The 20 February 2010 Madeira flash-floods: synoptic analysis and extreme rainfall assessment

Na . Haza ds Ea h Sys . Sci., 12, 715–730, 2012 www.na -haza ds-ea h-sys -sci.ne /12/715/2012/ doi:10.5194/nhess-12-715-2012 © Au ho (s) 2012. CC A ibu ion 3.0 License. Na u al Haza ds and Ea h Sys em Sciences The 20 Feb ua y 2010 Madei a lash- loods: synop ic analysis and ex eme ain all assessmen M. F agoso1, R. M. T igo2, J. G. Pin o3, S. Lopes1,4, A. Lopes1, S. Ulb ich3, and C. Mag o4 1IGOT, Uni e si y o Lisbon, Po ugal 2IDL, Facul y o Sciences, Uni e si y o Lisbon, Po ugal 3Ins i u e o Geophysics and Me eo ology, Uni e si y o Cologne, Ge many 4Labo a ´ o io Regional de Engenha ia Ci il, R.A. Madei a, Po ugal Co espondence o: M. F agoso ([email p o ec ed]) Recei ed: 9 May 2011 – Re ised: 26 Sep embe 2011 – Accep ed: 31 Janua y 2012 – Published: 23 Ma ch 2012 Abs ac . This s udy aims o cha ac e ise he ain all ex- cep ionali y and he me eo ological con ex o he 20 Feb u- a y 2010 lash- loods in Madei a (Po ugal). Daily and hou ly p ecipi a ion eco ds om he a ailable ain-gauge s a ion ne wo ks a e e alua ed in o de o econs i u e he empo al e olu ion o he ains o m, as i s geog aphic inci- dence, con ibu ing o unde s and he lash- lood dynamics and he ype and spa ial dis ibu ion o he associa ed im- pac s. The excep ionali y o he ains o m is u he con- i med by he e u n pe iod associa ed wi h he daily p ecip- i a ion egis e ed a he wo long- e m eco d s a ions, wi h 146.9mm obse ed in he ci y o Funchal and 333.8mm on he moun ain op, co esponding o an es ima ed e u n pe- iod o app oxima ely 290y and 90y , espec i ely. Fu he - mo e, he synop ic associa ed si ua ion esponsible o he lash- loods is analysed using di e en sou ces o in o ma- ion, e.g., wea he cha s, eanalysis da a, Me eosa images and adiosounding da a, wi h he ocus on wo main issues: (1) he dynamical condi ions ha p omo ed such anomalous humidi y a ailabili y o e he Madei a egion on 20 Feb u- a y 2010 and (2) he upli mechanism ha induced deep con- ec ion ac i i y. 1 In oduc ion On he 20 Feb ua y 2010 he island o Madei a was hi by o en ial ain all ha igge ed ca as ophic lash- loods, ac- coun ing o a dea h oll o 45, wi h oughly hal o he ic- ims (22) alone occu ing in he capi al ci y o Funchal, while 6 o he pe sons we e decla ed missing. I was he deadlies hyd o-me eo ological ca as ophe in he Po uguese e i o y in he las ou decades and he economic damage cos s we e es ima ed o be US$1.9 billion (EM-DAT CRED, 2010). The island o Madei a is qui e densely popula ed, pa icula ly in i s sou he n coas , wi h ci ca 267000 inhabi an s in 2011 census, wi h 150000 (app oxima ely 40%) li ing in he Fun- chal dis ic , one o he ea lies ou ism ho spo s in Eu ope, cu en ly wi h app oxima ely 30000 ho el beds. In 2009, he a chipelago ecei ed 1 million gues s; his co esponds o an income o mo e han 255 Million Eu o (INE-Ins i u o Na- cional de Es a ´ ıs ica, h p://www.ine.p ). Madei a is a moun ainous island wi h 740.7km2, loca ed in he eas e n sub opical a ea o he No h A lan ic Ocean (Fig. 1a); I s o og aphy (Fig. 1.b) is domina ed by a igo - ous olcanic landscape, wi h deep alleys, s eeped slopes and sca ps, exhibi ing i s highes ele a ions along a E–W o ien ed ba ie , whe e he maximum al i ude is achie ed a he op o he eas e n moun ain ange (1861m a Pico Rui o). Loca edbe ween he 30◦and 33◦la i ude N,Madei a has a Medi e anean ype clima e mode a ed by he A lan ic Ocean, and since he island o og aphic ba ie (E–W) has an almos pe pendicula o ien a ion wi h he p e ailing wind di ec ion (NE), empe a u e and ain all a y ema kably be- ween he no he n and sou he n slopes. The no he n slopes a e mo e humid han he sou he n coun e pa s a he same al i ude, while he amoun o ain all inc eases wi h al i ude on bo h slopes (P ada e al., 2009). The mean annual p e- cipi a ion a ies be ween 600mm in he Funchal dis ic o close o 3000mm a he op o he eas e n moun ain ange (s a ions 43 and 25 in Fig. 1, espec i ely). Rele an ain-induced na u al haza ds in Madei a a e lash- loods, landslides, deb is lows and less equen ly - sunamis, which can be caused by coas al ock slides (Ro- d igues and Ayala-Ca cedo, 2003). Unlike he Azo es and Cana y Islands, Madei a lacks majo olcanic ac i i y and is p one only o ela i ely modes ea hquake e en s. Flash- loods cons i u e p obably he mos dange ous na u al haza d Published by Cope nicus Publica ions on behal o he Eu opean Geosciences Union. 716 M. F agoso e al.: The 20 Feb ua y 2010 Madei a lash- loods Figu es (a) (b) Fig. 1. Loca ion (a) and elie (b) o he island o Madei a (Po ugal). Do s a e loca ions o he ain- gauge s a ions used in his s udy (names and desc ip ion in Table 1). Limi s o he d ainage basins a ec ed by he 20 Feb ua y lash- loods and landslides a e also shown (names and desc ip ion in he ex ). Fig. 1. Loca ion (a) and elie (b) o he island o Madei a (Po ugal). Do s a e loca ions o he ain-gauge s a ions used in his s udy (names and desc ip ion in Table 1). Limi s o he d ainage basins a ec ed by he 20 Feb ua y lash- loods and landslides a e also shown (names and desc ip ion in he ex ). Table 1. Main lash- lood e en s in Madei a be ween 1800 and 2010 (SRES, 2010). Da e Mos a ec ed a eas Casual ies and damage 9 Oc obe 1803 Funchal 800–1000 casual ies 6 Ma ch 1929 S. Vicen e 40 casual ies, 11 houses des oyed 30 Decembe 1939 Madalena do Ma 4 casual ies 21 Sep embe 1972 San o An ´ onio 2 casual ies 20 Decembe 1977 Es ei o de Cˆ ama a de Lobos 2 casual ies and 45 dislodged 23 and 24 Janua y 1979 Machico, Po o da C uz, Camacha, 14 casual ies Canhas, Calhe a and Faj˜ a do Penedo 29 Oc obe 1993 All he island 4 casual ies, 4 missed people, 306 dislodged, 27 inju ed people, 76 houses des oyed 5 and 6 Ma ch 2001 Cu al das F ei as and S. Vicen e 4 casual ies and 120 dislodged people 22 Decembe 2009 Madalena do Ma and S. Vicen e Houses and oads des oyed 20 Feb ua y 2010 Funchal and Ribei a B a a 45 casual ies, 6 missed people in Madei a, whe e he phenomenon has he common name o “alu i˜ ao”, e e ing o lash- loods a ec ing s eams whose discha ges and ene gy inc ease d ama ically a e in ense ain all episodes, d agging mud, blocks and deb is, and p o- ducing a powe ul and des uc i e cu en (Ribei o, 1985). An in en o y o lash- loods esponsible o human casual ies o se e e damage compiled by Quin al (1999) e e s 30 lash- lood e en s a e he his o ical ca as ophe o 9 Oc obe 1803 – when mo e han 800 people we e killed, mos ly in Funchal – un il he end o 1998. Taking in o conside a ion he numbe o casual ies o damage, a mo e ecen compila ion (SRES, 2010), indica es 9 main e en s o disas ous lash- loods o e he 1800–2009 pe iod (Table 1). Simila o he occu ence in Oc obe 1803, he damage su e ed du ing he 20 Feb u- a y 2010 e en was pa icula ly ha m ul in he sou he n slope o he island, mainly in he Funchal ci y dis ic . Examples o damages and des uc ion associa ed wi h he lash- loods in he down own a ea o Funchal and Ribei a B a a, wo o he mos a ec ed a eas, a e shown in Fig. 2. An ex ao di- na y olume o solid ma e ial – deb is and huge blocks – was d agged and deposi ed by he o en ial lows. A sys ema ic su ey o he landslides p oduced in 20 Feb ua y 2010 based Na . Haza ds Ea h Sys . Sci., 12, 715–730, 2012 www.na -haza ds-ea h-sys -sci.ne /12/715/2012/ M. F agoso e al.: The 20 Feb ua y 2010 Madei a lash- loods 717 a) (b) c) Fig. 2. Pho os showing he des uc i e e ec s o he 20 Feb ua y 2010 lash- loods and landslides in: (a) down own and (b) uplands; (c) Se a de Água, Ribei a B a a. Fig. 2. Pho os showing he des uc i e e ec s o he 20 Feb ua y 2010 lash- loods and landslides in: (a) down own and (b) uplands; (c) Se a de ´ Agua, Ribei a B a a. on emo e-sensing analysis and alida ed by ield da a collec- ion allowed he iden i ica ion o 3291 landslides in Funchal d ainage basins and 5172 landslides in he Ribei a B a a sec- o (SRES, 2010). Acco ding o he same epo , as a esul o his la ge numbe o landslides, i was es ima ed ha a ol- ume o a leas 250000m3o solid ma e ial was deposi ed in he Funchal u ban a eas. The hyd ological and geomo phological impac s ela ed o his ex eme e en we e concen a ed o e i e d ainage basins (limi s shown in Fig. 1) belonging o he sou h- e n lank o he island, h ee o hem loca ed in he Fun- chal egion (Ribei a de S˜ ao Jo˜ ao, Ribei a de San a Luzia and Ribei a de Jo˜ ao Gomes) while he emaining belong o he Ribei a B a a sec o (Ribei a da Tabua, Ribei a B a a). These d ainage basins a e e y small (wi h a eas a ying be- ween 8.8km2and 15km2), and hei shape is elonga ed, excep in he case o Ribei a B a a basin, he la ge one, wi h 40.9km2and exhibi ing a unnel shape. The i e d ainage basins a e also cha ac e ised by he dominance o s eep slopes (mean slopes a ying be ween 28 and 37 %, wi h some a eas wi h slopes o e 50%) and sho imes o concen a- ion (in e io o 3.3h in all cases, es ima ions made using he Temez me hod, SRES 2010). These wo pa ame e s a e ele an ea u es o lash- looding p oduc ion. Acco ding o SRES (2010), he es ima ed peak low o he 20 Feb u- a y 2010 e en eached 234m3s−1in he Ribei a de Jo˜ ao Gomes, close o 300m3s−1in S˜ ao Jo˜ ao and San a Luzia s eams and 663m3s−1in Ribei a B a a. The complexi y o he phenomenon s udied he e and he ex eme se e i y o i s impac s, esul ed om he combi- na ion o deb is- lows, landslides and lash- looding, ig- ge ed and p oduced in a sho ime lapse as a consequence www.na -haza ds-ea h-sys -sci.ne /12/715/2012/ Na . Haza ds Ea h Sys . Sci., 12, 715–730, 2012 718 M. F agoso e al.: The 20 Feb ua y 2010 Madei a lash- loods o an in ense p ecipi a ion episode ( he “pa oxysm”, in 20 Feb ua y 2010), bu also con olled by an eceden and longe du a ion ain alls, p o iding c ucial p e-exis ing con- di ions o he disas e ( he anomalous we 2010 win e in Madei a). In ac , he in ense ain all e en esponsible o he 20 Feb ua y 2010 lash- loods in Madei a occu ed du - ing a pa icula ly anomalous we win e a ec ing mos sec- o s o sou he n Ibe ian Peninsula (And ade e al., 2011; Ball, 2011; Vicen e-Se ano e al., 2011), and also he Madei a a chipelago. As desc ibed in de ail below, se e al s a ions egis e ed mo e han he iple o he a e age p ecipi a ion du ing he win e (DJF) mon hs. Mo eo e , he ex eme ain- all e en o he 20 Feb ua y was p eceded by e y high p ecipi a ion du ing he p e ious mon hs and, among o he s, an in ense p ecipi a ion episode occu ed h ee weeks ea lie (2 Feb ua y), a ec ing mos ly he cen al moun ain ange wi h se e e looding impac s being el in a s eam o he no he n coas (Faial). Undoub edly, he accumula ed p e- cipi a ion egis e ed h oughou he win e con ibu ed o sa - u a e soils, inducing a ou able condi ions o inc ease and accele a e su ace uno , agg a a ing lash- lood suscep i- bili y in he Madei a d ainage basins (SRES, 2010). The e- o e, he excep ionali y o he 20 Feb ua y ains o m episode should be analysed wi hin he con ex o he whole win e , as he impac s obse ed du ing he e en we e clea ly ampli ied by he con inuous moun ing o hese p edisposing ac o s o lash- lood occu ence. This in es iga ion is ocused on he me eo ological ea- u es o his ex eme e en , wi h he ollowing main objec- i es: 1. To p o ide a de ailed spa ial and empo al cha ac e i- sa ion o he p ecipi a ion obse ed on he 20 Feb u- a y 2010 ha igge ed he lash- loods and landslides. Daily and hou ly p ecipi a ion eco ds om he a ail- able ain-gauge s a ions ne wo k a e e alua ed in o de o econs i u e he empo al e olu ion o he ains o m, as well as i s geog aphic incidence, con ibu ing o un- de s and he lash- lood dynamics and he ype and spa- ial dis ibu ion o he associa ed impac s. 2. To es ima e he excep ionali y o he ains o m, using he longes mon hly, daily and sub-daily p ecipi a ion se ies a ailable o Madei a, wi h a special emphasis on he compu a ion o es ima es o he e u n pe iods o daily p ecipi a ion obse ed du ing he 20 Feb u- a y 2010 e en . 3. To desc ibe he mos ele an a mosphe ic ac o s in- ol ed in his ex eme ain all e en , iden i ying he main a mosphe ic synop ic condi ions and also ele an physical mechanisms ha a ou ed i s de elopmen . 2 Da a and me hods The p esen esea ch esul s om he collec ion, compila ion and analysis o di e se ypes o da a, in o ma ion ha can be conside ed o be included in wo main opics: p ecipi a ion da a and a mosphe ic da a. 2.1 P ecipi a ion da a The Island o Madei a is well co e ed by wea he s a- ions wi h ain-gauges. Un o una ely hese s a ions a e main ained by h ee di e en o ganiza ions, namely: (1) IM/Ins i u o de Me eo ologia, (2) IGA/In es imen os e Ges ˜ ao da ´ Agua and (3) LREC/Labo a ´ o io Regional de En- genha ia Ci il. Since ou aim is o ob ain de ails on he ex ension and in ensi y o he e en , a ca e ul p elimina y wo k o da a assembling was pe o med o e ie e a ho- mogenous p ecipi a ion da ase , allowing a wide geog aph- ical co e age o he island and o be su icien ly ep esen a- i e o i s complex o og aphy. Thus, daily and hou ly p e- cipi a ion da a om a o al o 47 me eo ological and ain- gauge s a ions we e ga he ed. Thei loca ion is depic ed in Fig. 1b and hei main cha ac e is ics a e p esen ed in Ta- ble 2. We a e con iden ha he aingauge ne wo k densi y (exceeding 6 s a ions/100km2)is su icien ly ep esen a i e, in spi e o he s eep o og aphy ha domina es he island land- scape. The p ecipi a ion da a om hese 47 s a ions was used o p oduce hou ly and daily p ecipi a ion maps ela i e o he 20 Feb ua y 2010 ollowing he applica ion o app op ia e geos a is ical echniques. In his ega d, he o dina y k iging was chosen as in e pola ion me hod, es ima ing speci ic a - iog am models o each map (e.g., Goo ae s, 1997; Showal- e , 2010). In addi ion o hese p ecipi a ion da a, long- e m se ies o annual maximum daily p ecipi a ion we e collec ed, al- lowing he e alua ion o he e u n pe iod, i.e., an objec i e measu e o he long- e m excep ionali y o he 20 Feb u- a y e en . Un o una ely, only wo se ies wi h app op i- a e empo al leng h (wi h ongoing obse a ions and a leas mo e han 60y o eco ds) – Funchal/Obse a ´ o io (no. 43, Fig. 1b) and A eei o (no. 25, Fig. 1b) – we e a ailable and used o his analysis. Mo eo e , bo h se ies a e no comple e and hei quali y was also a ec ed by changes in he ins u- men s along he o al pe iod o collec ed in o ma ion, s a - ing in 1936 and ending in 2010. Ne e heless, he numbe o gaps was 5% (8%) o missing da a o Funchal (A eei o), which was conside ed o be accep able when compu ing he e u n pe iod es ima ions. 2.2 Synop ic and a mosphe ic da a The synop ic con ex o he 20 Feb ua y 2010 ex eme e en was analysed using di e en sou ces o in o ma ion. Wea he cha s co e ing he Eas e n A lan ic we e ob ained om he Ge man Wea he Se ice (Deu sche We e diens ; DWD) Na . Haza ds Ea h Sys . Sci., 12, 715–730, 2012 www.na -haza ds-ea h-sys -sci.ne /12/715/2012/ M. F agoso e al.: The 20 Feb ua y 2010 Madei a lash- loods 719 Table 2. Desc ip ion o me eo ological s a ions and aingauges used in his s udy. Code Designa ion Longi ude La i ude Al i ude (see Fig. 1) (W) (N) (m) 1 Seixal (LREC) 17◦0602100 32◦4901200 70 2 S. Vicen e, Pos o Flo es al (LREC) 17◦0201900 32◦4703500 120 3 Pon a do Sol (LREC) 17◦0504400 32◦4005600 130 4 Lombo dos Palhei os (LREC) 16◦5005200 32◦4601800 212 5 S˜ ao Gonc¸alo (LREC) 16◦5202400 32◦3804100 220 6 LREC – EMA (LREC) 16◦5601500 32◦3805300 260 7 Achada do Til (LREC) 17◦0101300 32◦4603900 300 8 Massapez (LREC) 17◦0901700 32◦4204700 300 9 T apiche (LREC) 16◦5603400 32◦4002900 590 10 Faj˜ a Noguei a (LREC) 16◦5400800 32◦4402200 629 11 P aze es (LREC) 17◦1105200 32◦4500100 632 12 Faj˜ a O elha (LREC) 17◦1303800 32◦4602700 635 13 Faj˜ a do Penedo (LREC) 16◦5701900 32◦4701800 637 14 Camacha (LREC) 16◦5003600 32◦4000900 675 15 Po o Moniz, Es . Zoo . – EMA (LREC) 17◦1005200 32◦5002900 675 16 Pinhei o Fo a (LREC) 17◦0702800 32◦4302300 750 17 Cu al das F ei as (LREC) 16◦5702000 32◦4403100 800 18 Casa Velha (P.F.) (LREC) 16◦5001700 32◦4203600 880 19 Pico das Ped as (P.F.) (LREC) 16◦5303700 32◦4602800 920 20 Pico Ve de – EMA (LREC) 17◦1003000 32◦4505400 1020 21 T ompica, Pos o Flo es al (LREC) 17◦0002200 32◦4201500 1188 22 PEF EMA (LREC) 16◦5304800 32◦4105600 1300 23 Pico da U ze (LREC) 17◦0604800 32◦4404600 1365 24 Es anquinhos (LREC) 17◦0401500 32◦4600000 1590 25 A eei o-LREC 16◦5404500 32◦4300000 1590 26 Machico - EMA (LREC) 16◦4602600 32◦4301700 170 27 Meia Se a (IGA) 16◦5200200 32◦4202000 1100 28 ETA Aleg ia (IGA) 16◦5500000 32◦4003900 611 29 Encumeada (IGA) 17◦0101500 32◦4404700 854 30 Bica da Cana (IGA) 17◦0301200 32◦4501000 1585 31 Co a G ande (IGA) 17◦0505000 32◦4401400 1340 32 O il (IGA) 16◦5005600 32◦4202900 1014 33 Ros´ a io, Faj˜ a Rod igues (IGA) 17◦0104800 32◦4503800 575 34 Fon e do Bispo (IGA) 17◦1003500 32◦4701700 1245 35 San o Se a, Lamacei os (IGA) 16◦5002400 32◦4401600 784 36 San o da Se a, Quin a (IGA) 16◦4804600 32◦4302000 660 37 Achada da Madei a (IGA) 16◦5803200 32◦4604100 521 38 Ch˜ ao das Fei ei as (IGA) 16◦5205400 32◦4302900 1180 39 Ch˜ ao dos Lou os (IGA) 17◦0004900 32◦4501100 900 40 Cu al das F ei as ETA (IGA) 16◦5702900 32◦4301200 743 41 Poiso, Pos o Flo es al (IGA) 16◦5205800 32◦4203200 1360 42 Ribei a do Alec im (IGA) 17◦0702200 32◦4405800 1293 43 Funchal/Obs. (IM) 16◦5301800 32◦3803800 58 44 Luga de Baixo (IM) 17◦0501700 32◦4003900 15 45 Pon a do Pa go (IM) 17◦1503100 32◦4803700 312 46 S˜ ao Jo ge (IM) 16◦5400600 32◦4905200 185 47 Funchal/Lido (IM) 16◦5505300 32◦3705800 25 and we e use ul o he ecogni ion o he main ea u es o he associa ed synop ic a mosphe ic ci cula ion. A - mosphe ic ields we e ex ac ed om he ERA-In e im e- analysis (ECWMF) da ase , a ailable on a 1.5◦×1.5◦la - i ude/longi ude mesh and a e used o analyse he dynami- cal aspec s o he me eo ological e en . The he modynamic en i onmen associa ed wi h his e en is cha ac e ised by wo ins abili y pa ame e s, he Li ed index (LI) and CAPE (Con ec i e A ailable Po en ial Ene gy). The LI is he em- pe a u e di e ence be ween an ai pa cel li ed adiaba ically and he en i onmen empe a u e a 500hPa (e.g., Galway, 1958). Nega i e alues o LI indica e ha he a mosphe e is uns able. The mo e nega i e alues o LI, he highe he p obabili y o hunde s o m occu ence. CAPE is he www.na -haza ds-ea h-sys -sci.ne /12/715/2012/ Na . Haza ds Ea h Sys . Sci., 12, 715–730, 2012 720 M. F agoso e al.: The 20 Feb ua y 2010 Madei a lash- loods Fig. 3. (a)Accumula ed daily p ecipi a ion (ADP) in A eei o (s a ion numbe 25 in Fig. 1) be ween 1 Oc obe 2009 and 28 Feb ua y 2010 (blue line) and compa ison wi h he long- e m median ( ed line) and 95 h pe cen ile (da k line) o he ADP o he same ime span, es ima ed o e he 1936/37–2008/09 pe iod. Indica ed is he occu ence o h ee e en s (1) 22 Decembe 2009 lash lood (2) 2 Feb ua y 2010 hea y ain all e en (3) 20 Feb ua y 2010 lash lood. No e: he ep esen ed p ecipi a ion da a we e eco ded in di e en me eo ological s a ions, howe e , jus sepa a ed by a ew me es and loca ed app oxima ely a he same heigh (20m o di e ence). (b) Hou ly e olu ion o he 20 Feb ua y 2010 ains o m in Madei a, in ou selec ed udog aphic s a ions om he Funchal ci y su oundings. Indica ed hou s in he abcissae axis a e he ending ime o he co esponding p ecipi a ion amoun . The al i ude (me es) o he ou ain-gauge s a ions is inse ed in he legend. amoun o ene gy a pa cel o ai would ha e i li ed e i- cally h ough he a mosphe e (Monc ie and G een, 1972; Doswell and Rasmunsen, 1994). Posi i e CAPE indica e posi i e buoyancy o an ai pa cel being, hus, an indica- o o a mosphe ic ins abili y. The humidi y anspo as- socia ed wi h his e en has been calcula ed based on his da a using 3-D backwa d ajec o ies (Me h en, 1997). This ool allows he acing o humidi y (o o he a mosphe ic con en s) o an ai pa cel and has been used, he e o e, in some s udies (e.g., Me h en e al., 2001). In addi ion, he diagnos ic o he in ense mid-la i ude cyclone sys em asso- cia ed o he 20 Feb ua y 2010 lash- loods in Madei a was also pe o med using app op ia e sequences o Me eosa -9 sa elli e images (in a- ed and wa e apou channels, and also om he de i ed p oduc Mul i-Senso P ecipi a ion Es- ima e, a ailable a www.eume sa .in ). Fu he , he e olu- ion o local condi ions o he oposphe ic column was e- cons i u ed using da a om he adiosounding s a ion in Fun- chal. Un o una ely, only one sounding pe day is launched (a 12:00UTC). Sounding plo s, he modynamic pa ame e s and s abili y indices we e compu ed using he RAwinsonde Obse a ion (RAOB) so wa e (Schewchuk, 2002). 3 Rain all analysis This sec ion is de o ed o he empo al and spa ial cha ac- e isa ion o he p ecipi a ion associa ed wi h his p ominen e en . We s a by analysing he p ecipi a ion du ing he an eceden mon hs leading o he ex eme e en . I mus be unde lined ha he accumula ed p ecipi a ion in he p e i- ous mon hs was e y impo an o he phenomenon s udied he e. The la ge numbe o landslides and deb is lows sud- denly p oduced in he 20 Feb ua y 2010, a e connec ed wi h he accumula ed ain all. This ac o agg a a ed he se e - i y o he impac s o he lash loods, p o iding an eno mous amoun o ocks, deb is and solid ma e ial o be d agged in o he alleys. A e wa ds, a de ailed desc ip ion o he spa ial and empo al e olu ion o he p ecipi a ion e en is p o ided. Finally, he excep ionali y o he e en is e alua ed wi h long eco ds ha allow compu ing he e u n pe iods associa ed wi h he e en . 3.1 The ex emely we 2009–2010 win e in he Madei a Islands The analysis o Fig. 3a allows he compa ison o he accu- mula ed daily p ecipi a ion (ADP) be ween he beginning o he 2010 hyd ological yea (s a ing 1 Oc obe 2009) and he end o Feb ua y 2010 (blue line) wi h he long- e m median ( ed line) and 95 h pe cen ile o he ADP (black line) o he same 72-y long pe iod (i.e., 1937/38–2008/09). The p e- cipi a ion da a shown in his igu e (A eei o s a ion, no. 25 in Fig. 1b) is ep esen a i e o he p ecipi a ion eco ds in he uppe pa o he eas e n moun ains o he island, close o one o i s highes peaks. The win e was ex emely we a A eei o (abo e 4000mm o ain all be ween 1 Oc obe 2009 and 28 Feb ua y 2010), exceeding mo e han wice he median Na . Haza ds Ea h Sys . Sci., 12, 715–730, 2012 www.na -haza ds-ea h-sys -sci.ne /12/715/2012/ M. F agoso e al.: The 20 Feb ua y 2010 Madei a lash- loods 721 amoun o ADP ob ained be ween 1937 and 2008 o he same pe iod. Fu he mo e, he accumula ed ADP a he end o Feb ua y was also signi ican ly highe han he co - esponding 95 h pe cen ile; in ac , he win e o 2009/2010 was he ainies we season since 1937. Ne e heless, he ela i e ank o his win e was no cons an h oughou he empo al window analysed in Fig. 3a, s a ing wi h a we Oc- obe , he ADP emained sligh ly abo e he median un il mid Decembe , inc easing o highe pe cen ile only a e wa ds. In ac , a e he 22 Decembe 2009, he ADP begun o ex- ceed he 95 h pe cen ile and kep abo e his h eshold un- il he end o Feb ua y. The e o e, his compa a i e analysis clea ly demons a es ha he 2009/2010 ainy season was ex- cep ionally we a he moun ainous egion a ew kilome es abo e he Funchal bay a ea. In pa icula , hea y p ecipi a- ion episodes wi h ex emely high amoun s a daily scale oc- cu ed: 22 Decembe 2009, 2 Feb ua y 2010 and 20 Feb u- a y 2010 (he e desc ibed). These e en s a e ma ked 1–3 in Fig. 3a. In he Funchal a ea, he anomalously we 2010 win e (DJF) was e en mo e ex eme (in ela i e e ms) han wha has been desc ibed o he Madei a uplands. The accu- mula ed p ecipi a ion in 2010 win e was 997mm in Fun- chal/Obse a o y s a ion (no. 43 in Fig. 1b), la gely exceed- ing he 95 h pe cen ile o i s his o ical eco ds o his sea- son, which is close o 700mm. In pa icula , he p ecipi a ion amoun obse ed a his s a ion du ing Feb ua y – 458.7mm – co esponds o he new absolu e all- ime eco d since 1865, i.e., since he beginning o egula obse a ions in he is- land. We would like o s ess ha his alue exceeds 8 imes he median alue o Feb ua y p ecipi a ion (ob ained o he no mal 1971–2000 pe iod). This analysis con i ms ha he 20 Feb ua y 2010 e en was p eceded by an equally ou - s anding anomalous accumula ed p ecipi a ion, pa icula ly on he uppe pa s o Eas e n Moun ains, which p obably lead o inc ease he slope ins abili y a he ups eam sec o s o he Funchal ca chmen s and, hus, inducing a ou able condi ions o igge ing landslides. The con ol o ain all on landslides di e s subs an ially depending upon landslide dep h and kinema ics and he a ec ed ma e ial. Shallow soil slips and apid deb is lows a e ypically ac i a ed by a sho pe iod o e y in ense ain, while deep-sea ed o a ional and ansla ional slides a e usually associa ed wi h less in ense ain all occu ing in a pe iod las ing se e al weeks o se e al mon hs (e.g., Zˆ eze e e al., 2005). 3.2 Spa ial ea u es and empo al e olu ion o he 20 Feb ua y 2010 ains o m A syn he ic o e iew o he 20 Feb ua y ain all o selec ed s a ions is depic ed in Fig. 3b, whe e we can app ecia e he e olu ion o hou ly p ecipi a ion h oughou he day ela i e o ou selec ed ain-gauge s a ions placed in he su ounding a eas o he Funchal u ban dis ic . Among hese ain-gauge s a ions, S˜ ao Gonc¸alo (220m o al i ude, no. 5 in Fig. 1b) is he lowes al i ude s a ion and is loca ed eas o Funchal. T apiche (590m, no. 9 in Fig. 1b) and Camacha (690m, no. 14 in Fig. 1b) a e ep esen a i e si es om he in e medi- a e sec o s o he slopes and a e espec i ely loca ed o he wes and eas o he Funchal ci y a ea. A eei o (1590m, no. 25 in Fig. 1b) is he mos ele a ed ain-gauge ep esen ed in Fig. 3b, illus a ing he ains o m on he ups eam sec o o he d ainage basin ha c oss he Funchal u ban dis ic , and whe e lash- loods occu ed. The o al du a ion o he ains o m was qui e unusual wi h oughly 14 consecu i e hou s o unin e up ed p ecip- i a ion o e mos o he Island, s a ing a 03:00UTC and ending only a 17:00UTC. The hou ly p ecipi a ion in en- si y inc eased g adually du ing he ea ly mo ning hou s o 20 Feb ua y in his sou he n sec o o he Madei a Island. In e es ingly, he mos in ense and c i ical pe iod o he s o m hi ini ially he lowe and midway sec o s o he slopes (Fig. 3b), whe e he hou ly ain all in ensi y eached a leas 60mmh−1be ween 08:00 and 09:00UTC. A his s age, he p ecipi a ion was much lowe in he uppe sec o s o he moun ainous a ea, such as he A eei o Peak. Howe e , du - ing he ollowing hou s, while he p ecipi a ion s a ed o dec ease a lowe al i udes, an inc ease can be obse ed a highe le els (e.g., A eei o), wi h a maximum a 11:00UTC, when landslides and loods we e al eady occu ing and ag- g a a ing hei impac s e en mo e. A e wa ds, and un- il 14:00UTC, e y abundan p ecipi a ion was accumula ed a he ups eam sec o s o he d ainage sys ems, especially hose ha low in o he Funchal coas al a eas, as i is demon- s a ed by he hou ly in ensi y a es abo e 30mmh−1in he A eei o ain-gauge s a ion (Fig. 3b). In o de o ob ain an app op ia e econs i u ion o he space- ime a iabili y o he p ecipi a ion associa ed wi h he 20 Feb ua y ains o m in Madei a, hou ly maps we e elabo- a ed using all a ailable in o ma ion om he 47 ain-gauges o he island (Fig. 4). These ele en hou ly maps illus a e he e olu ion o he ains o m be ween 05:00 and 16:00UTC, hus, allowing a de ailed ep esen a ion o he e en ( he i s h ee hou s a e omi ed as p ecipi a ion amoun s we e el- a i ely small). Figu e 4 documen s he ini ial phase o he e en , when p ecipi a ion began o ba e he sou he n lanks o he island, especially i s eas e n sec o s, including he Fun- chal dis ic . F om 08:00 o 09:00UTC, he ain all was mo e in ense a lowe sec o s o he sou he n slopes, wi h in ensi ies close o 35mmh−1. In he ollowing wo hou s, he p ecipi a ion a e inc eased (up o 60 o 80mmh−1) and was mo e concen a ed o e he in e media e ele a ion sec- o s a ound he Funchal egion, a slope le els be ween 500 and 1000m o al i ude. The mos in ense co es o se e e p ecipi a ion a e 10:00UTC mainly a ec ed a eas close o Funchal o i s su oundings. F om 11:00UTC onwa ds, he mos in ense p ecipi a ion mo ed no hwa ds and was mos ly con ined o he uplands and highe peaks o he cen al idge o he island. Th oughou his pe iod, he spa ial pa e n o p ecipi a ion dis ibu ion is gene ally ma ked by he majo www.na -haza ds-ea h-sys -sci.ne /12/715/2012/ Na . Haza ds Ea h Sys . Sci., 12, 715–730, 2012 722 M. F agoso e al.: The 20 Feb ua y 2010 Madei a lash- loods Fig. 4. Hou ly p ecipi a ion in Madei a, 20 Feb ua y 2010, be ween 05:00 UTC and 16:00 UTC. The maps a e based on da a om he same ain-gauge ne wo k ep esen ed in ig. 1 (loca ions) and lis ed in Table 2. Fig. 4. Hou ly p ecipi a ion in Madei a, 20 Feb ua y 2010, be ween 05:00UTC and 16:00UTC. The maps a e based on da a om he same ain-gauge ne wo k ep esen ed in Fig. 1 (loca ions) and lis ed in Table 2. opog aphic con as s (depic ed in Fig. 1b), sugges ing ha he o og aphy played an impo an ole o enhance and ocus he e en , esul ing in a longe and e en ainie s o m. The o al daily p ecipi a ion amoun s o he 20 Feb u- a y 2010 in Madei a a e shown in Fig. 5a. Since no el- e an showe s occu ed a e i s end 17:00 UTC, he p e- cipi a ion dis ibu ion is ep esen a i e o he whole e en . Two majo indings eme ge om he analysis o Fig. 5a: (a) he la ge a ea o he island (>50%) wi h o al p ecip- i a ion inside he 250mm isoye h limi s; and (b) he exis- ence o wo main co es o ex emely abundan p ecipi a- ion, wi h amoun s a ying be ween 250 and 370mm, one loca ed o e he uplands and sou he n slopes o he eas e n moun ains (A eei o pla eau), and ano he one cen ed o e he cen al pla eau sec o (Pa´ ul da Se a and Encumeada). A close look a Fig. 5a indica es ha he Funchal a ea was en- i ely su ounded by he c escen pa e n ha cha ac e ises he sou heas e n peak o in ense ain alls, which explains he se e i y o lash- loods, landslides and deb is- lows impac s on i s d ainage basins (see example in Fig. 2b). On he o he hand, he second p ecipi a ion peak o e he cen al pla eau is esponsible o he ca as ophic impac s o loods and land- slides in he Ribei a B a a a ea, loca ed u he wes in he sou he n lank o he Island (Fig. 1b). The main co es o excessi e ain all may be exp essed by mapping he o al numbe o hou s wi h p ecipi a ion abo e 10mm (Fig. 5b; SRES, 2010). The pe iod o sus ained in ense p ecipi a ion was conside ably long, a ying om i e o en consecu i e hou s o e ex ended a eas, which was, e y p obably, a c i - ical condi ion o igge he la ge numbe o landslides and o he complex slope mo emen s obse ed in he wo main a ec ed sec o s (i.e., Funchal and Ribei a B a a, c . Fig. 5b). Undoub edly, he e ec s o such a la ge numbe o land- slides, p o iding an ou s anding olume o solid ma e ial o be d agged along s eams, accoun ed o agg a a ing lash- looding impac s, esul ing in he ca as ophic and deadly e en on he lowe sec o s o he small d ainage basins. 3.3 Assessmen o he p ecipi a ion excep ionali y The es ima ion o e u n pe iods is a classical app oach in o de o e alua e he excep ionali y and magni ude o ex- eme me eo ological e en s. This echnique uses he ex- eme alues analysis, he b anch o p obabili y and s a is ics ha is used o make in e ences abou he size and equency o ex eme e en s (S o ch and Zwie s, 2003). The p op i- e ies o some o he mos used ex eme alues dis ibu ions in clima ology and hyd ological sciences we e desc ibed by Na . Haza ds Ea h Sys . Sci., 12, 715–730, 2012 www.na -haza ds-ea h-sys -sci.ne /12/715/2012/ M. F agoso e al.: The 20 Feb ua y 2010 Madei a lash- loods 723 Fig. 5. (a) To al daily p ecipi a ion in Madei a (00:00–24:00UTC), 20 Feb ua y 2010. The map is based on da a om he same ain- gauge ne wo k ep esen ed in Fig. 1 (s a ion names a e lis ed in Ta- ble 1). (b) Numbe o hou s wi h p ecipi a ion abo e 10mm and landslides dis ibu ion on Funchal and Ribei a B a a a eas. This map is based on a subse o he ne wo k p esen ed in Fig. 1, consis - ing only in udog aphic s a ions. Sou ce: SRES, 2010. Gumbel (1958) and ecen e iews documen a la ge num- be o applica ions (e.g., Coles, 2001; Alexande sson e al., 2001; S enson and Jones, 2010). The a ailabili y o long- e m se ies o he pa ame e o be analysed is a manda o y condi ion o ca y ou his ype o in es iga ion in an app o- p ia e way, which ep esen s in many cases a signi ican con- s ain . In he p esen s udy, he sca ce a ailabili y o long- e m se ies posed a p oblem. Ano he p oblem is ha he a chi es only con ain he p ecipi a ion accumula ion o ixed pe iods (e.g., 24h beginning a 09:00UTC), ins ead o accu- mula ions o e a mo ing window (e.g., 24-h pe iod). Wi h ega d o he p ecipi a ion se ies o annual max- ima wi hin 24h, app op ia e long- e m eco ds (1936–2010) a e only a ailable o wo me eo ological s a ions: A eei o and Funchal/Obse a o y. A numbe o expe imen s we e conduc ed in o de o i he mos commonly used ex eme alue dis ibu ions o he da a. The bes i ing model was ob ained wi h he GEV dis ibu ion, anking he se o e- sul s om h ee s anda d es s o e alua e he goodness o i : Kolgomo o -Smi no , Ande son-Da ling and Chi-squa ed es s (e.g., Wilks, 2005). Re u n pe iods o he 24h ain- all obse ed o he 20 Feb ua y 2010 o bo h si es con- i m he excep ionali y o he ains o m. In he case o he moun ainous A eei o s a ion, he 24h amoun de ec ed (333.8mm) co esponds o an es ima ed e u n pe iod o ap- p oxima ely 90y . In con as , he 24h amoun obse ed in Funchal/Obse a o y (146.9mm) co esponds o a e u n pe- iod es ima ed o be in he o de o 290y , hus, an e en less equen e en . Cau ion is equi ed when es ima ing such long e u n pe- iods, as hey a e much longe han he ime se ies used o compu e hem. In any case his s a is ical app oach clea ly sugges s ha he 20 Feb ua y2010 ex emee en can be clas- si ied as excep ional o bo h s a ions, al hough e en mo e ou s anding in he Funchal a ea han a he op o he Eas - e n moun ains. Mo eo e , i mus be s essed ha almos 80% o his ex eme daily p ecipi a ion obse ed in he Fun- chal/Obse a o y occu ed wi hin a six hou s pe iod (08:00 and 14:00UTC). This pe iod o sus ained in ense ain-bu s s p oduced 114.8mm o accumula ed p ecipi a ion, an imp es- si e amoun o 6h-du a ion in Funchal, as i was con i med a e a compa ison wi h p e ious co esponding annual max- ima o e he 1980–2009 pe iod: This alue exceeds he p e- ious maximum o his ime leng h (92.6mm, obse ed in 27 Sep embe 1989). 4 Synop ic con ex This sec ion is de o ed o he discussion o he main aspec s o he a mosphe ic ci cula ion and associa ed physical mech- anisms ha we e esponsible o he 20 Feb ua y copious amoun s o ain all in Madei a Island. I is impo an o ad- d ess i s he la ge-scale a mosphe ic condi ions associa ed wi h he p olonged p ecipi a ion anomaly in Madei a du ing he win e 2010. In ac , his win e was cha ac e ised by a clea p e alence o he nega i e phase o he No h A - lan ic Oscilla ion (NAO) (new win e eco d-b eaking, e.g., Osbo n, 2011), wi h s ong posi i e SLP anomalies in he Iceland low a ea and nega i e anomalies in he Azo es high p essu e egion (And ade e al., 2011; Vicen e-Se ano e al., 2011). In p e ious wo ks (e.g., Ulb ich e al., 1999; T igo e al., 2004; San os e al., 2005, 2009) i was demons a ed ha hese la ge-scale a mosphe ic ci cula ion condi ions as- socia ed wi h nega i e NAO phases induce an inc ease in he equency o mid-la i ude cyclones a ec ing he en i e wes - e n Ibe ian egion and a la ge sec o o he con iguous no h A lan ic basin, he e o e, a ec ing also he island o Madei a. 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Sci., 12, 715–730, 2012 www.na -haza ds-ea h-sys -sci.ne /12/715/2012/