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WIND STRESS OVER INDIAN OCEAN
Abhijit Sarkar, K Satheesan, Anant Parekh
Ocean Sciences DivisionSpace Applications Centre, INDIA
ISRO-CNES Joint Programme on Atmosphere, Climate & Oceanography
SAC, Ahmedabad, October 2005
IMPORTANCE
•The wind stress or the momentum flux drives the oceanic currents with the ocean acting as a sink for atmospheric momentum
•Wind stress fields are used to force global models
•Accurate fields of wind forcing are essential if realistic model simulations of ocean circulations are to be achieved
Sources of Wind Stress from Space
• ERS 2 AMI (1995)• QuikSCAT (1999)• Oceansat – 2 (2006)• METOP (2006)• T-P/JASON – I (1992/2003)• ENVISAT (2002)• TMI/SSMI (1997/1987)• Megha – Tropiques (2008)
DS 2
OB 8
STUDY AREA
Arabian Sea
Bay of Bengal
Data Source
Parameter Source
Surface Pressure
P Buoy
Air Temperature Ta Buoy
SST Ts Buoy
Humidity R NCEP Daily
Wind Speed U QuikSCAT (available from ifremer)
Wind Stress s
b
QuikSCAT (available from ifremer)
Calculated from Buoy
Wind stress estimation by IFREMER
To estimate surface wind stress, for each scatterometer wind vector, the bulk formulation is used
|| = CDW2
The 10 m neutral coefficient formulation over the ocean is (Smith 1988)
CD = a + bW
The values of a and b are determined for each wind speed range.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
-6 -4 -2 0 2 4 6
Tair-Tsst (C)
Un
/U1
0
WS 0.5
WS 2
WS 10
Effect of boundary layer stability on winds using LKB algorithm
Liu et al., 1996
Pre-monsoon months in the NIO we have mostly high insolation and wind condition Tair-Tsst can be large
This can also happens during monsoon season break conditions
During active condition in monsoon its the other way round
This leads to neutral stability winds to be very different from actual winds.
Air – Surface Temperature in NIO
Bulk aero-dynamic formulations
Where U= Wind speed at reference height
Us= Wind speed at surface
U*= Friction velocity
Z= Height of the Sensor Z0= Roughness length
U= Stability function
k = Karman constant
kzzUUU Us //ln(/ 0*
Annual cycle of Wind stress (N/m2) over the north Indian Ocean
1 Jan 1 Feb1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec 1 Jan0.00
0.04
0.08
0.12
0.16
0.20W
ind
Str
ess
X Axis Title
Bay of bengal Arabian Sea
Where first term of right hand side is determines generation and second is dissipation of turbulent kinetic energy.
G and D are non-dimensional constants (G-D = 1.25)
* = Friction velocity
= (s /w)1/2
s = Surface wind stress
w= Water Density
m = Depth independent background dissipation (2 10-8 m2/s3)
h = Depth
hATKE mwwDG 3*)(
Kim J. , 1976, Shetye 1986
1 Jan 1 Feb 1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec-10-505
101520253035
K E
Diff
eren
ce
Days
K E Difference
1 Jan 1 Feb 1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec0.660.690.720.750.780.810.840.870.90
Hum
idity
Humidity
1 Jan 1 Feb 1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec-1.4-1.2-1.0-0.8-0.6-0.4-0.20.00.20.4
Air
- S
ea D
iffer
ence
Air - Sea Difference
1 Jan 1 Feb 1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec10021004100610081010101210141016
Pre
ssur
e Bay of Bengal Arabian Sea
Pressure
Inferences & Discussions
Wind stress computations with/without boundary layer parameters made – differences were small
Turbulent Kinetic Energy computations for different
Significant differences were found during monsoon seasons, particularly in the Bay of Bengal
This shows large overestimation when without boundary layer
Need for synergy of wind data with surface boundary layer
0 2 4 6 8 10 12 14 160
2
4
6
8
10
12
14
16
Qui
kSC
AT
Buoy
Bay of Bengal
0 2 4 6 8 10 12 14 16 18 200
2
4
6
8
10
12
14
16
18
20
Qui
kSC
AT
Buoy
Arabian Sea