Comparison and Generalisation of Spatial Patterns of the Urban Heat Island Based on Normalized Values

Physics and Chemistry of the Earth 35 (2010) 107–114

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Physics and Chemistry of the Earth

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Comparison and generalisation of spatial patterns of the urban heat island based on normalized values

János Unger a,*, Zoltán Sümeghy a, Sándor Szegedi b, Andrea Kiss c, Róbert Géczi c a Department of Climatology and Landscape Ecology, University of Szeged, P.O. Box 653, H-6701 Szeged, Hungary b Department of Meteorology, University of Debrecen, P.O. Box 13, H-4010 Debrecen, Hungary c Department of Physical Geography and Geoinformatics, University of Szeged, P.O. Box 653, H-6701 Szeged, Hungary article info abstract

Article history: The studied medium-sized cities (Szeged and Debrecen, Hungary) are located on a low and flat plain. Data Available online 6 March 2010 were collected by mobile measurements in grid networks under different weather conditions between April 2002 and March 2003 in the time of maximum development of the urban heat island (UHI). Tasks Keywords: included: (i) interpretation and comparison of the average UHI intensity fields using absolute and nor- Urban heat island malized values; (ii) classification of individual temperature patterns into generalized types by cities using Szeged normalization and cross-correlation. According to our results, spatial distribution of the annual and sea- Debrecen (Hungary) sonal mean UHI intensity fields in the studied period have concentric shape with some local irregulari- Normalization ties. The UHI pattern classification reveals that several types of the structure can be distinguished in Cross-correlation UHI types both cities. Shifts in the shape of patterns in comparison with the centralized pattern are in connection with the prevailing wind directions. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction 1940s (Balchin and Pye, 1947). In order to show the detailed spatial distribution of the UHI within an urban area it is necessary to pos- Urbanization modifies materials, structure and energy-balance sess a temperature data set in an appropriate (small-scale) resolu- of the surface and composition of the atmosphere compared to the tion. To resolve this task, determination of the surface and near- surrounding ‘natural’ environments. These artificial alterations surface air UHI by moving observation with different vehicles determine a distinguished local climate in cities, which is called (bicycle, car, tram) is a common process (Schmidt, 1930; Duck- as urban climate. The climate modification effect of urbanization worth and Sandberg, 1954; Park, 1986; Unger et al., 2001b), but is most obvious in case of the temperature excess (urban heat there are some (but rare) examples for a dense station network island – UHI), which is quantified by the UHI intensity (namely DT, (Mikami et al., 2003) and for the evaluations of airborne and satel- the temperature difference between urban and rural areas). lite images taken in the appropriate wavelengths (remote sensing) Different level of the UHI can be observed from couple of meters (Voogt and Oke, 1997; Chen et al., 2006). under the surface to 200–300 m above the surface. In the present The first objective of this paper is to show certain differences in investigation, the urban modification of the near surface (1.5– the interpretation of the urban heat island using absolute and nor- 2 m above ground level) temperature will be discussed, which malized temperature values. The second objective is to make a can be detected in the immediate environment of humans living simple analysis to distinguish typical UHI patterns based on nor- in settlements. Generally, its intensity has a diurnal course with malized values and cross-correlation using 1 year comprehensive a strongest development at 3–5 h after sunset (Oke, 1987). temperature measurements in two cities (Szeged and Debrecen) Presentation of the UHI by isotherms showing the spatial distri- of Hungary. bution and magnitude of the temperature excess relative to the surroundings of the city is well illustrated: the more or less circular and closed lines remind us on the topographical appearance of is- 2. Question of interpretation and demonstration of the spatial lands or hills on contour maps. The first demonstration of such pre- structure of UHI sentation goes back to the 1920s (Peppler, 1929), whereas the name of the urban heat island itself was first published in the Beside the morphology and material of urban surface and anthropogenic activity, the strength and structural features of the UHI development depend – similar to other local climatic phenom- * Corresponding author. Tel.: +36 62 544857; fax: +36 62 544624. ena – on the prevailing weather conditions: above all on cloudiness E-mail address: [email protected] (J. Unger). and wind which influence the radiation processes and turbulent

1474-7065/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2010.03.001 108 J. Unger et al. / Physics and Chemistry of the Earth 35 (2010) 107–114 mixing of the air. The artificial urban surface can be characterized but slightly extreme example that the ‘general’ picture obtained by by numerous types of measures which indicate alterations from this method, is not an ‘average’ picture (Fig. 1). In this context, the the natural surface (e.g. material, covering, built-up features, 3D background of drawing of the UHI fields and the normalization will geometry, land use) (Bottyán and Unger, 2003; Eliasson and Svens- be discussed in Sections 3 and 4, respectively. son, 2003). These all affect the energy-balance and, as a conse- The question is based on the fact that the (absolute) UHI inten- quence, the value of the forming temperature at a given urban sities were very different (1.60 and 6.81 °C) (Fig. 1a and b); thus, place. the values of the second case dominate in the average values calcu- Nevertheless, merely on the basis of only a few case studies lated for the points of the investigated area (Fig. 1c). For this reason general deductions cannot be drawn on the intra-urban variation the obtained ‘average’ UHI pattern reminds us to the pattern of the of temperature. Naturally, the isotherms can be drawn in the second case. That is, the UHI fields of the two cases appear in the above-mentioned cases, too. Thus, it provides a picture on the ‘average’ picture with different weight. structure which reflects the spatial distribution of the UHI at a certain moment. This momentary situation is derived from the com- bined effect of the relatively permanent surface features 3. UHI database changing spatially and the weather conditions changing tempo- rally. These individual patterns, referring to the same urban area, 3.1. Study areas (Szeged, Debrecen), grid networks can be sharp (strong UHI – favourable weather conditions) or faded (weak UHI – unfavourable weather conditions), as well as shifted Szeged is located in the south-eastern part of Hungary (46°N, depending on the prevailing air flow. 20°E) at 79 m above sea level on a flat plain. River Tisza passes Therefore, if we would like to present a general (average) pic- through the city; otherwise, there are no large water bodies near- ture on the development of the UHI and the typical temperature by. The river is relatively narrow and – according to earlier inves- patterns in a given area, these patterns have to be determined tigations – its climatic influence is negligible (Unger et al., 2001b). not from one or two, but from numerous cases. In this case, the The number of inhabitants is 160,000. Debrecen (47.5°N, 21.5°E) question is how we can achieve the goal to make the resulted pat- lies in the north-east region of the country at 120 m above the terns reflect really the main spatial features of the UHI. sea level on a nearly flat terrain. It is the second largest city in Hun- The answer could come easily if we apply a simple and com- gary and has a population of 220,000 (Fig. 2). monly applied method on averaging of the values by points, then Regions of both cities are categorized in Köppen’s climatic re- the obtained temperature field can be regarded as a typical pattern gion as Cf (temperate warm climate with a rather uniform annual of the UHI referred on a given urban area (Park, 1986; Steinecke, distribution of precipitation). In Szeged and Debrecen the annual 1999; Mikami et al., 2003). Here, it is demonstrated with a simple, mean temperatures are 10.5 °C and 9.9 °C with annual ranges of

Fig. 1. Areal distributions of the absolute (°C) and the normalized UHI intensities on 24 February 2003 and on 24 March in Szeged: absolute values (a, b), average of absolute values (c), normalized average of absolute values (d), normalized values (e, f) and average of normalized values (g). Sites of maximum value of DTnorm (=1) are indicated by . J. Unger et al. / Physics and Chemistry of the Earth 35 (2010) 107–114 109

Fig. 2. Locations of Szeged and Debrecen in Europe (a) and in Hungary (b), as well as division of the study areas into 0.5 0.5 km grid cells in Debrecen (c) and in Szeged (d) urban areas are marked by grey and rural cells are indicated by R.

22.6 °C and 22.9 °C, the mean precipitation amounts are 495 mm where Tcell and TR are the temperatures of a given urban cell and cell and 566 mm, respectively. R, respectively (Fig. 2). The obtained DT values were related to the These environmental conditions make Szeged and Debrecen cell centre points. The rural cell (R) in Debrecen is a bit nearer to the favourable places for studying of clear urban climate development. urban area than in Szeged because of the deﬁciency in street net- As a consequence, results of systematic measurements and analy- work (not shown in Fig. 2) in the SE part of the city. sis in this type of cities can be regarded as an appropriate basis for the deduction of general conclusions (Unger et al., 2001a; Bottyán 4. Normalization et al., 2005). The study areas were divided into two sectors and subdivided The ﬁrst step for the grouping of the UHI patterns by shapes was further into 0.5 0.5 km cells (Fig. 2). They consist of 107 and the normalization of DT values in each pattern. In the course of 105 cells covering the urban and suburban parts of Szeged 2 2 normalization the absolute intensities of each cell were divided (26.75 km ) and Debrecen (26.25 km ), respectively. The outlying by the absolute value of the cell which had the maximum intensity parts of the cities, characterized by village and rural features, are at the given night: not included in the mesh except for some cells at the edges of the areas in order to determine urban–rural temperature contrasts. DTnorm ¼ðTcell TRÞ=TcellðmaxÞ TR

3.2. UHI intensity (DT) According to the earlier description, the spatial temperature structure obtained from one measurement night can be character- Mobile measurements were taken by cars in the two sectors of ized by absolute or normalized intensity values. 105 and 107 both cities at the same time on ﬁxed return routes; during a 1 year points (the above mentioned centre points) covering the urban period, altogether 35 in Szeged and 30 times in Debrecen, respec- parts of Debrecen and Szeged, respectively, provide an appropriate tively. As we already mentioned, moving observation in the UHI basis to interpolate isotherms applying the standard Kriging proce- detection with different vehicles is an often used process (see Sec- dure (linear variogram model application). The value in cell R is tion 1). The approximately 10-day frequency of car traverses pro- equal to zero in both (absolute and normalized) cases. The absolute vided sufﬁcient information under different, but rainless weather maximum values are different while the normalized ones are al- conditions. ways equal to 1 (Fig. 3). Return routes of about 3 h through all cells by sectors were In connection with normalization, the rather similar process of made to make time-based corrections. Readings were obtained standardization has to be mentioned as well. Sometimes, standard- using radiation-shielded resistance sensors connected to data log- ized temperature patterns are applied within the investigated ur- gers. Data were collected every 10 s, so at a car speed of 20–30 ban areas, too (Saz et al., 2003). Its disadvantage is that the range km h1 the distance between measuring points was 55–83 m. of DT is relative and the smallest value cannot be connected to only The sensors were mounted at 1.45 m above ground level. The one site. Here, there is no reference or background rural area (Park, logged values at forced stops were deleted from the data set. Hav- 1986). ing averaged the 15–20 measurement values by cells, time adjust- At the Fourth Conference on Urban Climate held in Sydney, Oke ments to a reference time (4 h after sunset, namely the likely time (1999) already pointed at the importance of investigation on the of the strongest UHI in the diurnal course, based on earlier mea- normalized DT. In the investigation of the diurnal cycle of the surements) were applied. DT values were determined by cells UHI, Runnalls and Oke (2000) applied normalization on the time referring to the temperature of the rural (R) cell: and the UHI variation based on the lengths of day/night and min- imum/maximum values of DT during the 24 h diurnal cycle. In his DT ¼ T T cell R study on satellite-measured growth of the UHI, Streutker (2003) 110 J. Unger et al. / Physics and Chemistry of the Earth 35 (2010) 107–114

Fig. 3. Absolute (°C) (a, b) and normalized (c, d) UHI intensities in Szeged on 13 June 2002 and on 27 January 2003. Sites of the maximum value of DTnorm (=1) are indicated by .

used normalized distribution in the histograms for comparison of uation raises a possibility for classification. Taking absolute values rural temperatures and UHI magnitudes because of the differing into account, Klysik and Fortuniak (1999) already made an attempt number of measurements in the investigated two periods. Accord- with a simple grouping. According to the running of isotherms they ing to the authors knowledge there are no attempts to employ the distinguished only two types: the case of an ‘ordinary’ heat island chosen or similar normalization procedure for studying of the and another of a heat archipelago with several local maximum structure of UHI patterns in the related scientific literature. values. In our opinion the determination of the ‘average’ UHI pattern In our case, cross-correlation was applied for normalized tem- referred to a period using normalized values contrary to absolute perature patterns. For this 105–107 DT values for each of the 30– ones is a more appropriate method, since the roles of the different 35 measuring nights in Debrecen and Szeged, respectively were cases are balanced: that is the weights of cases in the average pat- used. It means altogether 435–595 relationships between the dif- tern are the same. This process eliminates the alterations caused by ferent patterns. For practical purposes, the coefficients can be gath- differences in the UHI magnitude (Fig. 1). ered in a cross-correlation matrix (Montavez et al., 2000). At the In order to verify further the above mentioned statement, Fig. 3 applied element number (n = 105 or 107) the correlation coeffi- shows two individual UHI patterns with very different absolute cient is significant at the confidence level of 99% if its value is lar- intensities observed in Szeged. Maximum DT values (4.75 °C and ger than 0.25. The classification is based on the next simple 1.06 °C) appeared in the NE part of the city. There are significant common feature: those patterns are in one type which are in the differences – with some similarities – in the temperature patterns. afore-mentioned significant correlation with each other pattern However, the examination of the normalized patterns reveals that in the class. the two spatial structures are very similar. Therefore, the application of normalization offers us a useful tool to compare temperature patterns measured at different occasions. 5. Results and discussion After studying the 30 and 35 UHI patterns in Debrecen and Sze- ged, respectively, it emerged that some patterns repeated. This sit- Fig. 4 shows the annual average patterns of the normalized DT based on the investigated 1 year period (April 2002–March 2003)

Fig. 4. Annual average of the normalized UHI intensities based on a 1 year-period (April 2002–March 2003) measurement series in Szeged (a) and in Debrecen (b). J. Unger et al. / Physics and Chemistry of the Earth 35 (2010) 107–114 111 in Szeged and Debrecen. The Szeged pattern shows a more regular In Debrecen, the city core and the housing estates in the wes- form than in Debrecen, owing to the more concentric city structure tern sector of the city belong to the most developed part of the of Szeged. Therefore, in Szeged the pattern is almost concentric, UHI. The greatest intensity occurs west from the centre, possibly and values are increasing from the outskirts towards the inner in connection with the fact that the large housing estates of Debre- areas. A large deviation from this shape occurs in the north-eastern cen with 10–12 storey buildings can be found in the western part part of the city, where isotherms stretch towards the suburbs. This of the city. The forested recreational area to the north is the coolest situation can be explained by the effect of a large housing estate part of the city. The highest mean absolute value (not shown) is a with high concrete buildings and with high built-up ratio (Bottyán bit lower than in Szeged (2.3 °C). and Unger, 2003). Considering the mean pattern of absolute DT In order to reveal the occurrent seasonal differences, mean sea- (not shown), greatest mean intensity of 2.72 °C is found in the sonal normalized patterns were calculated for both cities, although centre. small numbers of cases were taken into account because of the shorter time periods (Tables 1 and 2). According to the Figs. 5

Table 1 Name, seasonal and annual number and range of observed maximum intensities (°C) of the UHI pattern types based on a 1 year-period (April 2002–March 2003) measurement series in Szeged.

Type Name Number of cases DT range Spring Summer Autumn Winter Heating season Non-heating season Year A Centralized 1 3 1 1 2 4 6 0.35–5.70 B Shifted to NE 3 3 2 3 5 6 11 0.97–6.82 C Shifted to SE 2 1 0 3 4 2 6 2.57–5.06 D Shifted to S 1 0 2 0 1 2 3 0.82–1.43 E Shifted to SW 1 1 0 1 1 2 3 1.60–4.26 F Shifted to NW 1 1 4 0 4 2 6 1.83–3.21 All – 9 9 9 8 17 18 35 0.35–6.82

Table 2 Name, seasonal and annual number and range of observed maximum intensities (°C) of the UHI pattern types based on a 1 year-period (April 2002–March 2003) measurement series in Debrecen.

Type Name Number of cases DT range Spring Summer Autumn Winter Heating season Non-heating season Year A Centralized 2 2 2 1 3 4 7 1.93–5.60 B Shifted to N 2 1 0 2 3 2 5 1.84–3.38 C Shifted to E 1 0 2 0 1 2 3 1.02–1.81 D Shifted to SE 1 0 0 1 2 0 2 0.71–1.20 E Shifted to SW 2 5 3 3 6 7 13 0.70–5.78 All – 8 8 7 7 15 15 30 0.70–5.78

Fig. 5. Seasonal averages of the normalized UHI intensities based on a 1 year-period (April 2002–March 2003) measurement series in Szeged. 112 J. Unger et al. / Physics and Chemistry of the Earth 35 (2010) 107–114

Fig. 6. Seasonal averages of the normalized UHI intensities based on a 1 year-period (April 2002–March 2003) measurement series in Debrecen.

Fig. 7. Averages of the normalized UHI intensities in the heating and non-heating seasons in Szeged (a) and in Debrecen (b) based on a 1 year-period (April 2002–March 2003) measurement series.

and 6, the main structural features of the seasonal patters remind information (name, number of measurements, range of measured us of the annual patterns in both cities (see Fig. 4). The same con- maximum intensities, form of patterns) according to types. In com- clusion can be drawn if we investigated the normalized UHI fields parison with the relatively regular, centralized pattern (A), the for the colder (or heating) and warmer (or non-heating) seasons irregularities (or rather shifts) in the shapes are reflected in the (Fig. 7). names of the types. Presumably, these shifts can be explained by According to the classification of the normalized individual UHI the prevailing wind directions at the measuring nights: this is sup- patterns using correlation coefficients, six and five types (marked ported by the sample nights for every types listed in Table 3 in the by A, B, ...) can be distinguished in Szeged and Debrecen, respec- case of Szeged. The table contains the prevailing wind directions tively. Tables 1 and 2, as well as Figs. 8 and 9 contain the necessary and mean wind speeds during the 3-h measurement times and J. Unger et al. / Physics and Chemistry of the Earth 35 (2010) 107–114 113

Fig. 8. Average normalized UHI patterns based on a 1 year-period measurement (April 2002–March 2003) in Szeged: A (centralized), B (shifted to NE), C (shifted to SE), D (shifted to S), E (shifted to SW), F (shifted to NW).

Fig. 9. Average normalized UHI patterns by types based on a 1 year-period measurement (April 2002–March 2003) in Debrecen: A (centralized), B (shifted to N), C (shifted to E), D (shifted to SE), E (shifted to SW).

Table 3 Sample nights by types with the prevailing wind directions and mean wind speeds during the 3-h measurement times and the preceding 3 h in Szeged.

Type Measurement time Name Wind speed frequency during the selected 6 h Mean wind speed during the selected 6 h (m s1) Direction (%) A 18 September 2002 Centralizált W–WSW 78.4 1.47 B 13 June 2002 Shifted to NE S–SSW–SW 89.5 1.69 C 18 February 2003 Shifted to SE NW–NNW 81.6 2.38 D 05 April 2002 Shifted to S NNE 81.6 5.03 E 26 June 2002 Shifted to SW E–NE–NNE 76.3 2.59 F 27 August 2002 Shifted to NW SE–SSE 64.9 3.91

the preceding 3 h (altogether 6 h). The 10-min wind data were ob- ized structure could develop. For the other types the shifts in the served at the top of a building of the University of Szeged near the UHI structures are adequately explained by the wind direction city centre. In the case of type A the wind was weak thus a central- and speed. 114 J. Unger et al. / Physics and Chemistry of the Earth 35 (2010) 107–114

After the investigation of the annual situation, an idea on the Bottyán, Z., Unger, J., 2003. A multiple linear statistical model for estimating mean seasonal comparison of the classes emerged, but the seasonally maximum urban heat island. Theor. Appl. Climatol. 75, 233–243. Bottyán, Z., Kircsi, A., Szegedi, S., Unger, J., 2005. The relationship between built-up 7–9 measurements did not allow a properly based classification areas and the spatial development of the mean maximum urban heat island in for these shorter time periods (Tables 1 and 2). Debrecen. Hungary Int. J. Climatol. 25, 405–418. Chen, X.-L., Zhao, H.-M., Li, P.-X., Yin, Z.-Y., 2006. Remote sensing image-based analysis of the relationship between urban heat island and land use/cover 6. Conclusions changes. Rem. Sens. Environ. 104, 133–146. Duckworth, F.A., Sandberg, J.S., 1954. The effect on cities upon horizontal and On the basis of the analysis presented, the following conclu- vertical temperature gradients. Bull. Am. Meteorol. Soc. 35, 198–207. Eliasson, I., Svensson, M.K., 2003. Spatial air temperature variations and urban land sions can be drawn: use – a statistical approach. Meteorol. Appl. 10, 135–149. Klysik, K., Fortuniak, K., 1999. Temporal and spatial characteristics of the urban heat (i) Utilization of the individual normalized patterns (contrary island of Lodz. Poland Atmos. Environ. 33, 3885–3895. Mikami, T., Ando, H., Morishima, W., Izumi, T., Shioda T., 2003. Investigation of to the absolute patterns) in determination of the average urban heat islands in Tokyo metropolis based on the ground monitoring system. UHI field and in comparison of with the UHI patterns mea- In: de Dear, R.J., Kalma, J.D., Oke, T.R., Auliciems, A. (Eds.), Biometeorology and sured at different occasions is verified. Urban Climatology at the Turn of the Milleneum. Selected papers from the Conference ICB-ICUC’99. WMO/TD No. 1026, pp. 491–495. (ii) Spatial patterns of the mean normalized UHI intensity have Montavez, J.P., Rodriguez, A., Jimenez, J.I., 2000. A study of urban heat island of almost concentric shape. The in Szeged pattern is more reg- Granada. Int. J. Climatol. 20, 899–911. ular than in Debrecen, owing to the more concentric city Oke, T.R., 1987. Boundary Layer Climates, second ed. Routledge University Press, Cambridge. 435p. structure of Szeged. Anomalies in the regularity are caused Oke, T.R., 1999. Closing Speech on the ICB-ICUC’99 on 9th November 1999. Sydney, by the alterations of urban surface features. Regarding the Australia. seasonal patterns, their main structural features are similar Park, H.-S., 1986. Features of the heat island in Seoul and its surrounding cities. Atmos. Environ. 20, 1859–1866. to the annual patterns in both cities. Peppler, A., 1929. Die temperaturverhältnisse von Karlsruhe an heissen (iii) Normalization of intensities and cross-correlation provide a Sommertagen. Deutsches Meteorol. Jahrb. Baden 61, 59–60. useful tool for classification of the individual UHI patterns. Runnalls, K.E., Oke, T.R., 2000. Dynamics and controls of the near surface heat island Several types of the DT pattern can be distinguished in both of Vancouver, British Columbia. Phys. Geogr. 21, 283–304. Saz, M.A., Vicente, S.M., Cuadrat J.M., 2003. Spatial patterns estimation of urban heat cities. Shifts in the shapes are, as was demonstrated in some island of Zaragoza (Spain) using GIS. In: Klysik, K., Oke, T.R., Fortuniak, K., sample nights, in connection with prevailing wind Grimmond, C.S.B., Wibig, J. (Eds.), Proceedings of the Fifth International directions. Conference on Urban Climate, vol. 2, Department of Meteorology and Climatology, University of Lodz, Lodz, Poland, pp. 409–412. Schmidt, W., 1930. Kleinklimatische Aufnahmen durch Temperaturfahrten. Acknowledgements Meteorol. Z. 47, 92–106. Steinecke, K., 1999. Urban climatological studies in the Reykjavik subarctic This research was supported by the Grants of the Hungarian environment. Iceland Atmos. Environ. 33, 4157–4162. Streutker, D.R., 2003. Satellite-measured growth of the urban heat island of Scientific Research Fund (OTKA K-67626) and of Bolyai Research Houston. Texas. Rem. Sens. Environ. 85, 282–289. Scholarship of the Hungarian Academy of Sciences (BO/00519/ Unger, J., Sümeghy, Z., Zoboki, J., 2001a. Temperature cross-section features in an 07). Cross-correlations were calculated by Z. Bottyán. The review- urban area. Atmos. Res. 58, 117–127. Unger, J., Sümeghy, Z., Gulyás, Á., Bottyán, Zs., Mucsi, L., 2001b. Land-use and ers’ helpful comments are greatly appreciated. meteorological aspects of the urban heat island. Meteorol. Appl. 8, 189–194. Voogt, J.A., Oke, T.R., 1997. Complete urban surface temperatures. J. Appl. Meteorol. References 36, 1117–1132.

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