Egypt. J. Remote Sensing & Space Sci., V. 9, pp. 15-30 (2006) SIMULATION OF ATMOSPHERIC TEMPERATURE INVERSIONS OVER GREATER CAIRO USING THE MMS MESO-SCALE ATMOSPHERIC MODEL .~ H. A. KandiJl, M.A. Kader 3, A. A. Moaty3, B. Elhadidi2, A. 0. Sherif3 1-Department of Mechanical Engineering, AlexandriaUniversity ,Alexandria, Egpyt. 2-Department of Aerospace Engineering, Cairo University, Giza, Egypt 3-National Authority for Remote Sensing and Space Sciences (NARSS), Cairo, Egypt Abstract: Air pollution episodes have been recorded in Cairo, during the fall season, since 1999, as a result of specific meteorological conditions combined with large quantity of pollutants created by several ground-based sources. The main reason for the smog-like episodes (black clouds) is adverse weather conditions with low and variable winds, high humidity and strong temperature inversions in the few-hundred meters above the ground. The two important types of temperature inversion affecting the air pollution are surface or ground (radiation) inversion and subsidence (elevated) inversion . . The surface temperature inversion is associated with a rapid decrease in the ground surface temperature with the simultaneous existence of warm air in the lower troposphere. The inversion develops at dusk and continues until the surface warms again the following day. Pollutants emitted during the night are caught under this "inversion lid." Subsidence inversion forms when warm air masses move over colder air masses. The inversion develops with a stagnating high-pressure system (generally associated with fair weather). Under these conditions, the pressure gradient becomes progressively weaker so that winds become light. These light winds greatly reduce the horizontal transport and dispersion of pollutants. At the same time, the subsidence inversion acts as a barrier to the vertical dispersion of the pollutants. In this study, the Penn State/NCAR meso -scale model (MM5) is used to simulate the temperature inversion phenomenon over Greater Cairo region during the fall season of 2004. Accurate computations of the heat transfer at the surface are needed to capture this phenomenon. This can only be achieved by high-resolution simulations in both horizontal and vertical directions. Hence, for accurate simulation of the temperature inversion over Greater Cairo, four nested domains of resolutions of 27 km, 9 km, 3 km and 1 km, respectively, were used in the horizontal planes. Furthermore, 42 levels were used in the vertical direction to capture the correct surface heat flux and to observe the small changes in the vertical temperature gradient. The results of the numerical model showed that it is possible to capture both types of temperature inversion during the night and early morning hours. This can be observed from the results of the vertical temperature profile and temperature gradient, which indicate that an inversion system was present over Cairo at a layer extending between 300 m and 800 m above the ground. 15 Kandil et al. INTRODUCTION Atmospheric Temperature Inversion (TI) is a very important phenomenon that affects air quality and pollution level over cities. The rate of temperature change with height is called the lapse rate. The standard atmospheric lapse rate is -6.5 °C/km(Colette et al., 2003) for wet adiabatic air which means that the temperature decreases with height. In the case of positive lapse rate, the atmospheric air temperature increases with height and this is called temperature inversion. Temperature inversion prevents the air from naturally being ventilated vertically which keeps any aerosol and pollution always below the temperature inversion layer which increases the concentration of pollutants in the air. Temperature inversions are categorized into three types which are radiation (surface or ground) inversion, subsidence (elevated) inversion and frontal (aloft) inversion.Ground temperature inversion (GTI) develops at low levels. It occurs due to the nocturnal ground radiation during night time where ground losses heat to the black sky by radiation as shown in Figure (1). Any pollutants emitted during night are caught under this inversion lid. This type of inversion is responsible also for the morning fogs. Noon Midnight 6 p.m. 6 a.m. N oon t - Temperatm e ~ Figure 1: Typical ambient lapse rates during a sunny day and clear night, during. (After Colette et al., 2003). Subsidence temperature inversion (STI) produced by adiabatic warming of a layer of subsiding air and it is enhanced by vertical mixing in the air layer below the inversion. The STI develops with a stagnating high-pressure system (generally associated with fair weather). Under these conditions, the pressure gradient becomes progressively weaker so that winds become light. The light winds greatly reduce the horizontal transport and dispersion of pollutants. At the same time, the subsidence inversion aloft continuously descends acting as a barrier to the vertical dispersion of the pollutants. These conditions can persist over large areas for several days, and the resulting accumulation of pollutants can cause serious health hazards. Figure (2) shows the mechanism of this temperature inversion. A frontal (aloft) inversion usually occurs at high altitudes and results when a warm air mass overruns a cold air mass below. This type of inversion is not important from a pollution control point of view. Many studies were carried out in order to simulate and predict the weather conditions of cities. These studies considered the effect of human activities within cities on the weather and air quality of the city. Some of these studies focu sed on the modeling of the temperature inversion and its impact on air pollution. The early attempts to understand the temperature inversion formation and breakup mechanisms were carried out using field observations followed by empirical correlations and simple thermodynamic models. Among the pioneering studies of 16 Simulation of Atomospheric Temperature ... Figure 2: Formation of subsidence temperatune inversion (After Colette et al., 2003). this type are those presented by Whitman (1982), who studied the breakup of temperature inversion using real time observations of vertical temperature profile at the western slope of the Rocky Mountains in Western Colorado. The observations showed three patterns of temperature inversion breakup. The breakup patterns were classified according to the convective boundary layer height and the top layer height of the inversion. The first is characterized by the upward growth from the ground of the convective boundary layer (CBL) which describes the inversion behavior over flat terrain. The second pattern is caused by the descent of the top of the inversion into the valley which is accompanied by a warming of the valley atmosphere. While the third pattern represents a continuum of intermediate situations in which both the growth of the CBL and the descent of the inversion top are present. Based on the observations of the temperature inversion breakup, the wind speeds within the inversion layer were weak (less than 2 meters per second). Above the top of the inversion layer, the wind speeds were moderated (greater than 6 meters per second) . Based on the wind and temperature observations, a hypothesis has been developed by Whiteman (1982-a) to explain the temperature structure evolution. Since energy is required to change the temperature structure, and the changes begin at sunrise. It is reasonable to hypothesize that solar radiation is the driving force. A fraction of the solar radiation, received on the valley floor and sidewalls, is converted to the sensible heat flux that provides energy to the valley atmosphere. Sensible heat flux from a surface causes a convective boundary layer to develop over the surface. Mass and heat are entrained into the convective boundary layer from the stable core above. Mass entrained into the valley floor and sidewall convective boundary layer, however, is carried from the valley in the up- slope flows that develop in the convective boundary layers over the sidewalls. This removal of mass from the base and sides of the stable core causes the elevated inversion to sink deeper into the valley and to warm adiabatically due to subsidence, and decreases the rate of growth of the convective boundary layer. Following this hypothesis, the rate of warming depends directly on the rate of energy input into the valley atmosphere. This energy may be used to deepen the convective boundary layer or to move mass up the sidewalls, allowing the stable core to sink .. A simulation for the nocturnal ground temperature inversion in deep mountains valley breakup was presented by Whiteman (1982-b ). This simulation was based on a hypothesis for inversion breakup which was presented by Whiteman (1982-a). A thermodynamic model was developed in order to evaluate an energy balance between the valley atmosphere, ground and solar radiation. The model results were 17 -----------------------------------------------------------------------------Kandil et al. compared to observations for the inversion breakup. These observations were collected from different valleys and studied by Whiteman (1982-a). The model was able to predict the vertical temperature structure during the inversion breakup. The results showed that breakup time increases by increasing the width of the valley . This results in larger breakup time for plain areas compared to valley at the same atmospheric conditions. The formation of stable stratification after sunset in Sofia, Bulgaria, was studied by Kolev et, al. (2000). The dynamics of atmospheric parameters (e.g. , the wind velocity and temperature profiles, height of various layers, etc.) associated with the interaction processes of mountain valley circulation and urban heat island was followed. The observations were carried out using lidars (aerosol and Raman), kytoon (tethered balloon) and pilot balloons over the city of Sofia and covered heights from 70 to 900 m. The presence of neighboring mountains and the urban heat island effect lead to the formation of three temperature inversions (ground, elevated and capping) and specific vertical profiles of the wind velocity. The formation mechanism was considered of two-layer aerosol structure in the atmosphere over the urban area and its dynamics observed by the lidar after sunset. Modern computers with enhanced processors and huge memory and storage capabilities enabled researchers to attempt to solve the governing equations of fluid dynamics with reasonable accuracy. The trials in the area of atmospheric modeling are still limited because of the domain size that requires huge amount of computational memory to handle the full-size domain. In the following studies, idealized domains with simple configurations and small sizes were modeled. For instance, Colette et al. (2003) presented a numerical simulation of inversion layer breakup in idealized steep valley using the Advanced Regional Predication System (ARPS) model. The valley width and depth and topography shade effects were investigated. In addition, typical up slope and down slope wind circulation were reproduced. Influence of atmospheric stability in the valley was analyzed and characterized. They concluded that the valley width and depth influence the lifetime of the temperature inversion layer and the topographic shading effect is eliminated by increasing the width of the valley. Recently, Hanjalik and Kenjeres (2005) presented a computational simulation of diurnal air movement and pollutant dispersion over complex terrain with heat and emission islands. The method, based on numerical solution of momentum, energy and concentration equations in time and space using an algebraic turbulence closure for subscale (unresolved) motion, can account for terrain topography and dynamics of meteorological conditions. The study presented a realistic scenario over a medium-sized town situated in a mountain valley during windless winter days when the lower atmosphere is capped by an inversion layer preventing any escape of pollutants. The results include the predictions of local values of air velocity, temperature and pollutant concentration. They suggested that the approach can be used for regulating emission during critical weather periods, as well as. for long-term planning of urban and industrial development, for optimum location of industrial zones and for design of city transportation and traffic systems. Atmospheric modeling systems such as the meso-scale model MMS enabled the prediction of weather conditions of cities, countries and continents with fair accuracy. Such models suffer from losing the accuracy at high resolution. Some trials were presented where corrective measures were applied to obtain reasonable results at high resolutions. For example, Hauge and Hole (2002) used MMS to simulate the surface temperature inversion in complex terrain of Hedmark County, 18 Simulation of Atomospheric Temperature ... Oslo, Norway. The MMS modeling system was modified to take into account the slope irradiance in the calculations of the short wave radiation model which is a part of the MMS model. They compared the MMS simulation results with soundings observations for the vertical temperature profile and wind speeds at certain point located at Hedmark County. They reported that the MMS model is capable of capturing the ground temperature inversion . . In addition, The MMS model results were improved by implementing the slope irradiance where the RMSE in temperature was reduced by 13% and it was reduced by 35% in wind field . They noted also that inversion breakup modeled by MMS was too fast compared with the measured data, Hole and Hauge (2003). In a recent study by Rantamki et al (2004), the results of MMS model were evaluated against the observational data during an air pollution episode that occurred in the Helsinki Metropolitan Area on27-29 December, 1995. The MMS model was used with triple-nesting configuration that had resolutions of 9 km, 3 km and 1 km. The model runs have been performed with 17 sigma levels; the highest model level is located at 100 hPa. They concluded that the MMS model was able to predict the observed ground temperature inversions up to 50 m from the ground; however, the predicted surface temperature was too high, compared with the measured data. It is noted from the literature review that the temperature inversion phenomenon was not fully understood and the numerical solutions of idealized valleys models did not answer all questions because of the scaling problems and the lack of required resources to simulate the full-size problem. Finally, the MMS model was used, in limited number of studies, to model the GTI while the . STI was not considered in any study. In the present study, the MMS is modified and optimized to simulate the temperature inversions over Cairo which affects the pollution episodes (black clouds) that have been observed during the fall seasons since 1999. Temperature inversion and the black cloud over Cairo According · to Environmental Information and Monitoring Program (EIMP) of Egypt, the main reason for the smog-like episode experienced by a large part of the Cairo population in October 1999 is adverse weather conditions with low and variable winds, high humidity and strong temperature inversions in the fewhundred meters above the ground. The emissions of pollutants released from a number of different ground sources in the Cairo area were added to a slow transport of particles emitted from open air waste burning in the Delta. Furthermore, Sivertsen (1999) presented that a high pressure area is situated north of Egypt with its centre in the Eastern Mediterranean, giving rise to a slow movement of humid air from the north-east across the Delta into the Cairo area. Subsidence of air in the high pressure caused the formation of a temperature inversion in the lower atmosphere, which created a "lid" on the Cairo air mass. Under this lid the wind speeds are decreasing during the afternoon of 23 October 1999 to near calm conditions. At the end the local surface winds were turning to slowly move air pollutants back into Cairo from the south in the evening . . The unusual high humidity together with high concentrations of suspended dust and other pollutants created what in Europe is called a "winter type smog episode" . The main sources of pollutants were traffic, open-air waste burning and a large number of small enterprises releasing air pollutants near the surface. Moreover, Cairo is affected by the unorganized burning of agricultural wastes in the north during the season of rice harvesting and also from industrial emission from steeland cement factories in the south. Low wind speed conditions combined with stable atmospheric conditions created the problem. High concentrations of S0 2 and PMl 0 were recorded by all monitoring stations in Cairo, Hussein et al. (2003) . 19 Kandil et al. In this study, the MM5 model, by Chen and Dudhia (2001), is used to simulate the weather conditions and capture the temperature inversion over Greater Cairo. MMS modeling system is coupled with NOAH (N: National Centers for Environmental Prediction (NCEP), 0: Oregon State University (Dept of Atmospheric Sciences), A: Air Force (both AFWA and AFRL- formerly AFGL, PL), H: Hydrologic Research Lab- NWS) land surface model, (Mitchel, 1996), to simulate the ground effects on temperature inversion. For accurate temperature inversion modeling, a horizontal grid spacing of 1 km is used and four nests centered at Cairo are constructed. Figure (3) shows the four nested domains used in simulating TI. The first domain has 167x99 grid points with 27-km grid spacing. The second domain has 175x175 grid points with 9-km grid spacing. The third domain has 124x1 1'2 grid points with a grid spacing of 3 km while the most fine grid has 100x136 grid points and 1 km grid spacing. All four nested domains have 42 vertical grid levels starting at 3.6 m above ground level with 7 levels below 100m. MMS is initialized using initial conditions from the FNL data base with accuracy of 1° x 1° ( 111 km) at 26 standard pressure levels under 100 hPa. The vegetation fraction dataset is updated using satellite observations from VHRR sensors on the Figure 3: Nested domains used in simulating temperature inversion over Greater Cairo 20 Simulation of Atomospheric Temperature ... NOAA observation system. The normalized difference vegetation index (NDVI) data is available from NOAA receiving station located at the National Authority for Remote Sensing and Space Sciences (NARSS), Egypt. The data is processed and validated at NARSS according to the methods described by Gutaman and lgnatov (1997), Gitelson et. al, (1998) and Wunderle et. al, (2003). Using satellite derived data enhanced the boundary conditions of the modeling system, which should result in accurate prediction of the weather conditions as reported by many investigators, Nicole et. al, and Oesch et. al, (2004). The sea surface temperature data is obtained from FNL data sets. The physics options of the model used were: shallow convection scheme, MRF boundary layer scheme and cloud radiation scheme. The modified MM5 modeling system coupled with the NOAH LSM with the boundary conditions upgraded by satellite observations was used by the present authors in building weather simulation models for Egypt, the Nile Basin, North Africa and the continent of Africa, Sherif et. al (2005-a, 2005-b, and 2005-c). The first simulation of the temperature inversion over Egypt was presented by the present authors in 2005, Sherif et. al, (2005 -d) In order to study the effect of simulating the ground surface processes on the prediction of the temperature inversion, two simulations were carried out starting on 28 September 2004, one with land surface modeling (LSM) and the other without any land surface modeling (NOLASM). The simulated time was selected because a temperature inversion was observed at that time at Cairo International Airport. The model was initialized using FNL datasets at 12:00 UTC. The vertical temperature gradient was calculated for the two simulations and the results are shown in Figure (4) for selected time frames. The horizontal axis shows the temperature gradient (Deg. m-1) while the vertical axis shows the altitude in meters. l-' t J -.- r-J [:. : : "' &~J _:::::-..:::=- i ~- -J - -- _ _J__ _ - - 'tSJ·""'. ~ " Time: :1004-09-28_12:00 e--· - --- :.. .. ... Dti.w· - ,. .. lima: :1004·09·28_15:00 e-- ---- . ~~- ...... - ~- .. ... oe;.... ... lime: 2004'19-28_19:00 Time: 2004.()9·28_22:00 r. _ -·Timo: 2004~29_02:00 l- -... -.Time: 200Hl9·29_01:00 --- ---- - :g--~ ·.· - ---- -_};. . _ _>___ -J t- - .- . . .. Tuna: :1004-09-28_18:00 __, _ r.. ___r ___ _ lime: :1004~9.07:00 Tmo: 2IJOI-09·2S_li:OO Figure 4: Vertical temperature gradient when using NOAH LSM and without NOAHLSM 21 Kandil et al. The simulations started with the same initial conditions at 12:00 UTC as shown in Figure (4 ). After three hours of simulation, a reasonable difference in temperature gradients was detected as shown at 15:00 UTC. The temperatur~ gradient was strongly changed for the LSM simulation in the lowest I 00 meters. At that time, the solar elevation was 8.63 degrees and the solar radiation was low. This lead to rapid decrease in ground temperature while the air temperature was relatively high and the temperature gradient was strongly changed with altitude. The NOLSM simulation did not show a large change in temperature gradient since the surface heat fluxes were not modeled. After 3hours from sunset, at 18:00 UTC, subsidence temperature inversions began to develop at an altitude of 400 meters for the LSM simulation and 700 meters for the NOLSM simulation. At 22:00 UTC, a ground temperature inversion was detected for the LSM simulation while the NOLSM simulation did not predict any ground temperature inversions. Because of the ground temperature was decreased by nocturnal radiation, the adjacent air temperature was affected. These processes were simulated in .the LSM simulation and resulted in detecting the ground temperature inversion. In the case of NOLSM simulation, the ground radiation w~s not simulated which leads to higher ground temperatures compared to the real case. These .conditions are not suitable for a temperature inversion. By 01:00 UTC, 29th of September 2004, the ground temperature inversion was totally developed at 60 meters above the ground level for the LSM simulation. This ground temperature inversion was destructed by 04:00 UTC just after the sun rise (3:46 UTC). Furthermore, the two simulations predicted subsidence temperature inversions since this type of inversion is not greatly affected by the surface processes. The effect of the surface processes appears only in the thickness and the elevation of the subsidence temperature inversion.In brief, simulating the surface processes, using a land surface model, has direct impact in predicting the ground temperature inversion. The subsidence temperature inversion is detected weather the surface processes are modeled or not. Temperature Gradients of Vertical Cross Sections In orderto study the temperature inversion phenomenon, the simulation results for a set of cross sections are plotted at different locations in Cairo. This includes longitudinal and latitudinal cross sections for Cairo domain. Temperature gradient contours ..are plotted from 0.0 deg.m-1 to 0.015 deg.m-1 for the considered cross .sections. The height is clipped at 1500 meters in order to focus on the inversion layers. The . results are shown in Figures (6) and (7) for the selected sections. Three latitudinal cross sections are considered to cover most of the terrain features of the region. The latitudes of the cross sections passing through North Cairo, Middle Cairo and South Cairo are 30.29, 30.116?- and 29.9, respectively. At the north of Cairo, the terrain is almost flat and the ground is vegetated while in the south of Cairo, the terrain is complex and the vegetation coverage is lower than that of the north of Cairo. At the middle of Cairo, the train is less complicated than that of the south of Cairo but the urban areas are larger than both the north and south sections. Figure (5) shows the location of the cross sections for the Cairo domain. 22 Simulation of Atomospheric Temperature ... Figure 5: Latitudinal cross sections locations for the Greater Cairo Figure (6) shows the temperature gradient contours for the three latitudinal sections where the results for each section are shown in a row containing 4 frames. Each column shows the results of the three sections at a selected time. The MM5 model was initialized at 12:00 UTC of 30th of September 2004 and a spin up time of 6 hours is used in all simulations . A subsidence temperature inversion (STI) was detected in the three cross sections at 17:00 UTC a:s shown in Figure (6). It covered large area at the north of Cairo while it covered small areas at the middle and south of Cairo. The strength of the STI at the north of Cairo was greater than for the middle and the south of Cairo. The STI elevation at the north of Cairo was below 500 meters. At the middle and south of Cairo, the subsidence inversion elevation was 750 meters. Ground temperature inversions (GTI) were also detected at the three cross sections with different strengths. This layer of inversion has a small thickness for the three cross sections (20 meters). This type of inversion covered large area at the north of Cairo and smaller areas at the · , middle and south sections. At the north of Cairo, the STI covered the whole domain while the elevation was not changed at 20:00 UTC. Its thickness and strength was increased from 150 meters to 250 meters and from 0.07 deg.m-lto 0.10 deg.m-1 respectively.The GTI thickness, strength and coverage area were increased form 17:00 UTC to 20:00 UTC at the north section. The GTI thickness was increased from 20 meters to 300 meters at 20:00 UTC while the elevation was maintained at 3.6 meters. The GTI and STI were matched at 300 meters at the north of Cairo. This may lead .to a stagnant area which increases the concentrations of aerosols and pollutants in this region at that day. The north of Cairo cross section intersects the River Nile at 30.92° east which leads to destruction of GTI at 30.92° east. Moreover, the STI covered the whole domain at the middle and south of Cairo at 20:00 UTC. The STI elevation was decreased while the STI strength was increased for both middle and south of Cairo. The GTI was not detected at this time at middle and south of Cairo. The results in the third column, 01:00 UJ'C, 1st October, the STI began in destruction and the strength was decreased at the north of Cairo while the strength didn't almost change at the middle and the south of Cairo. The elevation of the STI was increased from about 400 meters at 20:00 UTC to 500 meters at 01:00 UTC at the north of Cairo. While the STI elevation 23 - - - - -- - - - - Kandil et al. (w)z (w)z (W)Z twJz . ' t, j ~ f 'l :r l (W)Z 24 Simulation of Atomospheric Temperature ... was maintained at about 500 meters at the middle and the south of Cairo. The STI thickness was increased at the three cross sections with average of 500 meters. The GTI began in destruction with highest strength in the north of Cairo at 01:00 UTC. The GTI was constructed at the middle and the south of Cairo with average thickness of 20 meters and vegetated areas were present. Finally, the.STI and GTI was completely destructed by 07:00 UTC at the three cross sections. In brief, the STI has a strong strength, lower elevation and smaller thickness in construction time compared to destruction time at the flat vegetated areas. The STI has weak strength and higher elevation in construction time compared to the destruction at complex urban areas. The GTI has stronger strength, larger thickness in destruction time compared to construction time for flat vegetated areas. Three longitudinal cross sections are considered. The longitudes of the cross sections passing through West Cairo, Middle Cairo and East Cairo are 30.90, 31.25 and 31.383, East respectively. At the west of Cairo, the terrain is simple and the vegetation coverage expands from 30.26° to 30.43° north while in the east of Cairo, the terrain is more complicated compared to the west of Cairo and the vegetation coverage expands from 30.13° to 30.43° north. At the Middle of Cairo, the terrain is almost flat and the ground is vegetated across the section except in the region from 29.95° to 30.125° north which represents the urban areas in the Cairo city as shown in Figure (7). l ~ 1 k West ]'d iddle Cau(> C 1u\ > Nc>rth f : ~~~~,~-~~ -- i ~......, 1 ! I I xf ' I t ~~ ~-·---·-----· ·~ . ~~.·_j' r~ ·---.)': ~ ~ 1,_.4 , 1>'.t\0•1>tO<.~!dl r I ~ -= . .,.-=--~~·-J ll l. ,f ~ l__.,//\. "'-- Figure 7: Longitudinal cross sections locations for the Greater Cairo. 25 I L,._~~~ Kandil et al. The temperature gradient contours of the three longitudinal sections are shown in Figure (8) where the results of each section are shown in one column with four frames. As shown in Figure (8), a GTI was detected at the three cross sections at 17:00 UTC. It covered a limited vegetated area at the west and the east of Cairo. Its thickness was about 10 meters (the first two vertical grid levels) at the three cross sections. The average GTI strength was 0.015 deg.m-1. The STI was not detected at this time. At 20:00 UTC, the STI was detected and covered the whole cross section at the west and east of Cairo and it covered about 75% of the middle of Cairo cross section. The STI average strength at the west and east of Cairo was larger than the middle of Cairo. The elevation of the STI was 500 meters at the three cross sections. The STI average thickness was 200 meters at the three cross sections. The GTI was detected at middle and east of Cairo at 20:00 UTC. At the middle section, the STI matched the STI at 300 meters while the thickness of the GSI was 70 meters at the east of the Cairo. The GTI coverage at the middle of Cairo was largest coverage where vegetation coverage is high. At the first of October, 2004, 01:00 UTC, the STI covered the whole three cross sections. The STI strength was higher at the west and east of Cairo than the middle of Cairo. The maximum thickness was obtained at the west of Cairo with average of 500 meters. The average thickness was 300 meters at the middle and east of Cairo cross sections. The STI elevation was 300 meters at the three cross sections. l \V~st Cni.ro North l I ~J ~-~ 4 ¥1 ~ \ ~M r·-·----·~ E Figure 8: Longitudinal cross section for different locations at Greater Cairo. 26 Simulation of Atomospheric Temperature ... The GTI was detected in the three cross sections and its maximum strength was close to the ground surface. The GTI covered the vegetated areas at the south and north longitudinal cross section at the middle of Cairo while the urban areas were not covered by GTI. The vegetated areas at the northern east and west of Cairo were covered by GTI. At 07:00 UTC, the STI began in destruction at the three cross sections while the GTI was totally destructed. The STI was decreased at the three cross sections and the elevation was increased from 300 meters to 600 meters. At 08:00, the STI was totally destructed. The vertical temperature profiles at Cairo International Airport are presented in Figure (9). Referring to figure 9, at the start of simulation, the negative laps e rate is sustained until 16:00 UTC. By 18:00 UTC, the formation of an inversion layer begins at 500-m above the ground level. The thickness and strength of the inversion layer was continuously changing during the simulation which agrees with the results in Figures (6) and (8). After sunrise the inversion layer started to break up. By 10:00 UTC (12:00 CLT) this layer totally disappeared. Vertical T ompvraturv Profile Vertlcel Temperature Profile :U "" l'~C) Vertical Temperature Profile ~ , (~()Qoi - ~·'W~'I.o<J 2~ ~ ""' - ~~> ~ ~ ~ ~ -~C) ~ 3~ ,./_,~ < I O. O I _ (l.-6 1 VertiCal Tomporarura Protua VorttcaJ Tomporaturo Prot!le = '",.......,..'"'xo4~· 3-0K •• .¢0 ~ ~ ~ - . ,~. a ~ - - ~ ~ ~·~ · • o-o• ~o~ol!'i' Profli~ Vertical TumpuraturQ Promo Vurtlclll Tvmpvrature Vertlesl Temperature Promo vatt!CAJ TamporahJtA Protuo Figure 9 : Vertical temperature profiles at Cairo International Airport 27 Kandil et al. CONCLUSIONS -The MM5 model coupled with NOAH land surface model was configured and optimized to simulate the temperature inversion over the Greater Cairo region in Egypt. The model was optimized to get accurate results for the high horizontal resolution of 1 km used in the Cairo domain. The following conclusions are drawn from the results of temperature inversion simulation: - The thickness and strength of the STI increase in complex terrains while its elevation decreases. - The construction of the STI begins from higher elevation and descends to reach its maximum strength and minimum elevation. At the destruction time, the strength decreases and the elevation increases. The maximum strength is related to the maximum elevation of the complex terrain area. - The GTI covers the vegetated areas with the minimum elevation very close to the ground. The GTI thickness and strength at vegetated areas are larger than that in the urban areas. - The obtained results match the real observations and help in explaining the physics of the temperature inversion phenomenon. Simulating more days requires extensive computational resources. REFERENCES Chen, F. and Dudhia, J. 2001. Coupling an advanced land-surface/hydrology mo,del with the Penn State/NCAR MM5 modeling system. 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