C. J. Pan

Quasi‐periodic echoes observed with the Chung‐Li VHF Radar during the SEEK Campaign

The 52-MHz Chung-Li VHF radar, located in Taiwan, was operated during the SEEK (Sporadic-E Experiment over Kyushu) campaign to investigate the so-called quasi-periodic (QP) echoes that occur from nighttime sporadic E (Es) layers. Highlights of this study are presented in this paper, using measurements made on August 17, 1996, to illustrate the echo characteristics. QP echoes displaying 5 to 10 minute periods were observed throughout this night. A new finding is the occurrence of QP echoes whose striations were almost vertically oriented when plotted using a height-time-intensity (HTI) format. This observation provides the first unambiguous evidence for the existence of steep spatial tilts in Es layers that are responsible for QP echoes. Other characteristics from three periods during this night are described including striations with positively sloped striations. These characteristics are discussed in light of existing models for QP echoes.

New observational techniques for studying the dynamics of the middle atmosphere using the Chung Li VHF radar

Abstract Taking advantage of the newly developed volume scattering model for MST radar and the unique features of the Chung Li VHF radar, several novel observational techniques have been developed and implemented. Techniques such as oblique spaced antenna (OSA), the frequency domain analysis of spaced antenna data, the full spectrum analysis (FSA) and the multifrequency frequency domain interferometer (FDI) will be discussed and experimental results will be presented. Potential applications of the new techniques to study the dynamics of the middle atmosphere will be discussed.

Observations of QP radar echo structure consistent with neutral wind shear control of the initiation mechanism

The semidiumal variations in quasi-periodic echo structure parameters associated with midlatitude sporadic E layers observed with the Chung-Li VHF radar located in Taiwan (24.9 N, 121.2 E; 52 MHz) are presented. The results show that the periodicity decreases with increasing range rate and that the spread of Doppler velocities increases as the altitude coverage of the echoes increases. These characteristics are shown to be consistent with the variations to be expected if the shears in the neutral wind and the variations in the background mean flow are the dominant mechanisms controlling the initiation of the echo structures.

Semidiurnal behavior of quasi‐periodic echoes in the mid‐latitude Es region observed with the Chung‐Li VHF Radar

The range rate of the so-called quasi-periodic (QP) echoes from nighttime mid-latitude sporadic-E regions observed with the Chung-Li VHF radar during the SEEK (Sporadic-E Experiment over Kyushu) campaign displayed semidiurnal characteristics. The time periods before ∼2200 LT and after ∼0400 LT were dominated by the negative range rate QP (NQP) echoes. In contrast, a preponderance of positive range rate QP (PQP) echoes occurred between 2200 and 0400 LT. The slopes of NQP echoes found during the evening period steepened with time and became positive around 2200 LT. Similar behavior was also found prior to ∼0400 LT when PQP echoes were replaced by NQP echoes during the SEEK campaign. The sign of the range rate changed around 2200 and 0400 LT on the six consecutive nights. Semidiurnal neutral-wind variations are proposed to explain the slope sign reversals in the observations. These results, together with previous findings, suggest that semidiurnal tides and long-period waves (with 30 minute periodicity) may be playing a role, in addition to atmospheric gravity waves, in the formation of QP echoes.

E region observations over Chung‐Li during the SEEK Campaign

During the Sporadic E Experiment over Kyushu (SEEK) campaign E-region field aligned irregularities (FAI) were observed with the Chung-Li VHF Radar (operated on 52 MHz) in Taiwan between August 15-22, 1996. The characteristics of the quasi-periodical echoes from the E-region about 80 km north of the Chung-Li radar are studied. During the same period ionogram records were taken with the Digisonde located at the Chung-Li radar site. Strong sporadic-E layers were detected with the Digisonde when the FAI, observed with the VHF radar, were also very intense. The variations of sporadic E-layer critical frequency and of the field-aligned irregularities were studied. The latter were estimated to be located at altitudes 95-115 km occurring in layers of 5-15 km thickness, moving downwards at a rate of about 2.5 km/h. There seemed to be a change over of the FAI echoes from lower to upper E-region every night approximately at 23 hour LT. The distinction between post-sunrise and post-sunset FAI echoes observed earlier by MU radar was not clearly seen in Chung-Li. A preliminary examination of the morphology of the fine structures within the layers indicates quasi-periodic features with 5-10 minutes period. Many of these periodic fine structures were observed at the Chung-Li VHF radar to have opposite slopes than those detected earlier with the MU radar in Japan, which is located 1500 km north of the Chung-Li VHF radar.

Doppler velocities obtained by the EISCAT VHF radar and the dynasonde during the PMSE95 campaign

Abstract In this paper, we examine the Doppler velocities simultaneously derived from the EISCAT VHF radar and the dynasonde, under the PMSE conditions. Spectral and filtering analyses are applied to study the velocities in detail. It is found that the high- and the low-frequency motions derived from the two radars correlate differently, high for low-frequency and low for high-frequency component. The agreement/discrepancy between the two velocities are interpreted in terms of radar scattering/operation principles and characteristics of acoustic gravity waves.

VHF radar and MF/HF dynasonde observations during Polar Mesosphere Summer Echoes conditions at EISCAT

The EISCAT VHF radar (224 MHz) and the EISCAT dynasonde (1–10 MHz) were operated simultaneously during a Polar Mesosphere Summer Echoes (PMSE) event on 22 June 1994. We investigate the echo characteristics revealed by these two instruments. The antenna of the VHF radar was set 10° off-zenith to the north for both transmission and receiving, while the dynasonde was operated with vertical beam to record ionograms every 3 minutes. The Height-Time-Intensity (HTI) variation of the VHF radar PMSE is studied, which is characterized layered structures. The intensity profiles are quite intermittent which show similarity to those data recorded by the dynasonde. To study the ionospheric condition with the dynasonde, the quantities of signal-to-noise ratio, echo frequency, echo polarization, echolocation, and Doppler velocity are investigated. Variations in the quantities demonstrate that during the PMSE conditions, echoes of the dynasonde are mainly attributed by reflection of many small irregularities within the observed ionospheric volume.

THE MEAN DOPPLER VELOCITY DERIVED BY AN ADVANCED IONOSPHERIC SOUNDER - DYNASONDE

The lower ionosphere was examined by using a fixed sounding frequency 7.953 MHz of the EISCAT dynasonde. The phase parameter and Doppler spectrum techniques were applied on the echoes to obtain radial Doppler velocities. It is found that the two derived velocities are very different. A procedure is developed and introduced to apply on the Doppler velocities obtained from the phase parameter technique. Result shows that mean Doppler velocities derived by the weighted mean procedure are close to those of the typical spectrum technique.

Observation of Precipitation and Drop-Size Distribution Associated with a Typhoon using VHF Radar

This paper describes some of the microphysical and kinematic properties of precipitating systems associated with a typhoon using Chug-Li VHF radar. In order to gain a better understanding of these mechanisms and the vertical structure of the precipitation associated with a typhoon at different stages of development, an analysis has been carried out of the radar back-scattered signal in order to obtain the power, velocity and velocity width of the Doppler spectrum of clear air and hydrometeors. The vertical profiles of raindrop size distribution (DSD) parameters are estimated through model-based regression analysis. The study reveals that during a typhoon, different convective and stratiform types of precipitation occur at different times with varying intensities. This study also reports on some of the characteristic features of the convective systems observed during the typhoon.