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Richard G. French and Peter J. Gierasch

Abstract

We examine a propagating wave interpretation of the temperature profile features observed in the Jovian upper atmosphere by Veverka et al. Inertia-gravity waves with frequencies on the order of 3 × 10−3 sec−1 are consistent with the data. If the interpretation is correct, and if the waves carry energy upward, it implies 1) that there is excitation of such waves at lower levels, 2) that eddy diffusivities on the order of 106 cm2 sec−1 are probably generated by the waves, and 3) that the energy carried by waves is important to the upper atmospheric heat balance.

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Adam J. French and Matthew D. Parker

Abstract

Some recent numerical experiments have examined the dynamics of initially surface-based squall lines that encounter an increasingly stable boundary layer, akin to what occurs with the onset of nocturnal cooling. The present study builds on that work by investigating the added effect of a developing nocturnal low-level jet (LLJ) on the convective-scale dynamics of a simulated squall line. The characteristics of the simulated LLJ atop a simulated stable boundary layer are based on past climatological studies of the LLJ in the central United States. A variety of jet orientations are tested, and sensitivities to jet height and the presence of low-level cooling are explored.

The primary impacts of adding the LLJ are that it alters the wind shear in the layers just above and below the jet and that it alters the magnitude of the storm-relative inflow in the jet layer. The changes to wind shear have an attendant impact on low-level lifting, in keeping with current theories for gust front lifting in squall lines. The changes to the system-relative inflow, in turn, impact total upward mass flux and precipitation output. Both are sensitive to the squall line–relative orientation of the LLJ.

The variations in updraft intensity and system-relative inflow are modulated by the progression of the low-level cooling, which mimics the development of a nocturnal boundary layer. While the system remains surface-based, the below-jet shear has the largest impact on lifting, whereas the above-jet shear begins to play a larger role as the system becomes elevated. Similarly, as the system becomes elevated, larger changes to system-relative inflow are observed because of the layer of potentially buoyant inflowing parcels becoming confined to the layer of the LLJ.

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Vanda Grubišić, Stefano Serafin, Lukas Strauss, Samuel J. Haimov, Jeffrey R. French, and Larry D. Oolman

Abstract

Mountain waves and rotors in the lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part paper. Part I by French et al. delivers a detailed observational account of two rotor events: one displays characteristics of a hydraulic jump and the other displays characteristics of a classic lee-wave rotor. In Part II, presented here, results of high-resolution numerical simulations are conveyed and physical processes involved in the formation and dynamical evolution of these two rotor events are examined.

The simulation results reveal that the origin of the observed rotors lies in boundary layer separation, induced by wave perturbations whose amplitudes reach maxima at or near the mountain top. An undular hydraulic jump that gave rise to a rotor in one of these events was found to be triggered by midtropospheric wave breaking and an ensuing strong downslope windstorm. Lee waves spawning rotors developed under conditions favoring wave energy trapping at low levels in different phases of these two events. The upstream shift of the boundary layer separation zone, documented to occur over a relatively short period of time in both events, is shown to be the manifestation of a transition in flow regimes, from downslope windstorms to trapped lee waves, in response to a rapid change in the upstream environment, related to the passage of a short-wave synoptic disturbance aloft.

The model results also suggest that the secondary obstacles surrounding the Medicine Bow Mountains play a role in the dynamics of wave and rotor events by promoting lee-wave resonance in the complex terrain of southeastern Wyoming.

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Jeffrey R. French, Samuel J. Haimov, Larry D. Oolman, Vanda Grubišić, Stefano Serafin, and Lukas Strauss

Abstract

Two cases of mountain waves, rotors, and the associated turbulence in the lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part study using aircraft observations and numerical simulations. In Part I, observations from in situ instruments and high-resolution cloud radar on board the University of Wyoming King Air aircraft are presented and analyzed. Measurements from the radar compose the first direct observations of wave-induced boundary layer separation.

The data from these two events show some striking similarities but also significant differences. In both cases, rotors were observed; yet one looks like a classical lee-wave rotor, while the other resembles an atmospheric hydraulic jump with midtropospheric gravity wave breaking aloft. High-resolution (30 × 30 m2) dual-Doppler syntheses of the two-dimensional velocity fields in the vertical plane beneath the aircraft reveal the boundary layer separation, the scale and structure of the attendant rotors, and downslope windstorms. In the stronger of the two events, near-surface winds upwind of the boundary layer separation reached 35 m s−1, and vertical winds were in excess of 10 m s−1. Moderate to strong turbulence was observed within and downstream of these regions. In both cases, the rotor extended horizontally 5–10 km and vertically 2–2.5 km. Horizontal vorticity within the rotor zone reached 0.2 s−1. Several subrotors from 500 to 1000 m in diameter were identified inside the main rotor in one of the cases.

Part II presents a modeling study and investigates the kinematic structure and the dynamic evolution of these two events.

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R. S. Lieberman, J. France, D. A. Ortland, and S. D. Eckermann

Abstract

Recent studies suggest linkages between anomalously warm temperatures in the winter stratosphere, and the high-latitude summer mesopause. The summer temperature anomaly is manifested in the decline of polar mesospheric clouds. The 2-day wave is a strong-amplitude and transient summer feature that interacts with the background state so as to warm the high-latitude summer mesopause. This wave has been linked to a low-latitude phenomenon called inertial instability, which is organized by breaking planetary waves in the winter stratosphere. Hence, inertial instability has been identified as a possible nexus between the disturbed winter stratosphere, and summer mesopause warming. We investigate a sustained occurrence of inertial instability during 19 July–8 August 2014. During this period, stratospheric winter temperatures warmed by about 10 K, while a steep decline in polar mesospheric clouds was reported between 26 July and 6 August. We present, for the first time, wave driving associated with observed inertial instability. The effect of inertial instability is to export eastward momentum from the winter hemisphere across the equator into the summer hemisphere. Using a primitive equation model, we demonstrate that the wave stresses destabilize the stratopause summer easterly jet. The reconfigured wind profile excites the wavenumber-4 component of the 2-day wave, leading to enhanced warming of the summer mesopause. This work supports previous numerical investigations that identified planetary wave–driven inertial instability as a source of the 2-day wave.

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G. B. Burns, B. A. Tinsley, A. V. Frank-Kamenetsky, O. A. Troshichev, W. J. R. French, and A. R. Klekociuk

Abstract

Local temperature, wind speed, pressure, and solar wind–imposed influences on the vertical electric field observed at Vostok, Antarctica, are evaluated by multivariate analysis. Local meteorology can influence electric field measurements via local conductivity. The results are used to improve monthly diurnal averages of the electric field attributable to changes in the global convective storm contribution to the ionosphere-to-earth potential difference. Statistically significant average influences are found for temperature (−0.47 ± 0.13% V m−1 °C−1) and wind speed [2.1 ± 0.5% V m−1 (m s−1)−1]. Both associations are seasonally variable. After adjusting the electric field values to uniform meteorological conditions typical of the Antarctic plateau winter (−70°C, 4.4 m s−1, and 623 hPa), the sensitivity of the electric field to the solar wind external generator influence is found to be 0.80 ± 0.07 V m−1 kV−1. This compares with the sensitivity of 0.82 V m−1 kV−1 to the convective meteorology generator that is inferred assuming an average ionosphere-to-ground potential difference of 240 kV taken with the annual mean electric field value of 198 V m−1. Monthly means of the Vostok electric field corrected for the influence of both local meteorology and the solar wind show equinoctial (March and September) and July local maxima. The July mean electric field is greater than the December value by approximately 8%, consistent with a Northern Hemisphere summer maximum. The solar wind–imposed potential variations in the overhead ionosphere are evaluated for three models that fit satellite measurements of ionospheric potential changes to solar wind data. Correlations with Vostok electric field variations peak with a 23-min interpolated delay relative to solar wind changes at the magnetopause.

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G. B. Burns, A. V. Frank-Kamenetsky, B. A. Tinsley, W. J. R. French, P. Grigioni, G. Camporeale, and E. A. Bering

Abstract

Atmospheric electric field measurements from the Concordia station on the Antarctic Plateau are compared with those from Vostok (560 km away) for the period of overlap (2009–11) and to Carnegie (1915–29) and extended Vostok (2006–11) measurements. The Antarctic data are sorted according to several sets of criteria for rejecting local variability to examine a local summer-noon influence on the measurements and to improve estimates of the global signal. The contribution of the solar wind influence is evaluated and removed from the Vostok and Concordia measurements. Simultaneous measurements yield days when the covariability of the electric field measurements at Concordia and Vostok exceeds 90%, as well as intervals when significant local variability is apparent. Days of simultaneous changes in shape and mean level of the diurnal variation, as illustrated in a 5-day sequence, can be interpreted as due to changes in the relative upward current output of the electrified cloud generators predominating at low latitudes. Smaller average local meteorological influences are removed from the larger Vostok dataset, revealing changes in the shape of monthly average diurnal variations, which are similarly attributed to changes in predominantly low-latitude convection from month to month.

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