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Elnabawi M, Hamza N, Dudek S. Numerical Modelling Evaluation for the

Microclimate of an Outdoor Urban Form in Cairo, Egypt. HBRC Journal 2015,

11(2), 246-251.

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© 2016 Elsevier B.V. or its licensors or contributors with open access funded by

Housing and Building National Research Center

DOI link to article:

http://dx.doi.org/10.1016/j.hbrcj.2014.03.004

Date deposited:

27/06/2016

HBRC Journal (2015) 11, 246–251

Housing and Building National Research Center

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Numerical modelling evaluation

for the microclimate of an outdoor urban

form in Cairo, Egypt

* Corresponding author. Address: School of Architecture, Planning

and Landscape, Newcastle University, Newcastle upon Tyne NE1

7RU, UK. Tel.: +44 (0) 755 023 3393; fax: +44 (0) 191 222 6115.E-mail address: m.elnabawi-mahgoub@ncl.ac.uk (M.H. Elnabawi).

Peer review under responsibility of Housing and Building National

Research Center.

Production and hosting by Elsevier

1687-4048 ª 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center.

http://dx.doi.org/10.1016/j.hbrcj.2014.03.004

Mohamed H. Elnabawi *, Neveen Hamza, Steven Dudek

School of Architecture, Planning and Landscape, Newcastle University, UK

Received 7 January 2014; accepted 8 March 2014

KEYWORDS

Thermal comfort;

Urban microclimate;

ENVI-met;

Mean radiant temperature

Abstract In order to achieve outdoor thermal comfort it is necessary to understand the interac-

tions between the prevailing climate, the urban form and roughness. The near surface boundary

layer is directly influenced by local irradiative and convective exchange processes due to the pres-

ence of a variety of different surfaces, sheltering elements and obstacles to air flow leading to dis-

tinctive micro-scale climates. The paper presents a micro-scale numerical model for an outdoor

urban form for a hot summer’s day in Al-Muizz street located at the Islamic quarter of Cairo, where

a few studies have attempted to study these conditions in vernacular settings in hot arid areas where

the continuously evolving urban patterns and shaded environments were perceived to produce more

pedestrian friendly outdoor environments. In situ measurements are used to validate the ENVI-met

results which showed an overall agreement with the observed ones, representing adequate mean

radiant temperature (Tmrt) which is one of the most important meteorological parameters govern-

ing human energy balance and has therefore a strong influence on thermal sensation of the pedes-

trians using the open public spaces and generating a micro-climatic map as an initial step in

addressing the urgent need for a modelling platform accessible to urban designers, architects,

and decision makers towards sustainable urban forms.ª 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research

Center.

Introduction

The 4th IPCC Assessment Report (2011) indicated that Africais warming faster than the global average, and this is likely tocontinue. This warming is greatest over the interior of semi-

arid margins of the Sahara and central southern Africa [1].This was compiled with the ACED (2004) [2] studies thatreported changes in area averaged temperature and precipita-tion over Egypt which was assessed based upon a dozen recent

GCMs (General Circulation Models) using a new version of

Numerical modelling evaluation for the microclimate 247

MAGICC/SCENGEN. All climate models estimated a steadyincrease in temperatures for Egypt, with little intermodal var-iance. That mostly attributes to the large fractions of the

Earth’s surface modified by human changes, particularly in cit-ies through a large number and variety of constructions, lead-ing to a changed energy balance of the surface and to the

formation of local, man-modified microclimates.This caused the urban areas to warm up much faster and

cool down much slower than their rural surroundings due to

their sealed surfaces. This provokes the creation of warmerzones in urban regions. This is known as urban heat islandphenomenon and represents a feedback of the atmosphericvariables on the use of the ground and the degree of soil seal-

ing. This phenomenon and the effect it causes on air tempera-ture as a result of urbanisation are now well documented innumerous studies [3–5] stating that the world’s average air

temperature has risen between 0.3 and 0.6 �C since the late19th century [6]. As a result, the urban micro climate startsto gain increasing attention during urban design and spatial

planning [7], starting with the study and description of micro-climate processes, and then focusing on microclimate researchin relation to outdoor thermal comfort contributions such as

those of Oke et al. [37], Bosselmann et al., Katzschner, Mori-waki and Kanda, Katzschner et al. (2004) and Stathopoulos[8–12] which were interesting for urban design because theyaddressed factors that can be changed through urban design

interventions. As it is well explained in the practice-orientedliterature that Urban microclimate depends on the type of cityin terms of size, geographical location, population size and

density, and land use as well as the street design features suchas height of buildings, street widths and orientation, subdivi-sion of the building lots, etc., the urban design of each neigh-

bourhood in a city creates its own particular local climate [13],

Fig. 1 Al-Muizz

where with well-defined planning measures the micro climatecan be improved or negative effects can be mitigated [14,15].Givoni et al. [16] highlighted the designers’ need for urban cli-

matic predicting tools to evaluate the effect of urban microcli-matic changes on outdoor human thermal comfort and thesetools need to provide the ability to process detailed environ-

mental information according to time and location variationsand to generate analytical results to reveal the relationshipamongst the microclimatic environment, outdoor urban

design and thermal comfort. In this study the microclimaticeffects of an urban site located within the Islamic Quarter ofCairo are numerically assessed using the numerical modelENVI-met 3.1.

The study context

The Cairo zone lies between latitude 26�5000 N and 30�4500 N.In the middle of that area lies Al-Muizz street as shown inFig. 1, which divides Islamic Cairo down the middle. The streethas the greatest concentration of Islamic architectural trea-

sures in the world and offers a vivid example of how life in Isla-mic Cairo might have looked like thousands of years ago. Inthe late 90’s the UNESCO recognising that Al-Muizz and its

surroundings hold great historical and cultural value declaredIslamic Cairo a protected world heritage site. A massive resto-ration project was commissioned by the Egyptian government

to restore Al-Muizz street and its surroundings, transformingthe street into an open-air museum. The first part of the streetwas fully restored and was opened to the public in Early 2010.

The second part of the street is yet to undergo restorationwork.

The case study – according to Koppen classification – isclassified within group B and sub group BWh – arid or desert

alley location.

Fig. 2 The mobile weather station setup.

Table 1 Main input data used for ENVI-met.

Parameter Value

Ta, air dry bulb temperature 308�KRH, relative humidity 59%

V, wind speed soil temperature 3.5 m/s at 10 m height

302 at (0–0.5 m) and

299 at (0.5–2m)

U value walls 1.7

U value roofs 2.2

Albedo walls 0.4

Albedo roofs 0.15

248 M.H. Elnabawi et al.

with hot climate [17], which is characterised by sparse rainfalland a vast daily temperature range.

Methodology

Micrometeorology site measurements

Based on 30 years of data from the WMO Station No. 623660

records at Cairo international airport June and July are con-sidered the hottest months. The data also emphasised thehomogeneity of the climatic condition in the summer of Cairoas shown in Fig. 2, i.e. hot, sunny and cloudless. Therefore,

meteorological measurements were carried out between 26thJune and 2nd July 2012 representing one week pattern of Cairoextreme hot summer. As stated in ASHRAE fundamental

handbook (2009) [18] the four main parameters were measuredincluding air temperature, relative humidity, solar radiationand wind speed, in addition to globe temperature which is used

to calculate the mean radiant temperature (Tmrt).

Numerical simulation

The model simulations have been carried out with the three-dimensional non-hydrostatic climate model ENVI-met version3.1 [19]. ENVI-met is a computer programme that predictsmicro climate in urban areas [20]. They describe it as ‘‘a three

dimensional microclimate model designed to simulate the sur-face-plant-air interactions in urban environment with a typicalresolution of 0.5–10 m in space and 10 s in time. ENVI-met is a

prognostic model based on the fundamental laws of fluiddynamics and thermodynamics. The model includes the simu-lation of flow around and between buildings, exchanges pro-

cesses of heat and vapours at the ground surface and atwalls, turbulence, exchange at vegetation and vegetationparameters, bioclimatology, particle dispersion.’’ (ENVI-met

website: www.envi-met.com).ENVI-met has almost all the algorithms of a CFD package

such as the model navier stoke equation for wind flow,E-e atmospheric flow turbulence equations, energy and

momentum equation and boundary condition parameters.Yet it is more developed than a specialist CFD package as it

simulates the soil/plant/surface/air relation by applying anumerical model for each [21]. It almost has the complete abil-ity to simulate built environments from the microclimate tolocal climate scale at any location, regardless of overestimation

due to uncalculating soil heat storage [22], global radiationoverestimation by day and underestimation of nocturnal bynight [23]. Moreover, ENVI-met’s combination of bio-meteo-

rological output provides an in-depth understanding of climateurban canopy layer [24] and better assessment for the outdoorcomfort levels based on human biometeorology [25].

For the model simulations, Al-Muizz alley has been trans-formed into a model grid with the dimension 30 · 140 · 30with a resolution of 1 m · 1 m · 3 m for the renovated part.Note that the model area is rotated 15� out of grid north.

Table 1 shows the simulation input data for the 1st of July2012 which is the extreme summer day for Cairo. The snapshotreceptor was located at the same spots of the measurement

campaign to record Ta, RH, V, solar radiation and Globe tem-perature at 1.2 m above the ground level. Outputs were thencompared with the local climate scale averaged records for

the same parameters observed from the site measurement.

Results and discussion

As shown in Fig. 3, the model simulation outputs at 1.2 m (dueto the vertical model resolution) are plotted against the dataobserved at 1.1 m above the ground for the Al-Muizz alley.

ENVI-met estimation for the dry air temperature (Ta) was ina good approximation with the observed ones. As shown inFig. 3, the air temperature (Ta) for the observed and modelledreached their peak in the afternoon, between 11 a.m. and

3 p.m. with peak value of 37.48 �C observed air temperatureagainst 35.2 �C computed air temperature. The regressionanalyses between the measured and computed were calculated

with correlation coefficient R2 = 0.942 for a linear relation-ship. Thus, the measured and computed ENVI-met for airtemperature is well correlated and considered reliable in pre-

senting the current air temperature condition of Al-Muizzstreet. Relative humidity in Fig. 4 also showed a good agree-ment between the RH observed from the site measurement

campaign and the RH generated by the ENVI-met, as theyreached their maximum in the late morning between 6 amand 10 am where the observed RH was 72% and 50% corre-sponding to ENVI-met estimation which was 66% and 58%.

0

5

10

15

20

25

30

35

40

6:00

7:00

8:00

9:00

10:0

011

:00

12:0

013

:00

14:0

015

:00

16:0

017

:00

18:0

019

:00

20:0

021

:00

22:0

023

:00

Air

Tem

pera

ture

°C

Ta measured Ta simulated

Fig. 3 Comparison between the dry air temperature measured

and the ENVI-met output.

0

10

20

30

40

50

60

70

80

6:00

7:00

8:00

9:00

10:0

011

:00

12:0

013

:00

14:0

015

:00

16:0

017

:00

18:0

019

:00

20:0

021

:00

22:0

023

:00

RH

%

RH measured RH simulated

Fig. 4 Comparison between the relative humidity measured and

the ENVI-met output.

Fig. 5 The mean radiant temperature Tmrt simulated by ENVI-

met plotted against measured Tmrt.

Numerical modelling evaluation for the microclimate 249

Mean radiant temperature (Tmrt)

Fig. 5 shows the Tmrt calculated using the 25 mm flat black

globe thermometer compared to the Tmrt estimated byENVI-met. Tmrt summarises all short and long wave radiation

fluxes reaching the human body in urban settings. The theoryof the globe thermometer has been thoroughly explained byKuehn et al. [26]. It was originally developed for the indoorapplication then extended to include the outdoor settings

[27]. Simply, if the globe temperature, air temperature andair velocity are known then the Tmrt can be calculated todiminish the wind factor and the diameter of the globe accord-

ing to Eq. (1) given by ASHRAE [18] with empirical coefficientrecently refined by Thorsson et al. is [28]:

Tmrt ¼ ðTg þ 273Þ4 þ 1:1� 108V0:6a

egD0:4

� ðTg � TaÞ� �1=4

� 273

ð1Þ

where (Tg) is the globe temperature (�C), (Va) is air velocity(m/s), (Ta) is the air temperature (�C), D is the globe diameter(mm) (= 152 mm in this study), and (eg) is the globe emissivity

(= 0.95 for black-colour globe). The empirically derived

parameter 1.1 · 108 and the wind exponent (V0:6a ) together rep-

resent the globe’s mean convection coefficient (1:1� 108V0:6a ).

ENVI-met is found to represent well the trends of the Tmrt

with a calculated correlation coefficient (R2) during the day-time between the measured and computed equal to 0.916.

However, simulated values of Tmrt were underestimatedafter the sunset in comparison to field data. This may attribute

to the decrease in short wave radiation after the sunset as it cango down by 20 �C of the main radiant temperature [28]. Fur-thermore the material heat storage is not taken into account

during simulation, where the heat stored in the building andtransferred through walls and roof is calculated by conductingU-values, while in real cases, each material has its own thermal

properties expressed by thermal admittance l = (KC) 0.5where (K) represents thermal conductivity and (C) representsheat capacity, therefore the lack of heat storage in theENVI-met leads to an overestimation of the long wave radia-

tion emitted by walls during the daytime and underestimationduring the night where no heat can be released after sunset[25,27].

Radiation map

According to a well literature review, the mean radiant temper-

ature (Tmrt) is one of the most important meteorologicalparameters governing human energy balance and a key vari-able in evaluating thermal sensation outdoors under sunny

conditions regardless of the comfort index used [29–31]. In thisview the models show specific areas with high Tmrt a temper-ature where action is needed (Fig. 6). And because, the sun andshade are the most predetermined parameters influencing

mean radiant temperature [32], it was essential to generateradiation patterns to identify locations with solar energy con-version potential or areas in need of shading due to excessive

solar exposure and high Tmrt, which will need to be studiedand avoided in the future. The diva for rhino – which is sus-tainable analysis plugin for the Rhinoceros 3D Nurbs model-

ling programme – was used to identify locations with solarenergy conversion potential or areas in need of shading dueto excessive solar exposure through the radiation map devel-

oped in Fig. 7. Diva performs a detailed day lighting analysisusing Radiance/DAYSIM with thermal load simulations usingEnergyPlus within [33] which is a powerful tool that can beused on an urban or building scale.

Fig. 6 The spatial pattern for the Tmrt.

Fig. 7 The radiation map generated by diva for rhino.

250 M.H. Elnabawi et al.

Conclusion

The creation of new design guidelines based on scientific

knowledge has been identified to be very important to closethe manifold ‘utility gaps’ [34,35] between science and design

practice. To enhance the use of scientific knowledge in design,it is crucial to provide practitioners with easily applicable, ‘pre-processed’ scientific knowledge. In this paper, the application

of the ENVI-met averaging tool has been validated throughcomparing the simulation outputs of different parameters withthe in situ measurement outcomes, resulting in an overall

acceptable agreement of the model performance. The conclu-sions of the analysis presented herein are summarised asfollows:

1. The results obtained provide some problematic areas (UHI)concerning pedestrian’s thermal comfort even after the res-toration project, which need to be studied and avoided in

the future. Therefore, in any planning process, or architec-tural intervention spatial distribution maps for microcli-mate conditions should be presented before construction.

2. The design of outdoor public spaces without a specificawareness to the microclimate is usually caused by insuffi-cient knowledge about microclimate processes amongst

many landscape architects and urban designers [34,36].And in order to overcome the problem more efforts stillhave to be made. Easily applicable design guidelines should

be generated in order to encourage a better inclusion ofmicroclimate issues in urban space design.

3. Although simulations can be very useful tools for predict-ing microclimate situations, they are only as precise as their

underlying mathematical models and the way these are inte-grated in the simulations. ENVI-met still has some short-ages as mentioned earlier.

Even so, these simulation tools are in constant developmentand are calibrated to make better predictions. ENVI-met was

authorised by The EU-project BUGS (Benefits of UrbanGreen Spaces) and The Dutch Air Quality Innovation Pro-gram (IPL) initiated by the Dutch ministries for Transport,

Public Works and Water Management (Rijkswaterstaat) andfor Housing, Spatial Planning and Environment (Ministry ofVROM). Thus, in the future, it will be increasingly useful tointegrate simulations into outdoor space design processes.

Conflict of interest

None declared.

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