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Climate prediction on subseasonal to decadal time scales is a rapidly advancing field that is synthesizing improvements in climate process understanding and modeling to improve and expand operational services worldwide. Weather and climate variations on subseasonal to decadal timescales can have enormous social, economic and environmental impacts, making skillful predictions on these timescales a valuable tool for decision makers. As such, there is a growing interest in the scientific, operational, and applications communities in developing forecasts to improve our foreknowledge of extreme events. On subseasonal to seasonal (S2S) timescales, these include high-impact meteorological events such as tropical cyclones, extratropical storms, floods, droughts, and heat and cold waves. On seasonal to decadal (S2D) timescales, while the focus broadly remains similar, (e.g., on precipitation, surface and upper ocean temperatures and their effects on the probabilities of high-impact meteorological events), understanding the roles of internal and externally-forced variability such as anthropogenic warming in forecasts also becomes important. The S2S and S2D communities share common scientific and technical challenges. These include forecast initialization and ensemble generation; initialization shock and drift; understanding the onset of model systematic errors; bias correction, calibration, and forecast quality assessment; model resolution; atmosphere-ocean coupling; sources and expectations for predictability; and linking research, operational forecasting, and end user needs. In September 2018 a coordinated pair of international conferences, framed by the above challenges, was organized jointly by the World Climate Research Programme (WCRP) and the World Weather Research Programme (WWRP). These conferences surveyed the state of S2S and S2D prediction, ongoing research, and future needs, providing an ideal basis for synthesizing current and emerging developments in these area, The International Conferences on Subseasonal to Decadal Prediction on which this paper is based were sponsored by: US CLIVAR, NSF, UCAR, NCAR and its Climate and Global Dynamics Laboratory (CGD), NOAA’s Climate Variability and Predictability (CVP) and Modeling, Analysis, Predictions and Projections (MAPP) Programs, Copernicus Climate Change Service, IPSL, and WWRP/WCRP’s Subseasonal-to-Seasonal (S2S) Prediction Project., Postprint (author's final draft)

The accuracy of velocity measurements made using a monopole acoustic logging-while-drilling (ALWD) measurement tool is influenced by the eccentering of the tool due to complex drill string movements. We have used the velocity of collar flexural mode (at the source frequency range) as a reference and classified the fast formations into (1) fast-fast (FF) formations with compressional velocity far larger than the collar flexural velocity and (2) slow-fast (SF) formations with compressional velocity approaching that of the collar flexural velocity. We use a 3D finite-difference method to simulate the response of an eccentered monopole ALWD tool with different eccentering magnitudes (offsets) for the two types of formations to facilitate better interpretation of velocity measurements made in an actual drilling environment. We find that the collar extensional mode, existing in the centralized and eccentered tool cases, only affects the formation P-wave measurement and can be eliminated by using an isolator. The collar flexural mode, which is a shear motion in the collar and can only be excited in a centralized tool by a dipole source, is also excited when a monopole tool is eccentered, and it significantly affects the measurement of the compressional velocity in the SF formation and that of the shear velocity in the FF formation, even for small eccentering offsets. Thus, the uncorrected monopole ALWD tool provides unreliable formation velocities (either the compressional or shear velocities) in fast formations because of the significant influence of the tool offsets on the measurement. To minimize the influence of tool offset on the measurements, we compared the differences between the waveforms collected for different azimuths and tool offsets and the centralized monopole waveforms. Keywords: borehole geophysics; logging; measurement-while-drilling; P-wave; S-wave

In the paper “A numerical investigation of the acoustic mode waves in a deviated borehole penetrating a transversely isotropic formation” by Liu et al. [1] numerical experiments are reported for a dipole sonic logging scenario with a deviated borehole penetrating a VTI anisotropic medium. In such a situation it is important to clearly define the meaning of group and phase velocities as this has led to much confusion in the literature as discussed by Miller, Horne and Walsh [2].