Simultaneous Radar Measurements Research Articles

Dark/bright band of a melting layer detected by coherent Doppler lidar and micro rain radar

Observation of a melting layer using a 1.55 µm coherent Doppler lidar (CDL) is first presented during a stratiform precipitation event. Simultaneous radar measurements are also performed by co-located 1.24 cm micro rain radar (MRR) and 10.6 cm Doppler weather radar (DWR). As a well-known bright band in radar reflectivity appears during precipitation, an interesting dark band about 160 m below that in lidar backscattering is observed. Due to the absorption effect, the backscattering from raindrops at 1.55 µm is found much weaker than that at short wavelengths usually used in direct detection lidars. However, the CDL provides additional Doppler information which is helpful for melting layer identification. For example, a spectrum bright band with broadened width and sign conversion of skewness is detected in this case. After a deep analysis of the power spectra, the aerosol and precipitation components are separated. The fall speed of hydrometeors given by CDL is found smaller than that of MRR, with the differences of approximately 0.5 m/s and 1.5 m/s for the snow and rainfall, respectively. To illustrate the influence of absorption effect, simulations of the backscatter coefficient and extinction coefficient of aerosol and rainfall are also performed at the wavelength range of 0.3 ∼ 2.2 µm using the Mie theory.

Radar Scattering from Ice Aggregates Using the Horizontally Aligned Oblate Spheroid Approximation

AbstractThe assumed relationship between ice particle mass and size is profoundly important in radar retrievals of ice clouds, but, for millimeter-wave radars, shape and preferred orientation are important as well. In this paper the authors first examine the consequences of the fact that the widely used “Brown and Francis” mass–size relationship has often been applied to maximum particle dimension observed by aircraftDmaxrather than to the mean of the particle dimensions in two orthogonal directionsDmean, which was originally used by Brown and Francis. Analysis of particle images reveals thatDmax≃ 1.25Dmean, and therefore, for clouds for which this mass–size relationship holds, the consequences are overestimates of ice water content by around 53% and of Rayleigh-scattering radar reflectivity factor by 3.7 dB. Simultaneous radar and aircraft measurements demonstrate that much better agreement in reflectivity factor is provided by using this mass–size relationship withDmean. The authors then examine the importance of particle shape and fall orientation for millimeter-wave radars. Simultaneous radar measurements and aircraft calculations of differential reflectivity and dual-wavelength ratio are presented to demonstrate that ice particles may usually be treated as horizontally aligned oblate spheroids with an axial ratio of 0.6, consistent with them being aggregates. An accurate formula is presented for the backscatter cross section apparent to a vertically pointing millimeter-wave radar on the basis of a modified version of Rayleigh–Gans theory. It is then shown that the consequence of treating ice particles as Mie-scattering spheres is to substantially underestimate millimeter-wave reflectivity factor when millimeter-sized particles are present, which can lead to retrieved ice water content being overestimated by a factor of 4.

Plasma irregularities adjacent to auroral patches in the postmidnight sector

We demonstrate a close association between decameter‐scale plasma irregularities in the E region ionosphere and auroral patches in the postmidnight sector. In September 2009, campaign‐based measurements of the aurora were conducted in Iceland with a white light all‐sky camera (ASC) at Tjörnes (66.20°N, 17.12°W) and the SuperDARN radar at þykkvibaer (63.77°N, 20.54°W). On one night during the campaign period, the ASC observed the successive passage of auroral patches in the postmidnight sector after a small substorm‐like activity. The patches were drifting predominantly eastward across the field‐of‐view of the ASC with a speed of approximately 360–450 m s−1, which is consistent with the sunward convection in the postmidnight westward electrojet. The simultaneous radar measurements recorded strong radar backscatter echoes (>15 dB) within the gaps between adjacent auroral patches, while such echoes were not observed or were very weak in the region of the aurora. The Doppler velocity estimation showed that the electric field was clearly reduced within the patches, which was probably the result of the enhanced conductance associated with auroral precipitation. Thus, this reduction in the electric field suppressed the generation of irregularities (i.e., radar echoes) in the regions of auroral patches. This suggests that the conductance enhancement associated with precipitating electrons not only modified the electric field within the aurora but also affected the generation of small‐scale plasma structures in the vicinity of the patch‐type optical auroral forms.

Estimation of the Turbulent Fraction in the Free Atmosphere from MST Radar Measurements

Abstract Small-scale turbulence in the free atmosphere is known to be intermittent in space and time. The turbulence fraction of the atmosphere is a key parameter in order to evaluate the transport properties of small-scale motions and to interpret clear-air radar measurements as well. Mesosphere–stratosphere–troposphere (MST)/stratosphere–troposphere (ST) radars provide two independent methods for the estimation of energetic parameters of turbulence. First, the Doppler spectral width σ2 is related to the dissipation rate of kinetic energy εk. Second, the radar reflectivity, or C2n, relates to the dissipation rate of available potential energy εp. However, these two measures yield estimates that differ with respect to an important point. The Doppler width measurements, and related εk, are reflectivity-weighted averages. On the other hand, the reflectivity estimate is a volume-averaged quantity. The values of εp depend on both the turbulence intensity and the turbulent fraction within the radar sampling volume. Now, the two dissipation rates εp and εk are related quantities as shown by various measurements within stratified fluids (atmosphere, ocean, lakes, or laboratory). Therefore, by assuming a “canonical” value for the ratio of dissipation rates, an indirect method is proposed to infer the turbulent fraction from simultaneous radar measurements of reflectivity and Doppler broadening within a sampling volume. This method is checked by using very high resolution radar measurements (30 m and 51 s), obtained by the PROUST radar during a field campaign. The method is found to provide an unbiased estimation of the turbulent fraction, within a factor of 2 or less.

Evidence for direct solar control of the mesopause dynamics through dayglow and radar measurements

Abstract. The day-to-day measurements of the daytime intensities of hydroxyl (OH) Meinel (8-3) band airglow emissions at 731.6 and 740.2nm carried out from the equatorial station Thiruvananthapuram (8.5° N, 76.5° E, 0.5° dip) during the period of January-March 2001 have been investigated. This investigation provides evidence for the presence of a long period (≈16 days) wave modulating these intensities at the mesopause altitudes. Simultaneous radar measurements of zonal wind at ~87km, i.e. mesopause from Tirunelveli (8.7° N, 77.8° E, 0.33° dip), a location nearby, also reveal the presence of these long period oscillations. The daytime airglow and zonal wind undergo changes simultaneously. Similar modulations are seen in the solar 10.7cm flux also preceding dayglow and wind variabilities by 4-5 days. It is inferred in the present case that the changes in the solar flux are the cause of the generation of this long period wave in the atmosphere below the mesosphere. The oscillations in the measured dayglow intensities in the mesopause region and the winds at ~87km are resulting from the modulation caused by this wave in this region after a delay of 4-5 days.

Seasonal variation of crosspolarisation statistics observed at CS-2 experimental Earth station

Crosspolarisation statistics obtained by the CS-2 beacon signal (19.45 GHz) are compared with the CCIR (1986) prediction. The observation shows a notable seasonal variation, although the overall long-term statistics agree well with the prediction. Simultaneous radar measurements suggest that as bright band heights become lower, effects of ice are increased in the statistics.

Simultaneous Hα line profile and radar measurements at Arecibo

High resolution measurements of the geocoronal Balmer alpha (Hα) emission were made with a Fabry‐Perot interferometer at Arecibo, Puerto Rico, in six observing campaigns between March 1980 and November 1984. Four of these included simultaneous radar measurements of the topside F region. The nighttime variation of the zenith Hα intensity is highly symmetric about solar midnight. However, effective hydrogen temperatures near the morning exobase are generally hotter than near the evening exobase, implying a depletion of exobase hydrogen density in the early morning hours. The exospheric temperature is derived from the Doppler line profiles and is compared to the topside F region O+ temperature. The cooling of H relative to O+ due to the escape of hot, energetic H is not obvious in our data, and it appears that a downward flux of hot protons often causes heating of the bound neutral H population by charge exchange during low to moderate solar activity conditions. This downward flux is usually strongest between 0200 and 0600 LT, and occurs throughout the night in the late fall and early winter. We have detected a deficiency of inbound high‐energy hydrogen relative to the high‐energy outbound population in radial velocity Hα profiles. We have also detected large distortions of the Hα profile wings during low to moderate solar activity periods and during periods of large downward ion speeds. We attribute these wing distortions to efficient charge exchange of H with H+. A narrowing of zenith profile widths near midnight demonstrates the gravitational cooling of H due to the inverse relationship between geocentric distance and the kinetic energy required for escape. We have also searched for evidence that satellite hydrogen populations with small orbital eccentricities may be contributing to narrow line widths.

A case study of plasma processes in the dayside cleft

On November 28, 1983, between 1638 UT and 1642 UT, the HILAT satellite moving approximately along the noon meridian passed over northeastern Canada and encountered the dayside high latitude cleft. During the passage an upward current sheet bounded to the north and south by weaker downward current sheets was observed. The current sheets, particularly the upward current sheet, were immersed in a region of fairly intense low energy electron precipitation of the type commonly observed in the polar cleft. Simultaneous radar measurements were made in a region slightly to the east of the satellite track and in which beacon measurements between HILAT and a ground station at Sondre Stromfjord, Greenland, were being conducted. The radar indicated an extended, magnetic east‐west‐aligned region of small‐scale irregularities at the same magnetic latitude as the cleft‐related current sheets and precipitation. The HILAT beacon experiment showed the strongest scintillations within the same region. On the basis of the available data, it appears as if the convective flows from the dawn and dusk sectors were converging and rotating poleward within the observational volume. The irregularities producing the scintillations and radar backscatter may have been produced by the combined influence of the gradient drift and current convective plasma instabilities, the former process being excited by the poleward convection of plasma parallel to the precipitation‐induced density gradient and the latter process by the upward field‐aligned current. The apparent absence of irregularities further into the polar cap may be accounted for by the stabilizing effect of plasma drift antiparallel to the density gradient on the poleward edge of the cleft precipitation zone.