ENSO and Hydrologic Extremes in the Western United States

ENSO and Hydrologic Extremes in the Western United Statesompcsite Anomaly day-1 530mb Vien tos(Mg, omb)-Z'cont) Composite Ar omaly day 0 500mb Vients(M somb)-Z(con:)Compcsite Ancmaly L'ay-1 250mb Vientos(Mg, somb)-L(cont) Composite Aromaly Day 0 2b0mb Vientos(Mg somb)-i(con0:AFigure 4: Same as Figure 3, but at 500(top) and 250 h Pa(bottomComposite Anomay day-1 850mb Moisture Flux (Ng, shaded Composite anomaly day 0 850mb Moisture Flux (Mg, shadedPomposite Anomay Day 1 700mb Moisture Flux(Ng. shaded: Composite Anomaly Day 0 700mb Moisture Flux(Mg, shaded)igure 5. SarFigure 3, but for moisture flux(intensity is shaded3b. Regional orographic influences on theexplored. The circulation associated with theseprecipitationextreme events is an intense negative anomaly ingeopotential heights strenghtened on the day ofA visual inspection of daily precipitation series heavy rainfall and located off the central Chile coastarround the mountain revealed that the most frequent Besides, an anomalous northwesterly flow andduration of each heavy event was 4 days. moisture flux at low, and mid-levels impinges on thePrecipitations were mostly not recorded in lee-side high Central Andes. The u component(cross-barrier)stationsg the first 2 days of eachnt andmponent (along-barrier) of thflow(including the one of highest intensity), but some promoted an intense orographic lifting on windwardstations reported the occurrence of Zonda"wind slope and humidity transport from subtropical(warm and dry downslope wind) during these days in latitudes, respectively. Thus, extreme precipitations itmost of the events. This reveals the strong influence would generate over the Central Andes rangeof orography on precipitations inhibiting them leewardThe orographic influence on precipitation is verydue to the descending character of the airflow. Rain important in the Central Andes region Most of theshadows were recorded in low-lying stations on lee precipitation systems that impinges on the barrier areside mostly in the last days of each eventblocked and only a small fraction of them can crossIn order to examine the precipitation decrease in the mountain obstacle. The reduction on lee-sidelee-side lowlands of the northern sector according to precipitation with respect to highlands area was notedthe precipitation intensity on the highlands(Fig. 1), its in frequency andd also this reductiondaily precipitation distribution was divided by the accentuated when the precipitation intensity increasedpercentiles 0-80%(weak and moderate),80-95% on highlands, up to 99% for extreme events(intense), and higher than 95%(extreme). Then, thedays with precipitation on mountains were retained Acknowledgments. The authors would like to thankfrom low-laying precipitation series. It can be seen in the Oganizing Committee of the 13 Conference onTable 1 that the winter total snowfall, and almost the mountain Meteorology for helping m. viale to attendannual total snowfall, was recorded mostly during the congressintense and extreme events(e, events with a 20% offrequency). When calculating the reduction REFERENCESprecipitation rates, defined as the daily area averageprecipitations on the lowlands divided by it on the Falvey, M. and R. garreaud, 2007: Wintertimehighlands, it can be noted that there is a sharplyprecipitation episodes in Central Chilereduction of precipitation on lee side that increasedsociated meteorological conditions andthen the intensity increased on highlands. For theorographic influences. J. Hydrometeor, 8, 171extreme events on the mountains. the precipitation193was almost entirely inhibited on lee sideGrumm,R.h. and r. e. hart. 2001: StandardizedPrecipitation Freq. Total amount Reductions MaximumAnomalies applied to significant Cold Seasonconditions(days) in 7 wintersWeather Events: Preliminary Findings. WeaPercentil 0-80% on 293927mm14.0mmForecasting, 16.736-754the Andes80%Junker N. W.R.H. Grum R. Hart. L. F bosart, KOn lee side0-80%0de when101of23318,1mmBell. and F.J. Pereira: Use of NormalizedAndesanomaly fields to anticipate extreme rainfall inthe mountains of northern California. WeaPercentil 80-95% 571478mm(40‰)Forecasting, 23, 336-356on the andes95%On lee side when 32 of 57 79 mm250mmPandey, G.R.,D. R. Cayan, and K P. Georgakakos80-95%。nthe1999: Precipitation structure in the Sierrala duringPercentile s1244mm1316rrRes,104(D10),12019-12030the anSuart, N. A, and R. H. Grumm, 2006: Using windon lee side when 12 of 19 13 mm3,8mm95% on the Andesanomalies to forecast east coast winter stormsWea. Forecasting, 21, 952-968rainfall for different threshold on the andes and on Uppala, S M. and coauthors 2005: The ERA-40 Relowlands lee side when precipitates on the Andesanalysis. Quart. J. Roy. Meteor. Soc., 1312961-3014. CONCLUSIONSThe synoptic patterns associated with extremeorographic over the Central Andes( 30-35Swereexamined by standardized anomaly fields. In additionthe regional orographic influence on precipitation was