Characteristics of atmospheric waves in the upper troposphere observed with the Gadanki MST Radar—RASS

Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1020–1030

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Characteristics of atmospheric waves in the upper troposphere observed with the Gadanki MST Radar—RASS T.V. Chandrasekhar Sarma a,n, Yasu-Masa Kodama b, Toshitaka Tsuda c a

National Atmospheric Research Laboratory (NARL), Gadanki 517 112, AP, India Department of Earth and Environmental Sciences, Hirosaki University, Hirosaki, Aomori 036 8561, Japan c Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Kyoto 611 0011, Japan b

a r t i c l e in f o

a b s t r a c t

Article history: Received 30 November 2009 Received in revised form 6 August 2010 Accepted 11 August 2010 Available online 18 August 2010

Indian MST Radar at Gadanki (13.46 % N, 79.17 % E) working at 53 MHz was operated continuously in Radio Acoustic Sounding System (RASS) mode with six distributed acoustic exciters during 22–25 August 2007 for about 69 h in RASS and turbulence echo modes alternately, with one cycle spanning about 20 min. Temperature and wind velocity were observed in the altitude range 3.6–12 km most of the time and 3.6–20 km, respectively, with a range resolution of 150 m. Presence of an inertia–gravity wave in the lower stratosphere was identified. Perturbations in the wind and temperature fields in the troposphere, however, indicated a mixture of waves with a wide frequency range, within which a dominant periodicity of  8 h with a vertical wavelength of  4 km was revealed by two-dimensional spectral analysis. Further, signatures of diurnal tide were also observed and the temperature phase profile was found to exhibit a close match with the Global Scale Wave Model (GSWM02). From the RASS ¨ al ¨ a¨ frequency squared was computed to virtual temperature profiles, time–height section of Brunt-Vais deduce the background atmospheric stability, which showed stable layered structures with slow downward phase progression. Outgoing Longwave Radiation (OLR) over Gadanki and TRMM precipitation data over peninsular India was used as an indicator of convective activity. During the beginning of the observation period, even though lower OLR was seen, corresponding convective activity or precipitation was not seen in the TRMM data. On 24 August, enhanced temperature perturbations could be related to widespread precipitation shown by TRMM. & 2010 Elsevier Ltd. All rights reserved.

Keywords: RASS MST radar Gravity wave Atmospheric tides

1. Introduction VHF Radio acoustic sounding system (RASS) has been established as a reliable technique for simultaneous ground-based remote profiling of vector wind velocity and atmospheric virtual temperature in the lower atmosphere on a continuous basis (e.g., Marshall et al., 1972). RASS is a unique technique that can operate during day and night in nearly all weather conditions. Matuura et al. (1986) showed the capability of this technique to profile atmospheric virtual temperature up to the lower stratosphere for the first time, and it was also demonstrated in the tropics by Sarma et al. (2008). Continuous temperature profiles are used to clarify the behaviour of temperature perturbations associated with meteorological phenomena, atmospheric waves and the stability structure (Neiman et al., 1992; Tsuda et al., 1992; Alexander et al., 2007; Alexander and Tsuda, 2008a).

o

o

RASS consists of a wind profiling radar and collocated acoustic sources. Acoustic excitation is generated at a wavelength that is half of the radar transmission wavelength, so as to obtain echoes by means of Bragg scatter from the propagating acoustic wavefronts. As the sound speed generally decreases with an altitude in the troposphere, the frequency of an excitation required for obtaining radar backscatter at different altitudes is different in order to keep the wavelength relationship with the radar transmission (Masuda et al., 1992). Therefore, an FM-chirped acoustic pulse is normally employed to expand the height coverage of RASS observations (Sarma et al., 2008). The radar measures the wind velocity and the propagation speed of acoustic wavefronts, Ca (ms  1), which is related to ambient temperature T (K) T¼

 2 Ca kh

ð1Þ

The constant kh (JK  1 kg  1) in Eq. (1) is given by n

Corresponding author. Tel.: +91 8585 272008  272024; fax: + 91 8585 272021  272018. E-mail address: [email protected] (T.V. Chandrasekhar Sarma). 1364-6826/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2010.08.010

kh ¼



1=2

gR M

ð2Þ

T.V. Chandrasekhar Sarma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1020–1030

where g(1.4), R(8314.472 JK  1 kmol  1) and M (28.964 kg kmol  1) are ratios of specific heat at constant pressure to specific heat at constant volume, universal gas constant and mean molecular weight of dry air, respectively. For a dry atmosphere, kh works out to be 20.047. The ‘sonic’ temperature determined by RASS is very close to the ambient virtual temperature Tv, which is defined as the temperature that dry air would have if its pressure and density were equal to those of a given sample of moist air. Absolute temperatures T and Tv are related as Tv ¼ ð1 þ 0:608qÞT

1021

of temperature perturbations, including the behaviour of gravity wave activity, up to an altitude of 20 km at a tropical location. Signature of diurnal tide was also derived from the zonal and meridional components of the wind and also the virtual temperature, and was compared with the Global Scale Wave Model (GSWM) for tides (Hagan et al., 1995). Further, using the virtual temperature profiles, atmospheric stability signified by the ¨ al ¨ a¨ frequency squared was computed; which virtual Brunt-Vais exhibited stable layers showing slow downward phase progression over the observation period. We refer to the outgoing long-wave radiation (OLR) and the precipitation rate from the tropical rainfall measuring mission (TRMM) satellite as an index of cloud amount and cloud convection, respectively, and discuss their effects on perturbations of wind velocity and temperature.

ð3Þ

where q (kg kg  1) is the specific humidity which is normally the highest near the surface and decreases exponentially with altitude. Tsuda et al. (1994) show that under a sub-tropical condition, the difference between T and Tv at 1.5 km is about 2.0 K and becomes very small at around 4 km. There are a variety of sources that produce temperature perturbations in the lower atmosphere. In the tropics, convective clouds produce disturbances, which further generate atmospheric waves, such as gravity waves, equatorial Kelvin waves, etc. These waves play an important role in transporting energy and momentum through their upward propagation and they eventually act to drive atmospheric general circulation. Atmospheric tides are generated by the periodic forcing by solar heating with a wave period of 24 h and its harmonics. In particular, diurnal tides are effectively generated in the tropics mainly due to absorption of solar radiation by water vapour, which consists of migrating and non-migrating components (e.g., Tsuda and Kato, 1989). Using continuous RASS temperature data, it is possible to determine time–height structure of temperature perturbations in the troposphere and lower stratospheric altitudes, and separate them into various atmospheric wave components. A RASS system has recently been developed (Sarma et al., 2008) with the Indian MST radar at Gadanki (Rao et al., 1995). Using this system, observations of atmospheric virtual temperature and vector winds were carried out during 22–25 August 2007 over a period of about 69 h. This paper presents the data analysis

2. RASS observations in August 2007 2.1. Experimental setup of the Gadanki MST radar The Gadanki MST Radar was constructed in 1993 and operates at a frequency of 53 MHz with a peak rated power of 2.5 MW. The antenna array consists of 1024 Yagi elements arranged in a 32  32 square grid. The radar antenna array consists of two sets of Yagi-elements—one aligned along EW and the other along NS plane and is capable of steering the antenna beam within an angular region of 7201 with respect to zenith in these planes. The zenith beam formed by NS aligned dipoles is designated as Zx and that formed by EW aligned dipoles is designated as Zy. Fig. 1a shows the location of the RASS acoustic transmitters in and around the MST Radar antenna array. Since Sarma et al., 2008, four additional acoustic exciters were added, as shown, along the periphery of the radar antenna array to augment the two exciters that are located at the center of the array. Fig. 1b shows a block schematic of the MST radar system with RASS, where the radar and RASS system is shown on the left and right sides, Acoustic Horn

Radar Antenna

N

DUPLEXER

ACOUSTIC TRANSMITTER

TRANSMITTER

RF Gain Audio File player

RF LO

A pair of acoustic horns

MASTER OSCILLATOR

IF Gain

SYNTHESISER 1

TxIF

MST Radar Antenna Array

Program over Ethernet

PULSE MOD

Ref

1:2

Quadrature Mixer

Ref SYNTHESISER 2

PC

1:2 90o

I

Q

Fig. 1. (a) Location of acoustic exciters in and around the MST radar antenna array and (b) a block schematic of MST radar—RASS. On the left is a simplified block schematic of the MST Radar and on the right is that of an acoustic exciter.

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Table 1 Specifications of the Gadanki MST radar/RASS system. Radio system (monostatic pulse Doppler radar) Location RF frequency IF frequency Output power (peak) Antenna aperture Antenna beam directions Pulse width Pulse repetition frequency Maximum duty ratio Pulse compression Number of coherent integration Number of incoherent integration FFT points Acoustic system Signal generation Frequency Acoustic power Antenna Pulse repetition period Pulse width

Gadanki (13.461N, 79.171E) 53 MHz 5 MHz 2.5 MW 126  126 m2 within 7201 at 11 step along E–W and N–S planes 1–32 ms Up to 5 kHz 2.5% Biphase coding User defined User defined Maximum 512 for online spectrum Network audio receiver ONKYO NC-500 90–130 Hz 140 dBA Hyperbolic horn Programmable Programmable

respectively. The specifications of the radar system and acoustic system are shown in Table 1. Assuming a temperature range  90 to 40 1C and the radar wavelength of 5.66 m, the range of acoustic frequencies for measurements in the troposphere and lower stratosphere is approximately 94–125 Hz. Corresponding Doppler shift is in the range 94 to 125 Hz, as negative Doppler shifts indicate propagation away from the radar. The measured sound speed from the wind profiler is the apparent speed with respect to the radar. It is biased by the background wind field. Therefore, wind speed in the direction of propagation of the acoustic waves is to be subtracted to obtain true speed of sound. To accomplish this, the system is operated first in an RASS mode to obtain line of sight apparent sound speed followed by correction wind mode, in which line of sight wind speed is obtained in the same direction for subtraction. The radar experimental specifications used for this experiment are shown in Table 2. Following these two modes, the radar was operated in the background wind mode to obtain vector winds up to lower stratospheric altitudes, using the Doppler Beam Swinging technique commonly employed in wind profiler radars. The total duration of one cycle of radar operation was about 20 min.

bin after subtracting noise (Hildebrand and Sekhon, 1974) and by Gaussian fitting (Sato and Woodman, 1982). Due to the manual operation for switching the three modes, the time gap between successive cycles of observations is somewhat irregular. In order to make the observations uniform before further analysis, cubic spline interpolation was used along time at each altitude and values of Tv, u and v, that are separated exactly by every 20 min, were saved.

3. Background meteorological conditions 3.1. Mean profiles of temperature and wind velocity with MST radar—RASS and radiosondes Simultaneous GPS radiosondes (Meisei RS-06G) were launched at an interval of about 6 h starting at 11:30 LT on 22 August 2007. However, on 24th as the data from the flight at 11:30 LT could not be recorded due to a technical problem, another flight was launched at 15:00 LT in lieu of the failed one. The cold point tropopause was at about 16.5 km altitude. A mean profile of the Tv using the data throughout the observation period was computed and is shown in Fig. 3a along with standard deviation. Fig. 3b shows the mean over the entire period obtained from RASS compared with the mean of the radiosonde observations. The difference between mean RASS and mean radiosonde, Tv, is shown in Fig. 3c. The mean bias between the two observations between 3.6 and 14 km is  0.57 K. This bias may indicate a small difference in the height coordinate of the radar and the radiosonde, which is smaller than the radar range resolution (150 m). Range calibration of the radar could be done to a high degree of accuracy, using such a comparison. After removing this bias, the rms deviation of the RASS measurement from the radiosonde measurement was found to be 0.06, 0.14 and 0.26 K in the height ranges 3.6–6, 6–10 and 10–14 km, respectively. Fig. 4 shows the six hourly height profiles of zonal wind (u) and meridional wind (v) velocity obtained with the MST Radar and their comparison with the radiosonde observations up to an altitude of 20 km. The radar and balloon observations match well, indicating a stable background over a synoptic scale. The observation falls within the Indian South-West monsoon period, during which tropical easterly jet is strong. The zonal wind profiles show a maximum wind of 40 ms  1 at  16 km altitude, whereas the meridional wind is relatively weak. From this figure, one can see downward phase progression at lower stratospheric altitudes (16–20 km) related to an inertia–gravity wave activity. The downward phase progression has been indicated by straight lines overlaid on the figure. This aspect is elaborated later in Section 4.3.

2.2. RASS data analysis procedure The RASS observations started at around 10:00 LT on 22 August 2007 and were continued for about 69 h up to 07:40 LT on 25 August 2007 in the three modes shown in Table 2, viz., RASS mode, correction and background wind modes in a cyclic fashion. One cycle spans about 20 min with each of these modes lasting about 3, 7 and 7 min. There is an additional overhead for setting the parameters and mode switching. Fig. 2a shows one of the typical profiles of Doppler spectrum obtained in the temperature mode, and Fig. 2b that in the wind mode. The lowest altitude of observation is 3.6 km. The maximum altitude extent of the backscattered signal varied between 12 and 14 km most of the time. However, on a few occasions, the maximum altitude went up to about 16 km and minimum altitude was about 9 km. From the spectral data, first three central moments were computed (Woodman, 1985) at each range

3.2. Distribution of clouds and precipitation from OLR and TRMM data During the RASS observation, a localized cloud-system existed over the observation site. Throughout the observation, the sky was overcast with occasional showers. In order to examine the characteristics of synoptic- and meso-scale cloud systems simultaneous hourly OLR at Gadanki over a 0.11  0.11 was examined and is shown in Fig. 5a along with the average over the 31  31 area. This data was derived from cloud top equivalent blackbody temperature, called Brightness Temperature (TBB) from MTSAT1R (Multi-functional Transport SATellite) recorded in longitude/ latitude grids of 0.051 between 111N–151N latitude and 771E–811E longitude covering the location of Gadanki.

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Table 2 Experimental parameters of MST radar—RASS. Parameter

RASS mode

Correction wind mode

Background wind mode

Pulse width (ms) Coding used

16 16-bit, 1 ms baud , biphase complementary 150 1000 101E, 91E, 81E, Zy 24 256 4

16 16-bit, 1 ms baud, biphase complementary 150 1000 101E, 91E, 81E, Zy 64 512 1

16 16-bit, 1 ms baud, biphase complementary 150 1000 101E, 101W, Zy, Zxa, 101N, 101S 64 512 4

3.6 150  110 3

3.6 150 0 7

3.6 150 0 7

Range resolution (m) Inter pulse period (ms) Beam sequence Number of coherent integrations Number of FFT points Number of online incoherent integrations Start of observation range window (km) Number of range bins Second LO offset frequency (Hz) Time duration (minutes)

Zx and Zy—zenith beams formed using N–S and E–W dipoles, respectively.

18.6

18.6

15.6

15.6 Range (km)

Range (km)

a

12.6 9.6 6.6

12.6 9.6 6.6

3.6

3.6 -20

-15

-10

-5 0 5 Doppler Shift (Hz)

10

15

20

-6

-4

-2 0 2 Doppler Shift (Hz)

4

6

Fig. 2. (a) A typical Doppler spectrum of acoustic echo along the beam direction 101 East. Here, 0 Hz corresponds to  110 Hz due to shifting of second LO signal and (b) a typical Doppler spectrum of turbulence echo along the beam direction 101 East.

At the observation site, there were short showers at around 20:00 and 23:40 LT on 22nd and around 02:00 LT on 24th as shown by the optical rain gauge data at Gadanki in Fig. 5b. The light showers contaminated the turbulence Doppler spectrum and heavy showers, though brief in duration, resulted in loss of RASS echoes. Fig. 6 shows the daily TRMM—TOVAS generated precipitation plots over south India obtained from the website of NASA.—http:// gcmd.nasa.gov/records/TOVAS.html. On 22nd, a rather isolated strong precipitating system is seen over the west coast of India. Further on 23rd and 24th scattered precipitation, though weak, is seen over the region. On 25 August, the areas around Gadanki are occupied with scattered cloud convections. Fig. 7 shows plots from the TRMM sensors Visible and Infrared Spectrometer (VIRS) and TRMM Microwave Imager (TMI) operating in visible band (0.63 mm), infrared band (12 mm) and W band (85 GHz), respectively, on 23rd and 24th. A low level cloud-system is seen on 23rd, whereas on 24th tall cloud anvil can be seen over peninsular Indian region. The cloud-system originated from an isolated strong convective system on 22nd that developed near the west coast. It moved eastward, then probably due to interaction by topography, the convective system was enhanced and spread over the southern region of India on 24–25 August. 4. Temperature and wind velocity perturbations 4.1. Time and height distribution of temperature and wind velocity perturbations Time–height plots of the u, v and Tv were prepared and the background mean for each of these was computed. Contour plots of

these parameters after subtraction of the background mean, henceforth referred to as u0 , v0 and Tv0 , are shown in Fig. 8a–c, respectively. The chequered area in Tv0 above about 10–12 km is caused by an unstable temperature retrieval due to missing acoustic echo. The twelve ticks on the x-axis, following the first one, indicate the times about which radiosonde flights were launched. The activity in the plots of u0 and v0 can be classified into two regions—that above the tropopause and the other below the tropopause. There is a region of enhanced perturbations in Tv0 on the 22nd and also towards the end of the observation period on 24th and 25th as evident from Fig. 8c. Further, the contour plots of u0 , v0 and Tv0 show a consistent behaviour throughout the observation period. In the lower stratosphere, organized enhancements in the perturbations of wind field that can be seen in Fig. 8a and b between 16 and 20 km, indicate presence of monochromatic gravity wave activity. As the RASS temperature measurements are not available at these altitudes, hodograph analysis of the radiosonde wind and temperature was done to reveal vertical periodicity of 4 km and temporal periodicity of 34.5 h with the wave propagation direction towards south-west. Studies in the tropical regions based on intensive radiosonde campaigns (e.g., Ratnam et al., 2006) have shown that convective cloud systems that reach up to tropopause altitudes generate such gravity waves that propagate over long distances. ¨ al ¨ a¨ frequency square (N2) Fig. 8d shows the virtual Brunt-Vais computed, using the Tv observations and the following relation N2 ¼ ð9g9=TvÞð@Tv=@z þ Gd Þ

ð4Þ

T.V. Chandrasekhar Sarma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1020–1030

14

14

13

13

12

12

11

11 Altitude (km)

Altitude (km)

1024

10 9 8 7

10 9 8 7

6

6

5

5

4

4 -80

-70

-60

-50

-40

-30

-20

-10

0

10

20

-80

-70

-60

-50

Virtual Temperature (°C)

-40

-30

-20

-10

0

10

20

Virtual Temperature (°C)

14 13 12 Altitude (km)

11 10 9 8 7 6 5 4 -1

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

Virtual Temperature difference (°C) Fig. 3. (a) Mean profile of RASS Tv throughout the observation period and standard deviation; (b) a comparison of mean profiles of Tv from RASS (solid line) and radiosonde (broken line); and (c) the difference between mean Tv profiles of RASS and radiosonde.

where g (¼9.8 ms  2) is the acceleration due to gravity, z (km) is the altitude and Gd (¼9.8 K km  1) is the dry adiabatic lapse rate. The white patches in Fig. 8d at around 20:00 LT on 22nd and around 02:00 LT on 24th and that above about 11 km throughout the observation are due to large values of N2 that resulted due to unstable temperature retrieval, which was due either to missing acoustic echo or contaminated spectrum in correction wind mode related to rain showers. The layered structures of enhanced stability present below 10 km and descending over the observation period are discussed in Section 4.4.

4.2. Two-dimensional spectra of wind velocity perturbations in terms of wave frequency and vertical wave number in the troposphere In the tropical troposphere, the main source of gravity wave generation is cloud convection (e.g., Song et al., 2003). Cloud convection, as shown by the TRMM satellite precipitation data over the peninsular India (Fig. 6) was widespread and varied over the three days. Wind shear of the tropical jet stream could also be a wave excitation mechanism. Further, wind shear modulates the spectrum of convectively generated gravity waves (Beres et al., 2002). The observation period falls during the active south-west monsoon over India, which is characterized by the presence of a strong easterly jet with zonal wind speeds reaching 40 ms  1. These factors together possibly led to the medley of different wavepackets present over the site and their signatures in data existing over short durations. The perturbations in the troposphere may appear as superpositioning of a number of events with different frequencies and height scales. However, we can recognize a quasi-monochromatic wave structure, for example, near the beginning of the

observation period. In order to identify a dominant wave period, the mean-subtracted time–height distributions of u0 , v0 and Tv0 , in the altitude region of 3.6 to 15.6 km, were subjected to a twodimensional spectral computation. Contour plots of the spectrum of u0 , v0 and Tv0 are shown in Fig. 9a–c, respectively. It appears that waves with several periodicities and vertical wavelengths existed due to passage of several different wave packets that were possibly generated due to the convective activity seen in the TRMM precipitation plots over the peninsular India. In spite of the medley of waves, a clear dominant wave activity at a periodicity of 8 h with corresponding vertical wavelength of 4 km can be seen. Further processing was done to clarify this component by filtering the data by the application of band pass filters, as described below.

4.3. Hodograph analysis of an 8 h wave component The wind and temperature data in the tropospheric altitude region were processed through a bandpass filter (BPF) first along time, and then through another filter acting along the height. The BPF along time had the lower and upper cutoff periods at 6 and 12 h, respectively. Similarly the filter used along altitude had the lower and upper cutoff wavelengths at 1 and 6 km, respectively. It was observed that the wave with a periodicity of 8 h was present only during the initial observation period. Hodograph plots during the period beginning 6 h after the start of observation to 14 h after the start are plotted once every 2 h in Fig. 10. The first subplot corresponds to 1610 LT on the first day of observation viz., 22nd August 2007. Filtered values of u0 ,v0 and Tv0 are shown in the lower row. The upper row is obtained by plotting the wind velocity vectors and an ellipse has been fitted to

T.V. Chandrasekhar Sarma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1020–1030

20

18

18

16

16

14

14 Altitude (km)

Altitude (km)

20

12 10 8

12 10 8

6

6

4

4

2

2

0

0 -50 -20 0 20 0

0

0

0

0

0

0

0

0

0

0

-50

-40

-30

-1

-20

-10

0

10

-1

Six hourly Zonal Wind Speed (ms )

Mean Zonal Wind Speed (ms )

20

20 18

18

16

16

14

14 Altitude (km)

Altitude (km)

1025

12 10 8

12 10 8

6

6

4

4

2

2

0

0 -20 0 20 0

0

0

0

0

0

0

0

0

0

0

-10

-8

-6

-1

-4

-2

0

2

4

6

8

10

-1

Six hourly Meridional Wind Speed (ms )

Mean Meridional Wind Speed (ms )

Fig. 4. (a) Six hourly profiles of zonal wind using MST Radar (solid line) and radiosonde (broken line). Mean over the entire period is shown in (b). (c) and (d) are same as (a) and (b), respectively, but for meridional wind. Sloping lines between 16 and 20 km indicate downward phase progression of the stratospheric gravity wave.

80

280

70

Precipitation Rate (mm/hr)

290

OLR (W/m2)

270 260 250 240 230 220 210 200

60 50 40 30 20 10 0

22

23

24

25

Day in Aug 2007

22

23

24

25

Day in Aug 2007

Fig. 5. (a) OLR over Gadanki in 0.11  0.11 grid (thick line) and over 31  31 grid (thin line) derived from cloud top equivalent black body temperature using MTSAT-1R and (b) precipitation rate recorded by an on-site Optical Rain Gauge.

determine the dominant plane of propagation. Further, using the method outlined by Hamilton (1991), the direction of propagation is seen to be towards south-west of the site.

4.4. Effects of atmospheric waves on the stability structure The atmospheric stability structure designated by the virtual ¨ al ¨ a¨ frequency squared (N2) during the observation Brunt-Vais period is shown in Fig. 8d. In this figure, signatures of several stable layers can be seen descending from an altitude of about 9 km on 22nd to below 4 km on 25th. It is most likely that

atmospheric waves with short vertical wavelength passing overhead modulated the stability structure. Another process to which the downward phase progression of these structures could be attributed to is an enhanced humidity related to the descending cloud masses over the period of observation. Simultaneous measurements using a Mie lidar to detect the cloud layers would have clarified such a behaviour. Even though planned, the Mie lidar at Gadanki could not be operated due to slight drizzle that was present most of the time and the occasional showers. A future coordinated study of RASS and Mie Lidar system is planned to reveal the dynamics of the humidity layers and clarify the stability structures, such as those seen here.

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a

b

c

d

Fig. 6. TRMM precipitation rates on (a) 22nd; (b) 23rd; (c) 24th and (d) 25th. Gadanki is shown with a white square.

5. Diurnal tides In Fig. 8, a diurnal enhancement in fluctuations can be seen between 8 and 14 km altitude in u0 and v0 and between 8 and 12 km altitude in Tv0 plots. This behaviour suggests diurnal tidal activity. Periodogram analysis was done to find the dominant wave period by fitting a sinusoid at each height to u0 , v0 and Tv0 , using a least squares fitting procedure. The periodogram maxima of the amplitudes and the phase are plotted in Fig. 11 along with results from the numerical model of migrating tides, viz. Global Scale Wave Model (GSWM-2002) (Forbes, 1982; Forbes and Hagan, 1988; Hagan et al., 1995) at 121N. The GSWM data was downloaded from the NCAR website (http://www.hao.ucar.edu/modeling/gswm/gswm.html). It is observed that below 8 km the phases of u0 and v0 show a mean difference of about 5.9 h in the observed values, which is close to the expected value of 6 h. From Fig. 11, it is observed that the phase in the Tv0 matches closely with the numerical model results up to an altitude of about 11 km. Below 4 km, a close matching is observed between the model results and observations for phase of u0 , amplitude and

phase of v0 and Tv0 . Above the 4 km altitude, significant differences between the observations and the model results of amplitudes and phases of u0 , v0 and amplitudes of Tv0 could be due to the presence of non-migrating components, which might not be well represented in the numerical model. Results from intensive radiosonde campaigns over the equatorial latitudes and southern hemisphere (Alexander and Tsuda, 2008b) show that the phase of the temperature perturbations has an organized behaviour, whereas phases of u and v vary depending upon location. Sasi et al. (1998), Sasi et al. (2001) and Dutta et al. (2002) have reported studies of solar tides using the Indian MST Radar—derived winds during different seasons over the year and have compared the observations to results of some numerical models. They have suggested that diurnal tides in the lower atmosphere at Gadanki consist mainly of migrating components in general. However, significant variation from model values could be present due to non-migrating components, which are generated due to localized events of planetary boundary layer heat flux and latent heat release from clouds and convective sources (Dutta et al., 2002).

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Fig. 7. (a)–(c)—TRMM outputs using Visible (VIRS(VIS)) and Infrared Spectrometer (VIRS(IR)), and Microwave Imager (TMI) at 0.63, 12 mm and 85 GHz, respectively, on 23 August 2007 0700 LT; (d)–(f) same as (a)–(c), but on 24 August 1554 LT.

Fig. 8. Time–height sections of—(a) u0 (ms  1); (b) v0 (ms  1); and (c) Tv0 (1C) obtained after subtraction of respective background means; (d) Brunt-Vaisala frequency squared (rad2 s  2).

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Fig. 9. 2D spectra of period and vertical wavelengths of (a) u0 ; (b) v0 ; and (c) Tv.

Fig. 10. (Upper row) Hodographs of an  8 h wave component shown every 2 h during the first 6–14 h of observation (blue) with an ellipse fitted to curve (black). (Lower row) Filtered components of u0 (ms  1) (blue); (b) v0 (ms  1) (red); and (c) Tv0 (1C) (black). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Studies of diurnal tidal fluctuations at other tropical latitudes conducted using radiosonde flights, (e.g., Tsuda et al., 1997; Alexander and Tsuda, 2008b), suggest that present observations show good similarity to the Tv0 diurnal phase at tropical locations over the equator and in the southern hemisphere.

6. Concluding remarks In this study, we present results of a 69 h observations from the Indian MST Radar—RASS conducted during 22–25 August 2007. These are the first continuous RASS observation up to upper

Altitude (km)

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Fig. 11. Diurnal amplitude and phase of u0 , v0 , Tv0 obtained by periodogram analysis, using a least square fitting of a sinusoid at each altitude (thick line) along with the results from GSWM02 at (121N, 801E) (thin line).

tropospheric region in the tropical latitudes. In this experiment, four additional acoustic exciters were added to the existing two exciters installed by Sarma et al. (2008). The RASS wind and temperature data is supplemented by twelve simultaneous GPS radiosondes launched at about a 6-hourly interval. Simultaneous OLR measurements from MTSAT-1R and precipitation measurements from the on-site optical rain gauge and TRMM satellite are also used to aid in the interpretation of results. In the lower stratosphere, a monochromatic inertia–gravity wave activity with a period of 34.5 h and a vertical wavelength of 4 km was isolated from the data. In the troposphere, however, several wave packets passed overhead. Out of these, a clearly marked periodicity of  8 h could be extracted during the initial 14 h of the observation period. This wave propagated in the south-west direction of the site. The atmospheric stability structure computed from the RASS virtual temperature exhibited stable descending structures over the entire observation period. Signature of diurnal tides was derived from the perturbations of u, v and Tv. Phase of Tv perturbations was found to exhibit a close match with the Global Scale Wave Model (GSWM 02). This study demonstrates the utility of continuous temperature and wind profiling capability of a VHF RASS system in the elucidation of tropical atmospheric dynamic processes. Temperature profiles observed on a continuous basis with RASS, up to lower stratospheric altitudes, would be useful in improving the efficacy of numerical weather prediction models. Further value addition is envisaged by simultaneous observations with a Mie or Raman Lidar to map the height profile of cloud or humidity structures.

Acknowledgements The MTSAT-1R data obtained from the Japan Meteorological Agency (JMA) was provided, after processing for Gadanki site,

by Dr. Yoshiaki Shibagaki of Osaka Electro Communication University. This work was supported by projects support grant of National Atmospheric Research Laboratory, Gadanki, which is a Grants-inAid institution of Department of Space, Government of India. First author acknowledges the support of Japan Society for the Promotion of Science (JSPS) by way of RONPAKU fellowship to cover research visits to Japan. The study was partially supported by JSPS Grants-in-Aid for scientific research 18340140 and 19405030.

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