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Subramanian, AC, Hoteit I, Cornuelle B, Miller AJ, Song H.  2012.  Linear versus Nonlinear Filtering with Scale-Selective Corrections for Balanced Dynamics in a Simple Atmospheric Model. Journal of the Atmospheric Sciences. 69:3405-3419.   10.1175/JAS-D-11-0332.1   AbstractWebsite

This paper investigates the role of the linear analysis step of the ensemble Kalman filters (EnKF) in disrupting the balanced dynamics in a simple atmospheric model and compares it to a fully nonlinear particle-based filter (PF). The filters have a very similar forecast step but the analysis step of the PF solves the full Bayesian filtering problem while the EnKF analysis only applies to Gaussian distributions. The EnKF is compared to two flavors of the particle filter with different sampling strategies, the sequential importance resampling filter (SIRF) and the sequential kernel resampling filter (SKRF). The model admits a chaotic vortical mode coupled to a comparatively fast gravity wave mode. It can also be configured either to evolve on a so-called slow manifold, where the fast motion is suppressed, or such that the fast-varying variables are diagnosed from the slow-varying variables as slaved modes. Identical twin experiments show that EnKF and PF capture the variables on the slow manifold well as the dynamics is very stable. PFs, especially the SKRF, capture slaved modes better than the EnKF, implying that a full Bayesian analysis estimates the nonlinear model variables better. The PFs perform significantly better in the fully coupled nonlinear model where fast and slow variables modulate each other. This suggests that the analysis step in the PFs maintains the balance in both variables much better than the EnKF. It is also shown that increasing the ensemble size generally improves the performance of the PFs but has less impact on the EnKF after a sufficient number of members have been used.

Orsi, AJ, Cornuelle BD, Severinghaus JP.  2012.  Little Ice Age cold interval in West Antarctica: Evidence from borehole temperature at the West Antarctic Ice Sheet (WAIS) Divide. Geophysical Research Letters. 39   10.1029/2012gl051260   AbstractWebsite

The largest climate anomaly of the last 1000 years in the Northern Hemisphere was the Little Ice Age (LIA) from 1400-1850 C. E., but little is known about the signature of this event in the Southern Hemisphere, especially in Antarctica. We present temperature data from a 300 m borehole at the West Antarctic Ice Sheet (WAIS) Divide. Results show that WAIS Divide was colder than the last 1000-year average from 1300 to 1800 C.E. The temperature in the time period 1400-1800 C.E. was on average 0.52 +/- 0.28 degrees C colder than the last 100-year average. This amplitude is about half of that seen at Greenland Summit (GRIP). This result is consistent with the idea that the LIA was a global event, probably caused by a change in solar and volcanic forcing, and was not simply a seesaw-type redistribution of heat between the hemispheres as would be predicted by some ocean-circulation hypotheses. The difference in the magnitude of the LIA between Greenland and West Antarctica suggests that the feedbacks amplifying the radiative forcing may not operate in the same way in both regions. Citation: Orsi, A. J., B. D. Cornuelle, and J. P. Severinghaus (2012), Little Ice Age cold interval in West Antarctica: Evidence from borehole temperature at the West Antarctic Ice Sheet (WAIS) Divide, Geophys. Res. Lett., 39, L09710, doi: 10.1029/2012GL051260.

Skarsoulis, EK, Cornuelle BD, Dzieciuch MA.  2013.  Long-range asymptotic behavior of vertical travel-time sensitivity kernels. Journal of the Acoustical Society of America. 134:3201-3210.   10.1121/1.4818785   AbstractWebsite

Vertical travel-time sensitivity kernels (VTSKs) describe the effect of horizontally uniform sound-speed changes on travel times in range-independent ocean environments. Wave-theoretic VTSKs can be obtained either analytically, through perturbation of the normal-mode representation, or numerically, as horizontal marginals of the corresponding two-dimensional and three-dimensional travel-time sensitivity kernels. In previous works, it has been observed that wave-theoretic finite-frequency VTSKs approach the corresponding ray-theoretic sensitivity kernels as the propagation range increases. The present work is an attempt to explain this behavior. A stationary-phase approach is used to obtain a long-range asymptotic expression for the wave-theoretic VTSKs. The resulting asymptotic VTSKs are very close to the corresponding ray-theoretic ones. The smoothness condition, required for the stationary-phase approximation to hold, is used to obtain an estimate for the range beyond which the asymptotic behavior sets in. (C) 2013 Acoustical Society of America.