ON THE DYNAMICAL ASPECTS OF THE MONSOON BOUNDARY LAYER

(Research Note)

SAVITA B. MORWAL and SURENDRA S. PARASNIS Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune 411008, India (Received in final form 7 October, 1996) Abstract. Characteristic variations in the three forces, viz., pressure gradient force, Coriolis force and frictional force, in the monsoon boundary layer have been explored with the help of a one-dimensional model. The wind observations carried out at an inland station during MONTBLEX-90 have been utilised for this purpose. Variations were observed in the three forces during active and break monsoon periods. The height at which the Coriolis and pressure gradient forces showed balance ranged from 360–700 m and 500–600 m during break and active periods respectively.

1. Introduction Boundary layers observed during the summer monsoon are different from those of trade wind boundary layers. The monsoon boundary layer (MBL) is influenced by the large-scale monsoon features such as the monsoon trough, depression etc. Holt and Sethuraman (1987) made a detailed analysis of mean boundary-layer structure over the Arabian sea and Bay of Bengal during active and break monsoon periods using MONEX-77 and MONEX-79 data. During break and active monsoon phases the boundary layers showed different characteristics (Parasnis and Morwal, 1991; Parasnis, 1991; and Kusuma Rao et al., 1991). Over the monsoon trough region studies of the thermodynamic structure of the MBL indicate significant variation in boundary-layer heights (Parasnis et al., 1991). For the purpose of understanding the MBL features in the monsoon trough region, a field experiment called Monsoon Trough Boundary Layer Experiment1990 (MONTBLEX-90) was conducted. In the present study the wind observations carried out at an inland station during active and break monsoon periods were used to evaluate the three forces with the help of a one-dimensional (1-D) model. The purpose of the study is to examine the variation in the frictional force, pressure gradient force and Coriolis force in the MBL during different monsoon phases. 2. Data and Meteorological Conditions The Indian Institute of Tropical Meteorology installed a 3-axis monostatic Doppler Sodar system at Kharagpur (22 20 N, 87 20 E, 83 m ASL) as a part of the MONTBLEX Programme. This system, with a dish diameter of 1.2 m, was operated at a Boundary-Layer Meteorology 83: 163–171, 1997. c 1997 Kluwer Academic Publishers. Printed in the Netherlands.

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fixed frequency of 1.5 KHz. Vertical range was preselected between 60 and 1500 m AGL (Above Ground Level) with a 30 m range interval for data acquisition. Spatial resolution of the vertical wind component was around 10.67 m. East-West (EW) and North-South (NS) oriented antennae were tilted 30 from the horizon. Mean horizontal wind components, zonal (u) and meridional (v ) are averages of 30 minute sounding periods, each with 255 active pulses during the interval. The hourly values of zonal (u) and meridional (v ) wind components from 60 m (AGL) up to 1500 m (AGL) at an interval of 60 m for the periods 18–20 June, 26–31 July and 3–10 August, 1990, were used for this study. The surface pressure distributions for 29 July and 8 August are shown in Figure 1. The position of the monsoon trough plays an important role as far as the rainfall distribution is concerned. When the monsoon trough shifts to the foothills of Himalayas (Figure 1a) then there is a break in monsoon activity. The monsoon revives when the monsoon trough shifts southwards and regains its mean position (Figure 1b). During the periods 26–31 July 1990, break monsoon conditions prevailed and the period 3–10 August, 1990 belongs to active monsoon conditions. The period 18–20 June, 1990, has been considered as ‘undisturbed’. 3. Computational Procedure The one-dimensional model used in this study is similar to that of Estoque (1971) in which linear variation of geostrophic winds with height and stationarity of the wind have been assumed. The weather conditions for some of the assumptions made regarding stationarity and linear variation of geostrophic wind, are rarely observed in the real atmosphere. However, these assumptions are useful as far as the numerical computation of the different forces are concerned. The model equations are du dt

=

dv dt

=

1 @P

 @x 1 @P  @y

+

fv +

1 @xz

 @z 1 @yz fu +  @z

(1) (2)

where f is the Coriolis parameter; , the air density and xz , yz are the shear stresses associated with eddy transports of momentum along the vertical direction. The observed mean wind profiles can be analysed in terms of the following equations by assuming stationarity.

f (v vg ) +

1 @xz

=

 @z 1 @yz f (u ug ) +  @z

0 =

(3) 0;

(4)

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Figure 1. Surface pressure distribution for (a) 29 July, 1990 and (b) 8 August, 1980.

where ug , vg are the geostrophic wind components. Assuming a linear variation of geostrophic wind with height from the surface (z1 ) to zn the equations for ug and vg are

ug = un + (zn

z)

(5)

vg = vn + (zn

z );

(6)

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SAVITA B. MORWAL AND SURENDRA S. PARASNIS

where zn is the top of the boundary layer and z varies from the surface (z1 ) to zn . Here  and are constants to be determined. We assume that at z = zn , the actual wind is equal to its geostrophic value. The heights zn used in the computations are mixed-layer heights (i.e., atmospheric boundary-layer heights). The stress components at the surface are given by

q



U2 u2 = u22 + v22

(7)



U2 v2 = u22 + v22 ;

(8)

( xz )1 = ( yz )1 =

where

q

q

U = k0 u22 + v22 =ln

z

z 

0+ 2

; (9) z0 and U is the friction velocity, u2 and v2 are wind components at height z2 (i.e., 60 m AGL). Here z0 is the roughness length = 10 4 m and k0 is the von Karman constant = 0.4. In order to get the estimates of ug , vg , xz and yz by using Equations (3) and (4), the vertical derivatives are replaced by finite differences. Thus the finite difference form of Equations (3) and (4) for the intervals z1 to z2 , z2 to z3 : : : and zn 1 to zn can be obtained. There will be a total 2(n 1) simultaneous equations. The unknowns are , and 2(n 2) stress components at z2 , z3 : : : zn 1 . The stress components at z1 and zn are known. These 2(n 1) simultaneous equations have been solved using the technique of matrix inversion for 2(n 1) unknowns. The hourly means of u and v wind components for 0700–1000 hours, 1500–1800 hours Indian Standard Time (IST) for the periods 18–20 June, 26–31 July and 3–10 August, 1990, were used to compute the vertical profiles of three forces viz. pressure gradient force (PGF), Coriolis force (CF) and frictional force (FF). 4. Results and Discussion The vertical profiles of the magnitudes of three forces viz. PGF, CF and FF along with vertical profiles of u, v , ug and vg for 3 hours in the morning (0700–1000 hours IST) and in the afternoon (1500–1800 hours IST) during break (26–31 July, 1990) and active (3–10 August, 1990) monsoon conditions are shown in Figures 2 and 3. It is seen from Figure 2 that geostrophic balance, i.e., the balance between the pressure gradient and the Coriolis force, is observed between 500–650 m during active monsoon conditions (which is the boundary-layer height) and between 360– 700 m during break monsoon conditions (in the morning hours). However, during both the periods (i.e., active and break) all three forces are significant between 200– 300 m. In the afternoon hours the balance between the pressure gradient and the

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Coriolis force is seen at approximately 600 m during active monsoon conditions. During break monsoon conditions the balance between the pressure gradient and the Coriolis force is seen at slightly higher levels, i.e., at 700 m. However, the features observed in the morning hours during break monsoon conditions are retained in the afternoon hours also i.e. there exists a level where all the three forces are significant. This feature is not seen in the afternoon hours during active monsoon conditions. Solar insolation plays an important role in the development of the ABL. During break monsoon conditions, the decrease in cloudiness increases solar heating giving rise to dry convection. Thus the extent of the ABL is increased to higher levels. During active monsoon conditions the presence of clouds decreases the surface heating, hence the ABL height decreases. Kharagpur is situated in the region of the deep moist convection zone of the monsoon trough. Break and active monsoon conditions thus do not show any appreciable differences in the ABL heights. Figure 4 shows the vertical profiles of the magnitude of the three forces alongwith vertical profiles of u, v , ug and vg during 18–20 June, 1990 (Break monsoon period) for the hours 0600–1200 hours IST. It is seen from this figure that in general the frictional force showed a decreasing trend up to the ABL height where pressure gradient force and Coriolis force are approximately equal. It should be noted that there was an appreciable decrease in frictional force in the height interval 150–240 m, above this height it increased slowly and decreased at ABL depth. For the ATEX observations such fluctuations were observed by Brummer (1976). The pressure gradient force showed a decrease in the lower 500 m layer and then it showed increase. This feature has not been observed during July and August periods. This may be attributed to changes in the wind direction in the ABL during these periods. Also, comparison of the forces during all the periods showed that the magnitude of all the three forces is less during 18–20 June as compared to the other two periods i.e., in July and August. The possible explanation for this can be given as follows. Although the monsoon sets over India in the first week of June, it takes a long duration to spread all over the Indian regions. Also the period 18–20 June is associated with break monsoon conditions. In the absence of any weather system the wind speed is low and hence the low forces.

5. Conclusions The study of the variation in the magnitudes of the three forces viz. pressure gradient force, Coriolis force and frictional force, during the two periods of break monsoon conditions and one period of active monsoon conditions over Kharagpur revealed the following: The heights where the Coriolis and pressure gradient forces approximately balance varied from 360–700 m and from 500–600 m during break and active periods respectively. These heights are also associated with a minimum value of the frictional force.

Figure 2. Vertical profiles of Frictional Force (———), Pressure Gradient Force (— — — —), Coriolis Force (- - -- - -), Zonal wind (u, ——), meridional wind (v , – –  – –), ug (— — —) and vg (– – – –) against normalised ABL heights for break (26–31 July, 1990) and active (3–10 August, 1990) monsoon conditions during 0700–1000 hrs. IST. ABL height is indicated at the top.

168 SAVITA B. MORWAL AND SURENDRA S. PARASNIS

Figure 3. Same as Figure 2 during 1500–1800 hrs. IST.

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Figure 4. Same as Figure 2 for break (18–20 June, 1990) moonsoon conditions during 0600–1200 hrs. IST.

170 SAVITA B. MORWAL AND SURENDRA S. PARASNIS

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The pressure gradient force in June showed a decrease in the lowest 500 m and then an increase at higher levels. This feature was not observed during July–August where the pressure gradient force showed a gradual increase with height. ABL heights as determined from geostrophic balance did not show much difference between break and active monsoon conditions, since Kharagpur lies in the deep moist convective zone of the monsoon trough.

Acknowledgments The Monsoon Trough Boundary Layer Experiment (MONTBLEX-90) was a coordinated experiment conducted by the Department of Science and Technology, Government of India. Thanks are also due to Director, IITM and Dr A. S. R. Murty, and Shri K. G. Vernekar, Deputy Directors for support. Shri T. A. Disale and Shri M. I. R. Tinmaker helped in the preparation of the manuscript.

References Brummer, B.: 1976, ‘The Kinematics, Dynamics and Kinetic Energy Budget of the Trade Wind Flow over the Atlantic Ocean’, Meteorol. Forsch. Ergebniss. 11, 1–24. Estoque, M. A.: 1971, ‘The Planetary Boundary Layer Wind over Christmas Island’, Mon. Wea. Rev. 99,193–201. Holt, T. and Sethuraman, T.: 1987, ‘A Comparison of the Significant Features of the Marine Boundary Layer over the East Central Arabian Sea and North Central Bay of Bengal during MONEX- 79’, Mausam 38, 171–176. Kusuma, G. R., Sethuraman, T., and Prabhu, A.: 1991, ‘Boundary-Layer Heights over the Monsoon Trough Region during Active and Break Phases’, Boundary-Layer Meteorol. 57, 129–138. Parasnis, S. S.: 1991, ‘Convective Boundary Layer during Active and Break Conditions of the Summer Monsoon’, J. Atmos. Sci. 48, 999–1002. Parasnis, S. S. and Morwal, S. B.: 1991, ‘Convective Boundary Layer over the Deccan Plateau, India during Summer Monsoon’, Boundary-Layer Meteorol. 54, 59–68. Parasnis, S. S., Morwal, S. B., and Vernekar, K. G.: 1991, ‘Convective Boundary Layer in the Region of the Monsoon Trough – A Case Study’, Adv. Atmos. Sci. 8, 505–509.