Indian J . Phys. 6 6 B (2 ), 1 2 3 -1 2 9 (1 9 9 2 )
Acoustic detection of an elevated stable layer in pre-monsoon ABL over Kharagpur — a case study
Biswadev Roy, D K Rakshit , B Pal *nd B Chakravarty
Deparimeni of Physics Sc Meieorology, Indian Inshtutc of Technology, Kharagpur 721 302, India
Receisfed 7 May , accepted 23 Sepiember 1991
A b stra c t : A prc-monsoon dopplcr acoustic remote sounding of the atmosphere (operated ai a frequency of 1500 Hzj reveals an interesting echo pattern obtained over Kharagpur on M a y 24.
1990 Some meieorological parameters such as gradient Richardson Number, kinetic energy dissipation rale, and heat flux have been calculated for the given lime range and relevant micrpreiaiions of the activity have been discussed in the form of a case study
K e y w o rd s’ : Prc-monst>on atmosphere, sonic detection and ranging (S O D A R ), boundary layer profiler (B L P ), atmospheric data acquisSition system ( A D A S ) , lurbulonce, dissipation Atmospheric rate, Instnimenialion Research (A IR).
P A C S N o : 92 60 Fm
1. In tro d u c tio n
Many experimenis over the Indian subcontinent have been done by acoustic remote sounding of the near-sea-coasi atmosphere (Singal ei al 1986, Gera ei al 1990 and Sengupta e[ al 1990), and it appears ihal this technique has proven its capability to record sea breeze induced elevated scatier layer signatures.
Sea breeze activity over Kharagpur occurs for at least 30% of the pre-monsoon period. An early morning elevated scalier layer formation was delected from the sounder thermal echogram on May 24, 1990 and this feature is explained in terms of sea breeze occurrence in details in this paper. Possible thermodynamic, turbulence and physical parameters ol the Atmospheric Boundary Layer (ABL) during the occurrence period are analysed with rcibrcncc to case .siudies that have already been reported.
Brown ei at (1978) have pointed out that such acoustic returns are only caused by scattering from turbulence fields in the vicinity of the elevated inversion regions and cannot be attributed to any sort of ‘reflection’ of sound wave. Such envelope echo formation also
Presem address : Department of Physics, Jadavpur University, Jadavpur, CalcuTta-700 032, Ibdia
© 1992 I ACS
124 Biswadev Roy. D K Rakshit. B Pal and B Chakravariy
occurs because of the lifiing inversion (Cronewett et al 1972» Wycoff et al 1973, and Fukushima et al 1976).
Kharagpur (22.25N, 87.37E) is a region of deep moist convection because of its location within a disiance of about 80 kilometres from the coast-line of Bay of Bengal. Sea
breeze flow in the region is frequent in the pre-monsoon months mostly in the early post- noon and late afiern(X)n hours (Lohar ci al 1990).
2. In-situ m easurem ents, data acquisition and analysis
As a pan ol the Monsoon Trough Boundary Layer Experiment (MONTBLEX main experiment, Gupta 1990) a doppler acoustic sounder system (Aerovironment make) was operational at Kharagpur from May 16 to September 10, 1990. During its operation, processed data were recorded and continuous thermal echogram was archived in facsimile chart as shown in Figure 1 for the morning thermal boundary layer. Vertical range was preselected at 900 metres. Echo.was recorded from 05:00 to 12:00 hours, thereby revealing an a.scending inversion with two discrete temperature discontinuities. An examination of 30 minutes averaged data (Figure 1) showed an oscillatory pattern of the elevated stable
900
Vf X
) 1 1
1 1 '
... .. 1 l.j
r , i 1 1 . f : ! *r r 1
1 f r ^ '
' ’' M l
_-____ _.. . » r r 1 i '
HLV:. j y (j)j
— H
XCT
i S i i i t M J M i
:}ii] li! ‘ r-
tim e (HRS)IST
Figure 1. Facsimile records of conveciive pattern from 05:00 to 12:00 hours on May 24, 1990 at kharagpur. Undulating elevated inversion has maximum height at 05:20 hrs. Plumes are seen to intensify vertically with lime.
Structure with an approximate period of 65 minutes. A complete break-up of the layer occurred al around 10:45 hrs. while the surface suability ceased at 07:45 hrs. as recorded by SODAR. Simultaneous Kyioon measurements from 05:00 hrs, showed a positive potential temperature gradient in the surface layer with neutral stability above it. There was no
Acoustic detection o f an elevated stable layer etc 125
possibility of IVonlal passage or any radiation induced wave formation during this period.
Elevation of the stable layer is supposed to have been caused due to break-up of the nocturnal inversion under active solar healing of the atmospheric surface based layer and associated thermal plumes in the atmospheric boundary layer over Kharagpur. Large acoustic returns arc expected from buoyant thermal plumes rich in temperature structure parameters with giadicnt Richardson number as one of the indicators for the associated turbulence fields in the vertical range.
Overall structure of the associated elevated Inversion occurrence conforms to the data on the stratified layer structure (Singal et al 1984) i|nd are found to be as follows :
i) average thickness is ~ 150 metres, :
ii) average height of (xcurrcncc -'410 metres and,
iii) duration of the envelope echo persistence is (or 4 hours and 45 minutes.
It may therclbrc be assumed that the phenomenon is a sort of clcyatcd scatter layer Ibrmation during pre-monsoon pcriml as the probability of such Ibnnation is relatively high during this |Xjriod (Singal et al, 1984).
2./. Wind analyses
Wind shear and gradient Richardson number were calculated for three heights of interest i.c.
at 90, 420 and 690 metres respectively. It is to be noted that with the rise in solar heating rate, the velocity shear gained prominence with height (as shown in Figure 2).
t i m e (hrs) 1ST
Figure 2. Shows ihc lime evolution of the vector velocity shear for 90, 420 and 690 metres level in the ABL. It is to be noieJ that velocity shear acquires maximum iniensiijcsiat 90 ami 690 ineircs respectively where a sharp turbulence disconlimiiiy might exist.
126 Biswadev Roy, D K Rakshit, B Pal and B Chakravarty
Gradient Richardson number (/?,^) was calculated by incorporating the potential temperature and associated wind vector in the vertical pressure height range of 3 to 480 metres by hoisting a gas filled Kytoon alongwith a Boundary Layer Profiler (BLP) and the data were received, processed and recorded with the use of an AIR (Atmospheric Instrumeniaiion Research) make Atmospheric Data Acquisition System (ADAS) which is operable in the meteorological f requency range. The Doppler Sonic Detection and Ranging
MOIST ai r RICHARDSON NUMBER
Figure 3. A D A S acquired Giadieni Richardson number m the vemcal, wiihin a period from 05 1 / 10 06 20 hrs during ihc per«:isiencc of ihe morning elevated scalier layer and vertical prolile of the Richardson number as obiaincd during ihe period between 05.17 to 06.20 hrs from moisl sialic siabiltiy condiiion of ihe atmosphere A least square curve filling has been perlonned on the calculaied daiapoinis and ihe points are physically shown lo describe ihc degree of scalier
(SODAR) system was continuously operated during the entire flight period of the Kytoon.
The magnitude ol takes on values between 0.1 lo 0.37 wiihin a range height of 60-210 metres. It exceeds 0.4 above 240 metres and at a level below the elevated layer base, the values are usually higher than required (as shown in Figure 3). This clarifies that the turbulence field in the region dominates (Brown and Hall 1978) and that there is a natural
‘adjusubiliiy’ of the feedback mechanism whereby the thickness of the stably stratified shear regions changes so that the region is not intensely turbulent while retaining the turbulent properties.
A rapid rate of decrease ol the turbulent kinetic energy dissipation rate ( e ) as shown m Figure 4 may indicate that a differential acceleration is being added to the shear flow (Lcnschow 1984). This is also being shown Hy the Sodar data since even a .small differential
A cou stic detection o f an e le v a te d sta b le la yer etc 127
acceleration of the flow is sufficient to produce and maintain an elevated scalier layer which fades and merges fully with the plume siruciure at around 11:00 hrs. At the same lime, a possibility of an internal boundary layer formation cannot be fully ruled out because of the incursion of cool moist air during sea breeze flow over the region during ihc previous afternoon.
Fij*urc 4. Prolilc.s ol C7^. I z in ihc convcuivc conJilion (bclwccn 06:00 lo 06 30 hrs ) as obtained from doppicr SODAK wmd daia and Turbulcrtt Kmeiic Hncrgy Dissipaiion raic (Icasi square titled) plot for a height range ol 60 lo 600 metres tx:iween the time period ot 0 6 '0 0 to 06 30 hrs Curve shows two promineni peaks ol dissipation thereby revealing regions of sharp turbulence generation outside the elevated layers
2.2. Static stability (a„) and the verttcal distribution o f Richardson number :
Sialic stability in the vertical was calculated using a moist atmosphere approximation in the concerned time and height range from where local moist sialic stability was finally calculated (Saha and Singh 1972). Vertical distribution of Richardson number for moist air was dien calculated using the equation of moist sialic stability,
= a „ / [ {5U I 6 P y + i S V / S P y ] (1) where U and V are N-S and E-W components of the wind at each level under consideration.
Fine scale pressure (P) values were obtained from ADAS records.
Figure 3 also shows the lime scries of /?unoisi over the entire time range which are found to resemble the seasonal values of the monsoon regions (Saha and Singh 1972) as derived from moist static stability.
128 B isw a d ev R oy, D K R a k sh it, B P a l a n d B C h a k ra va rty
3. D isc u ssio n
A Convective Boundary Layer (CBL) might have been formed in agreement with the results (Singal 1989), whereby the sensible heat flux becomes minimum at the CBL height.
Sensible heat flux is proportional to o j /z i.e.,
o j / z = / e ) { l T 9 ' ) (2)
where is the standard deviation of the vertical wind velocity, z is the height of the level, a = 1.7 (constant of proportionality for moist atmosphere), g is the acceleration due to gravity, 0 is the potential temperature and w* &' is the covariance of potential temperature and the vertical velocity (luctuations. These values are plotted in Figure 4.
The turbulent kinetic energy dissipation rate was calculated using,
6 (3)
where is the velocity structure parameter in the vertical. A plot of a j /z for the early morning hours (Figure 4) shows that the heat flux has a sharp increase in its values at two discrete levels, one at around 275 metres and the other at around 6(X) meters representing the scatter layers in the vicinity of the surface based inversion and the elevated scatter layer.
However, Turbulent kinetic energy dissipation rate in the elevated scatter layer structure (Figure 4) is maximum at a height of 4(X) meues which is the elevated layer ba.se height signifying a minimum in the backscattcr reflectivity of the sound wave during its passage through the region. In the early morning, mechanical turbulence generated by wind shear was responsible for advection, and once the free convection started, the dissipation rate decreased in the surface layer but acquired a constant value in the deep convective layer where the wind shear was minimum.
Figure 5. Wind dircciion plot from 09:45 AM 23 May to 09:45 AM 24 May. 1990 as recoixled in the surface observaiury. A rapid veer of the wind is seen to be at around post-noon hours indicating sea-breeze activity. Time lag is assumed to be continuous.
A cou stic detection o f an e le v a te d sta b le la yer etc 129 The wind direction (surface) records of May 23 and 24 were smoothed and the data points were used for the time-lag vs. direction plot (Figure 5). This plot shows that at around post-noon of 23 May a sea breeze activity prevailed over Kharagpur that brought in cool moist air with a sharp decrease in temperature alongwith an increase in humidity. This gives a clear picture of the advected marine bou n d i^ layer formation, and it is supposed that ihe air mass laden with moisture formed a land bilteze. and its movement across the region during early hours of May 24 resulted in the form ^on of an elevated scatter layer.
4. C o n clu sio n s ‘
Sea breeze induced marine air incursion causing th<^ formation of the elevated scatter layer in an off coast station like Kharagpur has been revested for the first time by Doppler acoustic sounding of the atmosphere in the early morn|ng hours. These types of full fledged experiments can reveal more information about the elevated scatter layer and possible interactions with the atmospheric boundary laye^ structure. Routine observations for a prolonged period may also revetil the frequency of such occurrences in this region. Since onset of sea breeze is location dependent, a more accurate study of the sea breeze pattern over the region can yield more information about the elevated layer formation causes and its relationship with the terrain over which such occurrences are reported.
A ckn ow led gm en ts
The authors are thankful to the Director, Indian Institute of Technology,Kharagpur for enabling us to experiment with the dopplcr acoustic sounder. We are also thankful to Mr KG Vernekar of Indian Institute of Tropical Meteorology, Pune, for installation and operation of the SODAR system and for providing us with the data during the MONTBLEX-1990 experiments at Kharagpur. One of the authors (BR) is thankful to Department of Science and Technology (DST), New Delhi, for the financial support to pursue research work in this field.
R eferences
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