Storm Surge Forecasting at RSMC New Delhi : Current Status and Future Plans

1. Storm Surge Forecasting at RSMC New Delhi : Current Status and Future Plans M Mohapatra INDIA METEOROLOGICAL DEPARTMENT MAUSAM BHAVAN, LODI ROAD , NEW DELHI-110003 mohapatraimd@gmail.com

2. Presentation layout • Introduction • Storm Surge and coastal inundation Monitoring and Prediction • Storm surge and coastal inundation warning services • Problem areas and Prospects • Conclusions

3. Impact of Cyclones

4. Components of sea level elevation • Astronomical Tide • Wind waves • Wave set up • Storm surge • Tsunamis

5. Storm Surge • Abnormal rise in sea level along the coast in association with a landfalling tropical cyclone is popularly known as storm surge. • Storm surges can be defined as oscillations of the water level in a coastal or inland body of water in the time range of a few minutes to a few days, resulting from forcing from atmospheric weather systems. • According to above definition, the so-called wind waves, which have durations on the order of several seconds, are excluded • (WMO, 2011).

6. Characteristics of Storm Surge • Tides, storm surges and tsunamis belong to the class of long gravity waves (Gonnert et al., 2001). • Storm surges are centred at about 10–4 cycles per second (cps, or hertz, Hz), which gives a period of approximately three hours. However, the periods of oscillations may vary considerably. • That storm surges can occur over short periods (~ a few minutes) is generally well understood and recognized. However, the situations in which high water levels associated with storm surge events can last for two to three days are not. • Duration of storm surge depends on • topography of the body of water, • direction of movement of the storm, • strength of the storm, • stratification of the water body and • nature of tidal motion in the water body.

7. Historical Records of Storm Surge (India) The funnel-shape of the North Indian Ocean and shallow bathymetry act to focus some of the world's highest storm storms onto the low-lying area at the northern end of the Bay of Bengal. Cyclones Storm Surge Height (m) Hooghly river (WB),7 October 1737 : 13 Contai (WB),5 October 1864 : 10-13 Paradip, Orissa,October 1971 : 4-5 Balasore, Orissa, May 1989 : 3-6 Orissa Super Cyclone, October 1999 : 5-6

8. Observational data on storm surges • As part of the Indian Tsunami Early Warning System, a real time network of Tide gauges has been established by Survey of India (SOI) and National Institute of Ocean Technology (NIOT). • The network comprises of 50 (36 by SOI and 14 by NIOT) • Tide gauges transmitting real time data through satellite communication to INCOIS at Hyderabad, SOI at Dehradun and NIOT at Chennai simultaneously for processing and interpretation. • In addition, the real time tide data from the Global Sea Level Observing System (GLOSS), in Indian Ocean is being received at INCOIS via internet. • However, the tide gauge network is not dense enough to record the storm surge due to cyclone always. • As a result, the storm surge at a coastal point is mainly estimated by the post-cyclone survey team based on the saline water marks etc.

9. Observational data on storm surges A shallower bathymetry piles up the surge more

10. Factors influencing Storm Surges • They include the oceanographic and meteorological parameters • Storm characteristics • Hydrological factors, • Basin characteristics and coastal geometry, • Wind stress • Seabed friction and • Information about the astronomical tides.

11. Tide Surge Interaction STORM SURGE STORM SURGE STORM SURGE HIGH TIDE MEAN TIDE (MSL) LOW TIDE DUNE DUNE • The rise in sea level due to high tide may be as high as 4.5 metre above the mean sea level at some parts of Indian coast. • The worst devastation is caused when peak surge occurs at the time of high tide. When Storm surge is combined with the astronomical tide it is called as storm tide. • The storm surge modifies the tide while the tidal cycle produces alterations in the storm surge. • The main causes that produce these interactions are the effects of bottom friction and the variation of the wave propagation speed (which is dependent on total depth).

12. Surge–river interaction • River flows can influence considerably the development of storm surge in the mouth of a river, where pronounced temperature and salinity gradients are formed. • Density-driven forces interact with the motion caused by external forces, forming a complicated dynamic system. • Surge–river interactions have been successfully modelled within a proper mathematical formulation of motion, involving momentum, heat and salt (WMO, 2011) • The influence of river runoff on water-level oscillations at the open coast is relatively small. Even with very large river discharge (water level is raised by several metres in river), influence on sea level is quickly reduced to no more than a few tens of centimetres. • Storm surge propagation into estuaries and distance of sea water penetration along a river depends on bottom slope of river. If river bed is relatively flat, storm surges can propagate upstream for tens of kilometres.

13. Interaction of surge and wind waves • Contribution of Ocean waves to roughness of the sea surface has been extensively studied (see, for example, Donelan et al., 1993). • There is a dependence of drag coefficient on state of development of waves and various methods have been developed (Johnson et al., 1998). • Traditional approaches to the determination of surface stress used for storm surge calculations consider momentum transfer to be solely and directly from the atmosphere to the surface current. • Increased roughness of the sea surface due to wave growth during active stage of the storm can enhance momentum exchange, so that water levels vary in a more physically realistic way in an atmosphere, wave and storm surge coupled model. • Widely used wave models can provide parameters to facilitate wave–surge coupling through surface stress (Janssen and Bidlot, 2003; Tolman, 2002).

14. Interaction of surge and wave set-up • The “wave set-up” is the additional water level due to the transfer of wave related momentum during the wave-breaking process. • The contribution of wave set-up during extreme storm events can add up to 1 metre to the sea level. • It is normally difficult to distinguish by measurement wave set-up from the larger-scale storm surge, since both cause sea levels to be higher than tidal predictions and both are due to meteorological effects. • However, estimates of the set-up component can be made from a numerical modelling study(Brown et al, 2011)

15. Storm surge and Meteorological Factors • The higher the cyclone intensity, the higher the storm surge. • Maximum storm surge occurs to the right of the storm track, roughly at the radius of max winds. • Very small, compact cyclones cause less storm surge than do large-sized cyclones. • Faster-moving cyclones cause higher surges at the coast line than do slower-moving hurricanes. • For areas with gentle slopes of the continental shelf, storm surge is large but waves are small. • Areas with deep water just offshore experience large waves, but little storm surge. (Gray, 2005)

16. Storm Surge Forecasting in World • Before the computer era, the techniques used for storm surge prediction were • analytical, • empirical, • graphical (nomograms) and • statistical (regression relations). • Electrical analog methods • Various methods currently used world-wide for storm surge prediction are as follows. • Empirical methods for surge prediction • Statistical methods • Storm surge prediction through artificial neural networks • Numerical methods

17. Storm Surge Forecasting in World

18. Operational Storm Surge Forecasting in India:Forecast Parameters • The followings are the parameters for prediction of storm surge • Time of commencement of storm surge • Duration • Height of storm surge • Area to be affected

19. Operational Storm Surge Forecasting in India : Methodology • Empirical and analytical methods-Nomograms • Dynamical methods-IITD model

20. Operational storm surge forecasting in IMD Ghosh Model

21. Operational storm surge forecasting in IMD Ghosh Model • The meteorological inputs are: • Pressure drop, • Radius of maximum winds, • Angle of track with the coast and • the landfall point. • The first nomogram gives the peak surge as a function of the pressure drop and radius of maximum winds. • The peak surge gets modified by the shoaling and storm motion factors defined through second and third nomograms. • Total surge is the product of preliminary estimates of surge from the first nomogram, shoaling factor and motion factor.

22. First Nomogram : Peak Surge • Peak Surge as a Function of • Pressure drop and • radius of maximum wind

23. Second Nomogram : Shoaling Factor

24. Third Nomogram : Vector Storm Motion

25. IIT Delhi Storm Surge Model • Inputs • Meteorological Input • Location specific input • Hydrological and Oceanographic input • Model • Dynamical storm model • Wind Stress associated with Cyclones • Storm Surge Model Equations • Vertically Integrated Mass Continuity • Vertically Integrated Momentum

26. Features of IITD Model: • It is fully non-linear and is forced by wind stress and by quadratic bottom friction. • The analysis area of the model covers from 2 ºN to 25º N and 45ºE to 100ºE. • A uniform grid distance is taken along latitudinal and longitudinal direction. • Orthogonal straight-line segments represent the coastal boundaries in the model. • Finite difference techniques applied to the simulation of surges. • Smoothed bathymetry is used from the depth collected from naval hydrographic charts.

27. Performance of IITD Model • The performance of this model with respect to Indian coast has been satisfactory. • It has been made operational in IMD and other Panel member countries • It uses the meteorological inputs provided by IMD • However, it has some limitations • It gives residual storm surge only

28. Surge due to Gonu Surge due to SIDR IRAN PAKISTAN JASK • ● KALAK • ● OMARA • ● GWADAR • ● ●MONGLA ● Kuakata MUSCAT● ARABIAN SEA OMAN RMAX = 40 km, DP = 98 mb COMPUTED PEAK SURGE = 2.6 m OBSERVED SURGE= 2-3 m Rmax = 25 km, DP = 68 hPa Observed surge =4 - 5.5 m Computed surge = 5.8 m

29. Role of Meteorological inputs for storm surge prediction • Pressure drop • Radius of Max. winds • Vector motion of the cyclone • Place of landfall • Duration of the storm Hence, proper monitoring and prediction of cyclone is very essential

30. Storm Surge Warning : Product dissemination • Predictions of storm surges have shorter ranges, usually 24 hrs. • Products derived from the numerical models are diverse and include time-varying sea level (surge) forecasts at specified locations and also charts, including local peaks and maxima charts • Over Indian region, the storm surge forecast is usually issued in association with cyclone warning bulletin, every three hour • Textual in form and is mentioned as, ‘storm surge of x meters above astronomical tide is likely to be inundate the low lying areas of a, b, and c districts at the time of landfall. • Does not specify time of commencement and duration of storm surge and amount of coastal inundation in terms of area and height of water. • Storm surge warning is from warning stage of cyclone.

31. Storm Surge Warning (Product dissemination) Format for Cyclone Warning Bulletin for AIR/Press / Public Cyclone Alert / Warning Bulletin No. ______ issued by _____ at _____ Hrs. IST on _________ (Date) for repeated broadcast at hourly / half hourly intervals. Cyclone Alert / Warning for _______ Districts. Cyclone centred at _____ hrs. IST of ________ (date) about ____________ kms. ________ of (direction) _______(Place). Expected to intensify further and move in a _________ direction and cross _______ coast near / between ____________ (Place)________ (day/time). Under its influence heavy to very heavy rain/extremely heavy rain likely cause floods in _______ districts commencing from _________ (time/day). Gales speed reaching ______ kmph causing _______ damage ________in districts commencing from ____________ (Date/Time) Gale force winds reaching -- kmph likely extend into _________ Districts, causing damage ___________ in ________ districts. Tidal wave of ________ m Likely inundate low lying area of _______ Districts at the time of crossing coast. Fishermen advised not to venture out.Public advised to cooperate with the State authorities in disaster management efforts.

32. Storm Surge Warning Product Dissemination • Format for State/Central Govt. Officials/Vital installations / Registered User Cyclone Alert/ Cyclone Warning Bulletin No. • Date and Time of Issue: • (i)Information on cyclone : The cyclonic storm lay over........Bay of Bengal/Arabian Sea Center........kms......(Direction) of ......... place. • (ii)ForecastFurther intensification:Direction of Movement:Expected landfall area:Expected time of landfall: • (iii) Weather Warning • (a)Rainfall.........in..........Districts(Names)(b)Gales reaching.........in...........Districts(Names)(c) Gale force winds reaching 35 knots in ................... Districts(d)Tidal waves ............... in coastal areas of .................. Districts (Names)(e) Sea condition: • (f)Damage(As per IMD instruction).......Districts(Names)(g)Likely impacts as per IMD Monograph on “Damage Potential of Tropical • (iv)Advice

33. storm surge prediction over the WMO/ESCAP Panel region. • Till 2008, RSMC, New Delhi was issuing storm surge prediction for Indian coast only. • Based on the recommendations of WMO Expert Team to Myanmar after NARGIS and subsequent recommendation of WMO TCP, it has been decided to issue storm surge guidance to member countries based on IITD model. • It has been implemented since 2009 with effect from cyclone, BIJLI in April 2009.

34. Forecast Verification • The performance of operational storm surge models is monitored, in most cases, on a continuous basis. • The sea-level products considered for the validation are either complete time series, peak levels or levels at selected times (such as high and low waters). • The statistical parameters obtained, usually for different forecast ranges, are varied. • The bias, RMS, standard deviation, average percentage error, linear regression (correlation coefficient) and the relation of standard error to mean square deviation are chosen by the different forecasting agencies. • Statistics are provided either with a monthly or yearly frequency or may be related to the occurrence of major storms. • In IMD, the predicted peak storm surge is verified against the observed/estimated peak surge due to a cyclone.

35. Forecast Verification in IMD

36. Forecast Verification in IMD

37. Limitations : Intensity and track estimation • Lack of observational data over the oceanic region leading to uncertainty in location of the system. The mean best track error may be considered as 30 km. • In the absence of the observations over the north Indian Ocean, the best track of the cyclone is mostly estimated with the satellite imagery interpretation with the help of Dvorak’s technique. • However, this technique, which has been developed for north Atlantic Ocean basin needs to be validated for north Indian Ocean. Further automated Dvorak’s technique has to be validated/developed for north Indian Ocean to minimize the human error.

38. Limitations : Track forecast error (km)

39. Limitations : Mean landfall point forecast error (km) Average (2008-12) 12 hr- 42 km 24 hr- 91 km, 48 hr- 96 km 72 hr- 135 km Mean landfall time forecast error (hr) Average (2008-12) 12 hr- 2.5 hrs 24 hr- 5.5 hrs 48 hr- 7.3 hrs 72 hr- 1.1 hrs

40. Operational requirements and Future Plan • Prediction of total water envelop, taking into consideration the following: • Storm surge • Astronomical tide • Wind wave and wave set up • River discharge in the sea, and • Rainfall distribution. • Parameters to be predicted: • Height of TWLE • Coastal inundation modeling • Prediction of above parameters should be attempted so that forecasting is introduced in IMD

41. Limitations Oceanographic and hydrological data Data on oceanography are concerned with: (a) Bathymetry (b) Astronomical tides, and (c) Inshore currents in closed regions. Accurate bathymetry maps are needed for improved surge prediction. Better coastline representation is also needed. The main hydrological information needed is: (i) River discharge in the sea, and (ii) Rainfall distribution.

42. Schematic diagram showing various steps in volved in coastal inundation modeling (WMO, 2013)

43. Operational requirements and Future Plan • TWLE and Coastal inundation modeling and hydrological modeling will be taken up in a project mode in 2013/2014

44. Thank you