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IPCC - Fifth Assessment Report - Working Group 1:
PhD Thesis
Storm track biases and changes in a warming climate from an extratropical cyclone perspective using CMIP5
The Fifth Coupled Model Intercomparison Project (CMIP5) provides an unprecedented opportunity to comprehensively explore biases and changes in storm tracks across all seasons using storm-centered feature tracking methods. 29 CMIP5 models in fully-coupled and atmosphereonly configurations are compared to a reanalysis to assess biases, and two Radiative Concentration Pathways simulations are contrasted against the present-day climate to assess future projections.
The models are found to have a pervasive equatorward bias in storm track position, and a zonal bias in their tilt. The primary cyclogenesis and storm growth regions have severe negative biases, contributing to an overall lack in the simulated total number of storms. The models generally under-represent the extratropical cyclone intensity, in part related to low spatial resolution. A particularly novel result is that the coupling with ocean and sea-ice components often contributes to the equatorward bias in the storm tracks, but is found to reduce the negative cyclogenesis biases and produce more storms. Many of the storm track biases are associated with equatorward and slow jet biases, and temperature biases.
In general, the future projections show a small poleward shift in the storm tracks and a slight contraction in their width, which are strongly associated with changes in the mean flow. This work has given several new insights, for example in demonstrating the robustness of the equatorward decreases in the storm tracks across the seasons. Exceptions to this include the North Atlantic in the winter and spring; and the North Pacific in summer, with the latter example having large uncertainty associated with uniquely large biases. There is a global, year-round decrease in the total number of storms. Cyclone intensity distributions are generally projected to shift slightly towards stronger storms in the Southern Hemisphere while there are few changes in the Northern Hemisphere.
Undergraduate/Masters Dissertation
Storm Tracks and Reanalyses
The hypothesis that extratropical cyclones in different reanalyses are all the same is tested and is shown that this is not necessarily the case.
The spatial and intensity distributions of extratropical synoptic scale cyclones are compared between three reanalysis projects (ERA-Interim, MERRA, JRA-25) using an objective feature tracking method. Objective feature tracking was performed using both relative vorticity (ξ850) and MSLP fields. Larger and more significant differences were found when using the ξ850 field which picks out smaller spatial scales. Spatial statistics indicate a high level of correspondence in the Northern Hemisphere (NH), particularly in the main storm tracks but with some differences of location, intensity and density in the secondary tracks. In the Southern Hemisphere (SH) there is less agreement, particularly in the Antarctic circumpolar storm track and in the genesis regions in the lee of the Andes. Overall, the highest track densities were found in ERA-Interim whilst the lowest densities were found in JRA-25. Intensity distributions similarly indicate better agreement in the NH and worse in the SH. Significantly different intensities were indicated between all three reanalyses in ξ850 and MERRA was systematically stronger overall, particularly in the high-intensity tails in both fields.
A direct comparison of the track ensembles using system matching indicates a broad range of intensities that compare well, however there are a number of small-scale weak systems that show no correspondence. A similar proportion of track matches were found between all reanalyses in the NH. In the SH however, a greater proportion were found between ERA-Interim and MERRA (75% for SH ξ850) compared with matches against JRA-25 (66% and 65% against ERA-Interim and JRA-25 respectively, for SH ξ850) and between older generations of reanalyses in previous studies. Separation distances of directly matched storms show smaller distances between ERA-Interim and MERRA compared with JRA-25 in the NH (≤ 2°). The direct track comparisons reveal worse matches and larger separation distances between JRA-25 and the other reanalyses in the SH however ERA-Interim–MERRA compares almost as good as in the NH. Life cycle composites of the 100 most intense storms in the NH and SH Atlantic and Pacific indicate very similar life cycles but with more intense deeper composites for MERRA and with JRA-25 the weakest, reflecting the intensity distributions.
The results indicate that the largest differences between the reanalyses is most likely related to the data assimilation process and model resolution, and consequently that ERA-Interim and MERRA have systems of comparable quality. Other differences seen between the three reanalyses may be attributed to model parameterizations and orographic representations. More experimentation is needed however to attribute with greater confidence reasons for the differences observed.