mesoscale variability using saral/altika in the bay of ... · cyclone phailin as observed by rama...
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ISRO-CNES SARAL/AltiKa Science Meet 22-24 April 2014
Mesoscale variability using SARAL/AltiKa in the Bay of Bengal with emphasis on cyclone
Phailin
Rashmi SharmaOSD/AOSG/EPSA
Space Applications CentreAhmedabad
Contributors: Rajesh Sikhakolli, Neeraj Agarwal, Sujit Basu & Raj Kumar
Courtesy: D. Chelton (2008), presentation made during OST-ST Meet atRuedesheim an Rhein in Germany during 29-31 January 2008.
Mesoscale Dynamics
• Mesoscale representation: eddies, fronts, rings, currentsetc.
• Typical scales: 50-500 km and few days to one year.• Instability: is major source of eddy energy.• Eddy activity maximum in major ocean currents. Hence
mesoscale study includes eddy and currents both.• Sudden TC intensification/weakening linked to high/low
value of heat content associated with eddies.• Exchange of energy between mean flow and eddy flow is
still not known.
Source:http://veimages.gsfc.nasa.gov/3305/Canary.
A2002186.1155.250m.jpg
Indian Ocean Eddies
Arabian Sea -Somali eddy: (warm core): size ~ 400-500 km, Southwest monsoon season)
Bay of Bengal: Small in size, 50-200 km, slow propagation, present through out the year
Previous studies have suggested that to detect small eddies in Bay, cross-track distance should be less than 50 km.
AltiKa Tracks for studying the spatial decorrelation: Mapping eddies
Along-track SLA de-correlation scale from AltiKa
De-correlation Scale: Arabian Sea = ~ >100 km; Bay of Bengal = ~ <100 km
Arabian Sea
Arabian Sea
Bay of Bengal
Bay of Bengal
5
7
9
11
13
15
17
19
21
23
25
75 80 85 90 95 100
Eddy Life Time (Days) in 2000
020406080
100120140160180200220240260
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
No. of Eddies
Days
Eddy Trajectories in year 2000
Eddy Life Time in year 2000
SARAL/AltiKa Data and Cyclone Phailin
SARAL/AltiKa tracks : 08-12 Oct, 2013
SWH (m)
SARAL/AltiKa Passes during cyclone Phailin
Cyclone Phailin Affected SARAL Track
~80 Km
~110 Km
Waveform shapes observed during cyclone Phailin
Ocean Waveform
Moderately contaminated Waveform
Highly contaminated Waveform
20-30 Sep 2013 01-10 Oct 2013
11-20 Oct 2013 21-30 Oct 2013
20-30 Sep 2013 01-10 Oct 2013
11-20 Oct 2013 21-30 Oct 2013
20-30 Sep 2013 01-10 Oct 2013
11-20 Oct 2013 21-30 Oct 2013
Merged SSHA using Jason & Cryosat Merged SSHA using SARAL, Jason & Cryosat
Merged Product: Role of AltiKa(Impact on Phailin)
Merged SSHA product from AVISO
Objectively analysed SSHA fields:
J2 + Cryosat
J2+ Cryosat + SARAL/AltiKa
Cyclone intensification/weakening : Role of ocean eddies
•Detected Eddy cores identified by Okubo-Weiss parameter are denoted by Black circles.
•Eddies which persists for more than 3-weeks (21 days) have been considered
11 Oct., 2013
Cyclone Phailin as observed by RAMA buoy at 15o N and 90 o E
Phailin was close to this location on 10th
Oct., 2013
Synergy of scatterometer and altimeter in deriving ocean currents(AltiKa and OSCAT)
Relative contribution of geostrophy and Ekman during clear sky and disturbed condition
2nd ISRO-CNES SARAL/AltiKa Science Meet 15-17 March 2011
Surface velocity outside equator:
Surface velocity near equator:
NOAA - OSCAR Methodology (Bonjean et. al., 2002) :
SSH SSWSST
(Real-time, 5-day averaged,1X1 deg)
Motivation:•To use Oceansat-2 scatterometer wind data •To assess the regional validity of the global algorithm(NOAA-OSCAR)•To study the surface current sensitivity to Ekman,geostrophic and thermal
Ocean surface current (m/s) in the Bay of Bengal for 11, October 2013 (1) total current (top left figure) (2) Geostrophic current component (top right figure) (3) Ekman current component (bottom right figure) and (4) Buoyancy current component (bottom right figure).
11 Oct., 2013
12, October 2013
13, October 2013
Summary• Decorrelation scale analysis suggests smaller eddies in BOB as
compared to AS • Even with 2 altimeters (Jason-2 and AltiKa), it was only possible to
get three tracks which were affected by the cyclone. Hence in order to monitor cyclone and for studying the processes, a minimum of 3-4 altimeters are required.
• Alternatively, swath altimeter (SWOT mission by NASA), would be the best choice for monitoring such events and also for assimilation in the ocean model for better surge prediction.
• Altika pass no. 0051 dated 12 Oct, 2013: waveform data was unavailable from latitude 16.66 N to 17.42 N. Similarly for pass no. 0066 dated 12 Oct, 2013, waveform data was unavailable from latitude 20.52 N to 20.95 N.
• Circulation pattern show dominance of Ekman current during Phailin cyclone. Normal condition : It is mostly geostrophy.
Thank You
Eddy Identification Method applied in BOB
Ref: Okubo,1970;Weiss, 1991
Ref: Henson S.A., Thomas A. C.,2008
Standard Deviation of WHigh-vorticity
Threshold
Strain
Shear Stress vorticity
SLA
Satellite merged Sea Level Anomaly (SLA)
Bathymetry
SLA data (not having valueswhere depth are less than 1000m)
u=-(g/f)*(∂H/∂y) v=(g/f)*(∂H/∂x)
∂u/∂x, ∂u/∂y ∂v/∂x, ∂v/∂y
Sn=(∂u/∂x-∂v/∂y), Ss=(∂v/∂x+∂u/∂y),ω= (∂v/∂x- ∂u/∂y)
W= Sn2+ Ss2-ω2
W< -0.2 σw
W<-0.5
The Okubo-Weiss parameter, W, separates the flow field into either strain dominated (W > 0) or vorticity dominated (W < 0)