http://hdl.handle.net/1721.1/7804 Wed, 22 May 2013 00:36:57 GMT 2013-05-22T00:36:57Z http://hdl.handle.net/1721.1/78549 Ice shelf-ocean interactions in a general circulation model : melt-rate modulation due to mean flow and tidal currents Dansereau, Véronique Interactions between the ocean circulation in sub-ice shelf cavities and the overlying ice shelf have received considerable attention in the context of observed changes in flow speeds of marine ice sheets around Antarctica. Modeling these interactions requires parameterizing the turbulent boundary layer processes to infer melt rates from the oceanic state at the ice-ocean interface. Here we explore two such parameterizations in the context of the MIT ocean general circulation model coupled to the z-coordinates ice shelf cavity model of Losch (2008). We investigate both idealized ice shelf cavity geometries as well as a realistic cavity under Pine Island Ice Shelf (PIIS), West Antarctica. Our starting point is a three-equation melt rate parameterization implemented by Losch (2008), which is based on the work of Hellmer and Olbers (1989). In this form, the transfer coefficients for calculating heat and freshwater fluxes are independent of frictional turbulence induced by the proximity of the moving ocean to the fixed ice interface. More recently, Holland and Jenkins (1999) have proposed a parameterization in which the transfer coefficients do depend on the ocean-induced turbulence and are directly coupled to the speed of currents in the ocean mixed layer underneath the ice shelf through a quadratic drag formulation and a bulk drag coefficient. The melt rate parameterization in the MITgcm is augmented to account for this velocity dependence. First, the effect of the augmented formulation is investigated in terms of its impact on melt rates as well as on its feedback on the wider sub-ice shelf circulation. We find that, over a wide range of drag coefficients, velocity-dependent melt rates are more strongly constrained by the distribution of mixed layer currents than by the temperature gradient between the shelf base and underlying ocean, as opposed to velocity-independent melt rates. This leads to large differences in melt rate patterns under PIIS when including versus not including the velocity dependence. In a second time, the modulating effects of tidal currents on melting at the base of PIIS are examined. We find that the temporal variability of velocity-dependent melt rates under tidal forcing is greater than that of velocity-independent melt rates. Our experiments suggest that because tidal currents under PIIS are weak and buoyancy fluxes are strong, tidal mixing is negligible and tidal rectification is restricted to very steep bathymetric features, such as the ice shelf front. Nonetheless, strong tidally-rectified currents at the ice shelf front significantly increase ablation rates there when the formulation of the transfer coefficients includes the velocity dependence. The enhanced melting then feedbacks positively on the rectified currents, which are susceptible to insulate the cavity interior from changes in open ocean conditions. Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 121-123). Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1721.1/78549 2012-01-01T00:00:00Z http://hdl.handle.net/1721.1/78538 The effects of ocean eddies on tropical cyclones Miltenberger, Alexander Reid The purpose of this study is to understand the interactions of tropical cyclones with ocean eddies. In particular we examine the influence of a cold-core eddy on the cold wake formed during the passage of Typhoon Fanapi (2010). The three-dimensional version of the numerical Price-Weller-Pinkel (PWP) vertical mixing model has previously been used to simulate and study the cold wakes of Atlantic hurricanes. The model has not been used in comparison with observations of typhoons in the Western Pacific Ocean. In 2010 several typhoons were studied during the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign and Fanapi was particularly well observed. We use these observations and the 3DPWP to understand the ocean cold wake generated by Fanapi. The cold wake of Fanapi was advected by a cyclonic eddy that was south of the typhoon track. The 3DPWP model outputs with and without an eddy are compared with observations made during the field campaign. These observations are compared to model outputs with eddies in a series of positions right and left of the storm track in order to study effects of mesoscale eddies on ocean vertical mixing in the cold wake of typhoons. Thesis (S.M.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 40-41). Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1721.1/78538 2012-01-01T00:00:00Z http://hdl.handle.net/1721.1/78537 Trends and inferred emissions of atmospheric high molecular weight perfluorocarbons Ivy, Diane Jean Atmospheric observations and atmospheric observation-based global emission estimates are presented for the five high molecular weight perfluorocarbons (PFCs): decafluorobutane (C 4 F 1 0 ), dodecafluoropentane (C5 F1 2 ), tetradecafluorohexane (C 6 F14 ), hexadecafluoroheptane (C 7F 16 ) and octadecafluorooctane (C8 F 18 ). Their atmospheric histories are based on measurements of 36 Northern Hemisphere and 46 Southern Hemisphere archived air samples, collected between 1973 and 2011, using two of the "Medusa" cryogenic preconcentration gas chromatography-mass spectrometry instruments, which are part of the Advanced Global Atmospheric Gases Experiment (AGAGE). A new calibration scale was prepared for each PFC, with estimated accuracies of 6.8% for C4Fi0 , 7.8% for C5F12 , 4.0% for CF 14 , 6.6% for C7FE16 and 7.9% for CF8i . Based on our observations, the 2011 globally averaged dry air mole fractions of these high molecular weight PFCs are: 0.17 parts-per-trillion (ppt, i.e., parts per 1012) for C4 F 10 , 0.12 ppt for C5 F 1 2 , 0.27 ppt for CF 1 4 , 0.12 ppt for CFE16 and 0.09 ppt for CF 18 . Newly measured infrared absorption spectra are presented for C7F 16 and CF 1 8 , and using these, their radiative efficiencies and global warming potentials (GWPs) are estimated. We find that the radiative efficiency of C8 F 18 at 0.57Wm- 2 ppb-' is similar to that of trifluoromethyl sulfur pentafluoride's (SF 5 CF 3 ), which has the highest radiative efficiency of any known atmospheric species (Forster et al., 2007). Using their radiative efficiencies, the 2011 observed globally averaged atmospheric mole fractions of the above five high molecular weight PFCs combine to contribute a global average radiative forcing of 0.35 mW m-2, which is 6% of the total anthropogenic PFC radiative forcing (Montzka et al., 2011; Oram et al., 2012). Global emissions for C4 Fio, C5F 12 , C6 F 14 , C7 F 16 and CF 18 were estimated from the observations using a 3-dimensional chemical transport model and a Bayseian inverse method that included a constraint on the annual growth rate of their emissions, consistent with the knowledge of the relevant industries emitting them (Rigby et al., 2011). The derived so-called "top-down" emission estimates show that global emission rates were largest in the 1980s and 1990s for C4 F 10 and C5 F 12 and in the 1990s for C6F 14 , C7F16 and C8F1 . After a subsequent decline, emission rates have remained relatively stable, within ±20 % year-to-year variation, for the last 5 years. Using their calculated 100-year time horizon GWPs, the high molecular weight perfluorocarbons studied here contributed up to 15.4 % of the total PFC emissions expressed in carbon dioxide (C0 2)-equivalents in 1997 and 6 % of the total PFC emissions in 2009. Furthermore, we compare our atmospheric observation-based global emissions to the available so-called "bottom-up" inventories, which are based on production information and end usage. Bottom-up emission estimates are available from the Emission Database for Global Atmospheric Research version 4.2 (EDGARv4.2) for C4F1o, C5 F1 2, C6 F 1 4 and C7F16 , and emission inventories of C4Fio, C5 F1 2 and C6 F14 are also reported to the United Nations' Framework Convention on Climate Change (UNFCCC) by Annex 1 countries that have ratified the Kyoto Protocol (European Commission, Joint Research Centre (JRC)/Netherlands Environmental Assessment Agency (PBL), 2009; United National Framework Convention on Climate Change Secretariat, 2011). The atmospheric observation-based emission estimates are 20 times larger than EDGARv4.2 for C4F10 and over three orders of magnitude larger for C5F1 2 . In contrast, the top-down emission estimates for C6F14 largely agree with the bottom-up estimates from EDGARv4.2. Moreover, the top-down C7 F16 emission estimates are comparable to those of EDGARv4.2 at their peak in the 1990s, albeit with significant underestimation by EDGARv4.2 for the other time periods. There are no bottom-up emission estimates for C8Fi8 , thus the emission rates reported here are the first for this gas. In general, the emission inventories for C4Fio, C5F1 2 and C6F 14 reported to the UNFCCC are five to ten times lower than those estimated in this study from observations. This underreporting to the UNFCCC may be due to only Annex 1 countries reporting inventories and also that some of these countries report a total PFC mixture in C0 2-equivalents, instead of individual PFC emissions rates. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 115-119). Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1721.1/78537 2012-01-01T00:00:00Z http://hdl.handle.net/1721.1/78479 Assessing United States hurricane damage under different environmental conditions Maheras, Anastasia Francis Hurricane activity between 1979 and 2011 was studied to determine damage statistics under different environmental conditions. Hurricanes cause billions of dollars of damage every year in the United States, but damage locations and magnitudes vary from year to year. Seasonal hurricane forecasts predicting the strength of the upcoming hurricane season have the potential to be used by many industries and sectors to reduce and mitigate the effects of hurricanes. However, damage itself is not predicted by these forecasts. This work analyzed trends in hurricane damage due to atmospheric and oceanic conditions, and the results could be applied to and included in seasonal hurricane forecasts, thus increasing forecast applicability and value. This work used synthetic hurricane tracks generated from background climate conditions, a U.S. property portfolio, and a damage function based on wind speed to determine 1979-2011 hurricane damage. Damage was split into La Niña/El Niño and pre-/post- 1995 year sets to determine spatial and temporal trends in U.S. hurricane damage. This work concluded that different regions of the country experienced more or less hurricane damage under different environmental conditions. Knowledge of these trends can be applied to seasonal hurricane forecasts and can influence property owner, regulator, and insurer behavior across the nation. Thesis (S.M. in Atmospheric Science)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 47-53). Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1721.1/78479 2012-01-01T00:00:00Z