ALBERT

Description: This study examines two sets of high-resolution coupled model forecasts starting from no-tropical cyclone (TC) and correct-TC-statistics initial conditions to understand the role of TC events on climate prediction. While the model with no-TC initial conditions can quickly spin up TCs within a week, the initial conditions with a corrected TC distribution can produce more accurate forecast of sea surface temperature up to one and half months and maintain larger ocean heat content up to 6 months due to enhanced mixing from continuous interactions between initialized and forecasted TCs and the evolving ocean states. The TC-enhanced tropical ocean mixing strengthens the meridional heat transport in the Southern Hemisphere driven primarily by Southern Ocean surface Ekman fluxes but weakens the Northern Hemisphere poleward transport in this model. This study suggests a future plausible initialization procedure for seamless weather-climate prediction when individual convection-permitting cyclone initialization is incorporated into this TC-statistics-permitting framework.

Description: The distribution and fate of dichlorodiphenyltrichloroethanes (DDTs) and hexachlorocyclohexanes (HCHs) in sediments from the East China Sea (ECS) and the Yellow Sea (YS) were compared and studied in this work. The ECS has directly large river-dominated inputs of sediment-associated pollutants while the YS does not. The results indicated that these mud deposits of the YS and ECS were the sinks of land-originated DDTs and HCHs. The consistence of these OCP species and total organic carbon (TOC) and the sediment grain size in the YS indicated that the shelf mud depositional process was the dominant factor in controlling the distribution and fate of these organic compounds under the more homogeneous and hydrodynamic-based sedimentary conditions of the YS. The distribution of these chemicals in the coastal ECS, however, showed a much different pattern where the concentrations of DDTs and HCHs decreased with distance from the coast, and the correlations between DDTs and HCHs with TOC were very poor. This indicates that the continuous transferring of organochlorine pesticides (OCPs) into the coastal ECS by the direct riverine inputs and surface runoffs play a key role on the occurrence and fate of OCPs within this more heterogeneous environmental system.

Description: ABSTRACT Dynamics of the near-Earth magnetotail associated with substorms during a period of extended southward IMF is studied using a three-dimensional (3-D) global hybrid simulation model that includes both the dayside and night side magnetosphere, for the first time, with physics from the ion kinetic to the global Alfvénic convection scales. It is found that the dayside reconnection leads to the penetration of the dawn-dusk electric field through the magnetopause and thus a thinning of the plasma sheet, followed by the magnetotail reconnection with 3-D, multiple flux ropes. Ion kinetic physics is found to play important roles in the magnetotail dynamics, which leads to the following results: (1) Hall electric fields in the thin current layer cause a systematic dawnward ion drift motion and thus a dawn-dusk asymmetry of the plasma sheet with a higher (lower) density on the dawn (dusk) side. Correspondingly, more reconnection occurs on the dusk side. Bi-directional fast ions are generated due to acceleration in reconnection, and more high-speed earthward flow injections are found on the dusk side than the dawn side. Such finding of the dawn-dusk asymmetry is consistent with recent satellite observations. (2) The injected ions undergo the magnetic gradient and curvature drift in the dipole-like field, forming a ring current. (3) Ion particle distributions reveal multiple populations/beams at various distances in the tail. (4) Dipolarization of the tail magnetic field takes place due to the pileup of the injected magnetic fluxes and thermal pressure of injected ions, where the fast earthward flow is stopped. Oscillation of the dipolarization front is developed at the fast flow braking, predominantly on the dawn side. (5) Kinetic compressional wave turbulence is present around the dipolarization front. The cross-tail currents break into small-scale structures with k ⊥ ρ i  ∼ 1, where k ⊥ is the perpendicular wave number. A sharp dip of magnetic field strength is seen just in front of the sharp rise of the magnetic field at the dipolarization front, mainly on the dusk side. (6) A shear-flow type instability is found on the dusk side flank of the ring current plasma, whereas a kinetic ballooning instability appears on the dawn side. (7) Shear Alfvén waves and compressional waves are generated from the tail reconnection, and they evolve into kinetic Alfvén waves (KAWs) in the dipole-like field region. Correspondingly, multiple field-aligned current filaments are generated above the auroral ionosphere.

Description: Multiple plasma bubbles have been detected by CLUSTER satellites on August 15th 2001 in midtail (∼−18 Re). Those bubbles can be classified into two types. Type-I bubbles are similar to those bubbles in previous studies [ Sergeev et al., 1996; Walsh et al., 2009]. Unlike type-I bubbles, the trailing parts/tails of type-II bubbles are much more dynamic than their leading parts/heads and have larger the flux transfer rate. The leading parts of type-II bubbles were suffering a deceleration process and the interaction between the leading and trailing parts will lead to the intensification of |B| and Bz and also a flow shear layer at the trailing parts. Those shear flows cause the twist of magnetic field line, the enhancement of x and y components magnetic field and the generation of field aligned current system. Enhancement of electric field fluctuations also can be found at the trailing parts of type-II bubbles. The corresponding ionospheric signatures were also detected by ground geomagnetic stations. We suggested that the type-II bubbles are bubbles in their late evolution stage and our results are important in understanding the evolution of plasma bubble or fast flow and the transportation of energy from magnetotail to ionosphere.

Description: Abstract One of the most important properties of Earth's magnetotail thin current sheet (TCS) is that its current is predominantly contributed by magnetized electrons. The Hall electric field, normal to the TCS and generated by charge separation, is critical to the generation of this electron current as well as a dawn‐dusk asymmetry of the magnetotail, such as duskside preference of magnetic reconnection and related structures and phenomena. However, systematic investigation of the Hall electric field has so far been lacking. Utilizing observations of TCS by Magnetospheric Multiscale and Time History of Events and Macroscale Interactions during Substorm spacecraft, we study the properties of this field. Our results, from various, complementary methods, show that the Hall electric field Ez (in the geocentric solar magnetospheric, coordinate system) or En (normal to the TCS plane) can be clearly observed to point toward the center of the current sheet. The typical magnitude of this electric field is several tenths of 1 mV/m. Statistics of Magnetospheric Multiscale magnetotail TCS crossings show that the Hall electric field is stronger on the duskside, indicating a stronger Hall effect there, which confirms predictions from global‐scale hybrid and particle‐in‐cell simulations.

Description: [1]  Single column models (SCM) are useful testbeds for investigating the parameterisation schemes of numerical weather prediction and climate models. The usefulness of SCM simulations are limited, however, by the accuracy of the best-estimate large-scale observations prescribed. Errors estimating the observations will result in uncertainty in modelled simulations. One method to address the modelled uncertainty is to simulate an ensemble where the ensemble members span observational uncertainty. This study first derives an ensemble of large-scale data for the Tropical Warm Pool International Cloud Experiment (TWP-ICE) based on an estimate of a possible source of error in the best-estimate product. This data is then used to carry out simulations with 11SCM and 2 cloud-resolving models (CRM). Best-estimate simulations are also performed. All models show that moisture-related variables are close to observations and there are limited differences between the best-estimate and ensemble mean values. The models, however, show different sensitivities to changes in the forcing particularly when weakly forced. The ensemble simulations highlight important differences in the surface evaporation term of the moisture budget between the SCM and CRM. Differences are also apparent between the models in the ensemble mean vertical structure of cloud variables whilst for each model cloud properties are relatively insensitive to forcing. The ensemble is further used to investigate cloud variables and precipitation and identifies differences between CRM and SCM particularly for relationships involving ice. This study highlights the additional analysis that can be performed using ensemble simulations and hence enables a more complete model investigationcompared to using the more traditional single best-estimate simulation only.

Description: Although the ion temperature gradient along ( X GSM ) and across ( Z GSM ) the Earth's magnetotail, which plays a key role in generating the cross-tail current and establishing pressure balance with the lobes, has been extensively observed by spacecraft, the mechanism responsible for its formation is still unknown. We use multispacecraft observations and three-dimensional (3-D) global hybrid simulations to reveal this mechanism. Using THEMIS, Geotail, and ARTEMIS observations during individual, near-simultaneous plasma sheet crossings from 10 to 60 R E , we demonstrate that the ion temperature Z GSM -profile is bell-shaped at different geocentric distances. This Z GSM -profile is also prevalent in statistics of ~200 THEMIS current sheet crossings in the near-Earth region. Using 3-D global hybrid simulations, we show that mapping of the X GSM -gradient of ion temperature along magnetic field lines produces such a bell-shaped profile. The ion temperature mapping along magnetic field lines in the magnetotail enables construction of two-dimensional distributions of these quantities from vertical (north-south) spacecraft crossings. Our findings suggest that the ion temperature gradient is an intrinsic property of the magnetotail that should be considered in kinetic descriptions of the magnetotail current sheet. Towards this goal, we use theoretical approaches to incorporate the temperature gradient into kinetic current sheet models, making them more realistic.

Description: Recent observations upstream of Earth's bow shock have revealed that foreshock transients can not only accelerate solar wind ions by reflection at their upstream boundaries, but may also accelerate ions inside them. Evidence for the latter comes from comparisons of ion spectra inside and outside the cores, and from evidence of leakage of suprathermal ions from the cores. However, definite evidence for, and the physics of, ion acceleration in the foreshock transients are still open questions. Using case studies of foreshock transients from THEMIS observations, we reveal an ion acceleration mechanism in foreshock transients that is applicable to ~25% of the transients. The ion energy flux is enhanced between several keV to tens of keV in the cores. We show that these energetic ions are reflected at the earthward-moving boundary of foreshock transients, are accelerated through partial gyration along the convection electric field and can leak out both upstream and downstream of the foreshock transients. Using ions moving self-consistently with a generic 3D global hybrid simulation of a foreshock transient, we confirm this physical picture of ion acceleration and leakage. These accelerated ions could be further accelerated at the local bow shock and repopulate the foreshock, increasing the efficacy of solar wind-magnetosphere interactions.

Description: [1]  When observations are assimilated into a high-resolution coupled model, a traditional scheme that preferably projects observations to correct large scale background tends to filter out small scale cyclones. Here we separately process the large scale background and small scale perturbations with low-resolution observations for reconstructing historical cyclone statistics in a cyclone-permitting model. We show that by maintaining the interactions between small scale perturbations and successively-corrected large scale background, a model can successfully retrieve the observed cyclone statistics that in return improve estimated ocean states. The improved ocean initial conditions together with the continuous interactions of cyclones and background flows are expected to reduce model forecast errors. Combined with convection-permitting cyclone initialization, the new high-resolution model initialization along with the progressively-advanced coupled models should contribute significantly to the ongoing research on seamless weather-climate predictions.