Tie Yuan and Haiyan Jiang Department of Earth & Environment, FIU, Miami, Florida Margie Kieper

1. 65th Interdepartmental Hurricane Conference Forecasting Rapid Intensification of Tropical Cyclones in the Western North Pacific Using TRMM/TMI 37 GHz Microwave Signal Tie Yuan and Haiyan Jiang Department of Earth & Environment, FIU, Miami, Florida Margie Kieper Private Consultant

2. Motivation of this study • The prediction of rapid intensification (RI) of tropical cyclones (TCs) has always been a great challenge in tropical weather forecasting. • Compared with the progresses in RI forecast in the Atlantic and Eastern North Pacific, there are few works in RI forecast in the North Western Pacific (NWP). • Margie Kieper’s subjective forecast method (2009) using the 37 GHz microwave can predict the onset of RI over Atlantic and Eastern North Pacific, will this method be equally valuable for NWP?

3. Objective • 37 GHz ring pattern RI index (Ring-RII) • Environment RI index (Envi-RII) • Combined 37 GHz ring pattern RI index and environment RI index (Comb-RII)

4. Data and methodology • Data period: (1998.1-2009.12) • JTWC best-track data (6-h interval) • TRMM TMI 37 GHz data from FIU/UU Tropical Cyclone Precipitation Feature (TCPF) database (16 km×9 km) • ERA-Interim reanalysis (1.5°×1.5°, 6-h interval) • Reynolds Daily SST analysis (V2, 0.25°×0.25°) • Samples include cases south of 30°N and over water during 24-h period

5. RI definition over NWP • RI of TCs is usually defined as the 95th percentile of all 24-h over-water intensity changes. • RI threshold of 30 kt is employed for the North Atlantic Basin in Kaplan and DeMaria’s study (2003). • In this study, 30, 35, and 40 kt RI thresholds are used, which represent the 93th, 95th, and 97th percentiles Cumulative frequency distribution of the overwater 24-h intensity change

6. 37 GHz ring pattern RI index • Must have a ring (follow Kieper 2009) • Vmax is between 35 and 100 kt • Over water during 24-h period

7. Automatic ring detection method: Case 1 • TMI 37GHz Ring: • Color • bright cyan: • 37 GHz PCT >=270K • and 37 GHz Vertical • >= 265K • or pink: • 37 GHz PCT <= 270K • Filled area • >= 60 % • Max radius • <= 160 km • Ring thickness must be >= 50% of the diameter of the outer edge (different with ATL) pink bright cyan: corrected center Best-track center TRMM 37 GHz microwave imagery

8. Automatic ring detection method: Case 2 • TC center correction : • Automatic • Find the pixels with PCT at 37 GHz >= 280 K within 60 km of best-track center, then calculate the mean of latitude and longitude, which is the corrected center • Manual • Interactive selection Corrected center Best-track center • Able to process • missing data • partial scan TRMM 37 GHz microwave imagery

9. Environment RI index The definitions of climatological, persistence and synoptic variables.

10. Mean values and their differences of climatological, persistence and synoptic variables of RI and non-RI samples at t=0 h • Ne in RI and non-RI sample are the effective sample size after serial correlation following Aberson and DeMaria (1994). Selected RI predictors: DVMX, SHRD,SST, VMAX,T200

11. The probability of RI when the selected RI predictors were satisfied for the RI and non-RI samples. 30 ktRI 35 ktRI • Previous 12-h intensity change (DVMX) has the highest probability. • The followings are vertical wind shear (SHRD), and sea surface temperature (SST), and Maximum sustained wind speed (VMAX) 40 ktRI

12. The composite probability of Envi-RII 30 ktRI 35 kt RI 20.6% 14.1% • The probabilities are provided as a function of the total number of the five RI predictor (DVMX, SHRD, SST, VMAX, and T200)thresholds satisfied • The probabilities of RI are close to the sample mean value when two thresholds were satisfied • The highest probability of RI are 52% , 40% , 38% for 30, 35 and 40 kt RI threshold, respectively. 40 kt RI 10.6% Total sample size is 5270

13. The Brier score of Envi-RII This result is similar to the skill score of the SHIPS RII (Kaplan et al. 2010) for the Atlantic basin.

14. The verification of Envi-RII for independent forecasts 35 kt RI 35 kt RI Good years 35 kt RI 35 kt RI Not so good years

15. Evaluation of the three RI indices Probability Of Detection False Alarm Ratio • The performance of Environment RII is better than that of the ring RII • The combined index is better than both. • In North Western Pacific, the internal process is not as important as the environmental factors as in the Atlantic and East Pacific basins Perice Skill Score

16. Conclusion • The 37 GHz ring pattern in the North Western Pacific is more common than in ATL, therefore we have to use a higher constrain for the ring thickness. • Environment RI index including five predictors is constructed. This index is shown to be skillful relative to climatology and through the verification of independent forecasts. • The 37 GHz ring index has a smaller contribution to the combined index, which suggests that the large-scale environment maybe play a more important role for the RI of tropical cyclones in the North Western Pacific .

17. Future works • Refine the ring definition, such as color range, thickness. • Further investigate the effects of environmental condition on RI of TCs in NWP, and find out their inherent physical mechanism. Thanks for attention