Ching-Lun Su

Morphology of sporadic E layer retrieved from COSMIC GPS radio occultation measurements: Wind shear theory examination

On the basis of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)-measured fluctuations in the signal-to-noise ratio and excess phase of the GPS signal piercing through ionospheric sporadic E (Es) layers, the general morphologies of these layers are presented for the period from July 2006 to May 2011. It is found that the latitudinal variation in the Es layer occurrence is substantially geomagnetically controlled, most frequent in the summer hemisphere within the geomagnetic latitude region between 10° and 70° and very rare in the geomagnetic equatorial zone. Model simulations show that the summer maximum (winter minimum) in the Es layer occurrence is very likely attributed to the convergence of the Fe+ concentration flux driven by the neutral wind. In addition to seasonal and spatial distributions, the height-time variations in the Es layer occurrence in the midlatitude (>30°) region in summer and spring are primarily dominated by the semidiurnal tides, which start to appear at local time around 6 and 18-h in the height range 110-120-km and gradually descend at a rate of about 0.9-1.6-km/h. In the low-latitude (<30°) region, the diurnal tide dominates. The Horizontal Wind Model (HWM07) indicates that the height-time distribution of Es layers at middle latitude (30°-60°) is highly coincident with the zonal neutral wind shear. However, Es layer occurrences in low-latitude and equatorial regions do not correlate well with the zonal wind shear. Key Points Examination of Es layer summer maximum phenomenon Global distribution of COSMIC-retrieved Es layer Es layer formation and wind shear mechanism.

First measurements of neutral wind and turbulence in the mesosphere and lower thermosphere over Taiwan with a chemical release experiment

[1] Neutral winds and turbulence structure in the mesosphere and lower thermosphere were measured with a sounding rocket chemical release experiment carried out in Taiwan. Trimethyl aluminum (TMA) was used as a tracer of the drift of the background atmosphere. The results show that the measured neutral wind had a maximum speed of 154 m/s at a height around 105 km. The wind vectors were found to rotate clockwise with height in the altitude range from about 98 to 121 km. On the basis of the hodograph analysis of the measured neutral wind vectors, an upward propagating inertio-gravity wave with intrinsic period of 11.2 hours and vertical wavelength of 19.5 km was believed to be primarily responsible for the height variations of the neutral wind and the Richardson number. Large vertical wind shears were observed near 100 km and 106 km. Turbulent structures were observed in much of the trail below 110 km, but there were enhanced structures in the altitude range between the two large shears, i.e., at and near the altitude of the maximum wind speed. Comparing the positions of the turbulent features with expected atmospheric stability zones induced by an upward propagating gravity wave indicates that the turbulence structures were primarily located within the wave-induced convectively unstable zone. The structures are therefore interpreted as counterrotating vortices within the convective instability zone of a breaking gravity wave. Moreover, the relation between the horizontal separations l (about 1.8 km) of the turbulent structures and the vertical extent h (about 0.7 and 1.5 km) of the wind shear zones with Richardson numbers less than 0.25 did not conform to the predicted l/h ratio of a factor of approximately 8 predicted by simple linear Kelvin-Helmholtz instability theory for the primary billow structures associated with the instability. These results suggest that the observed structures were not the primary billows but were more likely associated with a secondary instability, such as the counterrotating vortices that develop later in the evolution of the instability. In general, the observations reported here support the interpretation that the turbulence evident in the trail was very likely generated in the convectively unstable zone induced by the inertio-gravity wave propagating through the region with large temperature gradients � 32 K/km produced by wave breaking processes.

An Investigation of the Slope–Shape Relation for Gamma Raindrop Size Distribution

Abstract The gamma drop size distribution (DSD) has been widely used in the meteorological community for years to model observed DSD. It has been found that the relation between the slope (Λ) and shape (μ) parameters of the gamma DSD can be empirically described by a polynomial of second degree. In this article, on the basis of disdrometer-measured DSDs from seven independent precipitation events associated with different weather systems, an empirical μ–Λ relation that is slightly different from those reported by other scientists is obtained by best fitting a quadratic polynomial to observed data. In addition to the empirical relation, a μ–Λ relation is derived based on theoretical relations between gamma DSD moments and Λ and μ. It is shown that the derived μ–Λ relation is independent of the order of the moment of the gamma DSD. The key factor dominating the μ–Λ relation is the ratio of the number density parameter N(Dm) to total number density of the raindrop M0, where Dm is the mean diameter of the DSD...

Reply to comment by Lei et al. on A new aspect of ionospheric E region electron density morphology

[1] Since the FORMOSAT‐3/COSMIC satellites were launched in April 2006, ionospheric electron density profiles have been retrieved from the excess phase of the GPS signal by using the radio occultation technique and can be accessed from the Web site http://www.cosmic.ucar.edu/. On the basis of these electron density profiles and the use of data quality control criteria developed by Yang et al. [2009], Chu et al. [2009] investigated E region electron density morphology and showed that the general properties of the COSMIC‐ retrieved E region electron density are in good agreement with the predictions of the Chapman layer theory that was developed in accordance with photochemical process and controlled by solar zenith angle. Nevertheless, Chu et al. [2009] found existences of salient enhancements in the noontime E region electron density not only at the geomagnetic equator but also in the geomagnetic latitude regions ±15°−35°, which cannot be explained by the Chapman layer theory. In addition, they also provided compelling evidence to show the presence of longitudinal wave number 3 and 4 structures of the equatorial electron density in a height range of 100–200 km, which is in excellent agreement with longitudinal structures of equatorial electrojet intensity derived from equatorial magnetic field data obtained by the Orsted, CHAMP, and SAC‐C satellites during the years 1999–2006. [2] On the basis of the simulation result obtained by Yue et al. [2010], Lei et al. [2010] question the validity of the E region electron density retrieved by the GPS radio occultation technique. They argue that because of the presence of the ionospheric electron density gradient in the horizontal direction that violates the spherical symmetry assumption of the Abel transform for inverting ionospheric electron density profile from calibrated total electron content along the GPS raypath, the COSMIC‐measured E region electron density enhancements in midlatitude regions were caused by the retrieval error of the GPS radio occultation process. From Figure 1 of Lei et al. [2010] (identical to Figure 2 of Yue et al. [2010]), the simulation‐retrieved E region electron densities around 100 km in equinox season are approximately 50– 100% and 150–200% greater than the “true” model values at the geomagnetic equator and in geomagnetic latitude regions ±30°−50°, respectively. In addition, the former are about 150–200% smaller than the latter in geomagnetic latitude regions ±10°−30°. If the simulation‐retrieved results obtained by Yue et al. [2010] were true and able to be representative of the general GPS occultation‐retrieved results, the morphologies of the COSMIC‐measured E region electron density should be in accord with those of the simulation results. Namely, the E region electron density retrieved by COSMIC satellites should be much greater (smaller) than true measurement made by the ground‐based ionosonde in geomagnetic latitude regions ±30°–50° (±10°–30°). In order to validate the simulation‐retrieved results, we compare peak values of E layer electron density NmE between COSMIC retrieval and global ionosonde measurement in the different latitudinal regions for July 2006 to July 2009. The COSMIC data were selected for comparison if the separation between COSMIC occultation point and ionosonde station is 10 min in time and 2.5° in space. Figure 1 presents an example of latitudinal variation in histograms of percent errors of NmE between COSMIC retrieval and ionosonde measurement for spring (March–May) 2007–2009, in which the percent error (PE) is defined by

Extended Application of a Novel Phase Calibration Approach of Multiple-Frequency Range Imaging to the Chung-Li and MU VHF Radars

Multiple-frequency range imaging (RIM), designed to improve the range resolution of radar echo distribution, is now available for the recently upgraded Chung-Li VHF radar (24.98N, 121.18E). To complete the RIM technique of this radar, a novel phase calibration approach, proposed initially for the Ostsee Wind (OSWIN)VHFradar,wasemployedtoexaminetheeffectsofphasebiasandtherange-weighting functionon the received radar echoes. The estimated phase bias indicated a time delay of ;1.83 ms for the signal in the radar system. In contrast, such a time delay is more difficult to determine from the phase distribution of twofrequency cross-correlation functions. The same calibration approach was also applied successfully to the middle and upper atmosphere (MU) radar (34.858N, 136.118E) and revealed a time delay of ;0.33 ms for the radar parameters employed. These calibration results for various radars demonstrate the general usability of the proposed calibration approach. With the high-resolution performance of RIM, some small-scale Kelvin‐ Helmholtz (KH) billows, double-layer structures, and plumelike structures in the troposphere that cannot be seen in height‐time intensity plots have been recognized in present observations. The billows and double layerswerefoundto beclosely relatedtostrongverticalwindshearandsmallRichardsonnumber,supporting the hypothesis of a dynamic process of KH instability. On the other hand, the plumelike structures were observed to grow out of a wavy layer and could be attributed to saturation and breaking of gravity waves. These fine structures have shown some remarkable features resolved by the RIM method applied to VHF radars in the lower atmosphere.

Improvement of GPS radio occultation retrieval error of E region electron density: COSMIC measurement and IRI model simulation

In this article, we use the International Reference Ionosphere (IRI) model to simulate temporal and spatial distributions of global E region electron densities retrieved by the FORMOSAT-3/COSMIC satellites by means of GPS radio occultation (RO) technique. Despite regional discrepancies in the magnitudes of the E region electron density, the IRI model simulations can, on the whole, describe the COSMIC measurements in quality and quantity. On the basis of global ionosonde network and the IRI model, the retrieval errors of the global COSMIC-measured E region peak electron density (NmE) from July 2006 to July 2011 are examined and simulated. The COSMIC measurement and the IRI model simulation both reveal that the magnitudes of the percentage error (PE) and root mean-square-error (RMSE) of the relative RO retrieval errors of the NmE values are dependent on local time (LT) and geomagnetic latitude, with minimum in the early morning and at high latitudes and maximum in the afternoon and at middle latitudes. In addition, the seasonal variation of PE and RMSE values seems to be latitude dependent. After removing the IRI model-simulated GPS RO retrieval errors from the original COSMIC measurements, the average values of the annual and monthly mean percentage errors of the RO retrieval errors of the COSMIC-measured E region electron density are, respectively, substantially reduced by a factor of about 2.95 and 3.35, and the corresponding root-mean-square errors show averaged decreases of 15.6% and 15.4%, respectively. It is found that, with this process, the largest reduction in the PE and RMSE of the COSMIC-measured NmE occurs at the equatorial anomaly latitudes 10°N–30°N in the afternoon from 14 to 18 LT, with a factor of 25 and 2, respectively. Statistics show that the residual errors that remained in the corrected COSMIC-measured NmE vary in a range of −20% to 38%, which are comparable to or larger than the percentage errors of the IRI-predicted NmE fluctuating in a range of −6.5% to 20%.

A Study on the Relation between Terminal Velocity and VHF Backscatter from Precipitation Particles Using the Chung-Li VHF Radar

Abstract The relationships between the mean Doppler terminal velocities VT of the hydrometeors with different phases and the range-corrected VHF backscatter P from the corresponding precipitation particles are investigated by using the Chung-Li VHF radar. The radar precipitation data employed for the analysis were taken from four independent experiments conducted on different weather conditions. They show that the observed α and β values in the power-law approximation VT = αPβ above the melting layer are generally smaller than those below the layer, while in the bright band the values of β (α) are enormously smaller (greater) than those above and below the bright band. Theoretical analysis shows that the mathematical relationship between α and β can be approximated very well by a simple exponential function, which is in excellent agreement with the observations. A new method for estimating the coefficient A and exponent B in the fall speed–diameter relationship V(D) = ADB with respect to still air on the ...

Coordinated observations of F region 3 m field-aligned plasma irregularities associated with medium-scale traveling ionospheric disturbances

Three meter field-aligned irregularities (3 m FAIs) associated with medium-scale traveling ionospheric disturbances (MSTIDs) that occurred on 5 February 2008 were observed by using the Chung-Li 52 MHz coherent scatter radar. Interferometry measurements show that the plasma structures responsible for the 3 m FAI echoes are in a clumpy shape with a horizontal dimension of about 10–78 km in a height range of 220–300 km. In order to investigate the dynamic behaviors of the plasma irregularities at different scales in the bottomside of F region, the VHF radar echo structures from the 3 m FAIs combined with the 630 nm airglow images provided by the Yonaguni all-sky imager are compared and analyzed. The results show that the radar echoes were located at the west edge of the depletion zones of the 630 nm airglow image of the MSTIDs. The bulk echo structures of the 3 m FAIs drifted eastward at a mean trace velocity of about 30 m/s that is in general agreement with the zonal trace velocity of the MSTIDs shown in the 630 nm airglow images. These results suggest that the observed F region 3 m FAIs for the present case can be regarded as the targets that are frozen in the local region of the MSTIDs. In addition, the radar-observed 3 m FAI echo intensity and spectral width bear high correlations to the percentage variations of the 630 nm emission intensity. These results seem to suggest that through the nonlinear turbulence cascade process, the MSTID-associated 3 m FAIs are very likely generated from the kilometer-scale plasma irregularities with large amplitude excited by the gradient drift instability.

Measurement of Range-Weighting Function for Range Imaging of VHF Atmospheric Radars Using Range Oversampling

Multifrequency range imaging (RIM) used with the atmospheric radars at ultra- and very high-frequency (VHF) bands is capable of retrieving the power distribution of the backscattered radar echoes in the range direction, with some inversion algorithms such as the Capon method. The retrieved power distribution, however, is weighted by the range-weighting function (RWF). Modification of the retrieved power distribution with a theoretical RWF may cause overcorrection around the edge of the sampling gate. In view of this, an effective RWF that is in a Gaussian form and varies with the signal-to-noise ratio (SNR) of radar echoes has been proposed to mitigate the range-weighting effect and thereby enhance the continuity of the power distribution at gate boundaries. Based on the previously proposed concept, an improved approach utilizing the range-oversampled signals is addressed in this article to inspect the range-weighting effects at different range locations. The shape of the Gaussian RWF for describing the range-weighting effect was found to vary with the off-center range location in addition to the SNR of radar echoes—that is, the effective RWF for the RIM was SNR and range dependent. The use of SNR- and range-dependent RWF can be of help to improve the range imaging to some degree at the range location outside the range extent of a sampling gate defined by the pulse length. To verify the proposed approach, several radar experiments were carried out with the Chung-Li (24.98N, 121.18E) and middle and upper atmosphere (MU; 34.858N, 136.118E) VHF radars.

Meteor radar wind over Chung‐Li (24.9°N, 121°E), Taiwan, for the period 10–25 November 2012 which includes Leonid meteor shower: Comparison with empirical model and satellite measurements

The neutral winds in the mesosphere and lower thermosphere (MLT) region are measured by a newly installed meteor trail detection system (or meteor radar) at Chung-Li, Taiwan, for the period 10–25 November 2012, which includes the Leonid meteor shower period. In this study, we use the 3 m field-aligned plasma irregularities in the sporadic E (Es) region in combination with the International Geomagnetic Reference Field model to calibrate the system phase biases such that the true positions of the meteor trails can be correctly determined with interferometry technique. The horizontal wind velocities estimated from the radial velocities of the meteor trails and their locations by using a least squares method show that the diurnal tide dominates the variation of the MLT neutral wind with time over Chung-Li, which is in good agreement with the horizontal wind model (HWM07) prediction. However, harmonic analysis reveals that the amplitudes of the mean wind, diurnal, and semidiurnal tides of the radar-measured winds in height range 82–100 km are systematically larger than those of the model-predicted winds by up to a factor of 3. A comparison shows that the overall pattern of the height-local time distribution of the composite radar-measured meteor wind is, in general, consistent with that of the TIMED Doppler Interferometer-observed wind, which is dominated by a diurnal oscillation with downward phase progression at a rate of about 1.3 km/h. The occurrences of the Es layers retrieved from fluctuations of the amplitude and excess phase of the GPS signal received by the FORMOSAT-3/COSMIC satellites during the GPS radio occultation (RO) process are compared with the shear zones of the radar-measured meteor wind and HWM07 wind. The result shows that almost all of the RO-retrieved Es layers occur within the wind shear zones that favor the Es layer formation based on the wind shear theory, suggesting that the primary physical process responsible for the Es layer events retrieved from the scintillations of the GPS RO signal is very likely the plasma convergence effect of the neutral wind shear.