Yasuyuki Maekawa

Internal Inertia–Gravity Waves in the Tropical Lower Stratosphere Observed by the Arecibo Radar

Abstract High-resolution upper tropospheric and lower stratospheric (5–30 km) wind data were obtained during three periods from 1979 to 1981 with the aid of the high-power UHF radar at Arecibo, Puerto Rico (18.4°N, 66.8°W). A quasi-periodic wind oscillation with an apparent period of 20–50 h was observed between 16 and 20 km in every experiment. The amplitude of both zonal and meridional wind components was ∼2 m s−1, and the vertical wavelength ∼2 km. The direction of the wind associated with this oscillation rotated clockwise with time, as seen for inertia–gravity waves in the Northern Hemisphere. The wave disappeared near 20 km where the mean zonal flow had easterly shear with height. This phenomenon is discussed in terms of wave absorption at a critical level. It is suggested that the, wave had a westward horizontal phase speed of 10–20 m s−1. The intrinsic period and the horizontal wavelength at the wave-generated height are inferred to be 20–30 h and ∼2000 km, respectively, from the relationship base...

Inertio-gravity waves and subtropical multiple tropopauses: Vertical wavenumber spectra of wind and temperature observed by the MU radar, radiosondes and operational rawinsonde network

Abstract We have carried out continuous observations of the tropopause region over Japan for three weeks during the Baiu (early summer rain “in Japan”) season in 1991, by using a VHF Doppler radar (the MU radar), radiosondes launched at the radar site and operational rawinsondes at five meteorological stations. Based on these observations, we try to examine the hypothesis that the multiple tropopauses and the dominant inertio-gravity waves are one and the same feature, and obtain some interesting results that are not inconsistent with this hypothesis. First, vertical wavenumber spectra and hodographs analyzed from the radar wind data in the tropopause region suggest that inertio-gravity waves with vertical wavelengths of ∼ 2 km are quasi-monochromatically dominant (with 2–3 day scale variabilities of 10–20%), and are in accordance with activities of the subtropical jet stream and mesoscale cyclone-front system activities observed by the operational network. Second, striking (potential) temperature fluctuations are detected simultaneously by the radiosondes and rawinsondes, which appear as multiple tropopauses in meridional cross-section analysis. Third, vertical wavenumber spectra analyzed from the radiosonde temperature data are consistent with the radar wind spectrum, if we assume that both wind and temperature fluctuations are mainly induced by the dominant inertio-gravity waves. Finally, we confirm that the dominant interio-gravity waves can be barely detected also from routine rawinsonde (1.5-km running-mean) wind data if the amplitude is larger than 1.5 m/s. However, the monochromatic wave structures are generally quite localized in space and time.

Ku-band long distance site-diversity (SD) Characteristics using new measuring system

This paper deals with the short (10 km) and long distance (300-1400 km) site-diversity (SD) characteristics by using a newly developed measuring system. In the proposed measuring system, six earth stations transmit 14-GHz band QPSK signals, and one measuring earth station receives 12-GHz band signals and processes them to determine SD characteristics. As a result, easy operation and maintenance, low-cost measuring system construction and highly accurate data have been obtained. By comparing those measured results with the SD joint probability approximation equation in ITU-R Rec.P.618-7, a good agreement can be obtained. Furthermore, the effect of typhoons on SD characteristics were measured.

Dual polarization radar observation of thunderclouds on the coast of the Sea of Japan in the winter season

A C-band dual polarization radar has observed wintertime thunderclouds peculiar to the coast area of the Sea of Japan. At the moment of lightning, graupel identified by low Z{sub DR} and high Z{sub H} is seen to be rapidly lifted up to 3 km height, corresponding to the {minus}10C level. In addition, the graupel encounters small ice crystals with high Z{sub DR} and low Z{sub H} above this height. This situation satisfies the recent thunderstorm electrification theory. The PPI radar display reveals a band-like structure for this type of wintertime thunderclouds. In this structure, the small ice crystals are primarily seen on the windward side of the clouds. Part of the ice crystals mixed with the graupel seems to be brought from the back of an adjacent cloud, which precedes the thundercloud carrying the graupel.

FIRST OBSERVATION OF THE UPPER STRATOSPHERIC VERTICAL WIND VELOCITIES USING THE JICAMARCA VHF RADAR

The Jicamarca VHF radar (50 MHz) has detected atmospheric echoes in the so-called ”gap region” from 30 to 60 km heights. These echoes are, for the first time, discriminated from clutter echoes, using both co-polarized (co-pol) and cross-polarized (x-pol) arrays to monitor the clutter component which may enter the antenna sidelobes. The atmospheric scatterings in this region are shown to be composed of refractivity layers with thickness as of 1–2 km as in other middle atmospheric height ranges, suggesting that these scatterings are caused by thin turbulent layers. The height profile of inferred vertical wind velocities indicates a wavy structure. The wave amplitude, as a whole, increases with height, suggesting the activity of upward-propagating gravity waves.

Dual polarization radar observations of anomalous wintertime thunderclouds in Japan

Dual-polarization radar observations of wintertime thunderclouds for the Sea of Japan are presented. The range-height-indicator (RHI) and plan-position-indicator (PPI) scans, respectively, reveal the height and horizontal distributions of ice particles, such as graupel and ice crystals. The overall shape of these ice particles is also confirmed on the ground. The ice crystals are, in general, found at high altitude near the cloud top, whereas the graupel is primarily seen near the center of clouds. The PPI display indicates a bandlike horizontal structure, and the lightning tends to occur around the bandlike gap where the ice crystals are, in advance, accumulated on the windward side of the preceding cloud. Simultaneous field mill observations indicate electric charge separation between these ice particles precipitating from the thunderclouds. >

Long distance site-diversity (SD) characteristics by using new measuring system

This paper deals with the short and long distance (10-1400 km) site-diversity (SD) characteristics by using a newly developed measuring system. In the proposed measuring method six Earth-stations transmit QPSK signals and one measuring Earth-station receives signals and processes them for site-diversity characteristics. As a result, easy operation and maintenance, low cost measuring system construction and high accuracy data can be obtained. By comparing these measured results with the site-diversity joint probability approximation equation in CCIR Rep. 564-4, relatively good agreement can be found.

Distribution of ice particles in wintertime thunderclouds detected by a C band dual polarization radar: A case study

Distributions of radar echo intensities (ZH) and differential reflectivity factors (ZDR) for ice particles in Japans wintertime thunderclouds are investigated using C band dual polarization radar observations conducted on January 12, 1990, as a case study. At the moment of lightning strokes, the percentage of low-ZDR data points is increased around the cloud center, while that of high-ZDR data points is increased near the cloud edge. These radar echoes indicate the backscattering properties from graupel particles and ice crystals, both of which play a critical role in thunderstorm electrification. The ice crystal echoes are primarily enhanced 5–15 min before the lightning stroke. At the moment of lightning stroke; however, they are slightly decreased, and the graupel echoes gradually increase the intensity. These characteristics are well described by the convective motion of the dominant ice particles in this observational case. In addition, graupel echoes of more than 40 dBZ are found for these thunderclouds on January 12, whereas the clouds which produced no lightning, but indicated the evidence of comparable electrification, did not show more than 40 dBZ on January 18, 1990. The lightning locations are seen along the edge of the cloud center with ZH ∼ 30 dBZ, where graupel and ice crystal echoes distribute at short range. Their distributions are then estimated by counting both low- and high-ZDR data points on the constant altitude plan position indicator display, combined with their large and small echo intensities, respectively. Using polarimetric information (ZDR, ZH), the product numbers of graupel and ice crystal echoes in each 5 × 5 km area are found to be fairly well correlated with the lightning location. These results are thus obtained for the one case of the present thunderstorm event, but they may give some insights on evolution of thunderclouds and lightning discharges.

Some further results on long term mesospheric and lower thermospheric wind observations by the Arecibo UHF radar

Abstract A second series of long term mesospheric and lower thermospheric wind observations was conducted at Arecibo (18.4°N, 66.8°W) between 6 and 20 March 1981 using the UHF Doppler radar, following the first observations in August 1980 ( Hirota et al., 1983). Zonal and meridional wind velocities were measured during the morning (8–10 LT) and afternoon (13–15 LT) periods. The mean wind profile averaged over the entire observational period shows the predominance of the diurnal tide. The fluctuating wind vector rotates clockwise relative to height with a characteristic vertical scale of about 10 km. The phase difference inferred by a cross correlation analysis between morning and afternoon profiles indicates that the dominant period is about 20–30 h. This oscillation is discussed in relation to internal inertia-gravity waves observed by the same radar in the lower stratosphere. On the other hand, wind fluctuation with a vertical scale larger than 20 km shows a substantial day-to-day variation with a period of 5–8 days. This long period oscillation shows a good correlation with the global scale geopotential height anomalies at 1 mb (46–48 km) observed by the Tiros-N satellite at 20°N. Our evidence suggests that westward travelling planetary-scale waves with zonal wavenumber one may propagate up to the lower thermosphere.

Vertical propagation characteristics of internal gravity waves around the mesopause observed by the Arecibo UHF radar

Abstract A high resolution wind observation of the mesosphere and lower thermosphere (73–95 km) was conducted with the aid of the high power UHF Doppler radar at Arecibo (18.4°N, 66.8°W). Zonal wind velocities were continuously observed during day-time hours on 1–15 August 1980. We discuss here the observed wind fluctuations with periods of 1–4 h in the light of internal gravity waves. The phase propagation associated with these fluctuations is, on average, shown to be downward, indicating an upward energy flux. A space-time spectral analysis shows that waves with vertical wavelengths shorter than 10 km disappear around the mesopause (about 85km), while those with longer vertical wavelengths exist throughout the observational height. This result is explained in terms of wave absorption at a critical layer where the mean zonal wind has a westerly shear with height. This feature is consistent with the behavior expected for internal gravity waves around the summer mesopause in order to explain general circulation models.