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

rashmi@sac.isro.gov.in

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)