Indian Journal of Radio & Space Physics
Vol 43, February 2014, pp 24-40
Comparison of vertical wavelengths of gravity waves emitted by convection in the
UTLS region at Koto Tabang (0.20°S, 100.32°E) and Gadanki (13.5°N, 79.2°E)
using radars
S K Dhaka1,$,*, V Malik1, Y Shibagaki2, H Hashiguchi3, S Fukao3, T Shimomai4, H -Y Chun5 & M Takahashi6
1
Department of Physics, Rajdhani College, University of Delhi, Raja Garden, New Delhi 110 015, India
2
Osaka Electro-Communication University, Neyagawa-shi, Osaka 572-8530, Japan
3
Research Institute for Sustainable Humanosphere, Kyoto University, Kyoto 611-0011, Japan
4
Faculty of Science and Engineering, Shimane University, Matsue-shi, Shimane 690-8504, Japan
5
Department of Atmospheric Science, Yonsei University, Seoul 120-749, Korea
6
Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwanoha, Kashiwa-shi, Chiba, 277-8564, Japan
$
E-mail: [email protected], [email protected]
Received 29 May 2013; revised 16 August 2013; 22 August 2013
Observations of wind components and convection systems were made using suite of instruments centered on the
Equatorial Atmosphere Radar (EAR) at Koto Tabang, Indonesia (0.20°S, 100.32°E) during April-May 2004 in the first
Coupling Processes in Equatorial Atmosphere (CPEA) campaign. Experiments were also conducted using Indian
Mesosphere Stratosphere Troposphere (MST) Radar in India at Gadanki (13.5°N, 79.2°E) during June 2000, which is highly
convective season after the onset of south - west monsoon over southern part of India. During convective events, radar
reflectivity showed the temporal evolution of convection with different vertical velocities and depth of penetration (seen
from mid troposphere to upper troposphere). Observations covered several convective events that enabled to present forcing
scale in the vertical direction by observing vertical wavelength (λz) associated with gravity wave structure and updrafts.
Analysis of five convection events over Indonesian region and two convection events over Indian region revealed that λz of
gravity waves mostly dominated in the range of 1-3 km between 10 and 20 km heights immediately after passing the
convective storm over the radar sites. On the other hand, vertical wavelengths computed during formation of convective
updrafts over a period of ~1 hour (typical time of storm) were about 5-8 km, which is a representative of updrafts
characteristics. At both locations, λz increases gradually after the convection moved away from radars. Dynamics in the
upper troposphere and lower stratosphere seems affected by the interaction of short λz (~1-3 km) gravity waves with then
prevailing easterly wind. Dominant wave periods were observed in the range of 10-60 min with preference of shorter wave
periods (~10-20 min) at Gadanki and longer periods (~30-50 min) at Koto Tabang possibly having a relationship with quasiperiodic behaviour of rainfall and updrafts formation patterns.
Keywords: Gravity wave, Upper troposphere lower stratosphere (UTLS) region, Convection, Rainfall, Zonal wind, Radar
reflectivity, Vertical wind profile
PACS Nos: 92.60.hh; 92.60.hk
1 Introduction
Both deep and shallow convection is known to
be a potential source of wide spectrum of gravity
waves1-10. Information on gravity wave characteristics
on short horizontal and vertical scale produced by
strong convection is important to understand their
contribution to the mean flow acceleration in the
lower stratosphere11,12. The impact of convection
induced waves has also been noted in the stratosphere
and mesosphere that can modulate the existing
dynamics in the respective regions13,14. In previous
studies15,16, measurements based on rockets and
balloons showed an enhancement of wind and
temperature variance in the equatorial region in
comparison to middle and high latitude indicating the
importance of convection. Recently, Chun et al.11
have further strengthened the importance of
convection-induced waves by showing improved
results of convectively forced gravity wave drag
parameterization in the National Center for
Atmospheric Research - Community Climate Model
(NCAR CCM3).
In recent years, efforts have been made to
observe the localized convection and associated
DHAKA et al.: COMPARISON OF VERTICAL WAVELENGTHS OF GRAVITY WAVES AT KOTO TABANG & GADANKI 25
gravity waves at a high temporal and vertical
resolution using very high frequency (VHF) radars at
Gadanki (13.5°N, 79.2°E), India17,18 and at Koto
Tabang (0.20°S, 100.32°E), Indonesia2,4,19-23. Most of
these studies at Koto Tabang came as a result of
the Coupling Processes in Equatorial Atmosphere
(CPEA)–I campaign during April-May 200424.
Yamamoto et al.4 have shown a unique study of
vertical motions of air mass confined in two different
regions, i.e. upward motions from 8 km to 14 km
height and downward motions above 14 km height.
The upward motions (0.09 ms-1) associated with a
synoptic scale convective envelope are observed.
There was a sudden drop in vertical wind speed
(< 0.05 ms-1) after cumulus activity was suppressed
showing a direct link of vertical wind and convective
activities. Dhaka et al.5,25,26 have also shown
some salient features of convection and associated
wave disturbances using CPEA-I campaign data.
For instance, during convection events, gravity waves
were produced near and above strong updrafts17,26.
Gravity waves inherent vertical features also showed
to have a close relation with vertically oriented
updrafts. Some of these overhead radar observations
had confirmed the presence of high frequency gravity
waves with dominant period of a few tens of minutes.
They are believed to be well connected with
convective sources.
It is important to mention that during monsoon,
all the precipitating clouds are not deep convective
in nature27. They highlighted that the percentage
occurrence of precipitation is found to be 55%
stratiform, 9% convective, and 36% transition,
whereas the total rainfall is 12, 54, and 34%,
respectively. In a more recent study by Xu28,
characteristic features, which correlate precipitation
and convective system of summer deep convection
over East Asia, are shown using TRMM data.
A few convection events, which are captured by
the radar system during monsoon period over
two locations (Gadanki and Koto Tabang), have been
taken.
Dewan et al.29 were the first to observe
thunderstorm generated gravity waves feature at
stratosphere heights, which were linked to deep
convection and thunderstorm event in the tropics.
These waves possessed a short horizontal wavelength
in the range of 25-50 km and had wave periods
of 9-15 min. Within 1-2 hours, gravity waves had
propagated to a level of ~ 40 km after being generated
in the upper troposphere. This confirms that
convection generated gravity waves have large
spectrum on vertical and horizontal scale.
Alexander & Barnet3 presented the characteristics
of gravity waves using satellite observations and
emphasized that there is a large variability regarding
vertical wavelengths, and time averaged measures
often do not provide the real features. Hence, new
methods of analysis are needed to separately identify
the properties of wave events and their intermittency.
For instance, they mentioned that Atmospheric
Infrared Sounder (AIRS) radiance measurements
show fast vertical group speeds and individual wave
events display large wave amplitude (~3 K or higher).
However, time-averaged maps of these wave
amplitudes do not exceed 0.2 K. The difference is due
to intermittency in the occurrence of these fast waves.
Observations of satellite may be used to validate long
and short vertical wavelengths present in different
gravity waves emissions.
The temporal and spatial variability of vertically
propagating convectively generated gravity waves is
poorly understood because all currently available
observational techniques have serious limitations in
either time or space. Radar observations have high
time resolution with fine vertical spacing, but are
limited to a few locations on the globe. Using VHF
radar data, there is a possibility to observe episodic
behaviour of convection events and associated wave
features. This is also one of the points precisely
emphasized by Alexander & Barnet3 that there
is a need to understand individual events and their
intermittency.
In this paper, the characteristics of gravity waves
over Indonesian region using data from CPEA-I
campaign and over Indian region during special
observation period of 3 days during 21-23 June 2000
have been presented. The main objective is to show
vertical wavelength distribution in different
convection events and to present the unified view
during convection and immediately after its
termination. The preferential vertical scale of wave
emissions by convective sources can be detected
using this methodology. Though the case studies
don’t cover longer duration to make climatology,
however, it is an attempt in this direction to present
distribution based on limited case studies. The
Equatorial Atmosphere Radar (EAR) observations
were taken in Indonesian region under a well
coordinated CPEA campaign during April – May
26
INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
200424. The height region between 10 and 20 km is
selected to examine the vertical wavelength due to
highly expected influence of convection from lower
troposphere in vertical wind. Six convection cases
have been investigated during April 2004 (10, 11, 18,
19, 20 and 26 April) at Koto Tabang (0.20°S,
100.32°E) and two convection cases on 21-22 June
and 22-23 June 2000 at Gadanki. In general, wind
component was obtained at an interval of 3 min,
however, this experiment of June 2000 made use of
large sampling of vertical winds (~35 s, periodically).
Therefore, this data set is unique for convection study.
convection, observations were taken using vertical
beams more frequently by making a sequence of
three sets of vertical beams (ZxZy, ZxZy, ZxZy) and
one set in between as normal (E W Zx Zy N S).
The effective sampling interval for each beam was
35 s. A large number of sampling of vertical winds
enabled to examine vertical wind variability at a
fine scale during convective storms. The data was
obtained from a height of 1.5 km above the ground
in the vertical direction at a resolution of 300 m. VHF
radar is capable of distinguishing the clear air echo
from the rain echo.
2 Data used
The Equatorial Atmosphere Radar (EAR) operates
at 47.0 MHz (VHF band) with maximum peak and
an average transmitted power of 100 kW and 5 kW,
respectively30. The EAR is located at the equator
in Koto Tabang (0.20°S, 100.32°E), West Sumatra,
Indonesia. Sumatra is located at the eastern edge of
the Indian Ocean as shown in Fig. 1. In the west side
of Sumatra, there are several mountains with a height
of >1000 m. Continuous radar data were obtained
during CPEA-I campaign from 10 April 2004 to
9 May 2004. The EAR is a Doppler radar of VHF
band, which has a quasi–circular antenna array of
approximately 110 m in diameter. One beam of
the EAR is pointed vertically and four others are
tilted to the north, east, south, and west with
zenith angle of 10° in standard observation mode.
Wind measurements were carried out almost
continuously on the days of convection (10, 11, 18,
19, 20 and 26 April 2004) to study the convection
events and the generation of gravity waves. Vertical
wind data were used in this study. Data obtained
continuously with vertical resolution of ~150 m and
time resolution of 3 min.
Indian MST radar, operating at a frequency of
53 MHz with an average power aperture product of
7x108 W m2, is located at Gadanki (13.5°N, 79.2°E)
as shown in Fig. 1. A detailed description of this
radar system is given by Rao et al.31. Radar vertical
wind data were used on 21-22 June 2000 (from 1920
to 0100 hrs LT) and on 22-23 June 2000 (from 2216
to 0100 hrs LT) to study the forcing scale of gravity
waves. The radar beams were pointed sequentially
along six preset directions, viz. East (E), West (W),
Zenith (E-W polarization, i.e. Zx), Zenith
(N-S polarization, i.e. Zy), North (N), and South (S).
The oblique beams were inclined at an angle of
10 degrees from the zenith. After initiation of the
3 Results and Discussion
3.1 Result from Indonesian region
In order to investigate the forcing scale of gravity
waves in the vertical direction during convection, six
convection events, as mentioned above, have been
selected. The 19 April 2004 case for vertical
wavelength analysis is not included, however, wave
period is computed in this case too as data quality was
suitable in and around 17-18 km heights. The vertical
wind data is used for analysis obtained from the EAR.
Besides the EAR, Boundary Layer Radar (BLR) and
X-band radar data were also employed to examine
horizontal and vertical growth of convective systems.
The horizontal and vertical growth of convection
observed on different days has been shown. Temporal
evolution on horizontal and vertical growth of
convection is measured using radar reflectivity
derived from X-band radar and BLR, respectively.
3.1.1 Temporal evolution of radar reflectivity patterns using Xband radar at 2.1 km altitude
Figure 2 represents the radar reflectivity on a
horizontal plane at an altitude of 2.1 km, which is ~3 km
Fig. 1 — Location of the Indian MST Radar (13.5°N, 79.2°E) and
the Equatorial Atmosphere Radar (0.20°S, 100.32° E)
DHAKA et al.: COMPARISON OF VERTICAL WAVELENGTHS OF GRAVITY WAVES AT KOTO TABANG & GADANKI 27
from mean sea level (MSL). It shows the zonal
distribution of the maximum echo over the region
extending 15 km north and south of the radar site at 3
km in altitude. X-band radar used in this study is
different than reported by Kawashima et al.19 The
radar used in this study is located near the EAR site,
while radar used by Kawashima et al.19 is located
about 20 km south from the EAR site with Doppler
ability measurement. They had shown echo data at
nearly 4 km MSL as the radar data possibly get
influenced at lower height due to mountain range. On
the other hand, echo height of 3 km from MSL is used
traditionally, as the EAR site is at 0.865 km from MSL.
Temporal evolution of convective systems on
10, 11, 18, 19, 20, and 26 April 2004 are shown
in Fig. 2. Local time is shown on y-axis, while x-axis
represents the distance (in km) from the X-band radar
in the east (positive) and west (negative) directions.
Local convection systems were developed usually in
the afternoon on 10, 11, 19 and 20 April 2004. On 18
and 26 April 2004, convection developed in the
early morning hours and as well as in the afternoons.
The convective cloud systems expanded horizontally
and passed over the radar site in the afternoon of
10, 11, 19 and 20 April 2004. This is a typical
convection system characteristics observed over
Indonesia20,21. Rainfall was also observed in most
of the days of such convective events as shown in
Fig. 3. Enhanced radar reflectivity (>45 dBZ) showed
the association of intense convection with local
circulation over the radar region. Since, the rainfall
regions were located within few tens of kilometers
from the radar site, vertical wind continuously
got affected over radar area most expectedly due
to gravity wave generation and their propagation from
the adjacent areas.
3.1.2 Vertical growth of convective systems using BLR derived
radar reflectivity
In order to investigate vertical growth of
convection and its temporal evolution, BLR data is
shown on all observation days. Figure 3 shows radar
reflectivity in time-height section from above 2 km
heights (rain rate is also shown jointly in this figure).
Left panel shows reflectivity patterns on 10, 11, and
18 April 2004 and right panel represents patterns
on 19, 20 and 26 April 2004, respectively. Vertical
growth of convective systems with time is quite
evident mostly confined in the altitude range of
1-10 km. On 10 April 2004, a strong convection
event was identified between 1630 and 1730 hrs LT.
This event was followed by a convective cloud patch
between 6 and 10 km altitudes. Intense and deep
convection were observed almost on all the days that
typically lasted for about an hour. Radar reflectivity
plots in the vertical direction have confirmed that the
depth of penetration is noted up to 8-10 km altitudes
with high reflectivity. On some days, for instance on
10 and 26 April, convection was deeper and reached
near 14 km height (not shown here). However, the
tops of the echoes were weaker at higher heights.
Kawashima et al.19 using X-band radar with Doppler
ability had shown that occasionally the convection
Fig. 2 — Radar reflectivity patterns on 10, 11, 18, 19, 20 and 26 April 2004 at 3 km from mean sea level; movement of the convective
cloud, temporal growth and formation of convection cells is seen using X-band radar that operated at 9.74 GHz
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INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
penetrated near 14 km height. This is also confirmed
by the EAR reflectivity data, for instance on 26 April
2004 (Ref. 5, Fig. 3). The enhanced radar reflectivity
observed by BLR above the bright band structure
(at ~5 km altitude) indicates that convection systems
were quite intense that may result pushing of moisture
content above 0°C levels, which seem important for
the deep cloud formation and its further growth in the
vertical direction.
3.1.3 Optical rain gauge data
Optical rain gauge data were used to determine
whether convection events were associated with
rainfall or not. Figure 3 shows the rain rate (mm h-1)
jointly with radar reflectivity on 11, 19, 20, and
26 April 2004. Scale is shown on right bottom side.
Below 2 km height, rain rate is shown simultaneously
with reflectivity. Timings of rainfall exactly match
with enhanced reflectivity derived from BLR data.
These patterns also correspond to increased vertical
velocities (not shown). It may be noted that there was
no rainfall on 10 and 18 April 2004 at radar site.
Rainfall observed with varying amount of 40-60 mm h-1.
It is important to mention here that convective
and stratiform rain with varying amount also play an
important role in building up different depth
of penetration and radar reflectivity20,21,33. Convective
and stratiform systems can induce different vertical
forcing scale of gravity waves. Above mentioned
rainfall lie under the category of convective rainfall
as it is associated with quite deep penetration of
convective storms. Such systems can be expected
to induce wave system in the upper troposphere and
lower stratosphere (UTLS) region.
Madden-Julian oscillations (MJO) inactive
conditions prevailed before ~23 April 2004 and
thereafter, super cloud clusters (SCCs) appeared
over radar site19,20. Background wind speed and
direction is also one of the key factors to modulate
and substantially control the propagation / dissipation
of gravity waves once these are induced.
Fig. 3 — Time-height section of radar reflectivity derived from boundary layer radar (BLR) that operated at a frequency of 1375.5 MHz
[temporal and vertical growth of convection on 10, 11, 18, 19, 20 and 26 April 2004 are shown above 2 km altitude, rain rate (mm h-1) is
also shown simultaneously using optical rain gauge on the same time scale, rain observed is shown below radar reflectivity peak;
corresponding units and scale of reflectivity and rain rate are shown in dBZ and mm h-1 on bottom right corner]
DHAKA et al.: COMPARISON OF VERTICAL WAVELENGTHS OF GRAVITY WAVES AT KOTO TABANG & GADANKI 29
3.1.4 Background zonal wind
Time-height section of zonal wind is shown in
Fig. 4 using radiosonde data at Koto Tabang during
April 2004. One can infer from the graph that strong
easterly wind prevailed near 17-20 km height
reaching maximum ~-30 ms-1. Westerly wind
gradually enhanced above ~20-22 km heights, thus,
creating a strong wind shear near 20 km heights.
Lower troposphere mostly dominated by weak
westerly until 30 April 2004, and thereafter, westerly
wind burst enhanced below 5 km height (not shown).
It may be noted that near 20 km height, after the
SCCs appeared over the Indonesian region covering
radar site, easterly winds turned weak for a period
of few days19,20. Strong wind shear combined with
convective instability suppresses the development
of deep convection cells by inhibiting updraft
strength19,34. The deep penetration of convection is
supposed to excite gravity waves strongly in the
UTLS region. If the convection is not strong or
suppressed then excited gravity waves may confine
only in the troposphere region as it depends on two
important factors: phase velocity and background
wind.
3.1.5 Computation of vertical wavelength
Computation of vertical wavelength and wave
frequency would enable to understand forcing of
gravity waves vertically originated from different
convection cells developed on a given horizontal
and vertical scale.
Radar vertical wind data were used to determine
the vertical wavelength (λz) from 10 to 20 km height
region. The region between 10 and 20 km heights
is selected to observe the response of convection from
below because this region is stable in comparison
to first ten kilometers from ground. Dhaka et al.25
have shown using radiosonde data during convection
in CPEA campaign over Indonesia that generally
the atmosphere is in stable condition above 12 km
height. Stability of atmosphere supports the
propagation of atmospheric waves. On the other
hand, turbulent region associated with convection
does not support wave motions. However, above
turbulent region, wave system can get excited,
for instance in the UTLS region, i.e. near top of the
convective penetration6,17.
In order to estimate λz of emerged gravity wave
system, following criterion has been adopted to
use vertical wind profiles for analysis. Due to low
power of the EAR, there are data gaps in
UTLS region. However, during convection, data
acceptance rate is better than normal conditions. Data
acceptance rate is nearly 80% between 10 and 20 km
heights during observations around convective events,
except that of 19 April 2004, which is not included
in the vertical wavelength computation. The adjacent
Fig. 4 — Time-height section of zonal wind during CPEA-I campaign over Koto Tabang using radiosonde data; tropopause height is
shown by squares near 17 km height
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INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
vertical averaging is applied, i.e. height averaged
with 300 m keeping 3 min time interval unchanged in
all five cases. Using this method, data acceptance
rate is further increased. It has been noted that data
acceptance rate is nearly 80% on 11 and 20 April
2004 and about 90% on 10, 18 and 26 April 2004.
Between 10 and 20 km heights, thus obtained vertical
wind profiles are interpolated to fill the gap for
computing λz. The gap at most of nearly 20% (on 11
and 20 April) is not appearing at one stretch but it is
scattered (at 2-3 height gaps), which is filled using
linear interpolation. Interpolation is done uniformly
only up to 6 min gap in time and maximum 600 m in
height in all the cases.
Figure 5 shows interpolated time-height section
of vertical wind during 10, 11, 18, 20, and 26 April
2004. Using this criterion, several complete vertical
wind profiles from 10 to 20 km height are obtained.
Vertical wind profiles are still left, which could not be
completed and some gaps existed in time-height
sections, such profiles are rejected for analysis.
Also, 19 April case, having large gaps and data
acceptance rate less than 80%, is rejected; other five
cases are included in the analysis.
Alexander et al.1 have discussed data acceptance
rate during clear air and convection time, and
also mentioned the details of the different type of
convection systems during CPEA-I. However, data
acceptance rate is more in a convective environment
especially for vertical wind. Yamamoto et al.4
discussed that time averaging improves data
acceptance rate in a height region of less data
points (~12-16 km).
A typical example of computed λz on 26 April
2004 in vertical wind during and after convection
event is shown in Fig. 6. Left panel shows
pronounced peak of λz corresponding to ~5.5 km.
Right panel indicates that primary peak of vertical
wavelength is about 2.2 km, which is lesser by
a factor of about two after convection. This point
is also examined by comparing averaged vertical
wind profiles for a period of 1 hour (time averaged
of 20 vertical wind profiles) during convection and
after its termination. A similar change is found in λz.
It is important to mention here that after
termination of convection, the wave motions are
not contaminated by updrafts in the troposphere.
Short vertical wavelengths are basically a
representative of characteristics of gravity waves
originated close to the termination of convective
system. Preference of short λz (~1-3 km) is possibly
due to following reason: an excitation mechanism
is responsible for it that takes place at the interface
of unstable region (from below due to convection)
and a stable region (above). During convection,
successive oscillatory behaviour of updrafts beneath
stable atmosphere can excite the gravity waves.
This process is commonly known as mechanical
oscillator effect, i.e. dry oscillation of the stable layer
above penetrating updrafts6,32,35. Once gravity waves
are excited at the interface of convective unstable
and stable region, these can propagate or dissipate
depending upon the background atmospheric stability
and wind speed. One should also note that Brunt
Vaisalla frequency increases above tropopause
(corresponding time period decreases from about
10 min in the troposphere to 5-6 min in the lower
stratosphere). Following wave dispersion relation,
keeping in view N2 (Brunt Vaisalla frequency
squared) transition, λz should decrease in the vertical
direction. However, another mechanism based
on latent heat profile (thermal forcing) cannot be
rejected completely to induce gravity waves in the
lower stratosphere as shown in several simulation
studies36. Radar data is limited to 20 km height, it is
difficult to examine the response of thermal forcing
in this height range and one needs data up to higher
heights that is beyond the limit of this data set.
Results for λz are summarized in the form of
histograms and shown in Fig. 7. In order to examine
the distribution of λz during convection (~1 hour),
immediately after termination of convection (~1-2
hour), two different sets of histograms are shown
jointly using all five events at Koto Tabang.
This is performed to show a unified view of all
events. Lower panel of Fig. 7 represents histogram
of distribution of λz during convection on 10, 11, 18,
20 and 26 April 2004. These convective events are
mostly followed with rainfall. Typically, convective
events lasted for about an hour. It is seen from the
lower panel that λz in all five events jointly showed
maximum preference in the range of 4-7 km. This is
mainly a characteristics of updrafts formed during
convective events over radar area. However, there
was a day-to-day variability in formation of
updrafts intensity and vertical structure (not shown).
Upper panel shows histogram after the termination
of convective updrafts. This distribution is shown
over a period ~1-2 hour jointly in five cases. There is
no contamination of updrafts in computing λz shown
DHAKA et al.: COMPARISON OF VERTICAL WAVELENGTHS OF GRAVITY WAVES AT KOTO TABANG & GADANKI 31
Fig. 5 — Time-height sections of interpolated ‘w’ component shown for 10, 11, 18, 20, and 26 April 2004 sequentially from top to
bottom [data interpolated with 6 min interval in time and 600 m vertical; data acceptance rate is shown for five cases in the bottom panels
of each case]
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INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
Fig. 5 (contd.) — Time-height sections of interpolated ‘w’ component shown for 10, 11, 18, 20, and 26 April 2004 sequentially from top
to bottom [data interpolated with 6 min interval in time and 600 m vertical; data acceptance rate is shown for five cases in the bottom
panels of each case]
in this panel. Hence, histogram represents λz distribution
of gravity waves, which are generated in close vicinity
of convection. One can see that the number of profiles,
with λz in the range of 1-3 km, have increased
significantly in comparison to lower panel. There is also
a decrease in the number of vertical wind profiles with λz
in 4-7 km range. This shows that convective systems
have induced wave spectrum significantly with varying
λz in the range of 1-3 km. It is obvious that the impact of
convection in the troposphere and lower stratosphere
decreased since the forcing source is terminated.
Zonal wind speed during these convection events is
shown in Fig. 4. Almost on all days, similar easterly
wind speed is observed in the UTLS region except
on 26 April, which was influenced significantly by
super cloud clusters (SSCs)20. It may be noted
that gravity waves, encountered with similar
background wind speed almost in all cases,
provide similar conditions for their vertical
propagation or dissipation. Hence, it is justified and
reasonable to show a unified view of distribution of
λz using histogram.
DHAKA et al.: COMPARISON OF VERTICAL WAVELENGTHS OF GRAVITY WAVES AT KOTO TABANG & GADANKI 33
Fig. 6 — Maximum entropy method (MEM) power spectrum of dominant vertical wavelengths (λz) on 26 April 2004: (a) during
convection, and (b) after convection
Fig. 7 — Distribution of computed vertical wavelengths of gravity waves on 10, 11, 18, 20, and 26 April 2004 over EAR, Indonesia: (a)
during convection (mostly contaminated by convective updrafts and downdrafts) covering about 1 hour; and (b) after convection
termination over a period of about 1 hour
3.3.6 Computation of dominant wave period
In order to estimate wave period associated with
wave disturbances, averaged time series are used at a
few range bins (about 6–7 range bins, each bin
corresponding to 150 m) to produce 1 km height interval
time series data in the tropopause region and above.
Data of ~3-min time resolution is used in all six cases, it
may be noted that data on 19 April was also included
analyzing wave period near and above tropopause.
Fig. 8 — Maximum entropy method (MEM) power spectrum of
dominant wave period (min) on 26 April 2004 at 16 km height
A typical example of computed wave period during
convection is shown at 16 km height in Fig. 8 on
26 April 2004. It is evident from Fig. 8 that wave
period with ~50 min period is pronounced. The wave
periods are also computed at several consecutive
heights between 16 and 19 km. There is good
consistency in the dominant wave periods at these
heights. In general, primary peak of dominant wave
34
INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
periods were centered on ~30–50 min. Gravity waves
with similar wave periods were dominant during other
convection events too. However, at few height range
bins, occasionally a secondary peak is also noticed
that corresponds to 10-20 min (not shown). Alexander
et al.1 have shown similar wave periods present in
their study using CPEA campaign data during some
of the convection days.
Analyses of these convection events revealed that
wider spectrum (varying vertical wavelength) of
gravity waves had generated overhead and away from
radar during convection. Characteristics and role of
gravity waves with vital information on variability of
λz is important to understand dynamics in the
UTLS region3. However, there is limitation of radar
data up to 20 km height in the present study. The
current observations provide valuable information on
convective sources and generated λz distribution in the
vertical direction.
3.2 Results from Indian tropical region
Radar observations were carried out on 21–22 and
22-23 June 2000 during convection. It may be noted
that there are no data gaps in the UTLS region due to
large power of the Indian MST radar. The radar data
provides information about reflectivity (echo power),
turbulence (Doppler width) and vertical wind
disturbance (w). In this study, radar data is primarily
used to determine vertical wind component in
the troposphere and lower stratosphere. Details of
radar experiment are already provided above.
Dhaka et al.17,18 have shown structure of turbulence
and behaviour of tropopause stability during
convection using this data set. The development,
maximizing and weakening of convective phenomena
within a time span of several hours on 21–22 and
22-23 June 2000 was successfully monitored.
The vertical wind in the troposphere during this
period was found to be ~6–8 ms-1. Variations of the
radar returned signal power for the vertical beam with
height and time showed the continuous growth of
high turbulence in the ambient air, ascending as
high as the tropopause. The satellite cloud images
exhibited the temporal growth and movement of cloud
clusters over the region of the radar site during this
period. Information on tropopause was obtained from
the nearby station Chennai, which is located about
100 km south-east from the radar site. Cold point
tropopause was detected at ~16.5 km height during
21-23 June 2000. However, detailed information
regarding distribution of λz using several vertical wind
profiles are not discussed and shown by Dhaka
et al.17. Here, mainly histograms are presented
that show distribution of λz among different vertical
wind profiles with passage of time starting from
convective event to the generation of gravity waves
and their dissipation.
Typical updraft formation with varying vertical
shape and intensity in the middle and upper
troposphere is shown in Figs (9 and 10),
respectively. Figure 9 shows the vertical wind
profiles during convection (a) and after its
termination (b). It is seen that between 8 and 16 km
heights, convective updrafts were strong of the order
of ~4 ms-1 seen in large thick layer of air mass. After
termination of updrafts, signature of wave motion
emerged above 10 km height as seen in the right
Fig. 9 — Vertical wind profiles on 21 June 2000: (a) between 20:01:57 and 20:08:31 hrs LT; and (b) after termination of convection
[each profile is shifted right side by adding 2 ms-1 to avoid overlapping]
DHAKA et al.: COMPARISON OF VERTICAL WAVELENGTHS OF GRAVITY WAVES AT KOTO TABANG & GADANKI 35
Fig. 10 — Vertical wind profiles on 22 June 2000: (a) between 22:43:07 and 22:47:56 hrs LT during convection; and (b) between
23:50:29 and 23:57:03 hrs LT [each profile is shifted right side by adding 2 ms-1 to avoid overlapping]
panel. Amplitudes of wave motions in vertical wind
are about 0.5 ms-1 with short λz. Similarly, Fig. 10
shows two panels of vertical wind profiles on
22 June 2000. Figure 10(a) is a representative of
convective updrafts focused around 12 km height.
One can see that updrafts seen in Fig. 10 were
centered around 12 km heights, while the updraft
structure in Fig. 9 had deeper coverage. Figure 10(b)
shows wave motions in the UTLS region
after termination of convection with small
amplitudes upward.
Rain data were also recorded at a fine resolution
of 1 minute at the radar site using a disdrometer.
Dhaka et al.18 had discussed that a quasi-periodic
pattern in the rainfall emerged during 6 hours
of experiment on 21-22 June 2000. In the first
30 minutes of observations, the rainfall pattern was
not coupled parallel on the same time scale as that of
the strong updrafts. A moderate rainfall of 2–7 mm h-1
was observed shortly after the strong updraft
formation in vertical wind, while rainfall rate was
higher ~30-60 mm h-1 on 22-23 June 2000 (Ref. 18).
However, there appeared to be an apparent
relationship between a periodic increase in the rainfall
and the enhanced vertical wind.
It is informative to mention here that the
updrafts formed earlier than the peaks in the rainfall
on 21-22 June 2000. On the other hand, on
22–23 June 2000, the formation of the updrafts and
the successive peaks in rainfall data were noted
around the same time.
Reddy et al.33 have discussed some of the
issues regarding convective and stratiform
rainfall over India. On the other hand, Koto
Tabang region is greatly affected by ocean-land
contrast and mountain effects, which generates
local convection in the afternoon and responsible
for rainfall37,38. The origin of rainfall and
surrounding environment depend heavily on
the season. These cited studies also indicate
and confirm that rain patterns show variability
depending upon season and evening/ morning
hours of the day. Hence, forcing scale of gravity
waves could be different on horizontal as well as
on vertical scale.
3.2.1 Zonal wind over Indian MST radar during June 2000
Time-height section of zonal wind during June
2000 at Gadanki, location of Indian MST radar, is
shown in Fig. 11. National Centre for Environmental
Prediction (NCEP) data is used to construct the
time-height plot. Weak westerly wind prevailed in the
lower troposphere (~10-12 m s-1). Above ~ 400 mb,
easterly wind dominated in the UTLS region
throughout the month of June. Maximum wind speed
noted near 100 mb level is ~-35 ms-1. Easterly jet
is quite strong over Indian southern sub-continent
during summer season. Troposphere wind conditions
and strengths were similar at Indonesian region too,
however, wind speed were lesser in comparison to
Indian tropical region. On the other hand, from 20 km
height to 30 km height, wind speed and direction
were different at two locations. In this height range,
westerly wind prevailed over the EAR radar region
during April 2004, while easterly wind dominated
during June 2000 over Indian MST radar region.
This change over in direction corresponds to different
36
INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
phases of quasi-biennial oscillation (QBO) in
zonal wind. Propagation conditions of gravity waves
would be different in opposite phases of QBO that
dominates in 20-30 km height range. However, in
the present case, study is confined up to 20 km
heights due to radar limitation. Below 20 km height,
there are almost similar conditions of easterly wind at
Gadanki and Koto Tabang during two different
timings of observations. Therefore, it is highly
expected that propagation and dissipation conditions
are similar for gravity waves at two locations; hence,
comparison of vertical wavelengths is reasonable
below 20 km height.
Fig. 11 — Time-height (pressure levels in mb) section of zonal
wind (ms-1) during June 2000 at Gadanki (13.5°N, 79.2°E), Indian
MST radar site using NCEP data [different shaded portion above
400 mb level shows easterly; color contrast around 400 mb level
corresponds to transition of westerly wind from lower troposphere
to easterly wind in the middle and upper troposphere]
3.2.2 Computed vertical wavelengths on 21-22 and 22-23 June
2000
In order to investigate in detail the forcing scale
in the vertical direction, λz were computed for
each profile between 10 and 20 km height over
a period of ~6 hours on 21-22 June and ~4 hours
on 22-23 June 2000 during convection and after its
termination. Note that vertical wind profiles were not
averaged as done in the case of Indonesia region as
data was consistently obtained throughout the UTLS
region. Then, computed λz were arranged in the
form of histogram as performed for Indonesian
region. In this case too, λz distribution is separated
during convection and after its termination on both
days of observations and shown in Fig. 12. Figure 12
(upper panel) shows the histogram of computed
λz jointly on 21-22 June and 22-23 June 2000 during
convective events, which were also accompanied
with rainfall. The x-axis represents scale of λz in km
and y-axis shows number of vertical wind profiles.
Numbers of profiles with λz in the range of 4-8 km are
larger in comparison to 1-4 km range. As mentioned
earlier that at Gadanki radar site, sampling of vertical
wind were at a higher resolution of time in that
experiment, hence, data on λz is more in comparison
to Indonesian radar. Distribution of λz seen focused
on 4-8 km in the upper panel is a representative of
convection updrafts characteristics.
As seen in Figs 9 and 10 convective updrafts are
of varying shape and intensity in the vertical
direction, hence, they produce different λz but mostly
concentrated on 4-8 km range. Histogram distribution
shows similar tendency and shape as seen in Fig. 7
over Indonesian region except near λz >7 km, where
Fig. 12 — Distribution of computed vertical wavelengths of gravity waves on 21-22 and 22-23 June 2000 over Gadanki, India: (a) during
convection (mostly contaminated by convective updrafts and downdrafts); and (b) after convection termination
DHAKA et al.: COMPARISON OF VERTICAL WAVELENGTHS OF GRAVITY WAVES AT KOTO TABANG & GADANKI 37
number of profiles is more over Indian region
indicating deeper penetration of updrafts. Lower
panel of Fig. 12 represents distribution of vertical
wavelength induced by convection jointly on
21-22 June and 22-23 June 2000. Numbers of vertical
wind profiles shown in this histogram are included
after the termination of updrafts and rainfall.
The main purpose of this histogram is to show
statistically the forcing scale of gravity waves
between 10 and 20 km height after termination of
updrafts. In the lower panel, numbers of profiles
increased significantly in the range of 1-4 km. This is
a clear response and scale of vertical forcing of
convection induced gravity waves observed at Indian
MST radar site. It is clearly seen that more than 50%
profiles show λz in the range of 1-4 km. A simple
comparison of upper and lower panels reveals that
distribution shape shifts from high λz to short λz soon
after the termination of convection. Transition point
of λz is at about 4-5 km. The temporal variability of
λz is also examined after termination of convection.
There is an increasing tendency of λz with passage
of time as noticed over Indonesian region. It has
been confirmed by power spectrum of time averaged
of vertical wind profiles every hour in sequence from
the termination of updrafts at both the location.
Histograms analyses of observations on 21-22 and
22-23 June 2000, examined each day separately, also
confirm that forcing scale in the vertical direction
dominates with λz in the range of 1-4 km. However, a
marginal day-to-day variability is noticed in the
distribution of λz that depends on the vertical wind
velocity and penetration depth of updrafts.
Mechanism of generating short vertical wavelength
gravity waves at two locations seems similar, as
discussed above, especially for short vertical
wavelengths. Easterly zonal mean wind prevailed over
Indian radar as well as over Indonesian radar sites
during observations. Tropopause at both the locations
was observed at similar heights, for instance it was
noted at ~16.5 km near the Indian MST radar and at
~17 km height at the EAR, respectively. Though,
wind speed was little larger at Indian radar site than
the EAR site. Therefore, background conditions were
almost similar at both the places; hence, comparison
of histograms is meaningful.
Wave amplitudes seen in vertical wind profiles
(Figs 9 and 10, right panels), are generally small
above ~16 km heights, which is a region of strong
wind shear. It seems that the waves, possessing short
vertical wavelength (~< 3 km), around the region
of strong easterly jet get dissipated easily. In the
tropopause region and above (up to 20 km height),
one can easily see the transition in the magnitude
of Brunt Vaisalla frequency (~from a time period of
10 min to 5-6 min, not shown here). This also favours
the decrease in vertical wavelength upward and
this tendency is often seen in vertical wind profiles in
UTLS region. However, gravity waves with large
λz (~> 4 km) could not be assessed in terms of
their wave amplitudes variability with height due to
limitation of data. The mechanism of generation of
gravity waves with large λz may have a link with
thermal forcing (vertical profile of heating).
There is some dissimilarity noted at two different
locations. It is not possible to compare these results in
depth given the limited number of convective events
and apparently different seasons of observations.
However, from the limited comparison, it is
concluded that wave periods on two days of
observations found in the range of 10-20 min in the
upper troposphere at Gadanki18, whereas the dominant
wave periods were of the order of ~30-50 min at Koto
Tabang observed in six cases. However, there
appeared a secondary peak at ~10-20 min in a few
height range bins only in two cases. But maximum
power corresponds to ~30-50 min. This reveals a kind
of dissimilarity of successive formation of convection
cells over a given interval of time and their upward
movement up to the stable layer in the UTLS region.
By looking at rain patterns at two different locations,
one can notice a difference. At Gadanki, rain patterns
were of quasi-periodic nature and lasted for longer
duration, whereas at Koto Tabang, rainfall patterns
showed a single strong peak. There was also an
association of updrafts with quasi-periodic nature of
rainfall at Gadanki at an interval of 10-15 min
(Ref. 18). This seems to be a cause of difference in
induced dominant wave periods at two locations.
A different speed of updraft movement is also
noted, for instance at Gadanki, which is larger by
a factor of 3-4. This suggests a kind of rapid growth
of intense convection over Gadanki region, which is
occasionally a part of convection systems formed over
Bay of Bengal.
4 Summary and Conclusions
The following points are summarized from the
analysis:
(i) Typically a single strong convective event with
rainfall lasted, generally, less than an hour at
Koto Tabang.
38
INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
(ii) Histogram analysis of five convection events
over Indonesian region and two convection
events over Indian region revealed that
gravity waves were generated during
convection that showed pronounced range
of vertical wavelength ~1-4 km (maximum
number increased with ~λz < 4 km) between
10 and 20 km height.
(iii) These ranges of vertical wavelengths were seen
dominated after termination of convection
within ~1 hour. Vertical wavelength showed
an increasing tendency gradually after ~2 hours
when convection moved away from the radar
sites.
(iv) On the other hand, vertical wavelength
computed during formation of convective
updrafts over a period of ~1 hour (typical time
of storm) is about 5-8 km, which is a
representative of updrafts characteristic and
identified in all convection events as seen in
histogram analysis. Histogram distribution
showed similar tendency and shape over
both Indian and Indonesian region except
near λz >7 km, where number of profiles is
more over Indian region indicating deeper
penetration of updrafts (Figs 7 and 12).
(v) Dominant wave periods were observed in the
range of 10-60 min with preference of shorter
wave periods (~10-20 min) at Gadanki and
longer periods (~30-50 min) at Koto Tabang.
Pronounced wave periods seem to have a
relation with quasi-periodic behaviour and
duration of rainfall.
(vi) Wave amplitudes seen in vertical wind profiles
(Figs 9 and 10, right panels), are generally
small above ~16 km height, which is a region
of strong easterly jet [also a region of strong
wind shear due to sharp change in zonal wind
speed above 100 mb level (Figs 4 and 11)].
It seems that the waves possessing short
vertical wavelength (~< 3 km) around the
region of strong easterly jet get dissipated
gradually upward.
In this study, a suite of instruments were used
that includes the EAR, BLR and X-band radar under
first CPEA campaign at Koto Tabang, Indonesia and
MST Radar observations at Gadanki, India to bring
out characteristic feature of gravity waves in
the UTLS region induced by convective systems.
Relationship between evolution of convection in the
lower and middle troposphere and induced gravity
waves in the UTLS region is examined. Characteristic
feature of updraft formation and gravity wave
generation is separately shown. Histogram analysis
has clearly shown the difference of vertical forcing
scale of the updrafts and gravity waves. Histogram
distribution of λz of gravity waves at both the radar
sites showed similar tendency and shape. Though this
study has not included large number of convective
events, however, it is an initial effort to examine
such features in the equatorial and tropical region.
Main aim of this paper was to determine forcing scale
of gravity waves in the vertical direction using
vertical wind profiles. More than 50% vertical wind
profiles supported that gravity waves were generated
with λz<4 km after termination of convective events
at both the radar observation sites.
Some information about the horizontal scale of
source distribution at Koto Tabang is obtained using
X-band radar data. Figure 2 shows the horizontal
structure of cloud system within 60 km radius from
the EAR site. In most of these cases, convection
systems were developed in the afternoon everyday
except on 18 April and 26 April 2004. It may be noted
that convection system on 26 April was different due
to the presence of SSCs at radar site. The tendency of
movement of convective systems was also similar, i.e.
oriented from west to east side. Radar reflectivity on a
horizontal plane shows a sort of patterns with varying
intensity. These patterns appeared quasi-periodically
in the afternoons. Such patterns are believed due to
organization of convection on horizontal scale, which
are certainly a few tens of km in dimension.
The decrease in the amplitude seen in UTLS region
(Figs 9 and 10) may be due to damping mechanism
that caused the dissipation of the gravity waves. Such
short vertical wavelength gravity waves seem
important for the dynamics of UTLS region as their
vertical propagation could not be so obvious due to
dissipation in the presence of horizontal wind shear or
due to presence of large amplitude Kelvin waves.
Kelvin waves with period 10-12 days and vertical
wavelength of 6-7 km is detected by Tsuda et al.22.
For short vertical wavelength gravity waves, such
large amplitude waves can act as a dissipating
component. Ratnam et al.23 have also shown gravity
waves with 2-3 days wave period and large scale
horizontal wave length during CPEA campaign.
In these several reported results, it may be concluded
that there was a large spectrum of wave system
present during CPEA campaign that had a link with
DHAKA et al.: COMPARISON OF VERTICAL WAVELENGTHS OF GRAVITY WAVES AT KOTO TABANG & GADANKI 39
the convection. However, it is important to mention
here that in close vicinity of convection, a major part
of gravity wave spectrum is induced with the unique
characteristics mentioned in this study.
Acknowledgment
The authors sincerely thank the reviewers and
guest editor for giving their critical comments that
helped a lot in improvement of the manuscript. One of
the author (SKD) gratefully acknowledges RESPOND
– ISRO, India for the financial support. The EAR is
operated by the Research Institute for Sustainable
Humanosphere (RISH) of the Kyoto University,
Japan, and the Indonesian National Institute of
Aeronautics and Space (LAPAN). MST radar is
operated by National Atmospheric Research
Laboratory (NARL) with the support of ISRO, India.
The authors thank Mr M Inoue who helped in making
zonal wind figure using NCEP data. NCEP data is
taken from http://www.cdc.noaa.gov.
9.
10.
11.
12.
13.
14.
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