Christopher Andrew M. Jeffery

A New Ionospheric Electron Precipitation Module Coupled with RAM‐SCB within the Geospace General Circulation Model

Electron precipitation down to the atmosphere due to wave-particle scattering in the magnetosphere contributes significantly to the auroral ionospheric conductivity. In order to obtain the auroral conductivity in global MHD models that are incapable of capturing kinetic physics in the magnetosphere, MHD parameters are often used to estimate electron precipitation flux for the conductivity calculation. Such an MHD approach, however, lacks self-consistency in representing the magnetosphere-ionosphere coupling processes. In this study we improve the coupling processes in global models with a more physical method. We calculate the physics-based electron precipitation from the ring current and map it to the ionospheric altitude for solving the ionospheric electrodynamics. In particular, we use the BATS-R-US (Block Adaptive Tree Scheme-Roe type-Upstream) MHD model coupled with the kinetic ring current model RAM-SCB (Ring current-Atmosphere interaction Model with Self-Consistent Magnetic field (B)) that solves pitch angle-dependent electron distribution functions, to study the global circulation dynamics during the 25–26 January 2013 storm event. Since the electron precipitation loss is mostly governed by wave-particle resonant scattering in the magnetosphere, we further investigate two loss methods of specifying electron precipitation loss associated with wave-particle interactions: (1) using pitch angle diffusion coefficients Dαα(E,α) determined from the quasi-linear theory, with wave spectral and plasma density obtained from statistical observations (named as “diffusion coefficient method”) and (2) using electron lifetimes τ(E) independent on pitch angles inferred from the above diffusion coefficients (named as “lifetime method”). We found that both loss methods demonstrate similar temporal evolution of the trapped ring current electrons, indicating that the impact of using different kinds of loss rates is small on the trapped electron population. However, for the precipitated electrons, the lifetime method hardly captures any precipitation in the large L shell (i.e., 4 < L < 6.5) region, while the diffusion coefficient method produces much better agreement with NOAA/POES measurements, including the spatial distribution and temporal evolution of electron precipitation in the region from the premidnight through the dawn to the dayside. Further comparisons of the precipitation energy flux to DMSP observations indicates that the new physics-based precipitation approach using diffusion coefficients for the ring current electron loss can explain the diffuse electron precipitation in the dawn sector, such as the enhanced precipitation flux at auroral latitudes and flux drop near the subauroral latitudes, but the traditional MHD approach largely overestimates the precipitation flux at lower latitudes.

Turbulence-induced spatial variation of surface temperature in high-resolution thermal IR satellite imagery

Atmospheric eddies cause transient spatial and temporal variations of surface temperature and can limit the precision of satellite surface temperature retrievals. If a thermal IR sensor has sufficiently high spatial resolution, the effects of these transient changes of temperature will be seen as variations of the thermal spatial pattern. Nine thermal IR images of a uniform emissivity area on Mauna Loa caldera are carefully compared to document spatial differences between them. These images were obtained from the Dept. of Energy Multispectral Thermal Imager satellite at about 20m GSD. Spatial patterns with a 1C - 6C magnitude are present but not repeated in any of the images. In order to better understand the characteristics and impact of turbulence induced temperature fluctuations for quantitative remote thermal IR sensing, an effort to model the spatial variation of surface temperature as driven by turbulent energy fluxes has been initiated. Stochastic models initially examined showed a close coupling between surface temperature and turbulent fluxes but were not successful. Traditional energy balance models used in this type of simulation are insufficient to model skin temperature because of the importance of the skin layer and its small depth compared to soil depths used in the models. A new treatment based on surface renewal theory is introduced.

On the dynamics of hot air plasmas related to lightning discharges: 1. Gas dynamics

In this paper, we first study the dynamics of hot shocks in air in cylindrical geometry coupled to multiband radiation transport and detailed air chemistry. The wide energy and length scale ranges which are covered herein includes and exceeds the ones of first and subsequent return strokes happening during lightning discharges. An emphasis is put on the NOx production and the optical power emitted by strong shocks as the ones generated by Joule heating of the air from intense current flows. The production rate of NOx, which is useful for atmospheric global modeling, is found to be between 4.5 × 1016 and 8.6 × 1016 molecules/J for all computed cases, which is in agreement with the literature. Two different radiation transport methods are used to characterize the variability of the results according to the radiation transport method. With the exact radiation solver, we show that between 15 and 40% of the energy is lost by radiation, with a percentage between 20 and 25% for averaged lightning energies. The maximal visible peak is between 7 × 108 W/m and 3 × 107 W/m obtained for, respectively, a 19 kJ/cm and a 28 J/cm energy input. The mean radiated powers in the visible range are found between 9 × 106 W/m and 2 × 105 W/m for the energies just mentioned. We discuss the agreement of these values with previous studies.

Comparison of Narrow Bipolar Events with Ordinary Lightning as Proxies for the Microwave-Radiometry Ice-Scattering Signature

Abstract The narrow bipolar event (NBE) is a unique lightning discharge that has a short (∼10 μs) overall duration, lacks a prior leader phase, and produces too little light output to be visible by optical lightning detectors on satellites. NBEs thus have basic differences from ordinary lightning discharges, which occur in flashes lasting up to a fraction of a second, carry significant current in a “stroke” only after a leader stage that prepares the conductive channel, and produce copious light that is recordable from space. Thus, the authors are motivated to determine whether the meteorological setting of NBEs differs from, or is similar to, that of ordinary lightning. A previous paper started this project of comparing NBEs with ordinary lightning by comparing the placement of either type of lightning within spatial structures of cloud depth, as revealed by infrared cloud-top temperature. That previous study employed lightning data from the Los Alamos Sferic Array (LASA) in Florida. The present paper ex...

Signature of cloud-base-height skewness in ARM microwave water radiometer data: implications for cloud radiative parameterizations in GCMs

The statistics of ground-based retrievals of cloud liquid water path using the microwave water radiometer (MWR) are typically assumed to be independent of the clouds absolute position in the column. Furthermore, translational invariance implies statistical parity, i.e. invariance under reflection, of cloud-base height (zbot) and cloud-top height distributions. This symmetry is necessarily broken, especially under conditions of high boundary-layer relative humidity for which a minimum large-scale lifting condensation level leads to the generation of a significant positive skewness in the distribution function of zbot. We suggest that the signature of this boundary effect is visible in ARM MWR time-series collected at the TWP site. Motivated by the MWR analysis, we incorporate a minimum lifting condensation level into the analytic model of unresolved low-cloud optical variability developed by Jeffery & Austin (J. Atmos. Sci., to appear). Preliminary results indicate that the effect of cloud-base height skewness on mean oceanic low-cloud reflectivity averaged over GCM spatial scales (order 100 km) is significant.

Assessment of Model Error in Limited-Area Simulations of Shallow Water Test Cases on the C-Grid Plane and Sphere

AbstractA suite of limited-area test cases for the solution of the shallow water (SW) equations on the plane and sphere is collected and evaluated using the Model for Prediction Across Scales (MPAS) modeling system. Included are regional simulations of standard test cases, as well as new viscous and linearized test cases with exact analytic solutions. Four different aspects of model error are isolated and thereby assessed: 1) error generated by grid nonuniformity on the plane and sphere, 2) time-independent (balanced) error and time-dependent (propagating wave) error, 3) lateral boundary implementation error, and 4) error reduction due to a viscous equation set or an absorbing sponge layer. Results show that the nature of model error for these test cases is specific to the geophysical regime: SW flows on a rotating sphere with Froude numbers of O(0.1) and Rossby numbers also of O(0.1). For SW simulations in this context, inward reflection of gravity waves at the domain boundary does not appear to be a dri...

Path length of curved RF trajectories through vertically stratified and isotropic ionospheres

Global Navigation Satellite Systems (GNSS) require the removal of ionospheric-induced range errors for high geolocation accuracy. At the GHz frequencies used by GNSS, ionospheric correction is calculated by perturbative expansion in plasma frequency assuming a vertically stratified [1] or isotropic [2] ionosphere. Currently, these techniques are limited by formal analytic difficulties in calculating ionospheric dispersion along a curved ray path. Building on the earlier work of Roussel-Dupre et al. [3], we have developed a new perturbation scheme that overcomes the current difficulties associated with refractive ray bending. In this short paper, we present exact perturbative expressions for excess path length in vertically stratified and isotropic ionospheres, accurate to 6th order in the ratio of plasma frequency to wave frequency.