Dynamical and radiative processes in the Upper Troposphere/Lower Stratosphere
The overall aim of this thesis is to gain a better understanding of certain key attributes and processes in the Upper Troposphere/Lower Stratosphere (UTLS). This work involves exploring the interactions between radiation and dynamics which ultimately affect the destination of chemical constituents. A recently identified feature of the extratropical upper troposphere and lower stratosphere is a region of stable air located just above the tropopause known as the tropopause inversion layer (TIL) which appears as a peak in the stratification. The first main chapter of the thesis focuses on elucidating the large scale dynamical mechanisms which lead to the formation of the TIL by exploring the downward influence of the stratosphere on the UTLS region. Model experiments illustrate that the TIL can arise as a result of the dynamics and the wave driving leads to an upwelling structure that results in a TIL. The maximum value and the shape of the TIL peak vary considerable depending on how the observational data is processed. A common method involves averaging with respect to the tropopause. The details of this averaging are found to affect the shape and strength of the TIL. There exist a double peak in the radiative heating (and correspondingly in the upwelling) in the tropical lower stratosphere and radiative calculations reveal that part of it can be considered as imposed. Radiative process are usually thought of as relaxational with waves being regarded as driving the circulation. Idealised model experiments show that imposing a heating leads to an upwelling and the structure of the latter depends on the aspect ratio of the imposed heating. In such experiments, the waves form part of the response. The last core chapter looks at the relatively large annual cycle in temperatures at around 70 hPa in the UTLS. The annual cycle has peak to peak amplitude of about 7 K in observational data. Using radiative experiments that take into account the time evolution of trace gases, the effect of ozone and water vapour on temperatures is quantified. Water vapour is found to play a significant role in this region, especially lower down in the region of the cold point, with important non local influences on temperature. A further set of experiments reveals how the temperatures are affected by the interactions between the dynamics and radiation.