THE IMPACT OF SATELLITE-DERIVED WINDS ON HURRICANE ANALYSIS AND TRACK FORECASTING ABSTRACT

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Emery Hampton
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1 TE IMPACT F SATELLITE-DERIVED WINDS N URRICANE ANALYSIS AND TRACK FRECASTING Christpher S. Velden University f Wiscnsin Cperative Institte fr Meterlgical Satellite Stdies 1225 West Daytn St, Madisn, Wiscnsin ABSTRACT While qalitative infrmatin frm meterlgical satellites has lng been recgnized as critical fr mnitring trpical cyclne activity, qantitative data are reqired t imprve the bjective analysis and nmerical, weather predictin f these events. In this paper, reslts are presented which shw that the inclsin f high-density, mltispectral satellite-derived infrmatin int the analysis f trpical cyclne envirnmental wind fields can effectively redce the errr f bjective track frecasts. 1. Intrdctin Analysis ncertainties are ften cited as a majr srce f frecast errrs in nmerical mdels. This is especially tre in hrricane track predictin, where lack f cnventinal trpspheric bservatins ver the ceanic regins can lead t errnes depictins f the envirnmental flw that accnts fr mch f the strm mtin. peratinal meterlgical satellites (particlarly gestatinary) have the ability t freqently sample the large scale ceanic envirnment, making them prime tls fr mnitring trpical cyclnes. At the University f Wiscnsin-Cperative Institte fr Meterlgical Satellite Stdies (UW-CIMSS), an effrt has been nderway t develp and incrprate high density, high qality satellite-derived wind bservatins int analyses f trpical cyclne envirnmental wind fields in hpes f redcing track frecast errrs. This prgram is a cperative effrt with the NAA Natinal rricane Center (NC), the Atlantic ceangraphic and Meterlgical Labratry-rricane Research Divisin (AML-RD), the Natinal Envirnmental Satellite Data and Infrmatin Service (NESDIS), and the Natinal Meterlgical Center (NMC). The impact f these satellite data n bartrpic mdel hrricane track frecasts is examined here. 2. Methdlgy Effrts at UW-CIMSS have fcssed n imprving the depictin f the wind fields in the trpical cyclne envirnment by enhancing the peratinally-derived cldmtin winds with high-density vectrs frm several different srces f GES satellite data. The enhanced satellite wind sets inclde: 1) high-density cldmtin vectrs prdced by atmated methds sing seqences f high-resltin infrared-windw imagery, 2) atmatically-derived vectrs frm animated imagery f tw VAS (VISSR Atmspheric Snder) water vapr channels (6.7 and 7.3 micrn), and 3) gradient winds derived frm mltispectral VAS-retrieved height fields. These 113

2 enhanced satellite winds are cmbined with peratinally-derived cld-mtin winds prdced at NESDIS (fll-disk, lw density) and rawinsnde pper-air reprts. The data set is qality cntrlled by bth atmated and manal techniqes, and then bjectively analyzed at the mandatry levels frm 850 t 200 mb. The final step is the creatin f a trpspheric ( mb), mass-weighted deep layer-mean (DLM) wind analysis frm these fields. The DLM analysis is a gd apprximatin f the envirnmental steering flw in mst cases, and can be sed qalitatively by NC, r t initialize bjective hrricane track frecast mdels. The freging prcedres were develped n the Man-cmpter Interactive Data Access System (McIDAS) at UW- CIMSS and have been implemented int peratins n the VDUC (VAS Data Utilizatin Center) system at NESDIS in Washingtn, DC. Rtine prcessing has been cndcted by NESDIS since Thrgh a telecmmnicatins link t NC, the winds and DLM have been transmitted dring hrricane seasn in a qasi-peratinal mde fr evalatin by NC frecasters. 3. Satellite Data Impact-Reslts In this sectin, cases frm 1989 Atlantic rricanes g, Gabrielle and Jerry are sed t examine the specific impact f the qantitative satellite data n mdel mean frecast errrs (MFE). Tw independent analysis and frecast schemes are tilized t determine the cnsistency f the impact. In additin, the relative cntribtins f tw satellite data cmpnents (cld-mtin and VAS gradient) twards the redctin in mdel MFE are examined. a. UW-CIMSS analysis and frecast system The enhanced satellite wind sets were prdced in an peratinal mde by NESDIS dring Nrth Atlantic and East Pacific trpical cyclne events in data sets (cases) prdced dring rricanes g and Gabrielle are investigated fr their impact n mdel track frecasts. The frecast prcedre cnsists f tw cmpnents: a nndivergent bartrpic frecast f the envirnmental flw field, and a pint (strm center) trajectry frecast based n the bartrpic frecast f the envirnmental flw. The trajectry begins at the specified strm center lcatin, and is calclated after each bartrpic frecast time step (30 min). 72-hr frecasts frm the 18 cases initialized with the satellite data-enhanced DLM fields are cmpared with frecasts initialized with DLM fields that d nt inclde the enhanced satellite winds (nly cnventinal data and peratinal satellite winds). The reslts f the track frecasts are tablated in Table 1. A 9-17% redctin in_ middle t lnger range MFE is fnd when the mdel is initialized with the satellite wind-enhanced analyses. In the cntext f simple bartrpic mdel track frecasting, this represents a ntable imprvement. Table 1. Mean track frecast errrs (km) frm the UW-CIMSS analysis and frecast system initialized n DLM analyses with (SATWINDS) and witht (NSAT) satellite-enhanced winds, fr a sample f cases frm Frecast Interval NSAT SATWINDS # f Cases % Difference 12 h 24 h 36 h 48 h 60 h 72 h / 114

3 Given the psitive impact f the enhanced satellite winds n the verall redctin f MFE, i t is f interest t examine the cntribtins by the individal wind cmpnents, r types. 72-hr track frecasts frm the 18 cases were prdced sing DLM analyses cntaining the e n t i r e wind set, and with i n d i v i d a l types (cldmtin and VAS gradient) taken t. The reslts are shwn in Table 2. The cldmtin vectrs are lcated mainly near 850 and near 200 mb, which are levels that are nt as well crrelated with strm mtin as the middle trpspheric levels, and d nt typically make a strng cntribtin t the mass-weighted DLM flw. Therefre, their impact n frecasts frm this relatively simple analysis and bartrpic frecast system is minimal. Table 2. Mean track frecast errrs (km) frm the UW-CIMSS analysis and frecast system, which incldes a cntrl cntaining a l l enhanced satellite wind vectrs (SATWINDS), cntrl mins enhanced cld-mtin vectrs (NCLDMT), and cntrl mins VAS gradient vectrs (NGRAD) fr a sample f cases frm V* Frecast Interval 12 h 24 h 36 h 48 h 60 h 72 h SATWINDS NCLDMT NGRAD # f Cases VAS gradient winds at 850, 500 and 200 mb were inclded in the enhanced satellite data sets prcessed in A smewhat srprising reslt frm Table 2 is the magnitde f the impact f the gradient winds. It is rather nexpected that gradient winds wld have sch an impact in trpical applicatins. An examinatin f the individal satellite wind sets, hwever, reveals that very few gradient wind vectrs sth f 20N passed thrgh the qality cntrl prcedres at NESDIS. In additin, rricane g tracked int the sb-trpics late in it's trek, and rricane Gabrielle mved nrthward int a mid-latitde regime fr mch f it's lifetime. Ths, cases frm these tw strms prvided an pprtnity fr the gradient winds t shw an impact. Table 3. Mean track frecast errrs (km) frm the UW-CIMSS analysis and frecast system initialized n DLM analyses which inclde all enhanced satellite wind vectrs (SATWINDS), and SATWINDS mins the VAS gradient wind vectrs (NGRAD), fr a sample f cases frm The cases are stratified by thse with initial strm psitin sth f 25N latitde (tp), and thse nrth f 25N (bttm). Sth f 25N Frecast Interval 12 h 24 h 36 h 48 h 60 h 72 h SATWINDS NGRAD # f Cases % Difference less than 5 % Nrth f 25N Frecast Interval 12 h 24 h 36 h 48 h 60 h 72 h SATWINDS NGRAD # f Cases % Difference

4 This impact is frther examined by stratifying the reslts in Table 2 int thse cases nrth and sth f 25 N. This latitde was picked smewhat arbitrarily t separate trpical cases frm sb-trpical r mid-latitde cases. The reslts are presented in Table 3. It is evident that the gradient winds have a majr impact n track frecasts nrth f 25N. In fact, p t a 41% degradatin in the MFE (at 60-h) reslts with the exclsin f these winds frm the data set. b. AML-RD analysis and frecast system In this sectin, the VICBAR track frecast mdel is sed t evalate the impact f the satellite data. The VICBAR frecasting system cnsists f a mltilevel analysis and a bartrpic predictin mdel. This frecasting system is mre sphisticated than the relatively simple UW-CIMSS system, with the likelihd f increased sensitivity t enhanced data. Cntrl frecasts sing nly the NMC peratinal data base were made fr 21 cases frm 1989 Atlantic hrricanes g, Gabrielle and Jerry. The enhanced high-density cld-mtin winds were then added t the data base and the frecasts re-rn (enhanced cld-mtin winds were available in 18 f the 21 cases). Figre 1 illstrates the impact f the enhanced cld-mtin winds n the VICBAR track frecasts relative t the cntrl. A psitive impact is fnd in the h frecast intervals, peaking near 14% at 48 h. The maximm impact at the middle and lnger frecast intervals is cnsistent with the UW-CIMSS reslts. Table 4 smmarizes the impact f the cld-mtin winds n the VICBAR frecasts. The redctin in frecast errrs at 48 h was fnd t be significant (paired t-test) at the 95% cnfidence level. The cnsistency f the redctin in frecast errrs is shwn by the fact that the track predictins are imprved 64-81% f the time at the later frecast perids ENANCED CLUD (CASES) (18) (18) (17) (16) "l2~ VERIFICATIN TIME CNTRI, DATA SET; PERATINAL DATA (RS) (15) (14) V Table 4. Impact f enhanced cld-mtin vectrs n VICBAR frecasts. Verificatin statistics are valid fr frecasts frm the NMC peratinal data base pls the enhanced cldmtin vectrs: nmber f frecasts (N), effective nmber f independent frecasts (N ), and mean frecast errr (MFE). h Als given are cmparisns with frecasts frm the peratinal data base nly: the mean frecast imprvement (MFI), expressed bth in km and as a % relative t the peratinal data-nly frecast errr, the standard deviatin f the imprvements (SDI), the freqency f imprved frecasts (FIF), and whether the imprvements are statistically significant at the 95% cnfidence level (SIG). Frecast N N* MFE MFI SDI MFI FIF SIG Interval (h) (km) (km) (km) (%) (%) (y/n) n n n y n n Figre 1. Impact n mean VICBAR track frecast errrs by adding enhanced cld-mtin vectrs t a cntrl data set. Errrs are shwn relative t the mean frecast errr f the cntrl. 116

5 The next experiment invlves the inclsin f the VAS gradient winds t the peratinal data base. Figre 2 illstrates the impact f the gradient winds n the VICBAR frecasts, relative t the cntrl rns. As was the case with the cldmtin winds, the greatest frecast impact ccrs in the h time perid. This is nt srprising, since gradient wind infrmatin is nt available in cldy regins near the strm. Psitive impact reslts mainly frm an imprved definitin f the synptic-scale envirnmental flw featres away frm the strm circlatin, especially nrth f 25 N as shwn earlier. Table 5 presents a smmary f the impact f the gradient winds n the VICBAR frecasts. The MFE are redced by 6-9% in the h frecast perid, and are statistically significant at the 95% cnfidence level. In additin, nearly 3/4 f the cases are imprved ver this perid by the inclsin f the winds, indicating a gd degree f cnsistency in the redctin f frecast errr. In agreement with the UW-CIMSS reslts presented earlier, the mst imprved frecasts were assciated with cases t f the deep trpics. G < «55 U < d CA CA CA «W Z < i GRADIENT (CASES) (19) (19) (19) 12 I CN I KI. DATA SKT: PERATINAL DATA (17) VERIFICATIN TIME (URS) Figre 2. Impact n mean VICBAR track frecasts errrs by adding VAS gradient vectrs t a cntrl data set. Errrs are shwn relative t the mean frecast errr f the cntrl. (16) 60 (15) - f 72 Table 5. As in Table 4, except the statistics reflect the inclsin f the VAS gradient winds nly. Frecast N N* MFE MFI SDI MFI FIF SIG Interval (h) (km) (km) (km) (%) (%) (y/n) n n y y y y Verificatins fr the cmbined satellite data set (enhanced cld-mtin pls gradient winds) against the cntrl are shwn in Fig. 3 and Table 6. The frecast errrs are redced at nearly all frecast perids, with the 23% redctin in MFE at 48 h significant at the 99% level f cnfidence. igh levels f cnfidence (> 95%) are als fnd at 36 and 60 h. An example f the cmbined impact f the enhanced cld-mtin pls gradient winds n an individal frecast frm rricane Gabrielle is shwn in Fig

6 15 CNTRL DATA SET: b J at Z 10 5 ENANCED CLUD, GRADIENT PERATINAL DATA (A «ai Z (CASES) (16) (16) (16) V ( 1 5 ) (14) (13) VERIFICATIN TIME (RS) _l 1 I I l_ Figre 3. Impact n mean VICBAR track frecast errrs by adding enhanced cld-mtin pls VAS gradient vectrs t a cntrl data set. Errrs are shwn relative t the mean frecast errr f the cntrl. Figre h VICBAR frecast tracks frm rricane Gabrielle at 00 UT 8 September 1989, inclding the cntrl rn ( -), cntrl pls enhanced cld-mtin vectrs ( ), cntrl pls VAS gradient vectrs (...), and cntrl pls enhanced cld-mtin and VAS gradient vectrs (- ). Verifying track is als shwn ( ). Table 6. As in Table 4, except the statistics reflect the inclsin f the enhanced cld-mtin pls gradient winds. Frecast N N* MFE MFI SDI MFI FIF SIG Interval (h) (km) (km) (km) (%) (%) (Y/n) n n y y y n 4. Smmary The reslts presented in this stdy shw a cnsistent psitive impact by the satellite-derived winds n the redctin f bartrpic track frecast errrs, especially ntable at lnger ranges (36-72 h). The redctin in MFE is as high as j 23% in 48 h VICBAR frecasts fr a sample f 1989 cases. The imprvement reslts j frm a cmplementary distribtin f the high-density cld-mtin winds, and the "clear-air" VAS gradient winds. The psitive impact f the gradient winds is mainl) limited t strms in, r mving int, higher latitde regimes (i.e. nrth f 25 N). The fact that the VICBAR frecast mdel is qasi - peratinal, and that these enhanced satellite data sets were prdced nder peratinal cnstraints, demnstrates the ptential fr real-time applicatins t NC frecast gidance. 118

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