Assessment of Mixing Height at Qena/Upper Egypt Based on Radiosonde Data

Radiosonde and surface meteorological data have been carried out in south valley university meteorological station at Qena / upper Egypt for estimation of maximum and minimum mixing height MH during the periods (2006 2008) and (October 2004 September 2005), respectively. The effect of some meteorological parameters such as temperature (T C), wind speed (WS m/s), and relat ive humidity (RH%) on maximum MH has been studied. It is found that average daily maximum MH ranges from (700 m) to (2367 m) in winter, and from (567 m) to (2700 m) in autumn and from (1133 m) to (4860 m) in spring, and from (1200 m) to (3750 m) in summer. Seasonal average value of maximum MH reaches its maximum (2425 m and 2481 m) during spring and summer, respectively, while, it attains its minimum ( 1418 m and 1504 m) during winter and autumn, respectively. Minimum MH has generally a value between 100 and 200 m. Seasonal average of minimum MH is characterized by small fluctuation. Where, it varies from (166 m and 197 m) during spring and summer, respectively to (161 m and 173 m) during winter and autumn, respectively. Monthly averages of maximum MH is associated forwardly with T and WS and associated backwardly with RH. A model relates monthly average values of maximum MH at afternoon with T, WS and RH has been constructed and can be used locally for estimation of monthly average MH.


Introduction
Air pollution models (Lagrangian, Eu lerian or Gausian) are useful tools for evaluating emission rates and quantifying adverse pollutant effects in certain region. Stability and MH are two important dispersion parameters for those models. Hence, th e need is raised fo r estimation o f th ese two parameters. MH is a fundamental parameter characterising the structure of the lower troposphere. It can be defined as the height of the layer adjacent to the ground over which pollutants or any constituents emitted within th is layer or entrained into it become vertically d ispersed by convection o r mech an ical tu rbu lence [1]. A ir th at is not mo v ing smoothly is termed turbulent. Turbu lence is described as eddies in a local air parcel that produce mixing v ia t wo mech an is ms . Th e first o f th ese is pu rely mechan ical turbulence, which is caused by irregular air movement over terrain such as trees or around obstructions such as buildings. Thermal turbulence, the second mechan ism occurs as the result o f rad iat ion fro m the sun heating various ob jects. Different ial surface heat ing occurs, wh ich leads to s mall parcels of air moving upward. The maximu m mixing height is the height of the convective layer associated with the maximu m surface temperature. The maximu m mixing height exhibits both local and seasonal variations. In addition it is affected significantly by topography and large scale air movements. Min imu m values of mixing height will be observed in the late hours of the night or in the early morning hours and may be the result of a surface based inversion [2]. The pollutants which released into the air fro m any source of pollution are being continually mixed. On hot days, they are taken up ext remely high away fro m the ground. In contrast to that, in cool days, the mixing height is very low and pollutants remain close to the ground.
There is an enormous uncertainty in estimat ion of mixing height since it is not a directly measurable variable [3,4]. Holzworth method is the most commonly used one for calculating M H fro m temperature profile which can be obtained fro m radiosonde data [5]. Rad iosonde is a balloon borne measurements of the vertical profiles of meteorological parameters, i.e., pressure, temperature and relative hu midity, it is the most common source of data for operational determination of the MH [6].
Qena is a s mall city in upper Egypt. It is a place of many factories such as Aluminiu m factory, two factories of sugar and a factory of cement. There is strong need for estimation of MH in this city to be used in pollution modelling in the future. So that, the main purpose of this study is to estimate afternoon (12 Z) and early morn ing (0 Z) M H in Qena during the periods from 2006 to 2008 and fro m October 2004 to September 2005, respectively. The effect of some meteorological parameters such as temperature, wind speed, and relative hu mid ity on maximu m MH has been studied and an emp irical model relates monthly values of maximu m MH with T, WS and RH has been constructed.

Location description
Qena City (26°17' N, 32°43' E, 97 m asl) is located in Upper Egypt, about 600 Km south of Cairo and 60 Km north of Lu xo r ( Figure 1). It located main ly within the narrow Nile valley which separates Egypt to two unequal dessert parts, the western and the eastern deserts. Climate of Qena is very hot, dry in summer and cold in winter. It rarely rains. Also, it receives large quantity of solar radiation, especially in summer [7]. Radiosonde data at (12 Z) represented 1041 records during the total period (1095dayes), while that at (00 Z) represented 346 records during the total period (365 days). The complete radiosonde system consisted of 1-A balloon-borne radiosonde instrument package, 2-A radio receiver, 3-A tracking unit, and 4-A recorder .
The balloon-borne radiosonde instrument package is the main co mponent of the radiosonde, Model M2k2-DC. comp lete specifications can be found in http://www.meteo m odem.co m/rproduit.php?nom=M 2K2-DC. The following weather sensing instruments are located within or attached to this package. resistance thermistor for measuring temperature, hygristor sensor for measuring humidity and an aneroid barometer for measuring pressure. The radiosonde package is carried aloft by a spherically shaped balloon. The balloon is made of a film of natural or synthetic rubber (neoprene). A radio receiver model SR2K2 Modem is used for receiving the telemetry signal transmitted fro m the radiosonde. Comp lete specifications can be found in http://www.meteo modem.co m/gproduit.php?nom=SR2 K2. Highly direct ional rad io direction finding antenna is used also to obtain the wind speed and direction at various levels in the atmosphere by tracking the radiosonde and determining the azimuth and elevation angles. The data are recorded automatically during the flight and then processed.
For determin ing MH, three preliminary categories are distinguished by vertical temperature profiles: an elevated inversion layer, a ground based inversion layer, and no inversion layer. The following describe the three cases: For an elevated inversion layer, the mixing height is taken as the base of the lowest inversion [8,9].
If a ground-based inversion has been detected in the radiosonde temperature profile, the mixing height has been identified with the ground based top height [10].
If an inversion is not present, the mixing height is taken as the altitude of level defined by the first intersection of the temperature profile with dry-adiabatic line this method is called Ho lzworth method or dry adiabatic method [11,12,13,14,15,16]    At night time, the radiat ion inversion is the most common form of surface inversion and occurs when the earth's surface cools rapidly. As the earth cools, so does the layer of air close to the surface. If this air cools to a temperature below that of the air above, it beco mes very stable, and the layer of warmer air impedes any vertical motion. Radiation inversions usually occur in the late evening through the early morn ing under clear skies with calm wind.

Effect of Some Meteorological Parameters on
Afternoon MH Figure 6 and Table 1  There is positive association between monthly averages of maximu m MH and both of T and WS, in the same t ime, there is negative association with RH. Where, T monthly average value reaches its minimu m value in cold months, January (19.8o C) and December (22.4oC), and reaches its maximu m in hot months, June (40.1 o C) and August (39.8o C). RH monthly average value reaches its min imu m value in June (9.1%) and May (9.5 %), and its maximu m in January (29.9 %) and December (27,9 %). WS monthly average value reaches its minimu m value in December (2 m/s), and January (2.9 m/s), and its maximu m in July (5 m/s), and June (5.7 m/s). The highest MH occurrence in summer, and spring months may be attributed to increasing the frequency of unstable and slightly unstable atmospheric stability conditions in summer and spring in Qena [17], also, high atmospheric temperature associated with strong solar radiation and significant atmospheric turbulence lead to high MH. Higher wind speeds lead to strong mechanical turbulence and therefore high MH. In winter and autumn, lo w temperature and high frequencies of inversion result in stable atmospheric conditions and less turbulence and hence lead to low MH. High RH also limits MH development and leads also to low MH [15,16]. Table 2

Conclusions
Radiosonde and surface meteorological data have been carried out in south valley university meteorological station