İstanbul İçin Enerji Dengesi Eşitliği Kullanılarak Yüzey Sıcaklığının Belirlenmesi

Abstract

Bu çalışmada, özellikle atmosferik sınır tabaka ve yüzey tabaka ile ilgili çalışmalarda önemli bir parametre olan yüzey sıcaklığı değerlerinin çeşitli etkilerle değişimi incelenmiştir. Hesaplamalar İstanbul (41. 1 K, 29.0 D) için yapılmıştır. Enerji dengesi yönteminin kullanıldığı modelde, seçilen bir gün için ortalama hava sıcaklığı, bağıl nem, rüzgar şiddeti, bulutluluk ve toprak nemi gibi atmosferik koşulları belirleyen parametreler, modele girdi verisi olarak tanımlanmışlardır. Yüzey sıcaklığı saatlik olarak, bitki örtüsü olmayan, killi-kumlu toprak tipi için açık ve bulutlu atmosfer koşullarında hesaplanmıştır. Çalışmada öncelikle yüzeyi etkileyen net radyasyon akısı, türbülanslı ısı akısı, buharlaşma gizli ısı akısı ve toprakta depolanan gizli ısı akısı hesaplanmış, daha sonra yüzey sıcaklığı ve yüzeye yakın toprak altı sıcaklıkları sonlu farklar yöntemi ile elde edilmiştir. Hesaplamalar için bir bilgisayar programı geliştirilmiştir.

The planetary boundary layer that controls the mean atmospheric phenomena is defined as a fluid layer where heat, mass and momentum exchanges between the earth surface and the above atmospheric layers take place. The height of the planetary boundary layer changes according to the surface properties, heating and cooling ratios, wind speed, heat and moisture advection. The range of the layer height is observed from a few hundred to a few hundred kilometers. The %50 of the atmospheric kinetic energy is spend in the boundary layer to produce weather prediction models, the information of the structure of the boundary layer and surface layer are very vital for the performance of predictability. For this reason, especially in recent years, the studies on the atmospheric boundary layer and surface layer are the main subject of the micrometeorology and climatology. Sun that is the main energy source emits radiation and the majority of this radiation is absorbed by the surface and it is transferred to the atmosphere by the boundary layer processes The energy transfer between the earth's surface and the atmosphere is highly depended on the thermal properties of the ground. The ground temperature and the temperature just above the ground must be known in the energy exchange processes. The lowest levels of Atmospheric General Circulation Models and Numerical Weather Prediction Models are the planetary boundary and surface layers. Therefore, the surface temperature must be prescribed or calculated for these models. The energy that drives the biogeochemical cycles can be found as radiant, thermal, kinetic and potential forms, and can also be transported from one form to another. The energy transfer in the climate system can be carried out by conduction, convection and radiation. The sunlight spectrum that comes to the top of the atmosphere resembles to this spectrum of a 6000 °K black body. Therefore, the sun can be assumed as a black body having 6000 °K. On the other hand, the earth's radiation spectrum in the absence of the water vapor, C02 and other trace gases resembles to the spectrum of the black body having a temperature of 287 °K. The main interval of the sun radiation spectrum is 0. 15 - 4.0 mm, while this interval is 3 - 100 mm for the earth's surface. In the science of meteorology the two intervals of the radiation is known a ultraviolet and Infrared radiations. vn For the case of clear atmospheric condition the graphics that is emerged owing to estimation of the surface temperature is showing us that the distribution function start to increase with the sunrise, reaches to its maximum at noon and decreases to a constant value at night. Generally the surface temperature and the air temperature intersected around six o'clock in the morning twenty o'clock in the night. The maximum temperature are observed around thirteen o'clock. It is found that these surface temperatures are in high correlation with results of the other similar studies. For the same days and the overcast conditions the low cloud cover types C=0.5 and C=1.0 is used to calculate the surface temperature also. As expected when the cloudiness increases, t!. coming solar radiation decreases and its observed that the surface temperai e decreases during the daytime. When we consider the night, the radiative eoo.mg decreases because of the cloud effect and the surface temperature are detected to be higher than the ones of the clear day. In this study, besides the estimations of the energy fluxes and the surface temperature values, the effect of wind, soil moisture, air temperature and relative humidity factor dining the clear atmospheric conditions are investigated. The calculations showed that the wind and the soil moisture are very important factors that effect the surface temperature. During the low wind speed the warming is higher so high values arc observed at noon, and during the high wind speeds cooling is effective thus low surface temperatures are observed. When the weather content of soil is high and since the water more efficient heat conductor than the air, the surface will not be able to warm up considerable. For the reverse conditions when the water content decreases, the air that is conducting heal poorer than the water, will cause surface to warm up. Although the air temperature are not effecting the surface temperature to a great extend, the high surface temperatures are associated with high air temperatures and the low surface temperatures are associated with low air temperatures. It is observed that the relative humidity is not playing a great role in determining surface temperature variabilities. Because of the unavailability of the instrumental observations of the surface temperature, the estimated values can not be compare with the observed values. But it is possible to compare the estimated subsurface temperature (20 cm). When we compare the observations of 7-14-21 hours with the estimated values, we found that they are almost equal. d\ In the equation Ts is surface temperature. CT is the constant related with thermal properties of the soil, G is the energy budget. T2 is the soil temperature just below the surface and x \s a period of one day. This equation is discrilicized using advanced differences approach and by the aid of finite differences method. And it finally became : 2% 2% At Ts,nl,) (I)'= At [CTG+ - - - T?n (I)] fTs"(I)[1- ] At T T2<»"> (I) = [T2n (I)+ T2n (I)] T+At At In this work the surface temperature at the 2400 is calculated by adding five degrees to the mean air temperature of the previous day and it is used as initial value (Warren and Partain, 1986). For the initial value of the soil temperature that is close to the surface, the mean soil temperature at the 2U cm that of the same day is used (NCAR Technical Note, 1986). The variability of the surface temperature is investigated for the months of January, April, July and October (These months arc chosen to represent the individual season). Initially the components of the energy budget is computed for all the seasons is observed that the net radiation is positive throughout the day and negative during the nighttime. Sensible heat flux and evaporation latent heat flux is related with the surface warming. For both of the components positive values are encountered throughout the day. Sensible heat flux increases as the surface warms up. During the nighttime the surface will cool more readily, when the air and the sensible heat flux is decreasing in magnitude and also its direction is changing. The latent heat flux acts like the sensible heat flux during the day time. At night lime the evaporation is very small so that it can be negligible. The heat flux that is stored in the soil is showing the general overall budget between the net radiation, sensible heat flux and evaporation latent heat flux. Our energy budget graphics is in close relationship with the other studies conducted on this subject. The solar constant is accepted as the intensity of this sun's ultra violet radiation, and it is defined as the radiation flux coming perpendicular to a unit area outside of the atmosphere per time. It is estimated to be, 1367 W/m2. The majority of ultraviolet radiation coming from the sun is reflected back to the space. The infrared radiation near the surface two components; one goes from the surface to atmosphere on the other one goes from the atmosphere to the surface. The long wave radiation (Earth's radiation) that is emitted by the Earth is absorbed by water vapor C02, nitrogen oxides (NOx), methane (NH4), ozone (03), etc. in the atmosphere. The atmospheric gases, aerosols and clouds also emit thermal radiation that they ones absorbed. In order to write the energy balance equation for horizontal, homogeneous, and transparent idealized surface, a vei / thin layer between the surface and the atmosphere is used. The energy balance c juation is Rn = H + Hi. + Ho Here Rn is the net radiation, H and Hi, are the sensible heat flux and the evaporation latent heat flux terms, respectively. Ho is the net heat flux that stored in the ground. Net radiation is the difference between the coming ultraviolet radiation and going and going infrared radiation fluxes. Sensible heat flux occurs owing to the temperature differences between the surface and the air just above it. The latent heat flux is due to the evapotranspiration, evaporation or the surface condensation. The real values of the components of the surface energy budget depend on many features. Examples are the surface type and its characteristics, geographic position, month or season, the daytime. The surface temperature in a region is determined by the surface energy balance. The surface energy balance is dependent on the radiation balance, exchange between the atmosphere and the surface, the presence of a vegetation covers and its type, and the properties of the surface soil. Since the soil surface is a discontinuity zone, it is not possible to monitor the surface temperature with instruments directly. But using the energy budget equation it is possible to estimate or calculate the surface temperature. In this study by using the equations of Noilhan and Planton (1989). The surface and soil temperature at the level of 20 cm one calculated. The daily variations of the surface temperature for clear and cloudy atmospheric conditions in Istanbul (40. 1 °N) is estimated. The system of the equation follows: For all seasons, net radiations flux takes pozitive values during the day. During nights the reserve is valid. The maximum value for the net radiation is 310.16 W/m2, 621.97W/m2, 682.45W/m2 and 481.88W/m2 for January, April, July and October, respectively. Latent heat and sensible heat takes pozitive values during the day. Sensible heat reaches the maximum value nearly at 1400. This maxsimum value is calculated as 45.57 W/m2, 143.87 W/m2, 107.73 W/m2 and 82.17 W/m2 for winter,summer,spring and autumn,respectively. The maximum latent heat flux values are calculated as 42 W/m2, 247 W/m2, 3 1 1 W/m2 and 1 1 8 W/m2 for January, Apriljuly and October,respectively. The heat flux stored in the soil implies the general total budget among net radiation, latent heat and sensible heat. The maximum values of the heat flux stored in the soil are calculated as 265.92 W/m2, 367.44 W/m2, 310.24 W/m2 and 294.45 W/m2 for January, April, July and October, respectively. The difference between daily minimum and maximum values of surface temperature is much more than these of air temperature. These differences are 14.1 °C and 4.7 °C for surface and air temperatures,respectively in January; 23 °C and 8.3 °C in April; 20.16 °C and 7.9 °C in July and 18.5 °C and 10 °C in October. The maximum and minimum differences between surface and air temperatures are observed in April and January, respectively. It is seen in the daily plots of air and surface temperatures that both lines intersect at 0600 and 2000. Therefore it can be assumed that air and surface temperatures are equal at these times.