ALBERT

Description: Changes in the stratospheric polar vortex (SPV) can remarkably impact tropospheric circulation. Based onthe diagnosis of reanalysis data, this study finds that the location shift rather than the strength change dominates the intraseasonal variability of SPV. Further analysis suggests that it coupleswell with the tropospheric circulation, forming an intraseasonal stratosphere-troposphere oscillation (STO). The STO shows periodic westward propagation throughout its lifecycle and has a deep structure extending from the troposphere to the stratosphere. It reflects the movement of the SPV toward North America, then North Pacific, Eurasia, and the North Atlantic, and causes significant changes in surface air temperature over North America and East Asia. The mechanism of the STO involves Rossby wave propagation between thetroposphere and stratosphere and cross-scale interactions in the troposphere. Upward Rossby wave propagation from thetroposphere over East Asia maintains the STO’s stratospheric component, and the reflection of these waves back to the troposphere contributes substantially to the STO’s tropospheric center over North America. Meanwhile, the linear and nonlinear processes explain the STO’s westward propagation in the troposphere, which facilities vertical wave propagationchanges. The STO unifies the SPV shifts, the retrograding tropospheric disturbances, and the wave coupling processes intoone framework and provides a holistic view for a better understanding of the intraseasonal stratosphere-troposphere coupling. Given its oscillating nature, time scale, and widespread surface response, the STO may be a potential source of predictability for the subseasonal-to-seasonal prediction.

Description: Multi-GNSS now routinely measures positions precisely at a high sampling rate, up to mm-level of precision within a few minutes or cm-level in the long term of hours and days. A high sampling rate becomes critical in GNSS determination of velocity and acceleration, since difference methods are always used to determine velocity and acceleration from GNSS precise positioning on the implicit assumption of a low sampling rate of GNSS measurements, because the random errors of GNSS-based velocity and acceleration are proportional to the sampling rate in the case of velocity or square-proportional to the sampling rate in the case of acceleration. There are a number of critical technological problems with GNSS accelerometry: (i) with the increase of sampling rate, measurement errors are significantly amplified such that computed velocity and acceleration are completely immersed into noise and of no physical significance. Thus, GNSS precise positioning with a sampling rate higher than a few Hz has to be first down-sampled to, say 1 or 2 Hz; (ii) down-sampling very high-rate GNSS precise positioning measurements directly results in a lower resolution of velocity and acceleration, a larger aliasing effect and signal distortion, the extent of which depend on that specific rate of sampling; and (iii) a low sampling rate also implies impossibility to provide instantaneous velocity and acceleration. We present a regularization method to compute velocity and acceleration and design its apparatus, which solves the mentioned critical technological problems and lead to the technological invention of accelerometers.

Description: Slip inversion is essential to understand source mechanics of earthquakes. Problems of inverting for static/kinematic rupture from surface measurements have been almost always stated to be inherently non-unique in the seismological literature. Given the geometry of a fault and the displacement functions on the surface of the semi-infinite elastic medium, the slip function on the fault is defined by Fredholm integral equation of the first kind. We prove that Fredholm integral equation of the first kind for slip inversion is mathematically of a unique solution, theoretically implying that earthquake slip/rupture can be properly reconstructed. The inherent non-uniqueness issue of slip inversion in the seismological literature is not mathematically true but is practically caused due to lack of measurements. We propose a new inequality-constrained regularized inversion of slip distributions on multiple faults, which implements physically more general inequality constraints to accept more complex dislocation models and/or a combination of complex dislocation models and can be applied to a large earthquake involving multiple faults with different rake angles. It is a natural extension of positivity and the 45 degrees constraints by Olson/Apsel and Hartzell/Heaton. We apply the proposed method to the 2016 Kumamoto Mw7.0 earthquake with GEONET GNSS measurements. The inversion results of slips show that the 2016 Kumamoto earthquake is only of magnitude Mw6.6-6.7 and severe rupture takes place very shallowly on Hinagu and Futagawa faults, with the maximum slips of 4.81m on Hinagu fault and 7.89m on Futagawa fault, respectively, which may well explain the largest damage in Mashiki town.

Description: Reliable and easy-to-implement prediction of hydrometeorological variables is essential for policymaking and public security. Direct prediction of disaster events like droughts may be more meaningful for an early warning system. In this study, we develop a principle of maximum entropy (POME)-and-copula-based framework (PC) for describing and predicting the drought-related high-dimensional dependence by first-time coupling the optimal-moment-based POME with C-vine copulas. Case studies with different research scales are investigated to evaluate the performance of the proposed PC framework fully. One case focuses on comparing the performance of the constructed model framework in single-station drought simulation and prediction performance by comparing the PC with different commonly-used data-driven models. And another focuses on the possibility of the PC in large-scale drought prediction based on grid data. Results reveal that: 1) the PC can integrate the law of drought transmission and reproduce the dependency relationship between droughts and related variables; 2) the PC produces more skillful drought forecasts with additional information concerning the predictive uncertainty than data-driven models; and 3) both visual comparisons and statistical results together prove that the PC can capture the spatio-temporal characteristics of droughts. In addition, the sensitivity of the framework to the impact of high-dimensional drought prediction dimension, input order of variables and time delay effect among variables is also discussed in the study. We believe that the proposed model framework is not only applicable to drought prediction but also can potentially be used for higher dimensional hydrological and meteorological modeling and extended to other fields.

Description: Waveguide teleconnections are quasi-stationary Rossby wave packets trapped by the atmospheric waveguide. Since their energy dispersion is seriously constrained by the atmospheric waveguide, they can generally propagate into very far regions and even become circumglobal, distinguished from the canonical teleconnection pattern that are often oriented in great circle and only has limited zonal scale. The amplified activities of waveguide teleconnections can induce violent meandering of the atmospheric jet stream in global and give rise to vigorous energy transfer between extratropics and tropics among the climate system such as atmosphere, ocean, and sea ice. Recently, interests in the waveguide teleconnections have been quickly expanded within climate community for their close connections to many concurrent high-impact extreme weather events in the middle latitudes. In this talk, I will introduce how the waveguide teleconnections can affect the extreme heat events over Eurasia from intraseasonal to decadal timescales. In addition, though they provide alluring predictive potentials for the heat waves on these timescales, I will introduce current challenges confronted by the community to fully use them in practice and possible ways ahead.