MF Radar Research Articles

Computation of clear‐air radar backscatter from numerical simulations of turbulence: 3. Off‐zenith measurements and biases throughout the lifecycle of a Kelvin‐Helmholtz instability

Previous papers by Franke et al. (2011) and Fritts et al. (2011) described the computation of radar backscatter power and vertical velocities from numerical simulations of turbulence arising due to Kelvin‐Helmholtz (KH) shear instability. Comparisons of backscatter power and inferred velocities with the distributions of turbulence and the true velocities revealed biases in the identification of active or intense turbulence and in the inferred Doppler spectrum and vertical velocities throughout the flow evolution. This paper extends these analyses to off‐zenith viewing angles typical of multiple‐beam MF, HF, and VHF radars. These reveal similar biases in the identification of turbulence occurrence, Doppler spectra, and inferred radial velocities, with additional sensitivity to the off‐zenith angle relative to the mean shear across the turbulence layer. Radial velocities are typically underestimated during turbulence generation and breakdown of the KH billows, except where turbulence refractive index gradients are strong. Doppler spectra are biased toward regions retaining strong refractive index gradients, implying strong aspect sensitivity at later stages in the evolution. Persistent tilted structures at late stages of the evolution contribute to radial velocity measurement biases that also are functions of off‐zenith angle and time.

Mean winds, tides, and quasi-2 day waves above Bear Island

A new meteor radar on Bear Island (74.5°N, 19.0°E) has been conducting horizontal wind observations on a regular basis since completion of its installation on 1 November 2007. We have analyzed wind data obtained with the meteor radar between 80 and 100km over 42 months from November 2007 to April 2011, and presented seasonal variations as well as year-to-year variations in mean winds, diurnal and semidiurnal tides based on 1-month averaged wind data. We also presented seasonal variations in quasi-2 day wave (Q2DW). Obtained results are compared with those from Tromsø (69.6°N, 19.2°E) MF radar for the same interval. There is a good agreement in the mean zonal wind, meridional diurnal tide, and semidiurnal tides. In particular, the agreement is excellent for the semidiurnal tides, suggesting the same mode is dominant between 70° and 75° between 82 and 91km in the northern Scandinavia sector. Seasonal variations in the Q2DW at both sites are similar, but the intervals of intermittent activity were not correlated well at the two sites except for the meridional component in summer. In general, the results obtained at Bear Island agree with published results at other high latitude stations in the northern hemisphere. We also showed variations in the mean winds and the semidiurnal tide during the January 2009 Sudden Stratospheric Warming event.

Interhemispheric dynamical coupling to the southern mesosphere and lower thermosphere

Wind observations obtained between 1995 and 2011 using the MF radar at Davis have been used to demonstrate the modifying role the quasi‐biennial oscillation (QBO) plays on some aspects of interhemispheric coupling identified by previous authors. The response of the meridional wind in the southern summer polar MLT to changes in winter stratospheric planetary wave activity is shown to change sign according to the phase of the QBO. The time delay associated with the coupling is also shown to vary with QBO phase, with an eastward QBO providing a more rapid response. Coupling to the MLT meridional winds is strongest in January. Parts of the mechanism currently proposed have been tested using UKMO assimilated observations. The signatures of some aspects of this mechanism are present in the data. However, some differences to the mechanism are also apparent, in particular the effectiveness of the mechanism near the equator. An explanation for the QBO modulation of the MLT wind response to interhemispheric coupling is proposed on the basis of these differences.

曲靖地区电离层准6日扰动与MLT大气行星波的相关性

MF radar derived wind measurements and ionosonde data taken above Qujing (25.6°N, 103.8°E) from September to November 2008 are used to investigate the relationship between the atmospheric planetary wave in the mesosphere-lower thermosphere (MLT) and the quasi 6 day oscillation in the ionosphere. The results show that there is strong correlation between the quasi 6 day oscillation in the critical frequency of the sporadic E layer (foEs) and that of the zonal wind, which is related to the atmospheric planetary wave. This quasi 6 day atmospheric planetary wave in the MLT indirectly influences the Es layer by modulating the semidiurnal tides. No obvious planetary type oscillations seem to occur in the F region. These results could act as the experimental evidence for the coupling between the atmosphere planetary wave and ionospheric Es layer, which allows understanding of the ionospheric space-weather properties in low latitude regions of China.

Observation of a mesospheric front in a thermal-doppler duct over King George Island, Antarctica

Abstract. A mesospheric front was observed with an all-sky airglow imager on the night of 9–10 July 2007 at Ferraz Station (62° S, 58° W), located on King George island on the Antarctic Peninsula. The observed wave propagated from southwest to northeast with a well defined wave front and a series of crests behind the main front. The wave parameters were obtained via a 2-D Fourier transform of the imager data providing a horizontal wavelength of 33 km, an observed period of 6 min, and a horizontal phase speed of 92 m s−1. Simultaneous mesospheric winds were measured with a medium frequency (MF) radar at Rothera Station (68° S, 68° W) and temperature profiles were obtained from the SABER instrument on the TIMED satellite. These wind and temperature profiles were used to estimate the propagation environment of the wave event. A wavelet technique was applied to the wind in the plane of wave propagation at the OH emission height spanning three days centered on the front event to define the dominant periodicities. Results revealed a dominance of near-inertial periods, and semi-diurnal and terdiurnal tides suggesting that the ducting structure enabling mesospheric front propagation occurred on large spatial scales. The observed tidal motions were used to reconstruct the winds employing a least-squares method, which were then compared to the observed ducting environment. Results suggest an important contribution of large-scale winds to the ducting structure, but with buoyancy frequency variations in the vertical also expected to be important. These results allow us to conclude that the wave front event was supported by a duct including contributions from both winds and temperature.

Observations of 2-4 day inertia-gravity waves from the equatorial troposphere to theFregion during the sudden stratospheric warming event of 2009

[1] We have observed strong 2–4 day wave activity in the F region peak critical frequency (foF2) at 7.5°N obtained from GPS radio occultation measurements of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites and the horizontal component of geomagnetic field variations over Tirunelveli (8.7°N, 77.8°E, 1.75°N dip angle) during the sudden stratospheric warming (SSW) event of 2009. A similar wave is observed strongly in the meridional winds observed by the MF radar at Tirunelveli. The meridional winds at 9°N and the equator obtained from ERA-Interim, which is the latest global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts, shows clearly the 2–4 day wave, propagating eastward with zonal wave number 6 over the central and eastern Pacific, both before and during the SSW event, and over the Indian sector during the SSW event only. The potential vorticity (PV) intrusion at 200 hPa to lower latitudes is near the central Pacific before the onset of the SSW event. However, during the SSW event, the PV intrusion is observed more strongly near the Indian sector in addition to the weaker intrusion near the central Pacific. It clearly indicates the Rossby wave propagation to equatorial latitudes in the form of PV intrusion, and it selectively enhances the periodicities generated owing to equatorial convective heating. These waves could propagate to upper heights and cause variations in the mesosphere-thermosphere-ionosphere system.

Long-term observations of the wind field in the Antarctic and Arctic mesosphere and lower-thermosphere at conjugate latitudes

[1] Mean winds, semidiurnal and diurnal tides, and trends and long-period oscillations spanning a solar cycle (from early 1999 through June 2010) measured by medium frequency (MF) radars at conjugate Antarctic and Arctic latitudes (Syowa, Antarctica, 69°S, 39.6°E, and Andenes, Norway, 69.3°N, 16°E) are described and compared. Zonal mean winds are stronger and more uniform from year to year over the Antarctic, with a stronger eastward winter jet spanning the range of altitudes presented (70 to 96 km). The summer westward jet is also stronger and maximizes at higher altitudes over the Antarctic than over the Arctic. The eastward winter jet over the Arctic, while generally weaker, exhibits a localized maximum in late winter at ∼2 to 3 year intervals. Meridional mean winds likewise achieve somewhat stronger maxima at higher altitudes over the Antarctic than over the Arctic. Semidiurnal tide amplitudes are typically somewhat larger over the Antarctic and similar in the two components, with maxima at ∼85 km or above and narrow responses that tend to cluster from ∼February to May and ∼September to November over the Antarctic and from ∼December to February and ∼June to September over the Arctic. Zonal diurnal tide amplitudes are quite similar between the sites, with maxima extending from ∼70 to 90 km and slightly stronger over the Antarctic. Meridional diurnal amplitudes display more significant growth with altitude, achieve stronger maxima at the highest altitudes presented, and typically exhibit a single and narrow maximum during December to February over the Antarctic and double maxima from ∼May to September over the Arctic. Also discussed are trends and long-period oscillations over a solar cycle observed in these mean and tidal wind fields.

Nightglow imaging of different types of events, including a mesospheric bore observed on the night of February 15, 2007 from Tirunelveli (8.7°N)

All-sky imaging observations on the night of February 15, 2007, from Tirunelveli (8.7°N), revealed a variety of short-period small-scale dynamical features comprising of atmospheric gravity waves, ripples and a mesospheric bore. A total of 11 wave events other than the convective ripple patches were identified. All the ripple features of wavelengths less than 10km were found to propagate nearly in the same direction and they were frequent in the early part of the night. The presence of a dynamical ripple in all-sky images was also inferred. One of the identified events displayed steepening of the leading front and subsequent evolution into a mesospheric bore. Apart from these events, there were several ‘quasi-coherent’ features that were not clearly discernable for deduction of propagation velocities or wavelengths. In order to have a better understanding of these events, we used winds measured by co-located MF radar to derive intrinsic wave parameters. The MF radar winds and average of zenith intensity values from the imager indicate the presence of medium frequency waves of time periods in the range 0.75–3h. In addition, two sets of temperature measurements made by SABER instrument onboard TIMED mission were also used. The temperature profiles implied strong tidal control on this night. An attempt was also made to infer the source of the wave activity observed at mesospheric altitudes and the effects of wind filtering at altitudes below nightglow emission altitudes.

Multi beam observations of cosmic radio noise using a VHF radar with beam forming by a Butler matrix

Abstract. The Leibniz-Institute of Atmospheric Physics (IAP) in Kühlungsborn started to install a new MST radar on the North-Norwegian island Andøya (69.30° N, 16.04° E) in 2009. The new Middle Atmosphere Alomar Radar System (MAARSY) replaces the previous ALWIN radar which has been successfully operated for more than 10 years. The MAARSY radar provides increased temporal and spatial resolution combined with a flexible sequential point-to-point steering of the radar beam. To increase the spatiotemporal resolution of the observations a 16-port Butler matrix has been built and implemented to the radar. In conjunction with 64 Yagi antennas of the former ALWIN antenna array the Butler matrix simultaneously provides 16 individual beams. The beam forming capability of the Butler matrix arrangement has been verified observing the galactic cosmic radio noise of the supernova remnant Cassiopeia A. Furthermore, this multi beam configuration has been used in passive experiments to estimate the cosmic noise absorption at 53.5 MHz during events of enhanced solar and geomagnetic activity as indicators for enhanced ionization at altitudes below 90 km. These observations are well correlated with simultaneous observations of corresponding beams of the co-located imaging riometer AIRIS (69.14° N, 16.02° E) at 38.2 MHz. In addition, enhanced cosmic noise absorption goes along with enhanced electron densities at altitudes below about 90 km as observed with the co-located Saura MF radar using differential absorption and differential phase measurements.

First observations of tidal oscillations by an MF radar over Kunming (25.6°N, 103.8°E)

One year's data of an MF (medium frequency) radar installed at Kunming (25.6°N, 103.8°E) station, China, are used to study the tidal structure over Kunming. The tidal parameters including amplitudes and phases are analyzed, which show a strong diurnal tide and weak semidiurnal tide for both zonal and meridional components. The maxima diurnal and semidiurnal amplitudes are approximately 40 and 20m/s, respectively. The diurnal phases and amplitudes show strong seasonal variability. Maximum amplitudes occur in March, and subsidiary maxima are attained in August and October, while the phase indicates an annual cycle. Semidiurnal amplitudes vary not so evidently. The vertical structures of the tidal parameters are compared with the Global Scale Wave Model (GSWM-09) values. The observed amplitudes of diurnal tide are smaller than the model values, but reversed for the semidiurnal tide. The observed phases of the diurnal tide show similar variations to GSWM-09 values. The agreement between the observed semidiurnal phase and model values is not so good.

Diurnal variation of short-period (20–120 min) gravity waves in the equatorial Mesosphere and Lower Thermosphere and its relation to deep tropical convection

Abstract. We study short period gravity waves (20–120 min) in the equatorial Mesosphere and Lower Thermosphere (MLT) using a Medium Frequency (MF) radar at Pameungpeuk (7.4° S, 107.4° E), Indonesia. In particular, we study local time and seasonal variation of the gravity wave variance and its relation to tropical convection. The gravity wave variance at 88 km enhances between 20:00 LT and 07:00 LT, with a peak at 02:00–03:00 LT. The enhancement is mainly observed during February–April and September–October and shows inter-annual variability. Convective activity over the same location persists from 16:00–21:00 LT with a peak activity ~18:00 LT and enhances between November–April. Time delay between the peak of convection and that of gravity wave activity ranges 1–15 h, which is consistent with theoretical calculations and previous reports based on reverse ray tracing analysis.

Variabilities of mesospheric tides during sudden stratospheric warming events of 2006 and 2009 and their relationship with ozone and water vapour

The present study demonstrates how the relationship between the high latitude northern hemispheric major sudden stratospheric warming (SSW) events of 2006 and 2009 and low-latitude mesospheric tidal variability in zonal winds observed by the MF radar at Tirunelveli (8.7°0N, 77.8°E) exists. It is found that the ozone mixing ratio increases at low latitudes during the SSW and it could probably be due to the SSW induced reversal of meridional circulation towards southward, which may aid the transport of ozone from high to low latitudes, but prevent the same from low to high latitudes. As semi-diurnal tide is produced due to solar insolation absorption of ozone and the increase in the ozone mixing ratio could be a reason for the increase in the semi-diurnal tidal amplitude. The variabilities of diurnal tide appear to be governed mostly by variation of specific humidity at 300hPa over equator and intraseasonal variability dominates the variabilities in both the parameters.

The 10-day planetary wave examined by Odin/OSIRIS ozone profiles during late March 2002: comparison with UKMO and MF radar data

Ozone profile data have been retrieved from the Optical Spectrograph and InfraRed Imager System (OSIRIS) instrument onboard the Odin Satellite, which was launched on 20 February 2001. Considering the irregular sampling of the OSIRIS, a least-squares fit method is employed to provide the wavenumber and frequency spectra of the waves affecting the OSIRIS ozone profile distributions. Meanwhile, criteria are developed and applied for reducing spectral biases. Then spectral analysis of the global OSIRIS ozone profiles data is perfomred at altitudes of 38.5 km and 40.5 km within two latitude bins 35˚–45˚N and 45˚–55˚N during days 75–105 of the year 2002. Specifically, a westward-propagating 10-day planetary wave is detected and validated by medium frequency (MF) Radar (MFR) at Platteville (40˚N, 105˚W) and Saskatoon (52˚N, 107˚W), and UKMO global stratospheric data (UK Meteorological Office assimilated model). The spatial and temporal distributions of the global OSIRIS ozone profile data are shown to allow identification of major planetary wave features in the middle atmosphere.

First observations of Southern Hemisphere polar mesosphere winter echoes including conjugate occurrences at ∼69°S latitude

[1] We present the first account of polar mesosphere winter echoes (PMWE) observed from the Southern Hemisphere (SH), using measurements from Davis, Antarctica (68.6°S; 78.0°E). PMWE were observed by mesosphere-stratosphere-troposphere (MST) radar during solar proton events (SPEs) and auroral substorms. We supplement the MST radar measurements with profiles of temperature and turbulent velocity from co-located Rayleigh lidar and medium frequency (MF) radar, respectively, as well as temperature data from the Microwave Limb Sounder (MLS) onboard the Aura satellite. We establish that SH PMWE exhibit similar characteristics to their well-studied northern hemisphere counterpart. Significantly our observations reveal that these radar echoes in the lower mesosphere (∼50–80 km) can occur year-round, and on occasions simultaneously in both hemispheres. We report the seasonal occurrence distribution of SH PMWE is linked to the seasonal distribution of atmospheric turbulence in the lower mesosphere. We hypothesise that given sufficient turbulent velocities in the neutral atmosphere and co-located gradients in electron density, PMWE can occur throughout the year. However they are more likely in winter when turbulent velocities in the lower mesosphere maximise.

Case study of a mesospheric wall event over Ferraz station, Antarctica (62° S)

Abstract. On 16–17 July 2007 during an observational campaign at Comandante Ferraz Antarctic Station (62° S, 58° W), a mesospheric wall was observed with an airglow all-sky imager. The wave appeared like an extensive dark region in the all-sky airglow images, with a large depletion in the OH emission. Simultaneous mesospheric winds measured with a MF radar at Rothera station and temperature profiles from SABER instrument, on board of TIMED satellite, were used to obtain the propagation condition of the wave. Wind measurements during four days, around the time of observation of the wave, are presented in order to discuss the type and consistence of the duct in which this wave was propagating. By using wavelet analysis and tidal amplitude components we found that 12 and 8 h components were the most important periodicities around the time interval of the wave observation. A collocated imaging spectrometer, for mesospheric temperature measurements, has been operated simultaneously with the all-sky imager. Direct effects of the mesospheric front have been seen in the spectrometric measurements, showing an abrupt decrease in both OH intensity and rotational temperature when the wave front passes overhead. The main contribution of the present work is the investigation of the type of duct in which the wall event was propagating. We found evidences for a thermal duct structure to support the mesospheric wall propagation. This result was obtained by two types of analysis: (a) the tidal components analysis and winds filtering (harmonic analysis), and (b) comparison between the terms of the m2 dispersion relation.

On the occurrence and variability of the terdiurnal tide in the equatorial mesosphere and lower thermosphere and a comparison with the Kyushu-GCM

[1] We studied the occurrence characteristics and variability of the terdiurnal tide (8 hour period) in the equatorial mesosphere and lower thermosphere (MLT), using a meteor radar at Koto Tabang (0.2°S, 100.3°E) and MF radars at Tirunelveli (8.7°N, 77.8°E) and Pameungpeuk (7.4°S, 107.4°E). These locations, one being located right over the equator and the other two at conjugate points around the equator within ±10°, form a unique experimental setup to study equatorial MLT dynamics. The terdiurnal tide exists as a distinct wave signature at all three locations. While the daily amplitudes can be as large as 15 m s−1, the monthly mean amplitudes lie between 1 and 10 m s−1. The amplitude of the terdiurnal tide at Pameungpeuk is generally smaller than that observed at Tirunelveli and Koto Tabang. The seasonal variation in amplitude shows both annual and semiannual oscillations of ∼1 m s−1 at all three locations. The present observations combined with previous reports indicate that the timing of the primary maximum of the terdiurnal tide amplitude shifts from autumn to late spring and early summer as one moves from high latitudes to the equator (all with respect to the Northern Hemisphere). The amplitudes and seasonal variation in the present observations show good comparison with that simulated by the General Circulation Model (GCM) developed by Kyushu University, Japan. This study supports the occurrence of nonlinear interaction between diurnal and semidiurnal tides and shows that gravity waves play an important role in the generation of the terdiurnal tide.

Investigation and Application of FCA Method for Langfang MF Radar Wind Retrievals

中频雷达是获得临近空间60～100 km范围内大气风场的重要探测设备之一. 全相关分析方法是空分天线模式中频雷达的一种重要风场反演方法. 通过介绍全相关分析方法算法的一般理论, 根据廊坊中频雷达的天线配置, 分别研究了利用三副天线数据与四副天线数据的风场反演算法FCA-3A和FCA-4A, 讨论给出了全相关 分析方法的剔除标准. 用廊坊中频雷达实测的回波相位和振幅, 得到由该算法反演 的大气风场. 为验证算法的可靠性, 将其结果与ATRAD公司的反演结果进行比较. 结果显示大气纬向风和经向风随时间和高度的变化特性基本一致. FCA-3A与FCA-4A的风场反演结果基本一致, FCA-4A略好于FCA-3A.

MF Radar in Kunming and Its Preliminary Observation Results

MF雷达是中层大气风场及低电离层电子密度观测的重要手段. 简要介绍了新建昆明MF雷达设备的工作原理、结构及工作模式, 并对观测结果进行初步分析. 对2009年1月观测数据的分析表明, 该月纬向风场最大可达80 m/s, 经向风场则较小, 一般不超过40 m/s, 且二者均呈现一定波动性. 相应的LS谱及谐波拟合分析表明,周日潮汐是80~100 km高度大气风场的主要扰动成分, 其振幅随高度改变, 相位向下传播, 且周日潮汐经向分量相位超前于纬向分量相位. 此外, 分析了MF雷达白天的电子密度观测结果, 并与IRI2000进行比较, 发现两者在变化趋势上有非常好的一致性, 但雷达观测结果小于IRI2000给出的参考值.

Observations of the Ionosphere Effects of the Solar Eclipse on July 22, 2009

For the first time,Langfang Medium-Frequency(MF) radar was used to simultaneously observe the ionospheric effects due to the solar total eclipse on 22 July 2009,with three GPS/TEC receivers operated at Wuhan,Jiaxing and Langfang respectively.Results show that during the solar eclipse,the D layer electron densities at 78 km level observed by the MF radar decreased by 67%and the altitude level of equivalent electron densities of 200 cm~(-3) uprised about 10 km. The ionospheric Total Electron Content(TEC) observed by the GPS/TEC receivers in the above three sites respectively decreased about 1 TECU.Their minimum reductions of the D layer electron densities and TEC are synchronous with the solar eclipse maximum phase.After the solar eclipse, a Traveling Ionospheric Disturbance(TID) was revealed with a period of larger than 2h in the GPS/TEC data.

Airglow imaging observations of small‐scale structures driven by convective instability in the upper mesosphere over Tirunelveli (8.7°N)

Airglow imaging observations of broadband OH Meinel band emissions made on the night of 20 January 2007 from the tropical Indian site Tirunelveli (8.7°N, 77.8°E) reveal large‐ and small‐scale wave features that are aligned perpendicular to each other and that are associated with gravity wave dynamics. The large‐scale waves were the signatures of propagating quasi‐monochromatic (QM) waves, whereas the small‐scale features were identified to be ripples generated by convective instability in the mesosphere‐lower thermosphere (MLT) region. The QM waves showed consistent propagation toward the northwest direction throughout the observation period of ∼4.5 hr, whereas the ripples that manifested as short‐lived patches in the airglow images were transient and did not show any prominent phase propagation. In addition to these well‐resolved structures in images, we infer the presence of waves with relatively longer observed time periods (compared with the observed time period of QM waves) from the spectrum of the time series generated with zenith intensity values derived from the images. From the complementary wind and temperature data obtained with co‐located MF radar and Sounding of Atmosphere with Broad band Emission Radiometry on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics mission, an attempt is made to understand the dynamic processes occurring in the vicinity of the mesopause during the night of 20 January 2007. The uniqueness of the present observations is that there were short‐lived ripple patches seen in various parts of the sky during the entire observation period of 4.5 hr. The precipitation rates obtained by the Tropical Rainfall Measuring Mission satellite mission suggest that the deep convective activity occurring over the equatorial eastern Indian Ocean provided a plausible source mechanism for sustained generation of the waves observed over Tirunelveli during this night.