Termik Konveksiyon Öngörüsü

Abstract

Bu tezin ilk bölümünde konvektif sınır tabaka içinde planör uçuşlarına uygun termik bölgelerinin belirlenmesi amacı ile sıcaklık ve nem değerlerinin ölçülebildiği bir sistemin kalibrasyonu ile ilgili çalışmalara yer verilmektedir. Bu amaçla, radyosonde gözlem sisteminde bir kez kullanılabilen iki ayrı bağıl nem ve sıcaklık sensörleri laboratuvar koşullarında kalibre edilmiş, bağıl nem sıcaklık sensörlerinin farklı atmosferik koşullan altındaki direnç değişimleri saptanarak, kalibre eğrileri oluşturulmuştur. Gerçek atmosfer koşullarında Wilga uçağı ve Puachz planörü ile Eylül 1994 döneminde yapılan gözlem sonuçlarının analizinden kalibre edilen sıcaklık sensörleri ile nem sensörlerine nazaran daha sağlıklı gözlem yapılabildiği belirlenmiştir. Termiklerin yükseliri akım bölgelerinin saptanmasında planörün iki kanadına monte edilen sıcaklık sensörlerinden sıcaklık değeri artma gösteren sensör tarafına doğru yapılacak manevra ile daha güçlü termik koşullan allında uçuş olanağı sağlanabileceği gözlenmiştir. Tezin kuramsal bölümünde, grafiksel yönteme göre Eylül ayı için geçerli abaklar kullanılarak, konvektif sınır tabakanın yüksekliği, termiklerin gücü ve termiklerin oluşumu için gerekli tetikleme sıcaklığının tahminine çalışılmıştır. Pearson' m son çalışmasında, Amerika'da farklı koşullarda test edilen bu yöntem, maksimum hava sıcaklığı değeri yerine ıslak hazne sıcaklığının kullanılması ile iyileştirilmiş olup, her iki yönteme ait sonuçlar karşılaştırılmıştır. [ 1 ]. Burada sunulan çalışmanı Analiz Bölümü'nde son yönteme dayalı tahmin çalışmalar gözlemlerle karşılaştırılmış ve iyileştirilmiş yöntemin daha iyi sonuçlar verdiği belirlenmiştir. Çalışmanın son kısmında, "Çoklu Tabaka" ve "Eddy" yöntemlerine dayalı olarak türbülanslı gizli ısı ve buharlaşma ısı (nem) akılan ile kinematik ısı akılarının düşey değişimlerinin hesaplanmasına ve sonuçlarının karşılaştırılmasına çalışılmıştır. Bu aşamada, 1 -Boyutlu model (K-Teorisi) ile parametrelenmiş düşey akı profilleri hesaplanmıştır. Sabah saatlerindeki gözlem verilerine dayalı olarak hesaplanan akı değerlerinin, termiklerin gecikmiş olarak organize olduğu öğleden sonraki saatlerde, yapılan planör uçuş gözlemlerine dayalı akı değerlerinden daha düşük olduğu belirlenmiştir. Bu çalışmanın kuru nemli termiklerden yararlanılarak yapılan planör uçuş koşullarının belirlenmesi ve uçuş programlarının hazırlanmasına yardımcı olması beklenmektedir. Bu amaçla çalışmada sunulan ölçüm yöntemleri ve modellerinin Eylül 1996' da İnönü'de (THK) yapılacak planör yarışması sırasında uygulanması planlanmaktadır.

The vertical and regional variations of some meteorological parameters in a convective boundary layer have been experimentally and theoretically analyzed in this study. The convective activities which are observed in the convective boundary layer are of importance for gliding and also for other sportive flights. These convective activities are generally observed in two different kinds, as thermals and plumes. Thermals are individual air masses and continually rise from heated surfaces. Plumes are characterized by continuous sources of heated surfaces. Their initial radius may vary from ten meters to a few hundred meters. Their strength is directly proportional to radius. While ascending thermals experience turbulent lateral entrainment of surrounding air, their moisture content and temperature become modified. In order to describe this convection, vertical profiles of temperature, moisture, and wind are needed. The profiles are changed by synoptic process such as large scale subsidence or ascent, horizontal advection and convection itself. Precise measurements of structure and dynamics of thermals are difficult, because of size of areas and column heights to be scanned through in the relatively short life time of a thermal, which may be of the order of 20 or 40 minutes. A statistical description of shape and velocity of thermals can contribute to the general view on their characteristics that have been measured by Lindemann (1984). Sailplane flights, surface observations and the thermal waves are analyzed by Lindsay (1970). The satellite data could produce worthwhile results useful in forecasting low level turbulence, the waves and their relation to clear air turbulence. Various characteristics of locating the thermals and improved instrumentation technology have been presented by MacCready (1970). Two basic types of thermals are discovered : one with a single core of maximum vertical velocity and one with several cores of Konovalov (1970). The average of extreme soaring conditions for three widely separated soaring areas have Vll been investigated in the USA by Lester (1976). The characteristics of dry thermals under various meteorological conditions have been recorded and analyzed in South Australia by Hancy (1976), Hindman and Young (1983) have investigated a winter time convergence zone with a sailplane in northeast Colorado. A convective plume could model which gives the linearly interpolated and calculated cloud parameters such as cloud temperature, vertical velocity and water vapor content has been studied by Baker and Jensen (1987). Using the parcel metod a simple lagrangian model has been constructed in order to investigate the relative importance of different parameters, governing the characteristic of dry convection by Olofsson (1987). This model shows that the initial acceleration of thermal is entirely due to the temperature difference between the thermal and the surrounding air. Pearson (1991) has presented a graphical method to forecast the thermal characteristics such as convective layer depth and thermal strength. Generally the idealized convective boundary layer may be divided into 3 parts. Over a shallow superadiabatic layer at the surface, a much deeper and well mixed layer with almost vanishing vertical temperature gradient can be found. During the daytime, the fluxes are large, and usually change linearly with height over the mixed layer. At night, the fluxes are much weaker. This thesis is an attempt to initiate the cumulus convection studies by means of flight observations in Turkey. The previous attempts have been carried out in Eskişehir (İnönü) between 1983 and 1987, (Öney, et al., 1987, Türksoy 1993). The main goal of the presented study is to investigate the micro physical and dynamically structure of thermals and plumes below cumulus clouds by using different measuring systems and theoretical models. The specific purpose of this thesis is given below : i) To investigate the thermic potential of İnönü where only and most the most important center for training and flying activities in Turkey. ii) To calibrate two continuous measuring sensors of relative humidity and temperature. iii) To compute heat and humidity fluxes in and around thermals by using K- Theory and Eddy Method. iv) To test an aerological forecasting model for predicting thermic convection vis-a-vis optimal flight plan. In order to accomplish the objectives of the study, the meteorological parameters such as dry and wet-bulb temperature and flight data, flight speed and vertical velocity of the glider are measured in and around the thermals and cumulus clouds by means of Wilga airplanes or Puchacz-SZD-50 gliders at Eskişehir-Turkish Air League Flight Training Center between the 13-14-15 and 16 of September, 1994. Ground measurements at Eskişehir are also considered. Vlll Soaring thermal forecasting method is a graphical method which gives the thermal strength ( or lift) and convective layer depth by Pearson (1991 and 1995). The model investigates the meteorological conditions from the point of view of soaring. The input parameters are : early morning soundings of air temperature, at three successive levels viz., 1800, 2700 and 3600 meters above MSL, the expected maximum surface temperature and difference between maximum and minimum air temperature of the previous day. Four different nomographs have been used for different periods of the year. The forecast trigger temperature, trigger time associated with forecast trigger temperature, predicted maximum flight altitude and vertical velocity (lift) are defined by using a nomogram (Pearson, 1991 and 1995). A case study related with this metod is given at the following paragraph. The results are compared with observations. The boundary layer soundings are performed between 7:30 and 9:00 am with a tow aircraft. The vertical variation of temperature is measured with the calibrated temperature sensors mounted on the frontal part of the aircraft. During the glider flights between 2:00 and 4:00 pm, vertical air velocities and available maximum flight heights are recorded. These measurements are compared with the model results. To compare the temperature and vertical velocity variations in and in the near vicinity of thermals, the program of constant flight height with Puchacz-SZD-50 glider are carried out and the data recorded every 30 seconds. All data is recorded on a tape. Some details related with the flight path and other meteorological observations are also recorded on a tape-cassette. The form of thermic forecasting based on the observations between 13- 16, September, 1994 actual data and model results are presented as below: Measurement and Model Results, September 22,1993 IX According to this study during a flight program with constant speed and constant altitude; temperature and vertical air velocity variations are almost similar. But in the top of the convective layer, near vicinity of the mixing regions of a plume and the layers below the cumulus clouds, variations of both parametres are not playing an important role to define an updraft. Thermals rise because they are more bouyant than the surrounding air. Warm air is less dense than cold air and has usually been assumed to be the only factor in soaring thermal height forecasting methods. However, several research flight tests have indicated that frequently the thermal temperature excess is negligible (0.2 ° C) or even noticeably negative. Water vapour in a thermal effects on the bouyancy because it is less dense, about 60 per cent that of dry air, and can result in a thermal that is cooler than but more bouyant than the surrounding air. It has been found that thermals over a few hundred feet above the ground often have more water vapour than the surrounding air. The bouyancy forces resulting from the resulting densitiy differences are investigated by converting the temperature of the air into a "virtual temperature". The following table gives the corrections based on the moisture consideration. The corrections based on the moisture consideration METRIC SYSTEM ( DEGREES °C ) Dew Point Virtual Temperature Range Correction * 5 to 9 0.5 10 to 14 10 15 to 20 1.5 *Add to maximum surface temperature before plotting thermal height * Subtract from trigger temperature after determination. Sailplane pilots preparing for recreational or local weekend soaring as well as for cross country flying, need or want to know trigger temperature and time of occurrence, height of thermals and thermal strength. The contest pilot has this information furnished by a formally trained meteorologist after an exhaustive examination of meteorological data and soundings usually not economical or available to the recreational soaring pilot. This paper includes an updated "Do-It- Yourself' forecasting system, with moisture effects and thermal strength modifications, presenting a simple, easy to use method, including a single page reproducible forecasting from, for nominally determining these needs, within 10 minutes before going to the airport, that is suitable for the recreational pilot with limited access to meteorological information. It is planned to be tested and applied the measuring techniques, theoretical grapical models given in this study during the world pre -air games- gliding competitions which will be hold İnönü (Eskişehir) between 2 and 20 September, 1996. The structure of convection within the mixed layer is characterized by narrow updraft areas, so called thermals, which can extend throughout the whole layer. Their diameters vary between 100 and 2000 m (depending on the boundary layer height) and the vertical velocity may reach 5 m/sn. Between the thermals, which having typical separation distances of 1.5z, there are large areas of associated downdrafts with comparatively small vertical velocities. Vertical exchange of proporties in the mixed layer (ML) is mainly due to the complete thermal circulation ( updrafts and downdrafts ). In the surface layer vertical exchange is effected by small-scale turbulance (plumes) whereas in the entrainment layer, mixing is mainly due to penetrative convection into the capping inversion. Secondary circulations produced by dynamical or thermal instabilities can cause thermals to organize themselves linearly in bands called cloud streets, which are oriented in the direction of the mean CBL wind. The asymmetry of the circulation in the CBL characterized by locally restiricted strong updrafts and extended weak downdrafts leads to sligtly positive lapse rates above 0.5z and pozitive heat fluxes at the sama time. These counter- gradient fluxes were already observed in the early laboratory experiments of Deardorff et al (1969). These features of the exchange process in the CBL lead to speacial problems in models which are not capable of resolving the large eddies of the CBL due to their grid structure (ID-models and 2D-models or 3D-models with large horizontal grid size). The direct transports caused by large eddies have to be accomplised by parametrized turbulant fluxes. Problems always arise when effects of large eddies have to be described by local quantities like eddy difusivity (K-Theory). In the mixed layer, K- theory fails since in this case vertical gradients of mean fields almost vanish although considerable turbulant fluxes exist. This would imply that the diffusivity tends to infinity (at least takes very large values ) and that in the case of counter-gradient fluxes, it becomes negative. This of course is unrealistic and points to the inadequancy of K-theory, because the transport is governed by nonlocal large-eddy motions, not by local gradients. The turbulance kinetic energy is one of the most important quantities used to study the turbulant boundary layer. Turbulance can be generated by bouyant thermals and by mechanical eddies. It is suppressed by statically stable lapse rates and dissipated into heat by the effects of molecular viscosity. During the daytime, bouyancy allows air parcels to accelarate in the middle of the mixed layer. On overcast days when there is little heating of the ground, wind XI shears and flow over obstacles create turbulance near the ground that gradually decreases intensity with height. For night, turbulance is produced primarily near the ground by wind shears, althogh the enhanced shears near the nuctural jet can also generate turbulance.