No description available.

No description available.

No description available.

Atmospheric water vapour-a natural greenhouse gas of importance in the climate system-remains poorly monitored for some regions of the Earth. This study presents an analysis of Global Positioning System (GPS) data that results in a new precipitable water (PW) data set for Antarctica. In order to obtain optimal estimates of PW, the most up-to-date models, including the VMFI tropospheric mapping function and absolute antenna phase centre variations, are used. Particular emphasis is placed on the accurate modelling of Ocean Tide Loading (OTL) displacements. Initially, a precise point positioning GPS•.'analysis is presented, In which three dimensional ground displacements are estimated at eight diurnal and semi-diurnal tidal frequencies. A global comparison with measurements from Very Long Baseline Interferometry (VLBI) and with estimates computed from numerical ocean tide models is undertaken. GPS is shown to be capable of measuring OTL displacements with comparable accuracy to VLBI, particularly for lunar tidal constituents, N2 and Ql. Systematic GPS biases remain for the Kl and K2 constituents, however. Comparison of GPS and modelled OTL displacements in Antarctica identifies TPX06.2 to be the overall best fitting model. A global reprocessing of a 60 station GPS network is undertaken for the 1995-2006 period. Estimated zenith delays are converted to PW for twelve, mainly coastal, Antarctic locations. Comparison with radiosonde derived PW time series shows the reprocessed GPS measurement technique exhibits good temporal stability. GPS I radiosonde biases are small, at the sub-millimetre level; correlations are of the order of 0.95. Comparison with PW data sets obtained from MODIS, AIRS and AMSR-E satellite instruments shows the AIRS instrument gives the best agreement with GPS, again with sub-millimetre biases. The PW time series capture the dry climate of Antarctica. Periodic signals observed include a strong annual signal at all locations, and a semiannual signal at coastal East Antarctic locations. A 24-hourlY (S 1) periodic variation is observed in summer PW measurements at many sites. There is an apparent increase in summer-time PW over the 1995-2006 period in coastal East Antarctica and at the South Pole. It is concluded that globally reprocessed GPS solutions can provide accurate measurements of PW that are potentially useful for meteorological and climatological applications, particularly in remote, data sparse regions such as Antarctica.

No description available.

No description available.

No description available.

No description available.

Predicting the precise location and timing of the initiation of convective showers and thunderstorms is one of the major uncertainties in numerical weather prediction forecasts of heavy precipitation:Th~ structure and dynamics of convective clouds have beem the focus of substantial research, but the mechanisms governing the initiation of the first cells are one of the least understood aspects. ':r:hree complementary field campaigns have taken place during the last five years, with the same overarching goal of improving forecasts of convective precipi~ation: the International H20 Project (IHOP) iIi 2002, the Convective Storm Initiation Project (CSIP) in 2005 and the Convective and Orographically-induced Precipitation. Study (COPS) in 2007. A major focus of all three projects was improving .the understanding of boundary-layer processes that lead to the formation and development of convective storms. . The main aim of this thesis is to present a detailed observational analysis of the development of the convective boundary layer (CBL) and to highlight precesses that are important for the initiation of convection. Case studies from CSIP and IHOP are examlned by integrating a large number oCdata sets from both routine measurements and state-of-the-art instruments. The work focuses on the variability in the temperature and humidity structure of the evolving CBL and its influence on the development of thermals and cumulus clouds. The first case presents observations of the gradual development of thermals and cumulus clouds as they are at first inhibited by and then grow through multiple stable layers of dry air, knoWn as lids. One particular lid inhibited the development of convection for about an hour, delaying the production of precipitation and causing it to fall about 30 km further north than it would have done without the influence of the lid. The initiation of convection during the second case is shown to be influenced by differential heating on sun-facing slopes of higher terrain and a mesoscale band of higher boundary-layer temperature and humidity. It . . is shown that the CBL is not well-mixed, especially below cumulus clouds. Large fluctuations in temperat~e and humidity are observed as a result of two interacting processes: thermals that carry warm, moist· air upwards, probably from the . surface layer, and descending intrusions of warm, dry air from above the CBL. Similar processes were observed during the third case, but within the framework of open-celled convection.

This study investigates the role of multi-dimensional (MD) solar radiation on the cloud radiative properties and cloud evolution for convective clouds using the UK Met Office Large Eddy Model (LEM) with an interactively coupled Monte Carlo (MC) radiation model. Simulations are performed with the MC scheme running in MD mode and in Independent Column Approximation (ICA) mode. These simulations have been studied to ascertain the impact and understand how MD radiation is influencing clouds differently from the lCA implementation. Since the difference between the ICA and MD radiation is greatest for clouds with large vertical extent, focus has been on warm phase shallow cumulus and mixed phase deep convective (DC) clouds. Using these two types of clouds, it . was possible to evaluate not only the effect of differing clo~d side extent but also the effect of microphysical phase on the evolution. For the cumulus case, the Small Cumulus Microphysics Study (SCMS) is used and for the DC cloud a new case study has been developed based on the Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE) observations. All simulations were performed in 2D. It is found that the MD radiation creates minor differences in the early phase of the first plume; however, these differences have an important role in affecting the secondary plumes for the warm phase cumulus SCMS caSe and cause significant changes in the mixed phase cloud properties (ice, snow, graupel) for the CRYSTAL-FACE case, altering them by as much as a factor of two in the cases studied. Detailed accounts of cloud evolution are presented and the process of how MD as opposed to ICA affects the evolution is presented in terms of radiation-cloud diminishing/enhancing effects that are developed in the work. Cloud radiative forcing (CRF) values were calculated to assess the effects of MD compared to lCA simulations for the upwelling fluxes at the TOA. A tool was developed that related the cloud geometry to the upwelling solar flux, called the effective cloud perimeter. Simulations were performed to calculate the total CRF as well as isolate effects due to geometry and cloud evolution differences. The CRF for geometry effects is up to 70Wm-2 for the SCMS case and 15vVm-2 for the CRYSTAL-FACE case. The impact of evolution is found to be up to 62\Vm-2 for the SCMS and 55\Vm-2 for the CRYSTAL-FACE case. The geometry effects are found to occur during the development and maturation stages and the evolution effects are most prominent in the later mature and dissipation stages. This research developed two tools which can be applied to other simulations. The first was the radiation-cloud diminishing/enhancing effect, which was used to help determine the impact of radiation modifying convective cloud evolutions. The second tool was the effective cloud perimeter, which was used to understand the impact of cloud features that modified the solar radiative properties. Both tools enabled a rigorous understanding of the interactive nature of radiation and cloud processes, and how these varied with ICA and MD solar radiation.