The work contained in this thesis is toward producing an objective cyclone climatology of the extra-tropical regions of the Southern Hemisphere using observational data. The data identified as being the most appropriate for this task was the high resolution multi-spectral imagery produced by the Advanced Very High Resolution Radiometer (AVHRR) on board the National Oceanographic and Atmospheric Administration (NOAA) series of polar-orbiting satellites. A series of algorithms are presented to perform the automatic detection and tracking of cyclones in the Southern Hemisphere. Cloud detection was performed using a number of spectral and textural measures. Individual cloud objects were segmented using thresholding techniques. These cloud objects were pre-processed using an image normalisation algorithm to create an image (feature vector) which was invariant to rotation, translation, scaling and skew. Identification and tracking of cyclones was performed by finding the maximum cross-correlation between the normalised target images and a set of pre-defined normalised templates. Problems with automatically distinguishing between cloud and the surface near Antarctica still remain, and subsequently segmentation of the imagery into individual cloud objects was limited to certain cases. Identification of cyclones by matching with the pre-defined normalised templates was also shown to have only very limited ability to distinguish between cyclone cloud objects and non-cloud objects. Tracking of the cyclones once they have been manually identified and measured was shown to be promising, with the algorithm successfully tracking the cyclone in 16 out of 22 pairs of satellite images taken from 4 case studies of real Southern Hemisphere cyclones. Limitations of the tracking scheme were identified and improvements have been suggested.

The importance of blocking stems from its persistence and from the abnormal weather patterns associated with it. The causes of blocking are, at present, not identified, but there is circumstantial evidence that planetary waves are connected with the phenomenon, viz: the longer mean wave rides cluster around areas where blocking is most frequent. This work attempts to find a firm connection between blocking and the planetary waves, and to examine some current blocking theories which have planetary waves as an essential element.

Internal gravity waves have an important effect on the large-scale circulation of the middle atmosphere, which is conditioned by the deposition of momentum due to their breaking. The propagation of gravity waves is influenced by the properties of the background wind. This thesis examines this influence: it uses stochastic techniques to study gravity wave propagation through a randomly fluctuating background wind. It begins by describing general features of the atmosphere and gravity wave propagation. The basic equations of fluid flow within the atmosphere are derived. These lead via the WKB approximation to a dispersion relation and to ray equations for gravity wave propagation. Propagation equations, such as the ray equations and dispersion relation, are derived in a general context. The notion of a Wigner matrix is introduced, and this is used to derive transport equations for a general Hamiltonian system that may contain random components. These results generalise earlier works by Ryzhik and Guo and Wang. Atmospheric gravity waves are described as an application and the equations derived via the WKB approximation are recovered. The major factor influencing the distribution of gravity waves is the spread of their wavenumber as they propagate through a wind. This is described by the Doppler spreading model. A one-dimensional system with a randomly fluctuating background wind, dependent on altitude only, is considered. The model revisits that of Souprayen by using an Ornstein-Uhlenbeck process to describe the wind. Simple equations for the energy spectrum induced by gravity waves are derived. Analytic forms of the energy spectrum are given and features of the spectrum such as the <i>m</i>^-3 spectral tail (where <i>m</i> is the vertical wavenumber), central wavenumber and scaling with the Brunt-Väisälä frequently are found to be consistent with observations. An equation for the force on the background, induced by gravity wave breaking is also derived.

No description available.

High-resolution (3 kyr) stable isotope analyses (d¹⁸O, d¹³C) have been conducted on late middle Eocene planktonic foraminifera from the western North Atlantic (Ocean Drilling Program, Leg 171B, Site 1052). The data indicate significant (>1‰) and abrupt changes in surface water d¹⁸O. This variability is greater than that seen in open ocean Pleistocene records and indicates that the middle Eocene climatic system was not consistently warm or stable. There were intervals when annual sea surface temperatures were 4 to 7°C greater than modern mean values. These temperatures are similar to those recorded in the early Eocene, suggesting increased carbon dioxide or other greenhouse gases may have forced warm intervals. Fourier analysis reveals Milankovitch frequencies consistent with the wavelengths of eccentricity, obliquity and precession in the stable isotopic record. The long-period eccentricity signal (400 kyr) governs the large fluctuations in middle Eocene d¹⁸O produced significant changes in water column stability and thermal stratification. Large oscillations in sea surface temperatures occurred with surface water temperatures periodically approaching those of deep waters. A possible explanation is that these abrupt shifts in surface water d¹⁸O represent orbitally forced variations in upwelling intensity, which greatly reduced sea surface temperatures. It is concluded that the oscillations in the stable isotopic profiles in the western North Atlantic are due to climatic controls on the intensity of upwelling. The prominence of Milankovitch frequencies in middle Eocene climate records indicates that orbital variations in solar isolation were an important parameter of climatic variability during the Eocene transition period.

This thesis presents the energy, water, and carbon budgets of Sitka spruce plantation forest in Scotland over the period 1997 to 2001. The site microclimate is observed to be strongly influenced by the site’s oceanic climate, and canopy development. Atmospheric structure is observed to affect temporal patterns of microclimatological variables while topography is observed to affect microclimatological and flux measurements. Eddy covariance flux measurement theory and methods are examined and specific inadequacies are addressed. Theoretical aspects of eddy covariance that were examined include signal despiking, coordinate rotation, low frequency contributions, as well as correlations for density fluctuations, angle of attack errors, and sonic temperature determination. An analysis of frequency response correction methods was used to determined if superior methods could be identified. Fluxes of momentum were used to verify existing measures of atmospheric turbulence and analysed to identify canopy structure and growth. Sensible heat fluxes were found to have an unexpected negative bias, only a portion of which can be attributed to instrument error. This bias is found to depend upon topography and wind speed but is apparently unrelated to katabatic flow. Large errors in latent heat flux were caused by enhanced tube attenuation and were corrected using improved frequency response corrections. Interannual variability of momentum and sensible heat flux were closely associated with wind speed variability, while interannual variability of NEE was attributable primarily to radiation. The source of variability of latent heat flux was not clearly identifiable. Missing values of latent heat flux were modelled using a canopy conductance model, which incorporated effects of canopy evaporation.

Investigations of the interaction between climate and ice sheets were originally based on the orbital theory of the ice ages, and arose through a desire to understand the glacial cycles. More recently, interest has been stimulated in this field, with concern of global warming and the role of ice sheets with respect to future climatic change. The work of glaciologists shows that ice sheet models are very sensitive to the climate of the ablation zone. Studies by climatologists tend to look at the larger scale climate, concentrating on accumulation, rather than ablation patterns. This work looks at the way in which climate models can be used to simulate the climate over an ice sheet, in order to assess the climatic variables most sensitive to ice sheet evolution. Data of the climate over the present day ice sheets in Antarctica and Greenland have been compiled and presented in a form suitable for comparison with GCM data. The work shows that GCM's cannot reproduce the temperature field, and boundary layer structure sufficiently accurately to provide boundary conditions for an ice sheet model. A slab model is developed and used over the glacial slopes to investigate the way in which the boundary layer may affect the ablation of the ice. Over the glacial slopes, the boundary layer flow is turbulent and characterised by entrainment of warm air from above. The rate of entrainment governs the depth, temperature and wind regime of the boundary layer. Two new types of entrainment parameterisation are investigated, and compared to the laboratory derived formulation of Ellison &38 Turner (1959). Experiments show that the method of parameterisation of entrainment is not important, although the coefficients derived in the laboratory tend to be too high; this suggests that there is more loss of TKE from the boundary layer on the glacial slopes, than empirical data from the laboratory suggests. The model is run using surface profiles from present ice sheets, as well as reconstructed profiles from the Laurentide ice sheet at the last glacial maximum. The model is used to look at the climatic parameters which influence the ablation of ice, as well as the way in which the ice sheet itself may influence the climate of the boundary layer. Conclusions show that the evolution of ice depends on a balance between the upper geostrophic wind, the boundary layer development, ice sheet shape and surface radiation budget. If an ice sheet steepens as it retreats, the air accelerates, producing a warmer boundary layer via entrainment. This will enhance the steepening process, and increase the rate of retreat of the ice.

This study addresses the role of polar semi-deserts in land-atmosphere exchange processes by characterising a typical high arctic polar semi-desert site in terms of the major micrometeorological parameters that control and interact with the exchange of energy and carbon dioxide between the soil, vegetation and atmosphere. During 1995 and 1996 at a field site near Ny-Ǻlesund, Svalbard, surface fluxes of radiation balance, evaporation, sensible heat, momentum and carbon dioxide were measured during the active period between spring snowmelt and late summer freeze-up. Eddy correlation systems were used to measure the fluxes of water vapour, sensible heat, momentum and carbon dioxide. Radiation and climatic variable measurements augmented the flux measurements and provided longer term continuity and driving data for modelling purposes. The two active seasons were markedly different with a much shorter active season in 1996 due to a very late snowmelt onset. The season in 1996 was also cloudier. The aerodynamic roughness length was 3.5mm and the average aerodynamic resistance was 70 sm^-1. Average surface resistance was calculated to be 97 sm^-1 in 1995 and 34 sm^-1 in 1996. The energy budget showed different partitions of latent and sensible heat at different times of the active season. Sensible heat was small and occasionally negative during the snowmelt period with the majority of the net radiation being used to melt the snowpack. In the 1995 active season, latent heat flux of 229 MJ m^-2 was 40 per cent of net radiation. Sensible heat flux was 35 per cent (194 MJ m^-2) leading to an evaporative ratio close to 0.5. In the shorter active season in 1996, latent heat flux increased to 55 per cent (105 MJ m^-2) of net radiation with sensible heat flux reducing to 27 per cent (50 MJ m^-2), leading to the much higher evaporative ratio of 0.68.

Studies of atmospheric change, including the climatic effects of ozone depletion require the coupling of chemistry and general circulation models (GCMs). Such work requires that both the chemical and dynamical processes be accurately represented. We begin by showing the information that can be gained from the 2D THIN-AIR coupled model of the atmosphere, and highlight its limitations. As part of the UGAMP modelling effort we are currently involved in implementing coupled chemical-dynamical modelling in the 3D Unified Model (UM). As a first step in this process, the ability of the GCM to reproduce known transport mechanisms in the stratosphere and upper troposphere is assessed with different configurations of the model. The UM is run in troposphere-stratosphere mode and includes simulation of passive and simple-source tracers. We find that the results are very sensitive to the advection scheme implemented, with the age of stratospheric air varying from 3 years with the Roe scheme to 5.5 years with the Heun scheme. Simulations of the stratospheric tape recorder signal reveal a signal which ascends at a good speed with the Heun scheme, but does not decay with height as expected due to amplification of the signal by spurious oscillations. We find barriers to tropical - extratropical exchange which have similar properties both above and below the tropopause, and these are consistent with recent work, suggesting that the tropopause should be conceived as a ‘layer’ of several kilometres, whose properties change from being wholly tropospheric at the lower boundary, to stratospheric at its upper limits. Work presented here provides a useful framework for understanding more recent studies, which have incorporated chemical routines into the UM.

A microwave limb sounder (MLS) is an instrument used to study the composition and temperature of the atmosphere by measuring the microwave radiation that is thermally emitted by atmospheric molecules. When such measurements are made in the upper troposphere, the extinction of radiation caused by cirrus clouds may affect the quality of the results obtained. The extent to which such extinction degrades the sensitivity of MLS measurements to tropospheric parameters has been estimated using computer simulations of radiative transfer in a cloudy atmosphere. Results have been derived specifically for the proposed EOS MLS instrument due to be launched in December 2002, but are applicable to microwave limb sounding in general. To calculate the microwave emission spectrum of the atmosphere, a forward model was developed which represents the absorption properties of atmospheric molecules, and solves the equation of radiative transfer along a one-dimensional path through the atmosphere. A simple scattering model was developed using Mie theory to calculate scattering and extinction by a distribution of spherical particles, and was used to extend the capabilities of the forward model such that it was able to simulate radiative transfer through a cloudy atmosphere. A review of existing literature on the microphysical structure of cirrus clouds has been conducted, allowing the parameterisation of their size distributions and a characterisation of their crystal shapes. Cirrus is composed of ice-crystals which take a variety of non-spherical forms. Therefore in order to make use of the forward model described, a method was developed for converting the geometrical forms of the cirrus crystals into spheres of the same projected area, on the assumption that such spheres will have similar gross scattering properties to their non-spherical counterparts.