Indian Journal of Radio & Space Physics
Vol. 30, December2001, pp. 315-318
Diurnal evolution of temperature profile of troposphere and
lower stratosphere using MST radar
K Revathy & G Mrudula
Department of Computer Science, University of Kerala, Kariavattom, Trivandrum 695 581
and
S R Prabhakaran Nayar & B V Krishna Murthy
Department of Physics, University of Kerala, Kariavattom, Trivandrum 695 581
Received 2 Jun e 2000; revised 4 April 2001; accepted 31 July 2001
The observation of the fluctuations in the vertical wind using MST radar provides an excellent opportunity to determine
the temperature profile of the troposphere-stratosphere region. The MST radar is operated in the temperature mode to
evaluate the diurnal evolution of the temperature profile. Using these values, the diurnal variation of the temperature at
different altitude ranges is calculated. The height variation of the tropopause within a day is also calculated from these
results.
1
Introduction
The atmospheric temperature vanahon with
altitude plays an important role in the turbulence
studies and the atmospheric stability. Regular
measurements of temperature profile are made using
radiosonde twice-a-day basis by the Meteorology
Department. Revathy et al. 1 have developed a new
method of evaluating atmospheric temperature using
fluctuations in the vertical wind measured by the
MST radar. The unique feature of the MST radar is
the capability of measurement of vertical motion of
the atmosphere, which is an important parameter in
the study of the dynamics of the middle atmosphere.
The MST radar has the capability of measuring the
wind in the troposphere and the lower stratosphere
with time scales of few tens of seconds. These
measurements provide a powerful means of
determining the Brunt-Yaisala frequency, which is the
natural frequency of the medium, and hence the
temperature with high temporal and altitude
resolutions. Using the method of Revathy et al.1. 2 , it
is possible to study the temperature profile of the
troposphere-stratosphere region with good altitude
and temporal resolution. In the temperature mode of
operation, the MST radar is operated for measuring
the vertical wind continuously with good resolution in
time so as to evaluate the Brunt-Yai sala frequency
from the time series of vertical wind at each hei ght
range.
In this work, the temperature profile of the
troposphere-stratosphere region is evaluated by
operating the MST radar in the temperature mode of
operation and also in diurnal cycle. The radar is
operated over a day, taking twelve samples of vertical
wind for each one-hour duration, centred around each
two hours. From the time series analysis of vertical
wind data for each height range and evaluating the
Brunt-Yaisala frequency, the temperature profile is
determined. Using the twelve samples of temperature
profiles evaluated, the diurnal evolution of the
temperature profile is studied. The features in the
diurnal evolution of temperature are studied for
different heights. The variation in the tropopau se
height determined from the temperature profile is also
analyzed.
2
Evaluation of temperature
The vertical wind velocity in the tropospherestratosphere region shows a wide variety of
fluctuation s. The spectra of vertical wind velocity
shows a typical pattern with a peak at the Bru ntYai sala frequency having a steep decrease in
amplitude in the high frequency side and a fairly
shallow decrease in amplitude in the low frequency
side. Based on earlier studies by Rottger 3 , Revathy et
al. 1 developed a method for determining tropospheric
and lower stratospheric temperature from MST radar
observations of vertical wind. In this method , usmg
INDIAN 1 RADIO & SPACE PHYS, DECEMBER 2001
316
vertical wind velocity data (w) sampled at about 45 s
intervals for duration of one hour the temporal
spectrum of w is obtained. From the spectrum, the
Brunt-Vaisala (B-V) frequency (N) is determined by
identifying the spectral peak corresponding to N based
on the following criteria.
(i) The spectral peak at the B-V frequency is the
prominent spectral peak.
(ii) The spectrum on the high frequency side of the
B-V frequency spectral peak shows a steep
decrease with no prominent spectral peaks.
The B-V frequency N is related to the temperature T
by
2
+ 1]
... (1)
T=r dT!d h]=[N2 T lg -[']
... (2)
N =(g/T) [ (d Tl dh)
or
where, g is the acceleration due to gravity, h the
altitude and 1 the adiabatic lapse rate (dry). Equation
(2) is integrated to obtain T(h) from N(h) determined
from the temporal spectra of vertical wind at different
altitudes with boundary value T0 at ho (the lowest
altitude at which N is obtained). The parameter To is
obtained from the ground temperature value as
described by Revathy et al. 1• The limitations of this
method have also been discussed in detail by Revathy
et a/. 2• It is basically applicable under convectively
stable conditions and when the wind and wind shear
are not very high. Any Doppler shift due to
background vertical wind will be insignificant and
negligible as the mean and vertical wind over t e
period of observation (-two hours) is zero. The errors
involved in the determination of temperature have
also been studied by Revathy et al.
3
Observation techniques and data
The present study is based on the observations of
the vertical wind at Gadanki (13.5° N, 79.2° E) usi ng
MST radar of the National MST Radar Facility duri ng
23 and 24 December 1998. This observation forms
part of the study of diurnal and seasonal variation of
the temperature profile for the period November
1997-December 1998. The MST radar at Gadanki is
highly sensitive, pulse coded VHF phased array,
operating at 53MHz and has an average power
aperture product of 7xl08 Wm- 2 . We have conducted
the diurnal experiment on one day in each month fo:r a
period of one year from November 1997 to December
1998. In this paper, the preliminary results pertaining
to December 1998 are presented as an illustrative
example of diurnal variation of temperature. Results
of a detailed study of the diurnal variation over the
one-year period will be presented later. The
observation of vertical wind in the temperature mode
of operation started at I 054 hrs LT on 23 Dec. 1998
and ended at 1047 hrs LT on 24 Dec. 1998. To study
the diurnal evolution of temperature profile, the radar
was operated in the temperature mode once in two
hours in a day. In the temperature mode of operation,
the observation is carried out with one-hour observation of profiles of moments of vertical wind from
the FFf spectrum at intervals of about 46 s in the
height range 3.6-30.6 km. The details of experimental
parameters associated with temperature mode of
operation of the MST radar is as shown in Table 1.
These data are processed using the atmospheric data
processing (ADP) software to eliminate spurious
values due to noise and to calculate the vertical
velocity.
In addition to the vertical wind, four cycles of fivebeam observations were carried out on either side of
the vertical wind observation. The four cycles of fivebeam observation is for the determination of background wind around the temperature profile to be
determined. It is observed that the background wind is
quite low, not exceeding 20 rn/s on the day of observation. For evaluating the diurnal evolution of the
temperature profile, the experiment is repeated,
Table 1-MST radar parameters associated with temperature
mode of operation
Date
29 Dec . 1997
Time
II hr 25 min
No. of range bins
No. of FFf points
256
No. of coherent integration
No. of incoherent integrati ons
128
I
IPP
Pulse width
1000
183
16
Code n ag
No.of observati on windows
Window I start
2
24jlS
Window I length
182 jlS
Window 2 sta rt
Window 2 length
0
0
Beam position
Scan cycle
I
Receiver attenuatin
OdB
Data type
3
Comments
Zx
REV ATHY et al. : TEMPERATURE PROFILE OF TROPOSPHERE & LOWER STRATOSPHERE
317
24
23
09
21
18
E
- 15
:::£
w
0
::::)
11-
12
_J
<(
9
6
3
200
210
220
230
240
250
260
270
280
TEMPERATURE, K
Fig. !- Diurnal evoluti on of temperature profiles during II 00 hrs LT (23 Dec. 1998)- 0900 hrs LT (24 Dec. 1998)
[numbers indicate the time of observ.ation].
centred on each two-hour of the day providing twelve
samples of one-hour vertical wind and background
wind. From the time series of data at each height, the
Brunt-Vaisala frequency corresponding to each 150m
altitude interval is evaluated. Figure 1 depicts the 12
profiles of temperature in the troposphere-stratosphere
region , i.e. in the height range 3.6-25 km. The altitude
is limjted to 25 km because, in the data above 25 km
the noise level is very high. The errors in the
temperature evaluation have been dealt with in detail
4
by Revathy et al. • The standard errors are typically 1
K for 10 km and 1.6 K for 21 km for an altitude
resolution of 150m.
4
Diurnal evolution of temperature profiles
Figure 1 depicts the 12 temperature profiles during
the 24-h period of observation on 23-24 Dec. 1998.
The profiles are similar and show a variety of
flu ctuations. It is clearly seen that the temperature
profile shows diurnal variation in addition to the
typical height profiles. To evaluate the pattern of
diurnal variation of temperature, samples of temperature values for few height ranges, viz. 7 km, 10 km,
270
7km
260
250
::.::
[\! 240
10km
::>
~:rn
w
"-
...~ 220
20km
210
15km
200
1 ~ ~-----L----~----~----~--~~--~
15
19
23
03
07
TIME , hrs LT
Fig. 2--Curves showing diurnal variation of temperature at
different altitudes
15 km and 20 km are selected and plotted in Fig. 2. It
is noticed that the temperature values at each hei ght
range show a typical diurnal variation. The diurnal
variation at all the altitudes is, in general, similar. For
the higher altitudes the range of fluctuation s in the
temperature is higher compared to that of lower
altitudes. It may be noticed that after 24 h the values
INDIAN J RADIO & SPACE PHYS, DECEMBER 2001
318
17 0
~ 165
w
0
:::>
>-
;=
;j_ 16.0
15 5
17
13
05
01
21
TIME. hrs LT
represent the tropopause, especially, its day-to-day
variation. Figure 3 gives the variation of the tropopause height during the 24 h of observation. It is seen
that the height of tropopause varies from 15.45 krn to
17.05 km within 24 h. It comes to a minimum altitude
at around 1500 hrs LT and reaches a maximum height
of 17 km at around 0300 hrs LT. Figure 4 depicts the
variation of the tropopause temperature with local
time. It is noticed that the diurnal variation of the
tropopause temperature is exactly similar to the
variation of the temperature at altitudes 15 km and 20
km as shown in Fig. 2. The tropopause temperature
shows a variation with a minimum of 190.5 K and a
maximum of 224.8 K.
Fig. 3---Curve showi ng the variation o f tropopause height
6
200
1 00 L------L------~------~----~-----J
13
17
21
01
05
TIME. hrs LT
Fig. 4-Curve showing the variation of tropopause temperature
of the temperature in all the altitude ranges considered
here come back to the original starting temperature
level. It is also seen that shorter period fluctuatio ns
are superposed on the diurnal variation. However, as
the present temperature values themselves correspond
to the duration of two hours, and are obtained at twohour intervals, the characteristics of the shorter pe riod
variations could not be delineated .
Acknowledgements
The authors wish to thank the Department of
Science and Technology, Govt. of India and the
National MST Radar Facility, Tirupati, and UGC cell
at the S V University , Tirupati, for the financial
support and help during the course of the work .
References
I
2
5
Diurnal evolution of the tropopause height and
temperature
Tropopause hei g ht is identifi ed as the altitude at
which the temperature becomes minimum. Though
this is different from the WMO definition of tropopause, the minimum temperature may be taken to
Conclusion
Analysis of temperature vartatwns at typical
altitudes shows that the amplitude of fluctuation in
temperature increases with altitude. The present study
reveals that the altitude of the tropopause varies with
time of the day in the range 15.45-17.1 km. Similarly
the tropopause temperature shows variation very
similar to that in the higher altitudes of 15-20 km. The
micro-fluctuations in the temperature profiles may be
studied in detail and understood using the observation
of the background wind. A detailed study of the
dependence of the diurnal and seasonal variation in
the temperature profile on the background wind may
also be carried out separately.
3
4
Revathy K, Prabhakaran Nayar S R & Krishna Murthy B Y.
Geophys Res Lett (USA), 23 ( 1996) 285.
Revat hy K, Prabhakaran Nayar S R & Kri shna Murthy B V,
STEP Handbook, edited by Be lva Edwards ( Illinois, USA),
1997, p. 180.
Ranger J, Determination of Brunt- Va isala frequency from
vertical wind spectra. MAP Handbook, USA, Vol. 20, 1986,
pp. 168-172.
Revathy K, Prabhakaran Nayar S R & Krishna Murthy B V,
Indian 1 Radio & Space Phys, 27 ( 1998) 24 1.