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Title: A Lagrangian trajectory and isotopic fractionation (Flexpart-MCIM) approach to modelling the isotopic composition of rainfall over the British Isles
Author: Eames, Katherine Ann Teresa
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 2008
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available