The growth to maturity of a baroclinic disturbance on an initial zonal sheared flow is examined by integrating the quasi-geostrophic equations using semi-analytic energy conserving methods. Agreement with many observed features of large scale flow in the atmosphere is obtained using solutions for which the energy of the motion is artificially restricted to only a small number of eddy scales, while points of disagreement may be attributed principally to physical defects of the quasi-geostrophic approximations.

Convection is a well-observed atmospheric phenomenon, which has a fundamental role in global weather and climate. The transport of heat, moisture and momentum that result from convection are significant at a range of temporal and spatial scales. Convective clouds have a wide range of non-linear interactions with other atmospheric processes which make them difficult to understand and model numerically. Due to resolution constraints in climate models, sub-grid convection is represented by a parameterisation. Many parameterisation schemes are based on an assumed temporal and spatial separation between convection and its forcing. The implied equilibrium relates current convection directly to the large-scale forcing. The validity of the temporal scale separation is directly tested in this thesis by examining the convective response to a time-varying forcing.

Tropical cyclones can cause substantial loss of life and an improved understanding of storm variability and their response to climate change can help inform preparation and future adaptation. The influence of the El Nino Southern Oscillation (ENSO) on global tropical cyclone activity is investigated in a high-resolution coupled climate model (Hi GEM) compared to an atmosphere-only simulation using the atmospheric component of Hi GEM (Hi GAM). HiGEM is able to capture the shift in tropical cyclone locations to ENSO in the Pacific and Indian Oceans but not in the North Atlantic. The vertical wind shear response over the Caribbean is not captured in HiGEM compared to HiGAM and ERA-Interim. Precipitation biases in HiGEM remain in HiGAM over the Western North Pacific, however HiGAM simulates a more accurate representation of the ENSO-tropical cyclone teleconnection. Model experiments are subsequently undertaken to investigate a contemporary issue on how different types ofEl Nino influence tropical cyclone activity in the Western North Pacific. The Hi GEM-Hi GAM central Pacific El Nino experiment simulates an increase of tropical cyclones that move towards South East Asia. This response is attributed to a large-scale anti-cyclonic anomaly over east China. In contrast, the low SST in the West Pacific during east Pacific El Nino reduces tropical cyclone activity. How tropical cyclone activity might change due to the influence of increased atmospheric carbon dioxide concentrations using HiGEM is finally investigated. Tropical cyclones are shown to decrease in frequency globally by 9 % in the 2XC02 (2C02) simulation and 26 % in the 4 x C02 (4C02) simulation. Tropical cyclones only become more intense in the 4C02. A decrease in mean ascent at 500 hPa contributes to the reduction of tropical cyclones in the 2C02 in most basins. The larger reduction of tropical cyclones in the 4C02 arises from further reduction of mean ascent at 500 hPa and a large enhancement of vertical wind shear.

This thesis describes the development, characterisation and use of a dataset of measurements made using 51 radiosondes equipped with accelerometers to measure atmospheric turbulence. Atmospheric turbulence, especially Clear-Air Turbulence (CAT) is hazardous to aircraft as it cannot be observed in advance. Pilots and passengers rely on CAT forecasts, which at best, are correct 60-70% of the time. The reason for this moderate performance in turbulence forecasts is due to a lack of quantitative unbiased observations needed to improve the turbulence theory. This work seeks to improve understanding of turbulence through a standardised method of turbulence observations that span the entire troposphere. To achieve this a sensing package is developed to measure the acceleration of the radiosonde as it swings due to its carrier balloon being agitated by turbulence. The accelerometer radiosonde is then compared against multiple turbulence remote sensing methods to characterise its measurements. From a comparison with a Doppler lidar in the boundary layer a relationship in terms of the eddy dissipation rate, a meteorological measure of turbulence, is found. A further relationship is found when compared with the spectral width of an Mesospheric Stratospheric and Tropospheric (MST) radar. The full dataset of accelerometer sonde ascents is analysed and with information from instrumental comparisons a standard deviation of 5 m s−2 is defined as a threshold for significant turbulence. The dataset spans turbulence generated in meteorological phenomena such as jet streams, clouds and in the presence of convection. The analysis revealed that 77% of observed turbulence could be explained by the aforementioned phenomena. In jet streams turbulence generation was often caused by horizontal processes such as deformation. In the presence of convection turbulence is found to form when CAPE > 150 J kg−1. Deeper clouds were found to be more turbulent due to the increased intensity of in-cloud processes. The accelerometer data were used to verify the skill of turbulence diagnostics, in order to assess which diagnostics are best at forecasting turbulence. It was found that turbulence diagnostics featuring the wind speed, deformation and relative vorticity advection predicted turbulence best. This work provides a new, safe and inexpensive method to retrieve in-situ information about the turbulent structure of the atmosphere. It can inform the aviation industry on where turbulence is generated and assess which are the most skilful diagnostics to predict this.

The main effects of tropical cyclones include high winds, extensive storm surges, and widespread flooding. These main effects are all closely related to the low-level wind structure, the height of which is about 2 km from the underlying surface. A physically based analytic model (λ model) is presented to describe the low-level wind structure of tropical cyclones in terms of one intensity measure, one size measure, and the storm meridional position. The λ model provides an accurate fit of the near-surface azimuthal wind field simulated with an idealized full-physics numerical model. This numerical model is further applied to investigate the tropical cyclone low-level wind structure evolution during the mature stage with observations. Three basic features at the mature stage are observed: the outward expansion of eyewall, the reduction of intensity and the increase of tangential wind in the outer spiral bands. Model simulations show that the outer circulation expansion is accompanied by the outward migration of diabatic heating at mid-level in the eyewall. The outward movement of eyewall is caused by the opposing angular momentum transports. Consequently, the intensity decays due to the angular momentum conservation and cyclostrophic adjustment, which is captured in a new analytic pressure-wind relationship derived from the λ model. To investigate the relationship between the tropical cyclone damage and low-level wind structure, the λ model is used to reconstruct the historical wind structure of hurricanes that allows us, for the first time, to calculate the correlation of damage with the integrated wind profile of all hurricanes at landfall since 1988. We find that those metrics, which include the horizontal wind structure, rather than just maximum intensity, are better correlated with the hurricane cost. The vertical wind shear over the main development region of hurricanes plays a more dominant role than the sea surface temperature in controlling these metrics and therefore also ultimately the cost of hurricanes.

No description available.

This thesis examines the influences on the development of high winds in a case of a damaging extratropical cyclone. Extratropical cyclones are a regular occurrence over the United Kingdom (UK) and much of western Europe: strong cyclones can cause major damage and disruption through high winds, flooding and wave damage. Cyclone Friedhelm occurred on the 7th-8th December 2011, and was observed as part of the "DIAbatic Influence on Mesoscale features in ExTratropical Storms" (DIAMET) field campaign. A case of explosive cyclogenesis, Cyclone Friedhelm exhibited a deepening of 44 hPa in 24 hours, almost double that necessary to be considered a meteorological "bomb". As the storm passed over the UK, strong winds caused widespread damage and disruption, particularly across Scotland. These high winds and their influences are the focus of this thesis, in particular the high-wind region to the south-west of the cyclone centre, in which two high wind phenomena can occur, the cold-conveyor belt (CCB) and sting jets (SJ). The Weather Research and Forecasting (WRF) model is used with a high-resolution setup to investigate the mesoscale wind features, while a lower-resolution configuration is employed to examine the large-scale storm development. The high-resolution simulation is compared to routine observations, and those made specifically as part of the DIAMET project. The WRF model is shown to recreate the storm with a good degree of accuracy. The dynamics of the high wind regions are investigated in order to identify the features and the processes that contribute to their evolution. Trajectories are used to identify parcel ensembles resembling both the SJ and CCB. The effects of forces and balanced flows are discussed and a novel way to calculate the gradient flow is presented. The results of this analysis show a systematic region of super-gradient flow to the south and south-west of the cyclone centre and a region of sub-gradient flow to the north. The SJ parcels are analysed in relation to the past literature including a number of different theories on the cause of the SJ’s descent. Unlike some theories on SJs, latent cooling is shown to have little effect on the wind field, although it does change trajectory evolution. The lower-resolution setup shows that latent heating is key to the deepening of the cyclone, but little change in the track is observed when it is removed. The low-level PV anomaly is shown to be key in driving the deepening of the storm. Varying the time during the simulation at which latent heating is switched off shows the importance of the cyclone’s deepening rate on the wind speed development. Other novel analyses are conducted in order to examine the relationships between circulation and the high winds, showing a strong relationship between the circulation energy and cyclone pressure.

Severe windstorms regularly inflict damage throughout Europe. This research examines a set of 31 midlatitude cyclones, investigating each in depth but encompassing a wide variety of such storms. The set is selected using the Storm Severity Index, and categorised using two methods. The first method is based on the relationship between the storm and the jet stream. Four groups emerge: storms that move along the edge of the jet; that cross the jet stream early; that cross later; or have a split jet. The second method is based on the Pressure Tendency Equation. This establishes whether horizontal temperature advection or diabatic processes dominate during storm development. These two approaches are linked: storms in the first two jet groups tend to be driven by horizontal temperature advection, and the other two groups by diabatic processes. This work then studies the storms’ forecast quality and spread using ECMWF data. It finds that storm intensity tends to be under-forecast, the forecast storms move too slowly and are too far south. Forecast quality improves and spread decreases earlier in storms that cross the jet early, compared to those that cross later, suggesting a link between jet interaction and forecast error. Storms where horizontal temperature advection dominates are on average less well forecast than their diabatic counterparts, but diabatically driven storms tend to have greater forecast spread. Finally, this study proposes metrics for storm-prone situations, examining the configuration of the atmosphere prior to the development of the storms. These describe a variety of key factors for cyclogenesis, such as baroclinicity, barotropicity and moist stability. Of the 31 storms, 29 are associated with a value greater than the 98th percentile of one or more metrics. There is a large overlap between storms where baroclinicity is strong and those where horizontal temperature advection dominates the deepening, confirming that the two approaches are dynamically linked. This relationship between the storms, the dynamics, and the metrics will allow future work to identify sources of uncertainty in modelling severe European windstorms.

Abstract Data from a dense network of autographic gauges has yielded a record of 5-minute interval rainfall over sixty square kilometres at Dar as Salaam on the Indian Ocean coast of Tanzania, permitting detailed analysis of storm development and movement for a 29-month period. A total of 671 storms of varying form, intensity and size was analyzed to determine morphology, mode of development, movement and rainfall characteristics, the inter-relationships between these parameters and their relationship to prevailing wind fields an a seasonal and diurnal basis. Models of storm development and form have been evolved which may equally be applied to other, similar, tropical coastal locations. Two basic storm types have been identified with either a linear or elliptical shape. Further subdivisions of each have also been identified but in general linear storm are associated with land- or sea-breeze features, are aligned approximately parallel to the coast and the gradient wind at 850 millibars, and move to the left of the storm axis in a direction dictated by the relative strengths of surface winds on either side of the axis. Elliptical storms on the other hand are more closely related to overall air motion and move, and are aligned, on a majority of occasions, in the same direction as the general wind field. With both major storm types in situ development can occur according to prevailing atmospheric conditions. Generally high storm incidence is associated with on- or offshore gradient winds. Diurnal variation in storm occurrence is also very closely related to gradient wind, and therefore also varies seasonally with changes in the trade wind systems; storms occurring earlier when gradient winds are onshore.

In this thesis, the role of mid-latitude cyclones in air pollution transport in the Northern Hemisphere is quantified. The storm tracking model, TRACK, is used to study the mechanisms through which pollution, specifically ozone (O3) and carbon monoxide (CO), are vented from the boundary layer to the free troposphere and thus transported over large distances, as well as the introduction of O3 from the stratosphere into the troposphere. The relationship between mid-latitude cyclones and air pollution transport of O3 and CO is explored for the first time using the Monitoring Atmospheric Composition and Climate (MACC) reanalysis, a combined meteorology and composition reanalysis dataset. A comparison between springtime surface ozone measurements at rural background sites on the west coast of Europe and cyclone track frequency in the surrounding regions was used to first establish the correlation between cyclone location and surface air quality. The focus is on spring as it tends to be the season of maximum intercontinental transport of O3. The surface observations were compared to the MACC O3 values at the same locations and case studies of how cyclones can influence surface O3 measurements are described. When cyclones track north of 53°N, there is a significant probability that the surface O3 will be high (> the 75th percentile), due to the close proximity to stratospheric intrusions and the transport at low levels across the North Atlantic Ocean. The most intense spring cyclones (95th percentile) were selected for two regions, the North Atlantic and the North Pacific, for further investigation into the mechanisms which impact O3 and CO concentrations near cyclones. These intense cyclones ( 60 over each region) often tracked over the major emission sources of eastern North America and East Asia. The distributions of MACC O3 and CO within a "typical" intense cyclone are examined by compositing the cyclones together. The cyclone-centered composites were compared to background composites of "average conditions" created by sampling the reanalysis data of the previous year to the cyclone locations. Mid-latitude cyclones are found to redistribute concentrations of O3 and CO horizontally and vertically throughout the cyclone. This is clearly shown to occur through two main mechanisms: (1) vertical lifting of CO-rich and O3-poor air isentropically from near the surface to the mid- to upper-troposphere in the region of the warm conveyor belt; and (2) descent of O3-rich and CO-poor air isentropically in the vicinity of the dry intrusion, from the stratosphere toward the mid-troposphere. This work was expanded to identify the links between teleconnection patterns, mainly the North Atlantic Oscillation (NAO), that affect the major storm track pathways in the North Atlantic sector and the distribution of MACC O3 and CO throughout the troposphere and lower stratosphere. For this analysis, TRACK was used to calculate seasonal weighted-average O3 and CO distribution maps based on the monthly NAO index. During positive NAO phase, the persistence of low pressures over the North Atlantic coupled with the Azores High promotes transport across the North Atlantic throughout the troposphere. During negative NAO phase, blocking high pressure in the eastern North Atlantic are known to occur, which shifts transport pathways to a more southerly zonal flow. This work demonstrates the complex relationship between the horizontal and vertical distribution of pollution, including surface concentrations, and synoptic-scale systems.