Marmara Denizindeki sıcaklık profilinin modellenmesi

Başlık çevirisi mevcut değil.

Tez No: 75259
Yazar: E.GÜL EMECEN
Danışmanlar: DOÇ. DR. YUNUS BORAN
Tez Türü: Yüksek Lisans
Konular: Meteoroloji, Meteorology
Anahtar Kelimeler: Belirtilmemiş.
Yıl: 1998
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Fen Bilimleri Enstitüsü
Ana Bilim Dalı: Meteoroloji Mühendisliği Ana Bilim Dalı
Bilim Dalı: Belirtilmemiş.
Sayfa Sayısı: 78

Özet

ÖZET Denizlerin düşey sıcaklık yapısının incelenmesinde en önemli tabaka karışım tabakasıdır. Bu tabaka atmosfer ile denizin temas halinde olduğu ve her iki ortamın birbiriyle karşılıklı etkileştiği tabakadır. Karışım tabakası yapısının nasıl değiştiğini bilmek, denizlerin tüm yapışım anlamak ve bu tabakanın iklim üzerindeki etkilerini belirlemek açısından önemlidir. Mevsimlere göre deniz ile hava arasında enerji alışverişindeki değişimler bu tabakanın altında mevsimsel termokli oluşmasına sebep olur. Bu çalışmada Marmara denizinin üst katmanlarının düşey sıcaklık yapısı çıkartılarak, bu yapının mevsimsel değişimi araştırılmıştır. Çalışmada, Karaca- Müller Modeli adı verilen ve Kuzey Atlantik'te farklı konumlarda gemilerden alman verilerle yapılan çalışmalarda geçerliliği kanıtlanmış bir model kullanılmıştır. Bu modelde karışım tabakası sıcaklığı, derinliği ve efektif derinliği vasıtası ile akı ve kinetik enerjiler belirlenmektedir. Denizlerin üst katmanlarındaki sıcaklık profili enerjinin korunumu prensibine göre atmosferden gelen ısı akısı ve rüzgarın getirdiği kinetik enerji ile değişmektedir. Modelde Ro,bu tabakadaki ısı miktarı olup Ro'ın zamanla değişimi sadece atmosferden gelen ısı akısı ile olmaktadır. Rı ise potansiyel enerji olup R1'in zamanla değişimi hem termal hem de mekanik enerji ile olmaktadır. Çalışmada kullanılan veri Marmara denizindeki dört farklı istasyonda, farklı derinliklerde ölçülmüş sıcaklık değerleridir. Bu istasyonlardan ikisindeki (İnceburun, Tekirdağ) veriler bir yıllık, diğer iki istasyondaki veriler ise (Yeşilköy, Çanakkale) 1948-1992 yıllan arası aylık ortalama değerlerdir. Böylece Marmara denizinde sıcaklık profillerini oluşturan bir yıllık ve uzun süreli atmosferik güdümlemenin ne olacağı sorusuna cevap aranmıştır. Modelden bulunan sonuçlar genel ısı akısı formüllerine göre aynı dönemlere ait Devlet Meteoroloji İşleri Genel Müdürlüğü'nden alman ölçülmüş değerler kullanılarak elde edilen akı ve kinetik enerji değerleri ile karşılaştırılmıştır. vuı

Özet (Çeviri)

SUMMARY MODELLING OF TEMPERATURE PROFILE IN THE MARMARA SEA The atmosphere supplies momentum, heat and fresh water to the sea and the sea yields moisture and heat to the atmosphere. In the upper sea, air-sea interactions are controlled by the vertical turbulence and buoyancy fluxes. A characteristic feature of the upper sea is that its storage capacity is subject to systematic variations due to wind induced mixing and entrainment processes (Oort and Vonder Haar, 1976; Meehl, 1984). Thus, a parameter like the sea surface temperature depends on the dynamics of the entire upper sea. Besides local air-sea interactions, the state of the upper sea is affected by advection associated with the large-scale sea velocity field. For the interior sea in midlatitudes advective fluxes play a minor role. The surface of sea compared to the air has different properties. Water is very much denser than air. Thus the interface between air and water is very stable because of strength of the gravitational restoring force when it is displaced from its equilibrium position. The large difference in mass between air and water also implies a large difference in heat capacity. In fact, the specific heat (heat capacity per unit mass) of water is four times larger than that of air. The sea absorbs solar radiation very rapidly. The rate of absorption varies with wavelength and with the amount of suspended material. The total energy falls off exponentially with depth. In coastal areas where a lot of suspended material is present, the absorption rate can be much greater. In the atmosphere, long-wave radiation is absorbed much more rapidly than solar radiation by the principal absorber of water vapour. It is hardly surprising, therefore, that long-wave radiation in the sea is absorbed very rapidly. The result is that the emission of long-wave radiation takes place from a very thin layer. The large heat capacity of the sea is of importance for seasonal changes. Although in the long term each hemisphere loses by radiation about as much heat as receives, this is not true of an individual season. The excess heat gained in summer is not transported to the winter hemisphere, but it is stored in the surface layers of sea and returned to the atmosphere in the winter. Because of this ability to store heat, the sea surface temperature changes by much smaller amounts than the land surface, which cannot store much heat. Thermal storage in the sea is also important at longer time scales, and therefore is of significance for climatic variations (Gill, 1982). The temperature of the sea decreases with depth. Generally, the decrease is more rapid near the surface than deeper sea. A typical temperature-versus-depth profile has a surface layer tens of meters thick, generally referred to as the mixed IXlayer, because surface winds usually play an important role in keeping the water well mixed and maintaining it in a nearly isothermal condition. Below the mixed layer there is a region of rapidly changing temperatures referred to as the thermocline. The characteristics of the thermocline vary with the season, becoming“stronger”in the summer when the mixed layer is warmer, and“weaker”in winter when the surface layer cools. In the mixed layer, temperature, salinity, and the mean horizontal velocity are nearly uniform, while the quantities change rapidly below the thermocline. The average properties in mixed layer and its depth are strongly influenced by the atmospheric conditions above. The mixed layer has time variations from the diurnal scale to climatic scales. Since the atmosphere is very sensitive to the sea surface temperature, changes in the mixed layer will have more influence on atmospheric phenomena than changes in other regions of the sea. The mixed layer is forced primarily by solar heating by shortwave radiation, wind stress, and the vertical fluxes of latent and sensible heat through the sea surface. Smaller but in some circumstances important driving forces are precipitation and the mixing effects of internal waves. The presence of slicks affects both the heat fluxes and the influence of surface waves. The response of the mixed layer to such forcing is radiation back to the atmosphere at long wavelengths, downwards turbulent mixing, including entrainment at the bottom of the mixed layer by mechanical and buoyant forces. The modelling of mixed layers is actively evolving, after a big push in the mid-60's by a whole series of one-dimensional models, which are detailed in Kraus (1967). These one-dimensional models use as their basic premise the observed fact that vertical gradients of the sea water physical parameters are much stronger than horizontal ones at most placed in the ocean. Hence the mixed layer can be thought to have a vertical but no horizontal structure, and the forces at work can be considerably simplified. Kraus-Turner (1967) models are forced by surface buoyancy fluxes and the mechanical energy input. The Kraus -Turner model (1967) assumes step - type temperature profile. Its defining equations are, T(t,z)= T, Q>z>-hx T2 h]>z>-h2 (1) where Tj is mixed layer temperature, hi is mixed layer dept, 7^ is a reference temperature. The upper layer, extending from the surface to a varying depth, is considered to be well mixed. The profile parameters 7/ and hi are controlled by surface forcing, turbulent mixing and entrainment. The second layer connects the system to the deep ocean at some fixed level z - -h2. The original Kraus-Turner model assumes T-r T0 = constant. Kraus-Turner model was developed by other workers (Niiler, 1975; Gill and Turner, 1976; Niiler and Kraus, 1977; Zilitinkevich et al., 1978; Well, 1979). Niiler (1975) assumed that the thermocline can be represented by a piecewise linear profile. Well (1979) used a series of step-type profiles which needed several levels in the thermocline. Gill and Turner (1976) by analyzing weathership data, showed therelationships among sea surface temperature, heat content and potential energy by means of hysteresis loops. Meehl (1984) pointed out that the mixed layer depth is not a good approximation of the effective depth because the heat storage beneath the mixed layer is not negligibly small. Meehl (1984) derived the annual cycle of zonal- mean heat storage for the Northern Hemisphere oceans in the mixed layer, and in both the mixed layer and seasonal thermocline. The values of heat storage obtained by considering the mixed layer and thermocline show much better agreement with those obtained using observational data by Oort and Vonder Haar (1976) than their counterparts computed considering the mixed layer only. Karaca and Müller (1996) assumed that the step- wise distribution with a homogeneous thermocline in the original Kraus-Turner(1967) model is replaced by another with an exponentially decaying distribution below the mixed layer. The model, which will be referred to as ihe K-M Model, has two major component: a) it assumes a self similar temperature profile called“the model profile”; and b) it uses special parameterizations for the forcing functions in terms of surface heat flux and wind stress. The validity of the assumptions involved in the K-M model was supported by detailed comparisons between model predictions and temperature data collected by weather ships at five location in the Nort Atlantic (Karaca and Müller, 1989,1991). Using the K-M model permits a more natural calculation of heat storage in the upper ocean than Meehl' s (1984) technique. This is because the model profile represents more closely the observed vertical structure of the buoyancy profile of the upper ocean than that assumed by Meehl (1984). One of the basic assumptions of the K-M model is that the vertical buoyancy structure of the upper sea can be represented in terms of a self-similar, continuous temperature profile. For better agreement with observed temperature profiles, the step - wise distribution with a homogenous thermocline in the original Kraus - Turner (1967) model is replaced by another with an exponentially decaying distribution below the mixed layer. The temperature profile for the upper sea in the K-M model is described by the analytical expression T(z,t) = Tj(t) 0>z>-hi (2) T0 + Tio(t)eDM -hj>z>-h2 where 7/ is mixed layer temperature, hi is mixed layer depth, h2 is depth of the sea column, To is a reference temperature and Tw = Tj- To. The exponent D is given by D = (z+hi)/hi2 XIwhere ha is a scale depth of the seasonal thermocline. The temperature To, therefore, is the asymptotic temperature of the profile. According to (2), the parameters Ti, hi, To, and hu suffice to describe the profile completely. The temperature profile (2) will be referred to as the model profile. The scale depth of the thermocline can be expressed in terms of Tw and of T20 -Tb- To, where Tb is the temperature at z = -h2. It follows mat hi2= (h2-hi) /0 (3) where Q = ln(T10/T2o) In this way, the model profile is determined by the mixed layer temperature and depth, by the temperature (Tb) at a particular depth (h2) in the thermocline, and by the asymptotic temperature. The validity of the model profile under a wide range of ocean conditions has been well established empirically. e x t- û- u Q TEMPERATURE (*C) Fig. Model Profile (Karaca, 1996) xuThe total buoyancy and potential energy of the ocean column are given by the zeroth and first moments of the temperature profile, RQ=lh{T-TQyk (4) Rx= £ (T-T0)zdz (5) Lengths that can be taken as representative of the thermal depth (hq ) and centre of gravity (hp) of the ocean column are defined as *o = Twhq (6) *, = R0hp (7) According to the Kraus - Turner (1967) concept, the upper sea dynamics are governed in the one dimensional framework, without advection and salt effects and no solar flux penetration into the sea, by the equations, (8) (9) Appropriate parameterization for the forcing function Fp andF? are the main concern of the mixed layer theories. In the K-M model, Fq is approximated by the heat flux through the sea surface. In this study, vertical temperature profile of Marmara Sea and seasonal variation of this profile are investigated by using K-M Model. Heat flux and kinetic energy are calculated in terms of mixed layer temperature, depth and effective depth. The Findings obtained from the model was compared the flux and kinetic energy calculated by means of flux formula using the data observed by State Meteorological Service. Xlll

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