, encompasses a large variety of subjects, such as urban science, meteorology, urban climatology, environmental science, cities and sustainability, building science, energy, landscape and forestry, and urban planning.
Limitations were set to the minimum number of documents of a source as 3 and minimum citations as 15 to analyze the network of leading journals. The following overlay visualization in Figure 3 shows all the sources with 18 nodes and six clusters, the size of the scale of the circles represents the citation weights  According to the ranking based on the number of publications, Building and Environment published the most documents (46), followed by Sustainable Cities and Society and Urban Climate with 27 and 16 publications respectively. The link strength among the journals represents by the thickness of the connecting lines and the average normalized citations represents by the color (Eck & Waltman, 2020). In between the Building and Environment and the Sustainable Cities and Society has the highest link strength (56), Building and Environment and International Journal of Biometeorology (36), and Building and Environment and Landscape and urban planning (35). These journals are actively citing each other showing the highest link strengths. Thus, Building and Environment journal has the highest total linking strength (307) in the studies on urban microclimate and OTC in public space, irrespective of the climatic region. Table 1 quantitatively shows the ranking of the journals. Leading journals have been ranked according to the "average normalized citations" which is defined as the number of citations divided by the average number of citations published in the same year. This value modifies the misrepresentation that articled published earlier have more opportunity of being cited due to the time variations when compared with more recent articles. As such, the five most influential journals can be listed as Renewable Energy (2.5), Science of the Total Environment (2.0), Sustainable Cities and Society (1.7), Energy and Buildings (1.4), and the Building and Environment (1.3). The ranking of the fourteen most influential  journals is shown in Table 1. when taking into consideration the average number of publication per year, it is revealed that the Journal of Cleaner Production, Science of the Total Environment, International Journal of Environmental Research and Public Health have been recently active in publication of the articles in this particular subject.

Contributing Countries
Contributing countries in terms of publications were analyzed setting the restrictions to the minimum number of publications of a country as 1 and minimum citations of a country as 100. The following overlay visualization in Figure  4 shows all the countries with 22 items and 4 clusters. The size of the circles represent the document weights and the color represent the scores of average normalized citations. Since, the color of an item is determined by the score of the item, where by default colors range from blue (lowest score) to green to yellow (highest score) (Eck & Waltman, 2020), yellow color indicates the country with the highest normalized citations. Accordingly, studies on urban microclimate and OTC in public spaces are mainly based on China (34), United Kingdom (27), Germany (26), Greece (22), and Australia (20) in terms of the number

Influential Authors
Authors who make the highest contibution to this research domain were analyzed setting the restrictions to the minimum number of documents of an author as 1 and minimum citations of an author as 300. The following overlay visualization in Figure 5 shows 22 items in four clusters. Since the citation weights and scores of average normalized citations are represented by the size and the color of the circles respectively (Eck & Waltman, 2020), yellow colour indicates the author who has the highest normalized citations. Consequently, the most influential ten authors have been ranked in Table 2     The overlay visualization in Figure 7 shows the co-occurrence analysis based on author keywords and has been illustrated setting the weight as occurrences and scores as average citations to identify the most cited author keywords and number of occurrences in the research domain. Since the color of an item is determined by the score of the item, where by default colors range from blue (lowest score) to green to yellow (highest score) (Eck & Waltman, 2020), this representation helps to identify the most influential and frequent author keywords in the domain.
It is evident that most attention in this domain has been drawn to the author keywords namely; urban design, urban canyon, physiological equivalent temperature (PET), thermal sensation, evapotranspiration, and ENVI-met simulations. This demonstrates that the studies assessing human bio meteorology in terms of the cooling effect of vegetation and the impact of street morphology have been widely investigated. Further, the adopted methodologies mostly comprise of computational fluid dynamics (CFD) for simulations and calculation of PET values as a thermal comfort index to assess the bio meteorology of outdoor thermal environments.
In urban bio meteorological assessments, several bio meteorological indices  Model for Individuals (MEMI) and it has been defined as equivalent to the air temperature in a typical indoor setting at which the heat balance of the human body is sustained (Hoppe, 1999;Matzarakis et al., 1999). Table 3 shows the clusters of author keywords and the number of occurrences  According to the number of occurrences and average citations belonging to each cluster, the literature could be categorized into five main research trends (clusters) as mentioned in Table 4. Though there are eight clusters, considering the number of author keywords in each cluster, five main research trends were identified and cluster six, seven, and eight are not considered due to lack of appearance of author keywords.

Analysis B-Studies on OTC Assessments Conducted in Warm-Humid Urban Public Spaces
The second search was conducted adding the terms "warm-humid" OR "hothumid" OR "tropical" OR "tropics" to the first search for which specific climatic regions were not considered.

Evolution of Publications
Article publications and citations of the articles represent the growth and the attention received in the particular domain of research. Figure 8 shows  Urban heat gain and ambient temperature increase as a result of solar access and seeking strategies for thermal comfort using cooling materials to reduce the heat island effect.
Cluster 2 Approaches for urban space design using micrometeorological simulation modeling (ENVI-met) and comfort level (UTCI and PET) improvements in urban planning processes.
Cluster 3 Parameters of urban morphology and vegetation affecting thermal perception and urban microclimate.
Cluster 4 Urban heat gaining and cooling effects of greenery, vegetation, and green infrastructure improving thermal comfort levels.
Cluster 5 Thermal adaptation in public space design in terms of microclimatic enhancement.

Contributing Countries
The major contributing countries in terms of publication were analyzed setting the limitations to the least number of publications of a country and least cita-   Figure 11 illustrates the most contributing authors in this research domain and limitations for this analysis were set to the least number of documents of an author as 1 and least citations of an author as 100. Results illustrated seven items in three clusters in the following overlay visualization. The size of the circles represents the citation weights and the color represents the scores of average normalized citations. Since the color of an item is determined by the score of the item, color range from blue to green to yellow (highest score), with yellow color indicating the author who has the highest normalized citations.

Influential Authors
The six most influential authors (Table 6)

Research Topics-Document Analysis
The publications with the highest t impact in the domain of the microclimate and OTC in warm-humid urban spaces (34 documents) were analyzed by the VOSviewer. Limitations were set to the least number of citations of a publication as 1. The results yielded 20 documents in seven clusters as illustrated in Figure   12. Then the documents were ranked based on the normal citations as shown in Table 7. The content analysis has been conducted on 12 articles which are selected based on the normal citation above 1.00. Then each article was assessed and identified according to keywords and content analysis, because, the main focus and content of a study are represented by the keywords of the document considered within a specific knowledge domain (Su & Lee, 2010).
According to Table 7, the most influential study was published in 2015, The results revealed that urban morphology has received 50% of the total attention out of the total highest influential documents. Urban vegetation has received 33% and both impacts of water bodies and subjective thermal comfort assessment have received 8.3% of the most influential studies on microclimate and OTC in warm-humid urban public spaces. However, studies in warm-humid cities have considerable influence on the studies in other climatic regions. According to the most linked and frequent author keywords in this research domain, it is revealed that urban design parameters are significantly assessed to find the influence on OTC. Table 8 shows the urban design components and corresponding parameters assessed in 34 studies related.

Findings and Discussion
This review has revealed several urban design parameters that affect OTC and microclimate in pedestrian level, as discussed in previous sections. In this sec-   of facilitating comfortable outdoor spaces for urbanites (Morakinyo & Lam, 2016). The identified urban design parameters have significant impact on urban bio-meteorology irrespective of the climatic region. According to previous review studies, changes of urban geometry, inclusion of vegetation, cool surface materials, and incorporated water bodies have been identified as four major mitigation strategies to improve OTC in urban areas (Lai et al., 2019). Additionally, Setaih et al. (2013, August) (Taleghani, 2018;Lee & Mayer, 2018). Further, urban geometry and construction materials (Chatzidimitriou & Yannas, 2015) and material properties affect the energy balance of surfaces (Doulos et al., 2004). Besides, the surface energy balance is affected by urban hydrological processes such as runoff, infiltration, interception, or irrigation of the urban areas and especially improve the OTC level during heatwave conditions. Even though, water bodies increase the humidity levels (Broadbent et al., 2018;Wai et al., 2021), it is an effect yet to be considered in climatic responsive urban designs. Therefore, this review concludes that urban biometeorology is affected by four main components of urban design interventions namely; urban surfaces (vertical and horizontal), morphology and geometry, green infrastructure and blue infrastructure.
Blue infrastructure in urban spaces has received the lowest attention as a strategy of improving the OTC level (Wai et al., 2021). Nevertheless, Setaih et al. (2013) claimed that use of water features is a good heat sink technique in improving the pedestrian thermal comfort in hot dry urban environments, but not a suitable heat sink for climates with high humidity conditions. According to Lai et al. (2019), the cooling effect of vegetation, cool surface, and water bodies are less in compacted urban space when compared to an open area. Therefore, implementation of the cooling strategy depends on climate zone and the surrounding characteristics of the urban space. Yet, water surfaces act in a positive way in urban thermal process with the evaporative cooling effect due to its' low reflectivity and high thermal inertia (Chatzidimitriou & Yannas, 2015). Moreover, deep and dynamic water features partly shaded by trees could be a more effective way of reducing thermal stress (Fung & Jim, 2020). Future tropical OTC studies should consider the critical aspects of local humidity and wind conditions (Chow et al., 2016). Thus, investigation of urban water bodies as a cooling strategy is still in need of further research in warm-humid cities due to high humidity and increased ambient air temperature.
Green infrastructure improve the microclimate by reducing hot air flows, evapotranspiration, and shading as the most efficient way to reduce the negative effect of warming urban environments (Bartesaghi et al., 2019). Trees are the most effective factor for reducing long-wave radiation exchange by blocking short-wave radiation penetration to the surface, while turf lawns and shrubs provide only the surface shading (Shashua-Bar et al., 2011;Ng et al., 2012).
However, roof greening is not effective for human thermal comfort near the ground level (Ng et al., 2012). The foliage density makes 60% of temperature regulation as the most effective determinant of urban vegetation (Morakinyo et Hsieh, Jan, & Zhang, 2016;Morakinyo et al., 2018;Zaki et al., 2020;Teshnehdel et al., 2020). Therefore, further research is needed to find the best vegetation strategies in terms of climatic responsive urban design, especially in tropical cities.
Assessing the impact of urban surface albedo on OTC has received considerable research attention. Although, the use of high albedo materials is an effective strategy to reduce heat stress (Fintikakis et al., 2011), there is a debate on selecting urban surface materials depending on the albedo value as the results of the previous empirical studies. The reason is that the law albedo materials increase surface temperature while the high albedo materials increase the ambient temperature due to high reflected radiation, which causes negatively on thermal comfort (Chatzidimitriou & Yannas, 2015;Lee & Mayer, 2018;Taleghani, 2018;Falasca et al., 2019). Moreover, surfaces with high reflective materials create longer hot hours in day time compared to low reflective ones (Lin et al., 2010).
This should be overcome by identifying some cooling strategies that can reduce the negative effects of particular hard materials, since it is one of the major parameters of urban design.
Outdoor thermal comfort is influenced by due to altered alternative durations of incoming solar radiation and the mean radiant temperature (MRT) which is created by various urban forms (Taleghani et al., 2015). Urban morphology has been widely investigated in previous studies and this review has identified three main morphology parameters, namely; aspect ratio (H/W), sky view factor (SVF) and axis orientation of the spaces. Urban aspect ratio describes the building height (H) to road width (W) ratio. Aspect ratio and the orientation control the solar access into street canyons (Deng & Wong, 2020). According to the results of previous research in tropical regions, increased (H/W) ratio gives more significant cooling benefits (Morakinyo et al., 2017;Deng & Wong, 2020), as a result of due to the shade created by deeper canyons (Emmanuel et al., 2007).
Moreover, the canyons with higher H/W aspect ratios increase wind velocity and shading by improving thermal comfort at the pedestrian level. Nevertheless, the street length to building height (L/H) ratio had no significant effect on the thermal comfort level at the pedestrian level (Muniz-Gäal et al., 2020). Thus, the configuration of trees in street canyon is important, since trees act as a wind moderator (Hsieh, Jan, & Zhang, 2016;Yahia et al., 2018). Although, the asymmetrical streets canyons are mostly available in tropical countries, thermal comfort assessments related to such streets has received very low research attention.  Sharmin et al. (2015) found that traditional areas have lower air temperature and MRT than planned areas and people in diverse traditional areas perceive more comfort when comparing subjective (using TSV) and calculated MRT. Another study conducted by Chow et al. (2016) has found significant differences between measured and perceived thermal comfort. Therefore, as highlighted by Ruiz & Correa (2015), validation of the subjective approaches by quantifying thermal comfort levels using bio meteorological indices could be more rational to examine the local climate adaptation in urban spaces.
Adapted methodologies in objective approaches mainly comprise of two phases; 1) Onsite measurements of meteorological parameters, 2) numerical simulations. The meteorological parameters affecting thermal comfort; air temperature, humidity, wind speed, and wind direction are measured at the pedestrian level (1.5 m above ground level) in majority of the studies to ensure the pedestrian level thermal comfort. In recent studies, OTC is frequently estimated with parametric ENVI-met simulations employing physiologically equivalent temperature (PET) (Emmanuel et al., 2007;Muniz-Gäal et al., 2020;Fung & Jim, 2020;Fabbri et al., 2020;Shareef & Abu-Hijleh, 2020;Mahmoud et al., 2021;Wai et al., 2021).
Further, the simulation results have been validated by onsite measurements in many studies. Therefore, Computational fluid dynamics (CFD) simulations validated by field measurements is recommended for OTC assessments in urban outdoor spaces.
Findings of the review and potential research gaps, 1) There is inadequate research on the impact of blue infrastructure on OTC irrespective of the climatic zone, and especially in warm-humid cities. Considering the use of water bodies, irrigation and water spraying systems is yet to be explored.
2) Heat mitigation strategies to overcome the increased ambient temperature due to reflective urban surfaces (high albedo) need to be investigated.
3) Vegetation configurations in street canyons, and other critical areas in urban environments should be assessed taking ventilation into consideration. 4) Asymmetrical street canyons which are the most available street morphology type in tropical countries, have not received adequate attention.
5) How local climate zone (LCZ) classification approach could be used for assessing OTC in warm-humid climates require further exploration.

Conclusion
This