Dennis M. Riggin

Radar observations of a 3‐day Kelvin wave in the equatorial mesosphere

Mesospheric radars are used to investigate the characteristics of a Kelvin wave from two equatorial sites: Jakarta, Indonesia, in the western Pacific and Christmas Island in the central Pacific. Our study focuses on the time span from mid-January through mid-October 1993. A Kelvin wave with a period near 3 days was detected throughout this 9-month duration, although it underwent deep amplitude modulations on a ∼20-day timescale. A fitting procedure is applied to study the phase/amplitude behavior of the wave. The vertical wavenumber was measured by the radars and found to be small, wandering around zero with only a weak bias toward downward phase progression. The long vertical wavelength suggests that the wave was predominantly zonal wavenumber 1. The amplitude of the wave measured by the Jakarta meteor scatter radar was much larger than the amplitude measured by the MF partial reflection radar at Christmas Island. The smaller wave amplitude at Christmas Island could at least partially be due to a measurement bias associated with MF radars. The radar at Jakarta is a VHF meteor scatter radar and is not susceptible to this bias. However, the mean velocities and the amplitudes of the tidal and quasi 2-day wave components were in good agreement at the two sites. The estimated 9-month averaged zonal acceleration was ∼0.67 ms−1 day−1 over Jakarta at 94–98 km and only about half as large over Christmas Island. The magnitude of the zonal acceleration occasionally showed large enhancements which suggest the importance of refractive effects associated with vertical and temporal variations in the mean winds. The larger 3-day wave amplitudes and inferred acceleration at Jakarta may reflect its location in the western Pacific, a region of high convection, and hence an excitation region for equatorial waves. The relative phase of the wave between the two radar sites gradually shifted over a timescale of weeks. These smooth variations in relative phase are suggestive of a superposition of waves with different zonal wavenumbers, perhaps radiating preferentially from one longitude. The phase of the wave as a function of altitude and time was much more disordered at Jakarta than at Christmas Island. The conjecture can be made that the more chaotic phase structure observed over Jakarta is due to higher-order zonal wavenumber components which weaken as they propagate eastward.

MF radar observations of seasonal variability of semidiurnal motions in the mesosphere at high northern and southern latitudes

Abstract The semidiurnal tide (SDT) is investigated through comparative analysis of horizontal winds measured at Poker Flat (65°N, 147°W), Andenes (69°N, 16°E), Davis (69°S, 78°E), and Rothera (68°S, 69°W). At the northern hemisphere sites the SDT maximizes around the autumn equinox. Poker Flat and Andenes results from 1999–2001 are used to demonstrate that there is a clear repeatable enhancement in SDT amplitudes around the autumn equinox, and that the maximum is localized in height around 86 km . In the southern hemisphere seasonal dependence of the SDT during 1997–1998 is more complicated, and the autumn enhancement is less pronounced. Many competing mechanisms might contribute to the observed seasonal dependence of the SDT, but this study focuses on the refractive effects of shears in the mean zonal wind and gradients in temperature. The main evidence for a refractive influence is that the seasonal enhancement in the SDT amplitude is accompanied by a dramatic shortening in the waves vertical scale. This shortening of the vertical scale is consistent with refraction of the SDT energy into the horizontal wind component. Simplified linear tidal theory equations are used to estimate the expected magnitude of the refractive effects using wind and temperature fields observed over Andenes, Norway. The predicted refractive effects are shown to be potentially significant and qualitatively consistent with the observations. In addition to a seasonal dependence, the SDT amplitudes obtained at all the radar sites exhibit a deep amplitude modulation on a time scale characteristic of planetary waves. This sort of modulation has most often been attributed to nonlinear interactions between the tides and planetary waves. We suggest that refraction might instead produce, or at least contribute to, the observed modulation. Although the planetary waves are of weak ( m s −1 ) amplitude, the SDT (particularly the gravest S(2,2) mode) is only marginally propagating at high latitudes. Thus, small perturbations to the background are enough to periodically inhibit propagation of the SDT to higher levels.

High resolution Doppler imager observations of Kelvin waves in the equatorial mesosphere and lower thermosphere

The behavior of Kelvin waves in the mesosphere and lower thermosphere is examined in three 10-day sequences of high resolution Doppler imager mesospheric and lower thermospheric zonal winds at solstice. Eastward propagating signatures are observed in wavenumbers 1, 2, and 3 of the zonal wind field with periods near 3 and 5 days. These structures are coherent in latitude and altitude, and maximize near or on the equator. In the mesosphere, lines of constant phase move downward in time, implying that these waves are forced from below. Above 90 km the phase lines of Kelvin zonal wavenumbers 2 and 3 traverse upward in time, suggesting in situ or higher-level sources. Estimates of the Eliassen-Palm fluxes indicate that wavenumber 1 may significantly contribute to the eastward momentum budget of the equatorial lower thermosphere.

A climatology of tides in the Antarctic mesosphere and lower thermosphere

[1] A function that approximates atmospheric tidal behavior in the polar regions is described. This function is fitted to multistation radar measurements of wind in the mesosphere and lower thermosphere with the aim of obtaining a latitude-longitude-height description of the variation of tides over the whole Antarctic continent. Archival wind data sets are combined with present-day ones to fill the spatial distribution of the observations and to reduce the potential effects of spatial aliasing. Multiple years are combined through the compilation of monthly station composite days, yielding results for each month of the year. Despite potential problems associated with year-to-year variations in the tidal phase, a useful climatology of Antarctic zonal and meridional tidal wind components is compiled. The results of the fits reproduce the major features of the high-latitude tidal wind field: the dominance of the semidiurnal migrating mode in the winter months and the presence of a semidiurnal zonal wave number one component in the summer months. It is also found that the summer semidiurnal tide contains a zonal wave number zero component.

Global-scale tidal structure in the mesosphere and lower thermosphere during the PSMOS campaign of June-August 1999 and comparisons with the global-scale wave model

Observations of mean winds and semidiurnal and diurnal tides in the mesosphere/lower-thermosphere (MLT) region were made during the 3-month Planetary-Scale Mesopause Observing System Summer 1999 campaign. Data from 22 ground-based radars (and from two other instruments with measurements for the same period but in 1998) allow us to investigate the ability of the GSWM-00 to simulate the solar tides in the mesopause region (90-95 km). Here we have found that the GSWM-00 provides an increasingly reasonable estimate of most of the tidal characteristics in the MLT region. However, the representation of the 24 h tide appears superior to that of the 12 h tide. Some of these discrepancies are studied in detail. In particular, the observations reveal significant 12 h tidal amplitudes at high latitudes in the Northern Hemisphere summer. There is evidence for relation between the longitudinal variability of the mean zonal wind and the tidal characteristics seen from the radar wind measurements in the summer middle latitudes and a quasi-stationary planetary wave with zonal wave number one.

The 16-day planetary waves: multi-MF radar observations from the arctic to equator and comparisons with the HRDI measurements and the GSWM modelling results

Abstract. The mesospheric and lower thermospheric (MLT) winds (60–100 km) obtained by multiple MF radars, located from the arctic to equator at Tromso (70° N, 19° E), Saskatoon (52° N, 107° W), London (43° N, 81° W), Hawaii (21° N, 157° W) and Christmas Island (2° N, 157° W), respectively, are used to study the planetary-scale 16-day waves. Based on the simultaneous observations (1993/1994), the variabilities of the wave amplitudes, periods and phases are derived. At mid- and high-latitude locations the 16-day waves are usually pervasive in the winter-centred seasons (October through March), with the amplitude gradually decreasing with height. From the subtropical location to the equator, the summer wave activities become strong at some particular altitude where the inter-hemisphere wave ducts possibly allow for the leakage of the wave from the other hemispheric winter. The observational results are in good agreement with the theoretical conclusion that, for slowly westward-traveling waves, such as the 16-day wave, vertical propagation is permitted only in an eastward background flow of moderate speed which is present in the winter hemisphere. The wave period also varies with height and time in a range of about 12–24 days. The wave latitudinal differences and the vertical structures are compared with the Global Scale Wave Model (GSWM) for the winter situation. Although their amplitude variations and profiles have a similar tendency, the discrepancies are considerable. For example, the maximum zonal amplitude occurs around 40° N for radar but 30° N for the model. The phase differences between sites due to the latitudinal effect are basically consistent with the model prediction of equatorward phase-propagation. The global 16-day waves at 95 km from the HRDI wind measurements during 1992 through 1995 are also displayed. Again, the wave is a winter dominant phenomenon with strong amplitude around the 40–60° latitude-band on both hemispheres. Key words. Meteorology and atmospheric dynamics – waves and tides – middle atmosphere dynamics – thermospheric dynamics

The large‐scale dynamics of the mesosphere–lower thermosphere during the Southern Hemisphere stratospheric warming of 2002

An unprecedented major stratospheric warming occurred in the Antarctic winter of 2002. We present measurements of winds in the mesosphere-lower thermosphere (MLT) made with MF radars located at Davis (69degreesS, 78degreesE), Syowa (69degreesS, 40degreesE) and Rothera (68degreesS, 68degreesW). The mesospheric wind field in 2002 was found to be considerably different to other years due to increased planetary wave activity throughout the winter. Zonal winds were weaker than usual during the 2002 winter and also during the transition to the summer circulation. The MLT zonal winds showed a reversal about one week earlier than the stratospheric reversal associated with the warming. Meridional winds showed oscillations consistent with the presence of traveling wave-1 planetary waves with periods similar to14 days. The results are compared with similar mesospheric observations made during northern hemisphere stratospheric warmings. Some similarities between hemispheres were found, notably that the reversal in the mesospheric winds precedes the warming events.

Quasi 2-day oscillation of the ionosphere during summer 1992

The relationship between quasi 2-day (QTD) oscillations in the neutral wind near 90 km altitude, and in the critical plasma frequency (f o F 2 ) of the ionospheric F region, is investigated for the June-September 1992 period. Wind data are analyzed from MF radar stations at Christmas Island (1.9°N, 202.7°E), Hawaii (20.8°N, 203.5°E), and Saskatoon (52.1°N, 253.4°E) with emphasis on characterizing temporal evolutions of the amplitude and period of the QTD oscillation; four identifiable events are found, one each in the months of June, July, August, and September. Evidence for corresponding ±0.5-1.0 MHz QTD oscillations in f o F 2 is found in data from about half of the 24 ionosonde stations examined. Identification of such concurrent occurrences is complicated by the lack of consistency between QTD burst characteristics between station locations (for both wind and f o F 2 ) and between the meridional and zonal components of the QTD wind variations. These differences are probably due to the sensitivity of the QTD wind oscillation to the background wind structure, which contains latitude and longitude dependences. Evidence is presented for a zonal wavenumber one (s = 1) longitude dependence for the middle-latitude f o F 2 oscillation, which contradicts the expected s = 3 dependence of the mesosphere/lower thermosphere (MLT) neutral winds, but which is consistent with the first type of QTD oscillation in f o F 2 at middle latitudes described by Apostolov et al. [1995]. The data presented here are not able to shed light on which of several potential mechanisms lead to the QTD oscillation in f o F 2 .

Analysis of Ducted Motions in the Stable Nocturnal Boundary Layer during CASES-99

Abstract Data obtained with multiple instruments at the main site of the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99) are employed to examine the character and variability of wave motions occurring in the stable nocturnal boundary layer during the night of 14 October 1999. The predominant motions are surprisingly similar in character throughout the night, exhibiting largely westward propagation, horizontal wavelengths of ∼1 to 10 km, phase speeds slightly greater than the mean wind in the direction of propagation, and highly coherent vertical motions with no apparent phase progression with altitude. Additionally, vertical and horizontal velocities are in approximate quadrature and the largest amplitudes occur at elevated altitudes of maximum stratification. These motions are interpreted as ducted gravity waves that propagate along maxima of stratification and mean wind and that are evanescent above, and occasionally below, the altitudes at which they are ducted. Modal structures for ducte...

Gravity wave activity and dynamical effects in the middle atmosphere (60-90km): Observations from an MF/MLT radar network, and results from the Canadian middle atmosphere model (CMAM)

Abstract It has become increasingly clear that Gravity Waves (GW) have an essential and often dominant role in the dynamics of the Middle Atmosphere. This leads to them having strong impacts upon the thermal structure and the distribution of atmospheric constituents. However, the radar observations of GW have been limited in their latitudinal extent during the past decade, and although satellite observations are now significantly contributing, global-seasonal climatologies of important characteristics are still inadequate. With regard to models, the inclusion of GW-drag effects has been problematic. Usually no seasonal or latitudinal variation in the subgrid-scale GW-drag parameterization scheme is included, and varieties of parameterization schemes have been used. Although these often make conflicting assumptions, they generally produce similarly acceptable end-products, e.g. zonal-mean zonal wind fields. In this paper, we report upon the beginnings of a substantial program, using observations from a network of MF radars (North America, Pacific and Europe), and data from the Canadian Middle Atmosphere Model (CMAM). This model allows the tidal and planetary wave fields to be assessed, characteristics and climatologies of which are well known from the MF Radars. Here we focus upon the tides. There are useful similarities in the observed and modeled background wind and wave fields, and strong indications that the two non-orographic GW-drag parameterization schemes (Hines; Medvedev–Klaassen) have significant and differing effects upon the dynamics of the modeled atmosphere. It is shown that this comparison process is valuable in the evaluation, and potentially the optimization, of parameterization schemes.