Seasonal Evaluation of Temperature Inversion Kandil, H A 1, Kader M M. A 2, Moaty, A A. 2, Elhadidi, B 3, Sherif, A.O. 3 The seasonal evaluation of the temperature inversion over Cairo-Egypt is examined for a complete year [September 2004 to September 2005]. This area suffers annual episodes, known as Black Cloud, resulting from concentration increase of the trapped suspended Particulate Matter (PM) associated with temperature inversion at autumn and spring. The fifth generation of the Penn State NCAR Mesoscale Model (MM5) [ 1] was used to investigate two types of temperature inversion, the Ground Temperature inversion (GTI) associated with a rapid decrease in the ground surface temperature and simultaneous existence of warm air in the lower troposphere and Subsidence temperature inversion (STI) which forms when a warmer air mass moves over a colder air mass. The inversion develops in association with a stagnating high-pressure system (generally associated with fair weather) and higher water vapor content. Under these conditions, the pressure gradient becomes progressively weaker so that winds become light reducing the transport and dispersion of pollutants. The results of this study shows that the GTI strength increases in December, January, and February when the solar and ground heat fluxes are minimal. Strong anti-correlation coefficients of (-0.8 and -0.81) were obtained between the ground heat fluxes and the GTI strength and lifetime respectively. The STI appeared with higher frequencies and strength in September, October, April and May compared to the other months. The increase in STI strength resulted from large-scale convection from the Mediterranean Sea represented in increased sea level pressure and surface water vapor in warm season. The STI strength and elevation were correlated to satellite observations of Aerosol Optical depth (AOD) and resulted in strong correlation of (0.80 and -0.91) respectively. This showed that the higher strength and lower elevation of STI in addition to light wind speed increased the trapped suspended PM concentrations. 1. Introduction 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. These weather conditions represented in low variable winds, high humidity and strong temperature inversions in the few-hundred meters above the ground. The temperature inversion resulted from the subsidence of air in the high pressure area situated north of Egypt with centre in the Eastern Mediterranean. This high pressure area gives rise to a slow movement of humid air from the north-east across the Nile Delta into Cairo area causing the formation of the temperature inversion. These conditions create a lid on the Cairo air mass. Under this lid the wind speeds decrease during the afternoons to near calm conditions. As a result, the local surface winds 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 [ 8]. 1 German University in Cairo
References [ 9 4, 5, 14] used the MM5 model to study two types of temperature invasions over the Greater Cairo, which are GTI and STI, during an air pollution episode. The simulation results showed that the MM5 model was capable to predict the two types of temperature inversion. In addition, the results showed that most of Nile Delta area was covered by the two types of inversion from the sunset until the next day sunrise while the wind blew from the north-east of Greater Cairo. In this study, seasonal evaluation of the temperature inversion is presented for a complete year using the MM5 model. Two nested domains were used to cover Egypt (27km and 9km and 42 vertical levels) [ 9-12]. The selected grid spacing represents the general behavior of the temperature inversion due to limited availability of computational power [ 6, 11]. Thirteen months were considered [September 2004 to September 2005]. Black Cloud periods, which usually occur between the end of September and the beginning of November, were covered. To maintain the model accuracy within accepted limits [ 9-12], the simulation was divided into a sequence of runs each covering three days (72hours) and the model was reinitialized at the start of every three days. The boundary conditions were updated every six hours. A spin up time of twelve hours was used to avoid the effect of model initialization. A time step of 60 seconds was used for the coarsest domain in order to avoid any CFL model failure [ 1]. The model output fields hourly and was limited by the available storage system (one Terabytes). The vertical temperature profile was calculated [ 4-6] for a selected station located at the Cairo international Airport, and was analyzed to study the seasonal change of the temperature inversion. Three parameters for the temperature inversion were introduced which are the daily averages thickness, strength and lifetime based on hourly data. Each type of temperature inversion was identified based on observations of Whiteman [13], Kolev et al. [ 7]. The analysis for the GTI was calculated starting from ground level and up to 200 meters. The STI analysis was calculated starting from 200 meters up to 2000 meters. 2. Model Results (a) Ground temperature inversion analysis for whole year The obtained daily average values of the GTI layer for the thickness, the strength and the lifetime for December 2004 are shown in Figure 1. The detected average thickness of GTI was [20-250m], strength of [0.009-0.019 deg.m -1 ] and lifetime of [5-13 hours]. The maximum GTI
lifetime for this month was thirteen hours and was observed only in two days [27 th and 30 th ] of December. Figure 1 Ground temperature inversion daily averages, December 2004 Figure 1 shows that the GTI was detected for 15 days. The monthly average for the GTI thickness, strength and lifetime was 65m, 0.012181 deg.m -1 and 9.33 hours respectively. The daily averages for GTI thickness, strength and lifetime were calculated for nine months for the same location, Figure 2. It is clear that September and October recorded the minimum number of days that the GTI was detected while January recoded the maximum number (17 days). The maximum daily averages for GTI strength are obtained in December, January and February where the early morning fogs were present at the location of this study. The total surface heat flux, TSHF, increased in warm season where the solar radiation was high and associated with increase in ground temperature compared to cold season, Figure 3. The decrease in TSHF in the cold season was associated with decrease in ground temperature as well which increased GTI strength. Figure 4 shows the monthly averages for the GTI number of days, monthly average for the GTI strength and thickness. The maximum GTI strength was obtained at December, January and February leading to favorable condition for formation of GTI.
Figure 2 Ground temperature inversion analysis for nine months
Figure 3 Hourly total surface heat flux history at Cairo International Airport for complete year Figure 4 Ground temperature inversion monthly average analysis for appearance days, average strength and average thickness
(b) Subsidence temperature inversion analysis for whole year Figure 5 shows the STI daily averages for September 2004, detected for twenty days. The daily average thickness varied from [100-1014 m] in this month. The range of STI strength varies [from 0.0015 to.018 deg.m -1 ], The STI lifetime varies [from 4 to 22 hours]. The 6th day of September 2004 records STI maximum thickness of 1014 meters but the corresponding STI strength was minimal,.0015 deg.m -1. This was insignificant and hence did not contribute to the weather conditions. (Pasquill-Gifford classifications). Figure 5 Subsidence temperature inversion daily averages,september 2004 The same analysis had been carried out for the other months from. Figure 6 plots the analysis results nine months in the period [October 2004 to September 2005]. The STI was detected almost each month for the simulated period but the number of days in each month, the strength and the lifetime of the STI varies. The maximum number of STI days was detected in May, 2005, and was detected for twenty one days. The corresponding monthly average strength was 0.005 deg.m -1 while the average thickness was 280 meters.
The maximum STI monthly average strength was detected in September, 2004. The STI was detected in twenty days in September, 2004, Figure 7. The maximum monthly average STI thickness (was detected in January, 2005 but the corresponding monthly average STI strength was 0.003deg.m -1 which was weak STI. The STI was detected in five days only in January, 2005 as shown in Figure 6. The minimum STI monthly average strength was obtained at three months which are December, 2004, January 2005 and February, 2005. The corresponding number of days that STI was detected was 13 days for December 2004, 5 days for January 2005 and 11 days for February 2005 as shown in Figure 7. In brief, a complete year simulation was carried out to investigate the annual frequency of GTI and STI. Based on the model results, the GTI strongly appeared in the months December, January, and February when the early morning fogs are exist. The GTI strength increased in the months when the solar inclination and solar fluxes are minimal. In addition, the STI appeared with higher frequencies and strength in months September, October, April and May compared to the other months. The high strength of the STI was associated with decrease in elevation at September, October, April and May. It was noticed that the STI strength increased in the months when the air pollution episodes exist at the station of study. These air pollution episodes include the Black cloud phenomenon in months September, October and November [ 8] and the Khamasin sand storms (April and May).
Figure 6 Subsidence temperature inversion analysis for nine months
Figure 7 subsidence temperature inversion monthly average analysis for appearance days, average strength and average thickness (c) MODEL VALIDATION USING REMOTELY SENSED DATA FOR THE AEROSOL OPTICAL DEPTH Aerosol Optical Depth (AOD) is a measure of the opaqueness of air, where high values of AOD indicate poor visibility. AOD data from the Moderate Resolution Imaging Spectroradiometer MODIS sensor is used in to measure the relative amount of aerosols suspended in the atmosphere and hence how much light is blocked by the airborne particles. Among of these detected aerosols are the dust, sea salts, volcanic ash, and smoke either being solid or liquid particles. Dust and smoke are the main constituents of the vertical column in this case where they reflect the visible and near infrared radiation preventing them from passing through the atmospheric column. The presence of the dust and smoke particles during certain time periods through the year results in increase of optical depth.
The MODIS sensors are used to retrieve various surface parameters such as the ground surface temperature, vegetation coverage, surface emissivity and AOD. Furthermore, the STI has direct impacts on the weather conditions that it may trap the pollutants underneath as well. This will decrease the opaqueness of the air which will increase the AOD. In this section, the AOD of the MODIS sensors will be used in order to validate and correlate the STI to the AOD. The monthly averages for the STI strength was obtained for a selected station (31.38 E and 30.12 N) based on the year simulation results. The AOD is obtained for the same station based on monthly MODIS products [ 2]. Figure 8 Comparision between AOD[ 2] and STI strength for selected station Figure 8 shows that the STI is highly correlated to the AOD with correlation coefficient of 0.91145. This correlation increased in winter months where minimum values for the STI were obtained. As shown in Figure 8, the AOD values were high at September, April and May compared to other months. At April and May, the Greater Cairo episodes the Khamasin sand storm which decrease the visibility and hence increase in AOD. As shown in Figure 8, the increase in AOD at April and May is associated with increase in STI strength which shows the impact of the STI on the sand storms events. At September, the Greater Cairo episodes the Black Cloud event which decreases the visibility and increases the AOD. Figure 8, shows that the STI is high at September where the AOD is high and Black Cloud exists. This shows also that the STI has direct impact on the Black Cloud phenomenon.
The STI elevation was obtained based on a year simulation over the Greater Cairo. As shown in Figure 9, the elevation of the STI is anti-correlated to the AOD with correlation coefficient of -0.9005. This shows that the increase in AOD is caused by trapped particles under STI layers at lower elevation. This concludes also that the STI strength is associated with STI layers vertical movement and causes a particles trapping when the STI strength is high at lower elevations. Figure 9 Comparision between AOD[ 2] and STI elevation for selected station 3. Conclusions The seasonal evaluation of the temperature inversion has been studied over the Greater Cairo. The results of this study showed that the GTI strength increased in December, January, and February when the solar and ground heat fluxes were minimal. Strong anticorrelation coefficients of (-0.8 and -0.81) were obtained between the ground heat fluxes and the GTI strength and lifetime respectively. This shows that the GTI development strongly depends on the ground heat fluxes. The STI appears with higher frequencies and strength in September, October, April and May compared to the other months. The increase in STI strength resulted from largescale convection from the Mediterranean Sea represented in increased sea level pressure and sea surface water vapor in warm season which is giving rise to a slow movement of humid air from the north-east across the Nile Delta into the Cairo area causing the formation of the STI.
The STI strength and elevation was correlated to satellite observations of Aerosol Optical depth (AOD) and resulted in strong correlation of (0.80 and -0.91) respectively. This showed that the higher strength and lower elevation of STI in addition to light wind speed increased the AOD index. This resulted in air pollution episodes represented in Black cloud in September, October and November and Khamasin sand storms in March and April. References 1. Dudhia J., G. Dave, M. Kevin, W. Wei, B. Cindy, K. Sudie," PSU/NCAR Mesoscale Modeling System, Tutorial Class Notes and User's Guide", NCAR Technical Notes, January 2005. 2. El Askary H.,"Air Pollution Impact on Aerosol Variability over Mega Cities Using Remote Sensing Technology: Case study", Proceeding of 41 st U.S. Egypt Workshop on Predictive Methodologies for Global Weather- Climate and Related Disasters, Cairo, Egypt, 13 th 15 th March, 2006. 3. El Askary H. and M. Kafatos, Potential for Dust Storms Detection Through Aerosol Radiative Forcing on Atmospheric Parameters, IEEE International Geoscience and Remote Sensing Symposium IGARSS2006, Denver, USA, July 31 st - August 4 th, 2006. 4. Kandil A. H., M. M. Abdel Kader, A. Abdel Moaty, B. Elhadidi, A. O. Sherif, Simulation of Atmospheric Temperature Inversions over Greater Cairo Using the MM5 Meso-scale Atmospheric Model, Proceeding of 41 st U.S-Egypt Workshop on The Predictive methodologies For Global Weather-Climate and Related Disasters, 13 th 15 th March, 2006. 5. Kandil A. H., M. M. Abdel Kader, A. Abdel Moaty, B. Elhadidi, A. O. Sherif, Simulation of Atmospheric Temperature Inversions over Greater Cairo Using the MM5 Meso-scale Atmospheric Model, The Egyptian Journal of Remote Sensing and Space Sciences, ISSN 1110-9823, Volume IX, pp. 15-30, 2006. 6. Kandil A. H., M. M. Abdel Kader, A. Abdel Moaty, B. Elhadidi, A. O. Sherif, Building an Accurate Weather Prediction Model for Egypt, The Egyptian Journal
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