Height–time–structure of VHF back-scatter from stable and turbulently mixed atmosphere layers at tropical latitudes

Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1455–1463www.elsevier.com/locate/jastp

Height–time–structure of VHF back-scatter from stable andturbulently mixed atmosphere layers at tropical latitudes

A.R. Jain ∗, Y. Jaya Rao 1, N.S. MydhiliNational MST Radar Facility, P.O. Box No. 123, Tirupati 517 502, AP, India

Received 29 January 1999; accepted 1 March 2001

Abstract

Observations carried out over several diurnal cycles using the Indian MST Radar located at Gadanki (13:5◦N; 79:2◦E) showlayered structures of enhanced echo intensity at Zenith as well as oblique (Zenith angle �=10◦) beams. These structures areobserved to last for more than 4 h and often last as long as 18 h. Simultaneous MST radar observations and radiosonde datafrom Chennai (13:1◦N; 80:2◦E) are used to understand the causative mechanism of these echoes.

Results show that at some heights, these echoes are from sharp gradients in radio refractivity structures of horizontalcorrelation length of 10–20 m whereas at some other heights, these are from back-scatter frommore or less isotropic refractivitystructures, associated with turbulence. Thus, both mechanisms are e@ective depending upon prevailing atmospheric conditionsat di@erent heights. c© 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Discrete layers of high echo intensity; MST radar; Tropical latitudes

1. Introduction

Radar echoes from lower and middle atmosphere, at UHFand VHF, arise mainly due to coherent back-scatter fromrefractive index irregularities associated with atmosphericturbulence and Fresnel reDection=scattering from sharp gra-dients in radio refractive index. It is generally understoodthat for VHF radar echoes, at oblique incidence, with beamzenith angle �¿ 10◦, arise mainly due to back-scatter fromrefractive index irregularities associated with the turbulence(Tsuda et al., 1986; Hocking et al., 1990; Jain et al., 1997).

At vertical incidence, enhanced radar echoes could alsobe due to Fresnel reDection=scattering from sharp gradientsin radio refractive index due to the presence of atmospherestable layer structures associated with sharp temperatureinversions. Thus, echoing mechanism at VHF makes the

∗ Corresponding author.E-mail address: [email protected] (A.R. Jain).1 Current aFliation: Indian Institute of Tropical Meteorology,

Dr Homibhabha Road, Pune 411 008, India.

radar echo aspect sensitive. This feature has been used bymany investigators to detect and monitor stable layers suchas tropopause (Gage and Green, 1978, 1979; Rottger andLiu, 1978; Gage, 1990; Rottger and Larsen, 1990, Jain etal., 1994; Jaya Rao et al., 1994). Gage (1990) discussedechoing mechanism and showed that for a typical VHFradar �∼6 m, antenna diameter D∼100 m, the Fresnele@ect becomes important if horizontal correlation length(lt)¿ 29 m. For lt∼10 m the Fresnel e@ect is relativelyless, but the scattering power is still aspect sensitive dueto the presence of anisotropic scatterers. The e@ectiveechoing mechanisms depend on radar wavelength, beamzenith angle and background atmospheric parameters suchas winds, temperature and humidity and characteristics ofthe back-scatterers as discussed here.

Recently, there have been considerable e@orts to under-stand radar back-scatter and echo aspect sensitivity (viz,Tsuda et al., 1997a,b; Jain et al., 1997; Hooper and Thomas,1998). It has been reported that even for radar beam with�∼10◦, there could be signiHcant contribution from en-hancement in N 2 (for example; Hooper and Thomas, 1998),

1364-6826/01/$ - see front matter c© 2001 Elsevier Science Ltd. All rights reserved.PII: S1364 -6826(01)00032 -3

1456 A.R. Jain et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1455–1463

Table 1

Date Start time End time ESF(IST) (IST) usedh:min h:min

August 23–24, 1994 10:34 10:05 ESF1, ESF2October 25–26, 1994 16:26 16:22 ESF1, ESF2November 08–09, 1994 16:07 16:35 ESF1, ESF2December 21–22, 1995 10:15 10:40 ESF3January 4–5, 1996 14:00 14:07 ESF3January 16–17, 1996 10:07 10:04 ESF3January 31–February 1, 1996 11:13 11:13 ESF3February 15–16, 1996 10:57 11:04 ESF3March 12–13, 1996 11:20 04:03 ESF3

where N is Brunt–Vaisala frequency and the parameter N 2

represents atmospheric stability. However, most of suchstudies have, so far, been limited to mid-latitudes andonly a few measurements are available at tropical and lowlatitudes.

In the present study, high range and time resolutionmeasurements have been carried out over several diur-nal cycles using Indian MST Radar located at Gadanki(13:5◦N; 79:2◦E), a tropical station in the Indian zone.Height–time–intensity (HTI) maps are drawn separately us-ing radar observed signal-to-noise ratio (SNR) for verticalas well as o@-vertical beams, to study the layer structuresin the troposphere and lower stratosphere. A comparisonof HTI maps drawn using vertical and oblique beam echointensities show discrete layers of high echo intensity. Suchlayers are observed to last for more than 4 h and some lastas long as 18 h. In the present paper, simultaneous obser-vations from MST Radar and nearest radiosonde stationare used to understand the causative mechanism of suchdiscrete layers of enhanced echo intensity.

2. Observations

The Indian MST Radar is a high power, sensitive, coher-ent pulsed Doppler operating at 53 MHz in the VHF range.A very brief description of Indian MST Radar is given byJain et al. (1994) and Rao et al. (1995). Table 1 givesthe list of dates of 24 h observations utilized in this study.Table 2 gives the experimental speciHcation Hles (ESF)used for these observations. Observations during 1994 weretaken using ESF1 and ESF2. A set of observations, consist-ing of four scans of ESF1 and one scan of ESF 2 is takenevery half hour. A diurnal observation of this series thusconsists of 48 such sets. Observations during 1995 and 1996have been made every hour using ESF 3, which consistsof two vertical beams (Zx and Zy) and four oblique beamswith a Zenith angle of 10◦. A diurnal cycle observation inthis series consists of 24 such sets.

Table 2Experimental speciHcations used to study the height and timevariations of stable layers using Indian MST Radar

Parameter ESF 1 ESF 2 ESF 3

Pulse width (�s) 16 16 16Pulse modulation Codeda Codeda Codeda

Inter pulse period (�s) 1000 1000 1000No. of coherent integs. 64 64 128No. of FFT points 256 512 128Nyquist frequency (Hz) ±8 ±8 ±4Doppler resolution (Hz) 0.06 0.03 0.06Beam dwell time (s) 16 32 16No. of beams 4b 2c 6d

Observational window (km)Lowest range bin (km) 3.60 3.60 3.60Higher range bin (km) 24.00 30.00 32.00No. of scan cycles 4 4 4

aBiphase complementary code with Baud length of 1 �s corre-sponding to a range resolution (Kr) of 150 m.

bBeams with 20◦ zenith angle in East, West, North and Southazimuth directions.

cVertical beams formed independently using E–W and N–Spolarization arrays (Zy and Zx).

dBeams with 10◦ zenith angle in East, West, North and Southazimuth directions and two vertical beams formed independentlyusing E–W and N–S polarization arrays.

The three low order moments, viz., zero, First and Sec-ond representing strength of the radar echo, Doppler shiftand width of the Doppler spectrum, respectively, are com-puted from the Doppler spectrum for each range bin. Thenoise power for each range gate is determined using theobjective method given by Hildebrand and Sekhon (1974).Height proHles of horizontal winds are obtained, from theDoppler velocities observed for three non-coplanar beams.These are in turn used to estimate vertical shear (S) ofhorizontal wind. In order to understand the causative mech-anism of long-lasting radar echoes, simultaneous tempera-ture data, from the nearest radiosonde station at Chennaiwas (13:1◦N; 80:2◦E) used to compute height proHle ofthe atmospheric stability parameter (N 2) where N is theBrunt–Vaisala frequency. The parameters N 2 and S2 areused to determine the height proHle of the Richardson num-ber (Ri =N 2=S2).

3. Results

3.1. Height–time–intensity (HTI) maps of vertical andoblique beam echo intensity (SNR)

Fig. 1 shows a typical HTI map drawn using vertical beamsignal-to-noise ratio (SNR) observations on 21–22 Decem-ber 1995. The height of tropical tropopause at 05:30 IST(00:00 UT) measured by radiosonde is marked by an ar-row. Multiple layer structure of complex nature is clearly

A.R. Jain et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1455–1463 1457

Fig. 1. A typical height–time–intensity (HTI) plot drawn using vertical beam signal-to-noise ratio (SNR) for one diurnal cycle.

evident and are seen more prominently at heights near thetropopause. Rottger (1980) and Tsuda et al. (1988) observedsimilar multiple layer structures in the upper troposphere andlower stratospheric region. These structures are observed tolast for quite some time (4 h to more than 12 h) indicat-ing persistence of these structures. However, such layersat various height levels were found to show considerableday-to-day variability.

The enhanced echo intensity structures for vertical beam,as mentioned above, are expected to arise due to Fres-nel reDection=scattering from stable layer structures. Itwould, therefore, be interesting to examine whether, suchlong-lasting echo layers are also observed at oblique beam(�=10◦). Fig. 2 presents HTI maps drawn using verticaland oblique beam SNR for 3 days. Long-lasting enhancedecho intensity structures in oblique beam HTI maps arefewer as compared to corresponding vertical beam HTImaps. However, layers of enhanced echo intensity are ob-served at the same height in oblique as well as verticalbeam HTI maps. There is an almost one-to-one correspon-dence in enhanced echo intensity structures observed atoblique and vertical beams, though SNR for oblique beamis 3–5 dB lower than that for vertical beam. For obliquebeam also, enhanced echo intensity structures are observedto last from 4 h to more than 12 h.

3.2. Contour maps of horizontal correlation length andwind speed

Vertical and oblique beam signal intensities are used fordetermining the echo aspect sensitivity and the same isin turn used for determination of the horizontal correla-tion length of the scatterer � using the standard method(Hocking, 1989; Hocking et al., 1990; Hooper and Thomas,1995; Jain et al., 1997). The parameter � gives an idea ofthe horizontal length of the scatterer and is given by

�=15:2�=�s;

where � is the radar wavelength and �s the e−1 half width ofthe polar diagram of the radar back-scatter expressed in de-gree. Small values of �s mean large anisotropy and its valueclose to 90◦ means purely isotropic scattering. The receivedsignal intensity (SNR) at vertical and oblique beams is usedfor determination of �s (Hocking et al., 1986, 1990; Hooperand Thomas, 1995). Fig. 3 shows the contour maps of hor-izontal correlation length for the three cases presented inFig. 2. Layers of enhanced horizontal correlation length canbe noticed clearly in each of these panels. Fig. 4 presentscontour maps of the wind speed for all the three days ofobservations presented in Fig. 2. Layers of enhanced windspeed can be seen from this Hgure.


Fig. 2. Height–time–intensity (HTI) plots drawn using oblique and vertical beam signal-to-noise ratio (SNR) for three diurnal cycles.


Fig. 3. Contour plots of horizontal correlation length (in m) for three diurnal cycle observations presented in Fig. 2.

3.3. Simultaneous radar and radiosonde measurements

Refractive index irregularities due to turbulence asso-ciated with the vertical shear in the horizontal wind giverise to enhanced back-scatter. In addition, enhanced radarechoes, at various height levels, may also arise due toFresnel reDection=scattering anisotropic scattering fromgradients in atmospheric stability parameter (N 2). FresnelreDection=scattering, as discussed earlier, is expected to beprevalent at many heights at Zenith beam. Simultaneousradar observations from Gadanki and radiosonde observa-tions from Chennai are examined in order to determine therole these twomechanisms have in giving rise to long-lastingechoes at various heights as seen in Figs. 1 and 2.

Fig. 5 presents height proHles of (a) SNR for obliqueand vertical beam (b) atmospheric stability parameter N 2

(c) Richardson number (Ri) and radar signal spectral width(SW) (corrected for beam broadening as well as uncor-rected). It should be remembered that small values ofRi (Ri ¡ 1) and enhanced SW are good indicators of en-hanced turbulence. Observations in this Hgure correspondto three days presented in Figs. 2–4 and refer to 17:00IST. Horizontal bands drawn in Fig. 5 represent regionsof enhanced echo intensity. These bands of enhanced echointensity, are limited to the upper height reached by ra-diosonde observation on each day. The mechanism givingrise to some of the layers of long-lasting enhanced intensityechoes as seen in Fig. 2 and marked by bands in Fig. 5 can


Fig. 4. Same as Fig. 3 but for horizontal wind speed.

be understood with the help of Figs. 3–5. The followingimportant points could be noticed.

(i) It can be noticed from Figs. 3 and 5 that some ofthe enhanced echo intensity bands (bands 1 and 2 inpanel a, bands 1 and 4 in panel b and band 5 in panelc) are accompanied by high aspect sensitivity. Hori-zontal correlation length of the scatterer, correspond-

ing to observed aspect sensitivity, is determined to bebetween 10 and 20 m. At the heights of these bandsS26 3 × 10−4 s−2 and Ri¿ 1. This indicates clearlythat in these cases enhanced echo arises due to gradi-ents in N 2 and is not associated to enhanced shears.

(ii) It can be seen from Fig. 5 that at some heights, low as-pect sensitivity is accompanied by enhanced shear anddistinct enhancement in SW. Enhanced shears could,


Fig. 5. Height proHles of vertical and oblique SNR, N 2; S2; Richardson number Ri; observed radar spectrum width (�o) and corrected radarspectrum width (�T):


however, occur at slightly higher or lower heights com-pared to the enhanced echo intensity (band 4 in panela, band 3 in panel b and band 2 in panel c). At theseheights, Ri is small (i.e. Ri6 1). Persistent wind gra-dients can also be noticed at these heights (Fig. 5).Therefore, back-scattering from refractivity irregulari-ties more or less isotropic in nature appears to be themain causative mechanism of enhanced echoes in thesecases.In case of band 3 of panel b (Fig. 5) peak signal inten-sity can be seen at 12:25 km accompanied by a peak inSW. A layer of enhanced shear is also seen below thislevel. Low aspect sensitivity, enhanced SW and en-hanced shears just below this level of 12:25 km conHrmthat scattering from isotropic refractivity structures, as-sociated to turbulence due to enhanced vertical shear,is the main echoing mechanism in this case. A peak inRi is also seen above 12:25 km level which is due to agradual increase in N 2 between 11.5 and 13 km.

(iii) There are some cases, however, which are morecomplex and do not Ht in any of the simple picturesas discussed above. For example, bands 3 and 4 ofpanel c (Fig. 5) are accompanied by large shears andenhanced SW. At the same time, signiHcant aspectsensitivity (10–15 dB) can also be observed. Thesecases are discussed in the next section of the paper.

4. Discussion

Interpretation of radar returns and associated echo-ing mechanism are important for application of the VHFradar data. Recently, many observations have been madeon aspect sensitivity of the radar echo, to determine thecontribution of scattering and specular reDections and tounderstand the e@ect of echo aspect sensitivity on the radarwind measurements (Hooper and Thomas, 1995; Jain et al.,1997; Tsuda et al., 1997a,b).

It is generally presumed that at radar beam zenith angle�¿ 10◦, the radar echo arises mainly due to back-scatterfrom refractivity structures associated with turbulence.Hooper and Thomas (1998), making use of United King-dom radar at Aberystwyth (52◦N; 41◦W), have shownthat radar echo, even for an oblique beam with �=12◦, isnot necessarily due to enhanced turbulence. Furthermore,these authors noted that perturbations of the signal altitudeproHles are associated with those of N 2 proHle and thus,inferred that the possibility that Fresnel scatter contributesto radar returns at such a large zenith angle (�∼12◦) can-not be discounted. Aspect sensitivity measurements, madeusing MU radar located at Shigaraki (34:85◦N; 136:10◦E),showed that echo intensity decreased to about −10 dB at�=6◦, and then gradually decreased to a constant levelbetween −15 and −25 dB at �¿ 20◦ (Tsuda et al., 1997a).This constant level is interpreted as isotropic level. Theseobservations showed that for beam zenith angle, �∼10◦,

the echo intensity could be signiHcantly larger than theisotropic level. Gage (1990) discussed echoing mechanismsand showed that for VHF Radar Fresnel e@ect is importantfor horizontal correlation length (lt)∼29 m. For lt∼10 m,the Fresnel e@ect is small, though echo could still be aspectsensitive due to the presence of anisotropic turbulence.

More recently, Hocking and Hamza (1997) examined theproblem of echo aspect sensitivity in quantitative terms andinferred that the angle �s should be greater than 5◦ in allcases of turbulent scatter. These authors also concluded thatsmaller values of �s are indicative of a reDection mecha-nism other than turbulence, and scatterers in such cases canbe truly called ‘Specular’. For Indian MST Radar, �s = 5◦

corresponds to a horizontal correlation length of 17:4 m.The results of the present study show that the enhanced

long-lasting echo layers with larger aspect sensitivity arisedue to enhanced N 2 with horizontal correlation length be-tween 10 and 20 m. These results thus suggest that the en-hanced echo in these cases is due to atmospheric scatteringfrom refractivity structures associated with larger values ofN 2. At some height levels, radar echo is noted to be due toback-scattering from refractivity structures associated withthe isotropic nature of scatterers (Ri6 1). At any one height,which of the two mechanisms is prominent depends on pre-vailing atmospheric conditions. However, the Hrst mecha-nism i.e. anisotropic scattering from refractivity structuresassociated with larger values of N 2 is prominent at greaterheights. These results have implications in the interpretationof radar returns. Hocking and Hamza (1997) have also men-tioned that even if �s¿ 5◦, it does not necessarily mean thatback-scatter is associated with turbulence or an indicator ofthe presence of turbulence.

The present observations also show that there are somecases where none of the two simple mechanisms invokedhere, explains the observations. In such cases back-scattercould be from corrugated surfaces (e.g. see Tsuda et al.,1997b). Part of the diFculty in interpreting such cases arisesdue to the spatial separation between locations of radarand radiosonde measurements. An intensive campaign of si-multaneous radar and radiosonde observations from radarsite itself could help in understanding the nature of radarback-scatterers in such cases.

Acknowledgements

Radiosonde data from Chennai, used in the present study,were made available by India Meteorological Department.National MST Radar Facility has been established jointlyby Council of ScientiHc and Industrial Research (CSIR),Defence Research Development Organisation (DRDO)and Departments of Electronics, Environment, Science andTechnology, and Space of Government of India with theDepartment of Space (DOS) as a Nodal agency. The facil-ity is presently operated by DOS with partial support fromCSIR. The authors thank Mr. V. Sivakumar for his help inpreparing some of the graphics.


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Height–time–structure of VHF back-scatter from stable and turbulently mixed atmosphere layers at tropical latitudes