Definizione quantitativa dei regimi pluviometrici nelle Alpi orientali italiane – A new method to quantitatively discriminate the pluviometric regimes of the Eastern Italian Alps

A detailed analysis of the p recipitation featu res in the Iralian Alpin e system, including the Resia Pass (Val Venosta), the Ad amello -Pr esanella gro up and the eas tern mar gin of Giuli e Pre-Alp s (Mr. Matajur), has shown tha t p resent classifications of plu viornerr ic regimes, though valuable and well structured , are not satisfa ctory in disc rimin atin g the different regimes types. This is mostly due to the physiographi c complexity of the area th at de termines di fferent effects of atmospheric perturbations at both genera l and local scale. In the study area, abo ut 25 ,000 km 2 wide, within relat ively sho rt distanc es both the rainfall and d rought peaks of the Italian terri to ry are record ed, with annual values above 3,000 mm in the Giulie P re-Alps, north of Udine, and 500 mm north-west of Bolzano, respectively. P revious stu dies have po inted out an abso lute pr ecipit ation minimum (mos tly snow) in winte r, a summe r maximum in the northern – most watershed s and a bimodal equinotial regimen in the Pr e-Alpin e area near the Po Plain . The pr eviou s regimen classifications are mainly qualitative and are not adequate to the complexity of the study area physiograph y. The quantitative classification proposed in thi s paper was obtained con siderin g mean monthly, seaso na l and annual p recipitation data as well as their pr op ortion to the annual value averaged on the 1961-1995 inte rval for 250 raingauges . Th e aim of this study is ro define a new me thodology to single out with higher accur acy areas omogeneo us in terms of precipit ation cha racteristic s and the boundaries amo ng the various pr ecipitation regimes in mountain areas characterized by marked orogra phic and local precipitation variations. For this purpose K-mean cluster analysis was used. The study area was subdiveded into six classes of stations with similar statistical characteristics. Stations within the same class may be groupe d far apart on the stu dy area confirming the irre gular distibution of precipita tion, irr espective of ph ysiographic similarities, in such complex mo untain territor y. Th rou gh specifically designed qu antitative indexes, obtained by normalised precipitation da ta, micro clima tic situations with regimes, hardly identified by the pr eviou s method s, pa rt icularly where precipitations are cos tant in the warm semester (transition regimes ), were pointed out. The study area resulted sub divided int o six main pr ecipitation types, ran ging from the continental summer unimodal to the sub -padan with mod est but constant year round rainfalls, and fo ur sub-cont inenta l subtypes . A significant relationship between such pluviornetric types and th e six cluster s obtained by the K-me an cluster ing resulted. In particular, the clusters chara cterizing the inn er areas of the Alp ine chain coincide with the continental regime po rtions of the territory, while the outer areas genera lly con cide with those cha racter ized by Pre-Alpine and Sub -Alpine equinoti al regimes. Th e procedure developed allowed to identify a distinctive tran sition regime, including th e stations loc ated in int erm ediate areas or on clust er boundaries, ch araterised by similar sp ring, summe r and aut umn totals , capabl e of clea rly discriminate between the continental and sub-med iterranean dominions. A parti cul ar situation, given by the geog rap hicposition , is th at of the subcont ine nta l type 4 , occ ur ing in the valleys of th e southern portion of O rtles-Cevedale gro up (wes tern Trentino ) and characterized by a peak in May followed by a con stant de cline of precipitation , down ro minimum in De cember-January. Th e location of the area, downwind of the mostly perturba ted flows , determines scarce precipitations. Mor eover , the occur rence of wid e areas covered with vedrettes and valley glaciers markedly limitates the diurnal heating of highly elevated basins and in the summer contrasts the forma – tion of convective cells and the thunder storms associa ted . Th e subco nt inental type 4 is supposed not to derive from a less rainy th an normal in the summer t ransition regime, but to rep resent a cont ine nta l regimen with a summe r peak, truncated by the above mentioned synoptic and thermodynamic factors .