Tie Yuan and Haiyan Jiang Department of Earth & Environment, FIU, Miami, Florida Margie Kieper Private Consultant 65 th Interdepartmental Hurricane Conference 1

2  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?

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) 3

4 Data period: ( ) 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

Cumulative frequency distribution of the overwater 24-h intensity change 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). 5  In this study, 30, 35, and 40 kt RI thresholds are used, which represent the 93th, 95th, and 97th percentiles

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 6

7 TRMM 37 GHz microwave imagery Best-track center corrected center bright cyan: pink 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)

TRMM 37 GHz microwave imagery 8 Best-track center Corrected center 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 Able to process  missing data  partial scan Automatic ring detection method: Case 2

VariableUnitsDefinition VMXm s -1 Maximum sustained surface wind speed LAT˚NLatitude LON˚ELongitude SPDm s -1 Storm speed of motion DVMXm s -1 Intensity change during the previous 12 h SST℃Sea surface temperature POTm s -1 Maximum potential intensity (MPI) - VMX SHRDm s –200 hPa vertical shear averaged from R=200–800 km USHRDm s –200 hPa zonal wind shear averaged from R=200–800 km U200ms hPa u component of wind averaged from R=200–800 km T200℃200 hPa temperature averaged from R=200–800 km RHLO%850–700 hPa relative humidity averaged from R=200–800 km RHHI%500–300 hPa relative humidity averaged from R=200–800 km 9 Environment RI index

VariableUnits RI (35 kt) (N=407 N e =208) Non-RI (N=4863 N e =1590) Diff= RI-Non-RI Significance level LAT ˚N˚N LON ˚E˚E VMAXm s e-005 SHRDm s e-014 SST ℃ e-012 RHLO% RHHI% SPDm s T200 ℃ USHRDm s U200m s DVMXm s e-027 POTm s N e 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

The probability of RI when the selected RI predictors were satisfied for the RI and non-RI samples. 11 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) 30 kt RI 40 kt RI 35 kt RI

 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. The composite probability of Envi-RII kt RI 40 kt RI 35 kt RI Total sample size is % 14.1% 10.6%

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 Good years Not so good years

15 Probability Of Detection  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 Evaluation of the three RI indices Perice Skill Score False Alarm Ratio

16 I.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. II.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. III. 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  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.