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.

EVOLUTION OF THERMODYNAMIC STRUCTURES DURING RAPID GROWTH AND DECAY OF EXTREMELY SEVERE CYCLONIC STORM CHAPALA(2015)

The structure and evolution of inner-core convective bursts and their differences associated with rapid intensification(RI) and rapid decay(RD) of tropical cyclone CHAPALA are examined. The inception of RI was associated with substantial increase of convective heating and its vertical extent in the inner core. Increase in diabatic heating was of the order of 12-21 oC, particularly in the middle and upper troposphere. Latent heat release produced a diabatically generated potential vorticity(PV) in vertical column. The immediate cause of RI was a significant increase of moisture flux from surface to 500 h Pa. This was accomplished primarily by updrafts of the order of 6-12 Pa s-1, representing the strong vertical motion distribution inside the warm core convective zone. The episode of deep convective bursts transpired during the period of RI. The evolving flow became highly symmetric and dominated by deep convective axisymmetric vortex structures. The RD coincided with the significant weakening in updraft of moisture flux consequently decrease of diabatic heating in the middle and upper troposphere and dissipation of upper and lower PV.

AN ANALYSIS OF RECURVATURE AND DECAY OF THE TROPICAL CYCLONE ‘MADI’(2013) OVER THE BAY OF BENGAL

The track of tropical cyclone‘Madi’showed almost 180° turn around,while immediately after recurvature it decayed rapidly in the following 24 hours.Sea surface temperature(SST)analyses rule out confrontation with low SST as cause of weakening.At 0000 UTC of 9 December 2013 diabatic heating(DH),potential vorticity(PV)and vertical wind shear(VWS)were favourable for intensification.However,a dry area close to the system centre at mid-troposphere subsequently engulfed the middle strata and the mid-tropospheric relative humidity(MTH)drastically reduced on 0000 UTC of 10 December 2013.The MTH further reduced on 0000 UTC of 11 December 2013.Lack of moisture in mid-troposphere apparently caused a mutation within the structure causing a sudden decrease in elevation.As the system dried up,reduction in convection above 400 h Pa hindered latent heat release.PV above 400 h Pa decreased significantly.Eventually,there was a spurt of VWS at 1200 UTC of the 10December 2013.The opposite wind flow between lower level and upper level whisked away the top of the system to northeast.Under the influence of the net northeasterly flow in lower to middle troposphere,the miniature vortex recurved southwestward and the vertical distortion due to shear weakened the system into a depression.

DEVELOPMENT OF NWP BASED OBJECTIVE CYCLONE PREDICTION SYSTEM(CPS) FOR NORTH INDIAN OCEAN TROPICAL CYCLONES——EVALUATION OF PERFORMANCE

An objective NWP based Cyclone Prediction System(CPS) has been developed and implemented at IMD for operational cyclone forecasting over the north Indian Ocean(NIO). The five forecast components of CPS are(a) Genesis Potential Parameter(GPP),(b) Multi-Model Ensemble(MME) technique for track prediction,(c) Statistical Cyclone Intensity Prediction(SCIP),(d) rapid intensification and(e) decay model to forecast intensity after the landfall.Verification shows GPP had higher probability of detection(0.98) and lower false alarm ratio(0.27) with higher critical success index(0.72). Mean track error of MME ranged from 74 km at 12 h to 200 km at 72 h and reduced by 27% to 52% for 36 h to 72 h forecast during 2009-2013. The mean forecast errors of landfall position ranged from 56 km at 24 h to 137 km at 72 h and landfall time error ranged from 3.6 h at 24 h to 6.1 h at 72 h. Mean intensity errors of SCIP ranged from 5.4 kt at 12 h to 16.9 kt at 72 h. The probabilistic rapid intensification forecast was skillful compared to climatology. The 6-hourly decaying intensity(after landfall) errors ranged from 3 kt to 4.9 kt. Results demonstrate the potential of CPS for operational cyclone forecast over the NIO.

Improvement of wind field forecasts for tropical cyclones over the North Indian Ocean

This paper demonstrates a modification method for real-time improvement of wind field forecasts for a typical cyclone VARDAH,which formed over the Bay of Bengal(Bo B)in 2016.The proposed method to improve the wind field forecasts associated with tropical cyclone consists of two components.The first one is the relocation method,which relocates the wind field forecasts obtained from the Global Forecast System(GFS)data of National Centres for Environmental Prediction(NCEP).The relocation of the model forecasts wind field is made on forecast locations generated by Multi Model Ensemble(MME)track forecast of India Meteorological Department(IMD).The second one is the modification of wind speed,which directly modifies the NCEP GFS wind speed forecasts based on intensity forecasts by Statistical Cyclone Intensity Prediction(SCIP)model of IMD.Applying these two methods,the displacement of wind field and underestimation/overestimation of wind speed in the model forecast field can be improved.Both modification methods show considerable improvements in the displacement and speed of wind field forecasts.The displacement error of wind field is found to have improved by about 51%at 48 h and about 80%at 72 h forecast.Overestimation of maximum wind speed in the forecast field is found to be improved by about 88%at 48 h and about 38%at 72 h forecast.The spatial distributions of corrected wind speed forecasts are also found to be more analogous than direct model forecasts with the corresponding analysis wind at all forecast hours.Two proposed modification methods could provide improved wind field forecast associated with tropical cyclones in real-time.

APPLICATION OF FUZZY CLUSTERING TECHNIQUE FOR ANALYSIS OF NORTH INDIAN OCEAN TROPICAL CYCLONE TRACKS

A fuzzy, c-means(FCM) clustering technique is explored to investigate the track of tropical cyclones over the North Indian Ocean(NIO) for the period(1976-2014). A total of fi ve clusters is objectively identifi ed based on partition index,partition coeffi cient, Dunn Index and separation index. The results obtained during analysis emphasized that each cluster has the unique features in terms of their genesis location, landfall, travel duration, trajectory, seasonality, accumulated cyclone energy and Intensity. Analysis of large scale environmental parameters, constructed preceding day of genesis show some of these parameters to be potential precursors to TC formation for almost all the clusters, most prominently, mid-tropospheric humidity, zonal wind,vorticity and outgoing long wave radiation of the main developing regions. The individual clusters have the several distinct features in their seasonal cycles.The cluster C5 shows distinct bimodal distributions where as other clusters are formed throughout the year. ENSO infl uenced the cyclone frequency in two of the fi ve clusters. The MJO is found to play an important role in the genesis of the cyclone. The post monsoon season cyclone frequency is more in MJO phase 2, 3 and 4. The technique(FCM) can be used as a guideline in terms of the probable affected zone of TC Tracks by the operational forecasters.

OPERATIONAL PERSPECTIVES ON TROPICAL CYCLONE INTENSITY CHANGE PART 2:FORECASTS BY OPERATIONAL AGENCIES

This review summarizes experiences at operational centers to forecast tropical cyclone(TC) intensity change as presented to the International Workshop on Tropical Cyclones(IWTC-9) in Hawaii in 2018. Some operational forecast centers have been able to leverage advances in intensity guidance to increase forecast skill, albeit incrementally, while others have struggled to make any significant improvements. Rapid intensity changes continue to present major challenges to operational centers and individual difficult cases illustrate the forecasting challenges.It is noteworthy that the realization of a recommendation from IWTC-8 in 2014, to adapt guidance initially developed for the North Atlantic and North-East Pacific to other basins, has led to improved forecast skill of some agencies. Recent worldwide difficult cases are presented so that the research community can further investigate, potentially leading to improved intensity forecasts when similar cases are observed in the future.

Improvement of displacement error of rainfall and wind field forecast associated with landfalling tropical cyclone AMPHAN

Spatial distribution of rainfall and wind speed forecast errors associated with landfalling tropical cyclones(TC)occur significantly due to incorrect location forecast by numerical models.Two major areas of errors are:(i)over-estimation over the model forecast locations and(ii)underestimation over the observed locations of the TCs.A modification method is proposed for real-time improvement of rainfall and wind field forecasts and demonstrated for the typical TC AMPHAN over the Bay of Bengal in 2020.The proposed method to improve the model forecasts is a relocation method through shifting of model forecast locations of TC to the real-time official forecast locations of India Meteorological Department(IMD).The modification is applied to the forecasts obtained from the operational numerical model,the Global Forecast System(GFS)of IMD.Application of the proposed method shows considerable improvement of both the parameters over both the locations.The rainfall forecast errors due to displacement are found to have improved by 44.1%–69.8%and 72.1%–85.2%over the GFS forecast locations and over the observed locations respectively for the respective forecast lead times 48 h,72 h,and 96 h.Similarly,the wind speed forecasts have improved by 27.6%–56.0%and 63.7%–84.6%over the GFS forecast locations and over the observed locations respectively for the respective forecast lead times 60 h,72 h,and 84 h.The results show that the proposed technique has capacity to provide improved spatial distributions of rainfall and wind speed forecasts associated with landfalling TCs and useful guidance to operational forecasters.

SOME SPECIAL CHARACTERISTICS OF TRACK AND INTENSITY OF TYPHOONS OVER THE WESTERN NORTH PACIFIC

Joint Typhoon Warning Center(JTWC) Best Track data from 1995 to 2014 are processed to examine some specific patterns and trends shown by Typhoons over the Western North Pacific. With a multivariate dataset of 588 TC cases in hand, we carry out a sub-domain analysis by dividing the Western North Pacific region into domains of 2°x2° and find the preferred regions of genesis, favourable direction of movement, steep recurvature, rapid intensification, and rapid decay. The region from longitude 132°E to 134°E and latitude 16°N to 18°N showed the highest number of cases(19) for rapid intensification(RI) and a general pattern is found that the RI systems occurred mostly in the later half of the year with a negative Pacific Decadal Oscillation(PDO) index. Similarly, the domain from longitude 114°E to 116°E and latitude 26°N to 28°N had the highest probability of 0.857 for rapid decay. The probabilities of recurvature for each sub-domain were calculated for angles 30°, 45°, 60°, 90°, 120° and 150°. The sub-domain around longitude 118°E and latitude 12°N had the steepest recurve of 168.69°. It also had a high probability of 0.714 for a recurvature of greater than 90°. The most taken direction of movement of typhoons around the Western North Pacific were analysed in different ways and along the 16 points of compass, the direction from 270° to 292.5° was found to be the most preferred direction of movement.

OPERATIONAL PERSPECTIVES ON TROPICAL CYCLONE INTENSITY CHANGE PART 1:RECENT ADVANCES IN INTENSITY GUIDANCE

This review summarizes techniques used by operational centers to forecast tropical cyclone intensity change as presented to the International Workshop on Tropical Cyclones(IWTC-9)in Hawaii in 2018.Recent advances and major changes over the past four years are presented,with a special focus on forecasting rapid intensity changes.Although intensity change remains one of the most difficult aspects of tropical cyclone forecasting,objective guidance has shown some improvement.The greatest improvements are realized when consensus methods are utilized,especially those that blend statistical-dynamical based guidance with dynamical ocean-coupled regional models.These models become even more skillful when initialized with inner core observational data.Continued improvement and availability of intensity guidance along with associated forecaster training are expected to deliver forecasting improvements in the future.