Global observations of snow cover and snow water equivalent are vitally important for climatological and hydrological studies, at both global and local scales. Passive microwave remote sensing techniques have been used over the past 30 years to produce global estimations of snow water equivalent through empirical calculations. However, the uncertainties surrounding the influence of snow microstructure has led to large errors in snow water equivalent estimation. This study examined the extinction properties of the natural snowpack and produced a new extinction coefficient, for use with the semi-empirical multiple layer Helsinki University of Technology (n-HUT) snow emission model. Snow slabs from the natural snowpack were extracted and observed radiometrically upon bases of different reflectivities, as part of the Arctic Snow Microstructure Experiment (ASMEx). Snow parameters were characterised via traditional snowpit observation techniques, as well as with modern high resolution methods, such as with the SnowMicroPen and X-Ray Computer Tomography analysis. The ASMEx snow slab data were used with a flux coefficient model to calculate six flux absorption and scattering coefficients. The six flux scattering coefficients in the vertical polarization were used with a theoretical absorption coefficient model to create a new empirical extinction coefficient, eliminating the need to use subjective observations. The new extinction coefficient was compared to the original n-HUT extinction coefficient model, through the observation and simulation of snowpack brightness temperatures, obtained as part of the Sodankyl¨a Radiometer Experiment (SoRaX). The derived extinction coefficient produced more accurate simulated brightness temperatures at vertical polarizations, especially at 36.5 GHz. The ability to include snow specific surface area data within the n-HUT model has greatly increased its capability; by increasing the breadth of microstructure parameters to include objective observations of specific surface area, and by increasing the accuracy of simulations of the natural snowpack.

Clouds have a decisive impact on the Earth’s radiation budget and on the temperature of the atmosphere and surface. In global weather and climate models, however, cloud-radiation interaction is treated in an approximate way that contributes to the large uncertainty due to clouds in climate predictions. One of the simplifications is to only consider radiative transfer in one vertical dimension and neglect horizontal radiative transfer. This thesis provides the first systematic estimates of the global impact of 3D cloud-radiation interactions in the shortwave and longwave. We show that 3D cloud effects consist of both horizontal transfer through cloud sides and horizontal transfer within regions. We develop the longwave part of the SPARTACUS radiation scheme that incorporates treatment of these 3D effects in a one-dimensional radiation calculation at a numerical cost suitable for a global weather and climate model, and validate the scheme. SPARTACUS includes the effects of cloud internal inhomogeneity, of horizontal in-region transport and of the spatial distribution of in-cloud radiative fluxes. Algorithm evaluation is facilitated by an exact theoretical benchmark: for idealised optically thick cubic clouds, we can reason analytically that neglecting longwave 3D cloud side effects leads to an underestimation of cloud radiative effect (CRE) by exactly a factor of three. We introduce a new measure of the cloud geometry information relevant to radiation in the ”effective cloud scale” CS, which only depends on cloud type. Analysis of the effective cloud scale of various cloud types demonstrates that CS = 1.0 ± 0.4 km is a good estimate for the cloud scale of boundary-layer clouds, irrespective of their cloud type and of data source. More variety of cloud types at middle and high levels leads to a greater uncertainty range of CS = 5 to 20 km for clouds above the boundary layer, with a best estimate of CS = 10 km. We conduct offline radiation calculations on atmospheric states from a year of ERAInterim re-analysis. We estimate that overall 3D cloud effects warm the Earth by about 4 W m−2, with warming effects in both the shortwave and the longwave, of 3 W m−2 and 1 W m−2 respectively at top-of-atmosphere and both about 2 W m−2 at the surface. Longwave heating and cooling in vertical layers is increased by up to 0.2 K d−1 and −0.3 K d−1 respectively. In the shortwave, we have separated two different 3D effects. We find that the effect of transport through cloud sides has a cooling effect of around −1 W m−2. This cooling is dominated by the previously rarely investigated effect of in-region horizontal transfer that significantly decreases cloud reflectance and warms the Earth’s system by 5 W m−2. These 3D effects are neglected by current models, but are noticeably stronger than the effect of anthropogenic greenhouse gases and therefore definitely worth considering in climate simulations. We have shown for the first time how this can be achieved in a computationally affordable way.

Water is key to life on Earth. The distribution and quantity of precipitation controls the availability of water, yet little is known about past changes in precipitation. This is especially true at regional scales. The land region of the Northern Hemisphere, especially the mid-latitude region, offers an ideal opportunity to compare and contrast output from climate models with our longest and most comprehensive precipitation observations. This thesis develops current understanding, with the aid of climate models, to attribute changes in global mean precipitation to known key forcings. Perhaps the most obvious feature of twentieth-century global mean precipitation change is a decrease in response to mid-twentieth-century aerosol forcing. Changes in historical precipitation for the land mean of two regions in the Northern Hemisphere are shown to strongly resemble changes in the global mean, due to a greater sensitivity to aerosol forcing than greenhouse gas forcing. This aerosol response is predictable across models, which offer an ideal resource to test this constraint due to their large range in aerosol forcing. However, this aerosol response is not evident in these key mid-latitude precipitation observations. Observed runoff changes, derived from river discharge measurements, also contrast with observed precipitation changes in this region. This contrast is a consequence of an obvious breakpoint in the runoff-precipitation relationship. An ensemble of land surface models that are driven with observed precipitation data fails to simulate this contrast and breakpoint. Combined, these two lines of evidence strongly suggest that Northern Hemisphere mid-latitude precipitation observations are unreliable, at least in the early twentieth century. It is expected that the true trend is disguised by inhomogeneities. This should be recognised in future research that is reliant on these data.

Changes in the hydrological cycle are one of the most important aspects of climate change prediction. Globally, precipitation change is well understood in terms of the atmospheric energy budget, whereby the latent heat released is balanced by net atmospheric cooling. As a result, forcing agents affect precipitation directly through forcing-dependent adjustments, as well as through temperature-driven feedbacks. However, the physical processes driving regional precipitation changes are less understood, particularly over land, and regional projections exhibit significant uncertainties. The global energetic perspective can be extended to regional scales through incorporating horizontal transport of dry static energy. Therefore, the aim of this thesis was to utilize analysis of the local atmospheric energy budget to improve understanding of how forcing agents affect regional precipitation patterns through both forcing-dependent adjustments and temperature-driven feedbacks. The precipitation response to a range of atmospheric forcing agents was analysed using the Met Office Hadley Centre climate model, HadGEM2, as well as output from a large number of the latest generation of global climate models. Land-mean precipitation was shown to have a weak sensitivity to global temperature change. Therefore, adjustment processes have a strong influence on land-mean precipitation trends. During the historical period temperature-driven intensification of land-mean precipitation has been entirely masked by negative adjustments in response to anthropogenic sulphate and volcanic forcing. However, as projected sulphate concentrations decline, temperature-driven changes will soon dominate. The rapid land surface response to forcing was found to play a key role in driving regional precipitation adjustment patterns. Adjustment processes were found to be particularly important for precipitation in the eastern Amazon. Projected drying of the eastern Amazon was shown to be dominated by the physiological effects of CO2 on plant stomata, through reducing evapotranspiration. These results highlight the importance of short-timescale adjustment processes in understanding historical and future precipitation changes over land.

Mixed-phase layer clouds are common throughout the globe, from the tropics where detrainment from convection forms long-lived altocumulus layers, to the mid-latitudes where humidity is brought to the mid-troposphere by cyclonic activity, and in the Arctic regions where low-level mixed-phase stratocumulus clouds persist. Supercooled water is common in these clouds and so they have a strong impact on the radiative balance of the planet. Global climate and numerical weather prediction models fail to predict sufficient mid-level cloud and this leads to deficiencies in the representation of incoming solar radiation at the surface and thus large biases in surface temperature, notably in the Southern Ocean. High resolution and cloud resolving models do not perform significantly better, and in part this is due to large uncertainties in the nature of ice nucleation and the phase transition from liquid to ice. This study exploits new observations of mixed-phase cloud to attempt to better understand the processes that control their evolution. Observations of altocumulus clouds from an instrumented aircraft are presented that probe the nature of liquid and ice cloud particles, and the underlying aerosol population. The performance of cloud microphysics probes in measuring ice particles smaller than 100 microns when liquid cloud drops are present is assessed. New characterisation of SID2 (Small Ice Detector 2) and CIP15 (Cloud Imaging Probe, 15 microns) is presented. Calculations are performed that assess the ice nucleating particle budget in altocumulus, and the ice production rate in mixed-phase altocumulus and cumulus clouds.

This work is specially contributes to the knowledge in the field of efficient removal of entrained micron size liquid drops from air mist in a centrifugal field using bimetallic and bi-surface (low and high energy) porous discs. The available equipment was suitably modified, commissioned and utilised in this investigation. The variables investigated include: different rotational speeds; various air and liquid flow-rates; three materials of construction and five porosities of the rotating porous discs. The average residence time of two-phase two-component flow of fine air mist through the rotating porous disc was in the range of 0.7ms to 3.0ms. A high-resolution and magnification optical method has been used to record, explain and understand the mechanism of wettability of the bi-metal and bi-surface energy porous discs. Navier-Stokes equations of momentum have been used to develop a mathematical model to predict the width of a liquid film formed on top of the rotating porous disc. The predictions of the model compared very favorably with the experimental data. The final empirical equations that represents the separation efficiency, (45 to near 100 %) of the bi-metallic and bi-surface energy (high and low) porous disc rotating at, 0+ to 6100 r/min, are [equation 1] for inline arragement of openings/holes in the porous disc. And [equation 2] for staggered arrangement of openings/holes in the porous disc. Artificial neural networks have been successfully used for the modelling and correlation of experimental data, collected from six porous discs with five porosities and three materials of construction with high and low surface energies, on percent separation efficiency from mixture of air-water mist. The correlation coefficient between the experimental and predicted separation efficiency was found to be 0.95. The Lockhart- Martinelli approach has been used to develop an empirical equation for the constant B, as defined by Lockhart- Martinelli that may be used for the calculation of co-current plus cross-current two-phase two-component pressure drop in this system. [equation 3] An empirical equation that has been developed to predict the liquid film's width is [equation 4] The correlation coefficient between the experimental and predicted film widths was found to be 0.92. Equations 1 to 4 may be used to design a new system for the continuous removal/separation of entrained micron size drops from high velocity air mist.

A novel approach to modelling the oxygen and hydrogen isotope ratios of rainfall over the British Isles is presented. The model process involves two stages. First, a Lagrangian particle dispersion model (FLEXPART) that uses European Centre for Medium Range Weather Forecasting Reanalysis (ECMWF ERA-40) fields to produce ensembles of back trajectories of the three-dimensional path of air parcels prior to rainfall events. Second, physical atmospheric parameters along these trajectories are then input in to a Mixed Cloud Isotope Model (MCIM) to predict the isotopic ratio of rainfall. Models of the movement of oxygen and hydrogen isotopes through the hydrological system are vital to gain understanding of the isotopic systems so as to improve the use of isotopes as palaeoclimate proxies to uncover information about the past. A case study comparing daily observed isotopic values with modelled values for the same day is presented for Norwich for raindays in November and December 2005. The results of this comparison are very promising for the simulation of <>180, ID and deuterium excess for events where more than 3 mm but less than 15 mm of rain fell. A positive relationship is seen between the modelled and observed values, i.e. higher modelled values correspond to higher observed values. The regression equation of this relationship for <>180 is y =0.35x -4.18, which can be compared with the ideal case of modelled =observed (y =x), with an r value of 0.84, significant at the 95% confidence level. The gradient of this relationship and the similar ones seen for ID and deuterium excess reflect the fact that the model sensitivity is too low; the full range of observed values is not captured, though the pattern of variability is reproduced by the model. Natural variability in the observed data was seen when <>180 values from precipitation collected at 5 sites around Norwich during November 2005 were compared. However, insufficient observations (only 8 days in one city) were made to allow a general sampling error to be estimated. For the days where multiple samples were collected and analysed, the standard deviation of observed 8180 values varied between 0.11 and 0.92 %0. This factor should be considered in other studies where modelled values from a grid box are compared with point observations. Similarly, variability was seen across the modelled ensemble of values. For the model runs for Norwich on the 7th November 2005 at 1200 a range of 8180 values of 6.65 %0 was seen, emphasising the importance of an ensemble of runs being conducted rather than a single trajectory. Comprehensive sensitivity tests of the model were conducted. Case studies for other U.K. locations in Dublin and Birmingham during November 2005; and for sites at Driby, Lincolnshire and Stock Hill, Somerset during 1977 to 1982 are also presented. Positive correlations were seen between modelled and observed oxygen and hydrogen isotopic ratios and deuterium excess in all cases except for Dublin where there was an insufficient observed sample size. However, as for Norwich, the model sensitivity was too low (the maximum modelled range across all sites was 3.9 times less than that of the observed values for 8180 and 3.5 times too small for 80) . This approach shows promise for modelling the isotopic composition of rainfall for mid-latitude maritime climatic regions as a complimentary method to the technique of explicitly modelling the isotopic composition ofprecipitation in General Circulation Models (GCMs). The very nature of GCMs means that it is difficult to identify which processes involved have the largest impact on an individual atmospheric component. The simpler format of model used in this study more easily allows processes to be added or removed in order to investigate the relative importance of each one. Also, smaller scale features are accounted for using the Lagrangian approach used in this study, whereas the resolution of Eulerian GCMs is still limited by the computational times involved. However, more investigation is required into the problems seen in this study in producing a large enough modelled range before this study's approach could be widely used.

The mechanisms controlling the intraseasonal variability of the West African monsoon (WAM) are investigated. Knowledge of these mechanisms will help to improve forecasting capabilities over the West African region. There are two dominant modes of variability, one within the 25-60-day band and one with an average period of 15 days. Observations show that the 25-60-day variability in rainfall over West Africa is influenced by the Madden-Julian oscillation (MJO). A number of mechanisms have been suggested. However, previous observational studies are not able to easily distinguish between cause and effect. Intraseasonal convective anomalies over West Africa are simulated in an atmospheric general circulation model (AGCM) as a response to imposed sea surface temperature (SST) anomalies associated with the MJO over the equatorial warm pool region. Negative midtropospheric temperature anomalies, associated with negative SSTs and reduced convection, over the warm pool propagate eastwards as a Kelvin wave and westwards as a Rossby wave, reaching Africa approximately 10 days later. The negative midtropospheric temperature anomalies act to destabilise the troposphere resulting in .enhanced convection over West Africa. Variability in soil moisture has the potential to feedback on the atmosphere and hence rainfall, on a regional scale. The I5-day westward-propagating mode found previously in rainfall is detected in soil moisture observations. A set of AGCM experiments is performed to analyse the influence of soil moisture on the WAM. The I5-day westwardpropagating signal in precipitation is found to exist in the model independent of soil moisture. However, soil moisture is found to feedback into the atmosphere via changes in evaporation which ultimately result in anomalies in the low level circulation. The anomalies in low-level circulation can then change the supply of moist, unstable air to the region, influencing the convective anomalies.

A combination of modelling studies and synthesis of aircraft data was used to understand the formation and development of ice and precipitation in convective clouds. Two contrasting cases were studied to determine whether supercooled raindrops play an important role in the formation and development of ice and precipitation. It was found that the fast production of graupel by directly freezing of supercooled raindrops was crucial for the activation of the Hallett-Mossop (HM) process in the 18 May case. Low concentrations of raindrops and thus graupel caused the HM process to be ineffective in the 13 July case. Therefore, supercooled raindrops were critical to the ice development regarding the production of graupel. Furthermore, only a few ice particles were required (order of about 0.01 L-1) in order for the HM process to quickly dominate the ice process. Another important aspect regarding the research subject is the effect of aerosols. High concentrations of relatively small ice particles were observed in the 15 July case. The explanation for such high concentrations of ice is likely associated with the type of ice nuclei ingested into the cloud. Biological nuclei, oxidised organic aerosol particles vented from the Murg valley, and desert dust are all possible candidates. A model sensitivity test with biological nuclei produced similar concentrations of ice particles to the observations. Two stages were found in the development of a multi-cell storm that occurred over the Vosges mountains in the 24 August case. The response of cloud dynamics and precipitation to the changes in droplet concentration was negative in the earlier single-cell stage, while it was positive in the later multi-cellular stage. The simulations indicated that the aerosol-induced invigoration of convection and enhancement of precipitation were the result of the interaction between the effect of aerosols on freezing and the effect of cold pools on the cloud development.

This project makes a study of meteorological and hydrological processes operating in the Mawddach river catchment ofNorth Wales, with the objectives of recommending catchment management options to reduce the severity of flooding and to produce a design for a high resolution flood forecasting model for the catchment. An array of rain gauges installed across the catchment has allowed the detailed mapping of rainfall distributions. Two patterns are identified, with axes of high rainfall running NW-SE and N-S respectively. Within these zones, the locations of rainfall maxima do not necessarily correspond with the highest altitude. The approach direction of weather systems and the funnelling ofair flows along deep valleys appear to control rainfall distribution. A series of flood events are examined, particularly the convective squall line storm of3 July 2001 and the period ofintense frontal rainfall of3-4 February 2004. Modelling of rainfall is carried out using the MM5 meteorological model. It is found that frontal rainfall can be forecast to a high degree of accuracy. Convective thunderstorm events are less predictable, and different convective physics schemes within the MM5 package had differing degrees of success in forecasting the July 2001 Mawddach storm. The catchment is an area of hard, low permeability Palaeozoic rocks. Thick deposits of glacial and periglacial materials are locally present, particularly in valleys. Experiments to monitor hillslope throughflow and runoff show that these superficial deposits playa crucial role in controlling the antecedent conditions necessary for saturation-excess flood events. Deep blanket peat is found at a number of upland sites in the catchment. Watertable monitoring indicates that older peat has a low water storage capacity, with saturation possible within a few hours of heavy rainfall. Areas ofyoung Sphagnum peat can act as regulating reservoirs for flood water, and should be conserved. Field monitoring ofriver bed temperatures shows that resurgence of groundwater can occur in the deep river valleys of Coed y Brenin during storm events. However, resurgence occurs after the flood peak has passed and is not thought to influence the severity of flooding. Flood scenarios for the town ofDolgellau are investigated, including the effects of continued gravel deposition in the River Wnion. Gravel supply should be controlled through planting of native broadleaf woodland on riverbanks of(peri)glacial materials. A flood reduction scheme is proposed, with establishment of wet woodland and creation ofa flood interception basin in the lower Wnion valley. Field monitoring ofriver and tidal flows at the head of the Mawddach estuary indicates no additive effect ofriver flood and tidal peaks. Flooding at the head of the estuary is generally caused by river flows. Further reclamation of salt marsh could worsen upstream flooding. A new hillslope model has been written which allows for changing antecedent soil moisture conditions. The model generates a soil distribution based on the HOST (hydrology of soil types) scheme. A flood forecasting system is developed by combining the hillslope model with MM5 and existing river routing and floodplain modelling components. The system operates by a combination of parallel and distributed processing, to produce forecasts within an operationally useful timescale.