Documents

poveda 2015

Description
Description:
Categories
Published
of 13
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  The Diurnal Cycle of Precipitation in the Tropical Andes of Colombia G ERMÁN  P OVEDA , O SCAR  J. M ESA , L UIS  F. S ALAZAR , P AOLA  A. A RIAS , H ERNÁN  A. M ORENO ,S ARA  C. V IEIRA , P AULA  A. A GUDELO , V LADIMIR  G. T ORO ,  AND  J. F ELIPE  A LVAREZ Escuela de Geociencias y Medio Ambiente, Universidad Nacional de Colombia, Medellín, Colombia (Manuscript received 9 October 2003, in final form 7 July 2004)ABSTRACTUsing hourly records from 51 rain gauges, spanning between 22 and 28 yr, the authors study the diurnalcycle of precipitation over the tropical Andes of Colombia. Analyses are developed for the seasonal marchof the diurnal cycle and its interannual variability during the two phases of El Niño–Southern Oscillation(ENSO). Also, the diurnal cycle is analyzed at intra-annual time scales, associated with the westerly andeasterly phases of the Madden–Julian oscillation, as well as higher-frequency variability (  10 days), mainlyassociated with tropical easterly wave activity during ENSO contrasting years. Five major general patternsare identified: (i) precipitation exhibits clear-cut diurnal (24 h) and semidiurnal (12 h) cycles; (ii) theminimum of daily precipitation is found during the morning hours (0900–1100 LST) regardless of season orlocation; (iii) a predominant afternoon peak is found over northeastern and western Colombia; (iv) over thewestern flank of the central Andes, precipitation maxima occur either near midnight, or during the after-noon, or both; and (v) a maximum of precipitation prevails near midnight amongst stations located on theeastern flank of the central Cordillera. The timing of diurnal maxima is highly variable in space for a fixedtime, although a few coherent regions are found in small groups of rain gauges within the Cauca andMagdalena River valleys. Overall, the identified strong seasonal variability in the timing of rainfall maximaappears to exhibit no relationship with elevation on the Andes. The effects of both phases of ENSO arehighly consistent spatially, as the amplitude of hourly and daily precipitation diminishes (increases) duringEl Niño (La Niña), but the phase remains unaltered for the entire dataset. We also found a generalizedincrease (decrease) in hourly and daily rainfall rates during the westerly (easterly) phase of the Madden–Julian oscillation, and a diminished (increased) high-frequency activity in July–October and February–Aprilduring El Niño (La Niña) years, associated, among others, with lower (higher) tropical easterly wave (4–6day) activity over the Caribbean. 1. Introduction Colombia is located in the northwestern corner of South America, exhibiting complex geographical, envi-ronmental, and hydroecological features in which theAndes plays a major role. In terms of precipitationamount and distribution, Snow (1976, p. 362) describesthe Andes as “a dry island in a sea of rain,” but theatmospheric dynamics and precipitation variability as-sociated with the Andes are not well understood at awide range of time–space scales. Improving our under-standing of the atmospheric dynamics and precipitationvariability is crucial under current environmentalthreats faced by the tropical Andes, identified as themost critical “hot spot” for biodiversity on earth (Myerset al. 2000).On seasonal time scales, it is well known that thedisplacement of the intertropical convergence zone(ITCZ) exerts a strong control on the annual cycle of Colombia’s hydroclimatology (Snow 1976; Mejía et al.1999; León et al. 2000; Poveda et al. 2004, submittedmanuscript to  J. Hydrol. Eng. ). Central and westernColombia experience a bimodal annual cycle of precipi-tation with distinct rainy seasons (April–May and Oc-tober–November), and “dry” (less rainy) seasons (De-cember–February and June–August), which result fromthe double passage of the ITCZ over the region. Aunimodal annual cycle (May–October) is witnessedover the northern Caribbean coast of Colombia and thePacific flank of the southern isthmus, reflecting thenorthernmost position of the ITCZ over the continentand eastern equatorial Pacific, respectively (Hastenrath1990, 2002). A single peak is also evident over the east-ern piedmont of the eastern Andes. Moisture trans-ported from the Amazon basin encounters the oro-graphic barrier of the Andes, thus focusing and enhanc-ing deep convection and rainfall in the eastern flank of the Cordillera, with maximum rainfall occurring during Corresponding author address:  Germán Poveda, Escuela deGeociencias y Medio Ambiente, Facultad de Minas, UniversidadNacional de Colombia, Cra 80 x Calle 65, M2-315 Medellín, Co-lombia.E-mail: gpoveda@unalmed.edu.co 228  MONTHLY WEATHER REVIEW V OLUME  133© 2005 American Meteorological Society  June – August. The meridional migration of the ITCZ ishighly intertwined with atmospheric circulation fea-tures over the Caribbean Sea, the easternmost PacificOcean, and the Amazon River basin. The presence of the three branches of the Andes with elevations sur-passing 5000 m, containing tropical glaciers, and includ-ing diverse intra-Andean valleys in a predominantsouth – north direction introduce strong local orographiceffects that induce atmospheric circulations and deepconvection with heavy rainfall. Over the Pacific coast of Colombia, extreme precipitation rates are witnessed,including one of the rainiest regions on earth (Povedaand Mesa 2000). Other large-scale phenomena associ-ated with the seasonal hydroclimatological cycle overthe region include the low-level  “ Chorro del OccidenteColombiano ”  (CHOCO) jet that flows onshore fromthe Pacific Ocean (Poveda and Mesa 2000) and theassociated development of mesoscale convective com-plexes, which in turn exhibit characteristic diurnalcycles (Velasco and Frisch 1987; Poveda and Mesa2000; Mapes et al. 2003a). The interannual variability of the diurnal cycle is dominated by the effects of bothphases of El Ni ñ o – Southern Oscillation (ENSO). Sucheffects are due to influences of sea surface tempera-tures off the Pacific coast, but also to atmospheric tele-connections, which are enhanced by land surface – atmosphere feedbacks (Poveda and Mesa 1997, 2000;Poveda et al. 2001). At intraseasonal time scales, tropi-cal easterly waves are known to affect precipitation re-gimes over different regions in Colombia in their west-ward propagation during the Northern Hemispheresummer – autumn (Mart í  nez 1993).Previous studies regarding the diurnal cycle of rain-fall for Colombia and tropical South America arescarce and limited to small datasets. Trojer (1959) iden-tified a distinct effect of altitude on the diurnal cycle of precipitation maxima over the Cauca River valley, withan early-morning peak for stations located near the val-ley floor, while those at higher elevations showed af-ternoon maxima. Snow (1976, p. 365) found differentdiurnal-maxima characteristics at three localities lo-cated on each of the branches of the Colombian Andes:Quibd ó  (on the lowlands of the Pacific coast), showinga late-night and early-morning maximum; Chinchin á (central Andes), displaying an early-morning maxi-mum; and Bogot á  (high plateau on the eastern Andes),exhibiting an afternoon maximum. Other studies fornorthern South America rely on precipitation estimatesderived from satellite data (Kousky 1980; Meisner andArkin 1987; Velasco and Frisch 1987; Hendon andWoodberry 1993; Janowiak et al. 1994; Negri et al.1994, 2000; Garreaud and Wallace 1997; Imaoka andSpencer 2000; Lin et al. 2000; Soden 2000; Yang andSlingo 2001; Ricciardulli and Sardeshmukh 2002; So-rooshian et al. 2002; Mapes et al. 2003a). For instance,Negri et al. (2000) identified a strong diurnal cycle inAmazonian rainfall and pointed out the important roleof land – atmosphere feedbacks in controlling the inten-sity of the diurnal cycle. They also identified a rainfallmaximum offshore in the Gulf of Panama that occursduring the morning hours around 0900 LST and a to-pographically induced afternoon maximum in themountains of Venezuela (near 5 ° N, 63 ° W) that occursduring May – September. Over the eastern slope of theAndes, a nighttime maximum of precipitation has beenidentified (Garreaud and Wallace 1997; Negri et al.1994). The diurnal march of convection in a small re-gion of the Amazon was studied during the 1999 Tropi-cal Rainfall Measuring Mission (TRMM) Wet-SeasonAtmospheric Mesoscale Campaign of the Large-ScaleBiosphere – Atmosphere Experiment (WETAMC/LBA) (Machado et al. 2002). The three-part study of Mapes et al. (2003a,b) and Warner et al. (2003) reportsobservational and modeling results for the diurnal cycleof precipitation over western Colombia and the neigh-boring Pacific, using a short-period dataset (July – September 2000) that is based on the rain-rate satelliteestimates of Negri et al. (2002a). Most of the aforemen-tioned studies on the diurnal cycle of precipitation overthe region are based upon short-term datasets derivedfrom satellite information that, though spatially distrib-uted, may induce estimation errors. For instance, Negriet al. (2002b), showed that 3 yr of rainfall data derivedfrom the TRMM satellite are inadequate to describethe diurnal cycle of precipitation over areas smallerthan 12 °  12 °  because of high spatial variability in sam-pling.We aim to study and characterize the long-term sea-sonal diurnal cycle of precipitation (section 3) and itsrelationship with altitude on the Andes (section 4). Wealso aim to investigate the interannual variability of thediurnal cycle associated with both phases of ENSO(section 5). In section 6, the intra-annual variability of the diurnal cycle is studied for the easterly and westerlyphases of the Madden – Julian oscillation (30 – 60 days).In addition, high-frequency variability (less than 10days) of two contrasting ENSO years is studied, usinghourly data from rain gauges located throughout thetropical Andes of Colombia. Conclusions are summa-rized in section 7. 2. Data and methods The dataset consists of hourly precipitation rates at51 gauges, with records spanning between 22 and 28 yr.Percentages of missing data are generally less than 5%,which was ignored for estimation purposes. The datasetwas provided in paper logs by the Centro Nacional deInvestigaciones del Caf  é  (CENICAFE) and EmpresasP ú blicas de Medell í  n (EPM), from which the digitalfiles were created. The gauges lie between 1 ° 15  N and10 ° 20  N, and 77 ° 29  W and 72 ° 40  W, and are situatedbetween 260 and 2595 m in elevation. Details of thegeographical setting and exact location of the gaugesare shown in Table 1 and Fig. 1.The Andes are divided into three major branches in J ANUARY  2005 POVEDA ET AL.  229  Colombia, namely Cordillera Occidental (westernbranch), Cordillera Central, and Cordillera Oriental(eastern branch), which are separated by the two intra-Andean valleys of the Magdalena and Cauca Rivers.The  “ day, ”  or 24-h period, over which the hourly rain-fall values are examined is arbitrarily defined as run-ning from 0700 to 0700 LST to better capture the largequantities of precipitation observed during late-night – early-morning hours. Gauging stations are numbered inFig. 1 according to their location along different An-dean slopes: western slope of the Cordillera Occidental(1 to 3); southern  “ Macizo Colombiano ”  (4 to 6); east-ern slope of the Cordillera Occidental belonging to theCauca River valley (7 to 12); the western slope of theCordillera Central (13 to 32); the eastern slope of theCordillera Central, Magdalena River valley (33 to 40);the western flank of the Cordillera Oriental (41 to 48);the eastern flank of the Cordillera Oriental (49 and 50);and a single station at the isolated northern Sierra Ne-vada de Santa Marta (51). T ABLE  1. Location and details of the 51 rain gauges.No. Rain gauge Lat (N) Lon (W) Elev (m) Period of recordMissingdata (%)Elev fromvalley floor (m)1 Sireno 06 ° 23   75 ° 40   1210 Mar 1978 – Apr 1999 0.02 Santa B á rbara 06 ° 24   75 ° 43   2595 Mar 1978 – Dec 1999 0.03 Mand é  06 ° 27   75 ° 08   495 May 1978 – Apr 1999 0.04 Manuel Mej í  a 02 ° 25   76 ° 45   1700 Jan 1972 – Dec 1999 2.15 El Sauce 01 ° 37   77 ° 06   1610 Jan 1971 – Dec 1999 12.46 Ospina P é rez 01 ° 15   77 ° 29   1700 Jan 1972 – Dec 1999 0.77 Julio Fern á ndez 03 ° 48   76 ° 32   1360 Jan 1972 – Dec 1999 1.0 4808 Manuel M. Mallarino 04 ° 13   76 ° 19   1380 Jan 1973 – Dec 1999 2.5 5709 Santiago Gutierrez 04 ° 43   76 ° 10   1550 Aug 1971 – Dec 1999 5.7 82010 Alban 04 ° 46   76 °  11 1500 Jun 1973 – Dec 1999 1.7 78011 Rafael Escobar 05 ° 28   75 ° 39   1320 Oct 1970 – Dec 1999 0.112 Miguel Valencia 05 ° 36   75 ° 12   1570 Jan 1971 – Dec 1999 1.913 Rosario 05 ° 28   75 ° 40   1600 Jan 1971 – Dec 1999 0.9 108614 Luker 05 ° 12   76 ° 28   1420 Mar 1970 – Dec 1999 2.7 36015 Agronom í  a 05 ° 03   75 ° 29   2150 Jan 1972 – Dec 1999 1.1 148016 Santagueda 05 ° 05   76 ° 15   1010 Apr 1972 – Dec 1999 1.2 35017 Santa Ana 05 ° 01   75 ° 40   1250 Jan 1972 – Dec 1999 2.2 58018 Cenicaf  é  05 ° 00   75 ° 36   1310 Jan 1972 – Dec 1999 0.4 63019 Naranjal 04 ° 59   75 ° 39   1400 Jan 1971 – Dec 1999 0.6 72020 Santa Helena 04 ° 57   75 ° 37   1525 Nov 1980 – Dec 1999 2.1 84021 El Jazmı´n 04°55   75°38   1600 Jan 1972–Dec 1999 2.1 91022 Planta Tratamiento 04°48   75°40   1450 Mar 1969–Dec 1999 4.0 74023 La Catalina 04°45   75°45   1350 Nov 1976–Dec 1999 1.0 63024 El Cedral 04°42   75°32   2120 Jan 1973–Dec 1999 5.8 142025 Arturo Gómez 04°40   75°47   1320 Jan 1972–Dec 1999 1.5 59026 Bremen 04°40   75°37   2040 Mar 1972–Dec 1999 1.7 130027 Maracay 04°36   75°46   1450 Feb 1982–Dec 1999 0.6 71028 El Sena 04°34   75°39   1550 Dec 1971–Dec 1999 8.8 80029 La Bella 04°30   75°40   1450 Jan 1972–Dec 1999 0.4 69030 Paraguaicito 04°23   75°44   1250 Jan 1972–Dec 1999 0.5 47031 La Sirena 04°17   75°55   1500 Jan 1971–Dec 1999 1.0 70032 La Selva 03°40   76°18   1800 Jan 1979–Dec 1999 3.3 90033 Jorge Villamil 02°22   75°33   1500 Jan 1972–Dec 1999 1.134 El Limón 03°40   75°35   990 Jan 1971–Dec 1999 7.2 61035 Chapetón 04°27   75°16   1300 Jan 1971–Apr 1988 2.2 101036 La Trinidad 04°54   75°03   1430 Jun 1972–Dec 1999 0.6 119037 Llanadas 05°05   75°41   1020 Jan 1972–Dec 1999 10.0 122038 Inmarco 06°17   72°48   260 Aug 1968–Aug 1992 0.0 14039 Bizcocho 06°18   75°05   1070 Apr 1971–Dec 1999 0.0 112040 Peñol 06°24   75°51   1880 Apr 1960–Feb 1999 0.0 175041 Aguas Blancas 06°50   73°29   920 Jan 1972–Dec 1999 8.442 Bertha 05°53   73°34   1700 Jan 1972–Dec 1999 5.1 155043 Yacopí 05°28   74°22   1340 Jan 1972–Dec 1999 1.544 Santa Inés 04°43   74°28   1250 Jan 1972–Dec 1993 1.9 99045 Misiones 04°33   74°25   1540 Jan 1977–Dec 1999 3.0 126346 Granja Tibacuy 04°22   74°26   1550 Jan 1972–Dec 1999 1.4 125047 Luis Bustamante 03°54   74°34   1643 Jan 1972–Dec 1999 18.948 La Montaña 03°33   74°54   1260 Jan 1973–Dec 1999 15.949 Blonay 07°34   72°37   1235 Jan 1972–Dec 1999 0.750 Franciso Romero 07°46   72°40   1000 Jan 1972–Dec 1999 1.151 Pueblo Bello 10°25   73°34   1000 Jan 1972–Dec 1999 3.4 230  MONTHLY WEATHER REVIEW V OLUME  133  To examine interannual variability, we quantify thediurnal cycle of precipitation during both phases of ENSO: El Ni ñ o and La Ni ñ a. ENSO years were ob-tained from the multivariate ENSO index (MEI), de-veloped by the National Oceanic and Atmospheric Ad-ministration (NOAA), as follows: 1957/58, 1965/66,1972/73, 1982/83, 1986/87, 1991/92, 1994/95, 1997/98,and 2002/03 for El Ni ñ o, and 1964/65, 1970/71, 1973/74,1975/76, 1988/89, and 1998/2000 for La Ni ñ a (see http://www.cdc.noaa.gov/  kew/MEI/).Intra-annual variability of the diurnal cycle is inves-tigated by discriminating according to the phase of theMadden – Julian oscillation (MJO; Madden and Julian1994; Maloney and Hartmann 2000). Characterizationof easterly and westerly phases of the MJO was madeusing the index suggested by Maloney and Hartmann(2000), defined as the first principal component of zonal wind pentads over the eastern Pacific at 850 hPa,during the period 1949 – 1997, estimated with data fromthe National Centers for Environmental Prediction – National Center for Atmospheric Research (NCEP – NCAR) 40-Year Climate Reanalysis Project (Kalnay etal. 1996). The westerly (easterly) phase of the MJOcorresponds to those periods in which the index is F IG . 1. Geographical setting and detailed location of the set of rain gauges. Numbers correspond to rain gaugesdescribed in Table 1.J ANUARY  2005 POVEDA ET AL.  231
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks
SAVE OUR EARTH

We need your sign to support Project to invent "SMART AND CONTROLLABLE REFLECTIVE BALLOONS" to cover the Sun and Save Our Earth.

More details...

Sign Now!

We are very appreciated for your Prompt Action!

x