Forecasting of tropical cyclones ASANI(2022)and MOCHA(2023)over the Bay of Bengal-real time challenges to forecasters

This study examines the track and intensity forecasts of two typical Bay of Bengal tropical cyclones(TC)ASANI and MOCHA.The analysis of various Numerical Weather Prediction(NWP)model forecasts[ECMWF(European Centre for Medium range Weather Forecast),NCEP(National Centers for Environmental Prediction),NCUM(National Centre for Medium Range Weather Forecast-Unified Model),IMD(India Meteorological Department),HWRF(Hurricane Weather Research and Forecasting)],MME(Multi-model Ensemble),SCIP(Statistical Cyclone Intensity Prediction)model,and OFCL(Official)forecasts shows that intensity forecasts of ASANI and track forecasts of MOCHA were reasonably good,but there were large errors and wide variation in track forecasts of ASANI and in intensity forecasts of MOCHA.Among all model forecasts,the track forecast errors of IMD model and MME were least in general for ASANI and MOCHA respectively.Also,the landfall point forecast errors of IMD were least for ASANI,and the MME and OFCL forecast errors were least for MOCHA.No model is found to be consistently better for landfall time forecast for ASANI,and the errors of ECMWF,IMD and HWRF were least and of same order for MOCHA.The intensity forecast errors of OFCL and SCIP were least for ASANI,and the forecast errors of HWRF,IMD,NCEP,SCIP and OFCL were comparable and least for MOCHA up to 48 h forecast and HWRF errors were least thereafter in general.The ECMWF model forecast errors for intensity were found to be highest for both the TCs.The results also show that although there is significant improvement of track forecasts and limited or no improvement of intensity forecast in previous decades but challenges still persists in real time forecasting of both track and intensity due to wide variation and inconsistency of model forecasts for different TC cases.

Forecasting tropical cyclone coastal and marine hazards and impacts

The report highlights the significant progress over various regions with respect to understanding of coastal hazards,numerical modeling techniques and the generation&dissemination of coastal hazard warnings and products.The developments over various regions in the globe during 2014–18 have been discussed in this report as presented during 10th Session of International Workshop on Tropical Cyclones(IWTC-X)at Bali,Indonesia.More specifically,various regions have started to confront the uncertainty that cannot be removed from TC analyses and forecasts and further communicate those hazards within the context of risk[probabilistic]based information.Progress also includes impact-based forecasts such as communicating coastal inundation information relative to total water level instead of storm surge,specifically(i.e.,anomaly from as-tronomical tide and waves).Lastly,updates to model grid configuration,model resolution,and coupled dynamical systems continue to resolve the costal hazards more effectively.Those approaches have likely helped reduce loss of life relative to historical standards.However,regions agree that the generation and dissemination of coastal hazard information still need to be improved in view of growing population along the coast and thus increased exposure of life to coastal hazard.

REVIEW OF RECENT PROGRESS IN TROPICAL CYCLONE TRACK FORECASTING AND EXPRESSION OF UNCERTAINTIES

The Ninth International Workshop on Tropical Cyclones(IWTC-9) took place in Hawaii, USA in December 2018. This review paper was presented at the Workshop under the Tropical Cyclone Track topic.The forecasting of tropical cyclone(TC) track has seen significant improvements in recent decades both by numerical weather prediction models and by regional warning centres who issue forecasts having made use of these models and other forecasting techniques. Heming and Goerss(2010) gave an overview of forecasting techniques and models available for TC forecasting, including evidence of the improvement in performance over the years. However, the models and techniques used for TC forecasting have continued to develop in the last decade. This presentation gives an updated overview of many of the numerical weather prediction models and other techniques used for TC track prediction. It includes recent performance statistics both by the models and the regional warning centres.

EVALUATION OF CONE OF UNCERTAINTY IN TROPICAL CYCLONE TRACK FORECAST OVER NORTH INDIAN OCEAN ISSUED BY INDIA METEOROLOGICAL DEPARTMENT

India Meteorological Department(IMD) introduced cone of uncertainty(COU) in cyclonic disturbances(CDs) alongwith the 72 hr track forecast over the north India Ocean(NIO) in 2009. The track forecast for CDs is issued for +6, +12, +18, +24, +36, +48, +60 and +72 hrs time period from the stage of deep depression onwards. An attempt is made to evaluate COU forecast issued by IMD during 2009-2011(3 years). The size of the cone is deduced from climatological track forecast errors. The accuracy of COU forecast has been analysed with respect to basin of formation, season of formation, intensity and type of track(climatological/straight moving and recurving/looping type) of CDs by calculating percentage of total number of forecasts in each category lying within COU. The observed track lies within the forecast COU in about 60% of the cases over the NIO. The accuracy of COU forecast is about 66% in post-monsoon season and about 50% in pre-monsoon season. The observed track lies within the forecast COU in 90% cases of climatological/straight moving CDs and 39% cases of recurving/looping CDs. The observed track lies within COU forecast in about 71% cases of severe cyclonic storm and 37% cases of cyclonic storm/deep depression.

Characteristic features of Super Cyclone ‘AMPHAN’-observed through satellite images

Characteristic features of Super Cyclonic Storm(Su CS),AMPHAN which crossed West Bengal-Bangladesh Coast on May 20,2020 have been analyzed based on INSAT-3 D&passive microwave(PMW)images with special emphasis on eye characteristics and its relationship with intensity.These satellite images/products are analyzed to determine the centre of the cyclone,its intensity and the characteristics of the eye of the cyclone.It shows the characteristic variation of intensity of Su CS with geometric and thermal characteristics of the‘eye’.Precise changes in the eye features of the cyclone can be used for very short-range forecasting of the intensity of the cyclone.

An Evaluation of the Advanced Dvorak Technique (9.0) for the tropical cyclones over the North Indian Ocean

The Advanced Dvorak Technique(ADT)is used by tropical cyclone prediction centres around the world to accurately evaluate the intensity of tropical cyclones(TCs)from meteorological operational satellites.The algorithm development team has introduced new improvements to the objective ADT to further extend its capabilities and accuracy.A study has therefore undergone to evaluate the new edition of ADT(9.0)based on all the North Indian Ocean Tropical cyclones during 2018,2019 and 2020(Total 15 No.).It is found that ADT(9.0)performed well with the conformity of IMD’s best track T.No estimates.ADT is reasonably good in estimating the intensity for T≥4.0(VSCS to SuCS)and overestimate the intensity for T≤3.5(CS/SCS).