The Diurnal Cycle of Precipitation in the Tropical Andes of Colombia

Home , Gulf of Panama, Magdalena River Valley, Tropical Andes

228 MONTHLY WEATHER REVIEW VOLUME 133

GERMÁN POVEDA,OSCAR J. MESA,LUIS F. SALAZAR,PAOLA A. ARIAS,HERNÁN A. MORENO, SARA C. VIEIRA,PAULA A. AGUDELO,VLADIMIR G. TORO, AND J. FELIPE ALVAREZ Escuela de Geociencias y Medio Ambiente, Universidad Nacional de Colombia, Medellín, Colombia

(Manuscript received 9 October 2003, in final form 7 July 2004)

ABSTRACT

Using hourly records from 51 rain gauges, spanning between 22 and 28 yr, the authors study the diurnal cycle of precipitation over the tropical Andes of Colombia. Analyses are developed for the seasonal march of 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 and easterly phases of the Madden–Julian oscillation, as well as higher-frequency variability (Ͻ10 days), mainly associated with tropical easterly wave activity during ENSO contrasting years. Five major general patterns are identified: (i) precipitation exhibits clear-cut diurnal (24 h) and semidiurnal (12 h) cycles; (ii) the minimum of daily precipitation is found during the morning hours (0900–1100 LST) regardless of season or location; (iii) a predominant afternoon peak is found over northeastern and western Colombia; (iv) over the western flank of the central Andes, precipitation maxima occur either near midnight, or during the afternoon, or both; and (v) a maximum of precipitation prevails near midnight amongst stations located on the eastern flank of the central Cordillera. The timing of diurnal maxima is highly variable in space for a fixed time, although a few coherent regions are found in small groups of rain gauges within the Cauca and Magdalena River valleys. Overall, the identified strong seasonal variability in the timing of rainfall maxima appears to exhibit no relationship with elevation on the Andes. The effects of both phases of ENSO are highly consistent spatially, as the amplitude of hourly and daily precipitation diminishes (increases) during El Niño (La Niña), but the phase remains unaltered for the entire dataset. We also found a generalized increase (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–April during El Niño (La Niña) years, associated, among others, with lower (higher) tropical easterly wave (4–6 day) activity over the Caribbean.

1. Introduction On seasonal time scales, it is well known that the displacement of the intertropical convergence zone Colombia is located in the northwestern corner of (ITCZ) exerts a strong control on the annual cycle of South America, exhibiting complex geographical, envi- Colombia’s hydroclimatology (Snow 1976; Mejía et al. ronmental, and hydroecological features in which the 1999; León et al. 2000; Poveda et al. 2004, submitted Andes plays a major role. In terms of precipitation manuscript to J. Hydrol. Eng.). Central and western amount and distribution, Snow (1976, p. 362) describes Colombia experience a bimodal annual cycle of precipi- the Andes as “a dry island in a sea of rain,” but the tation with distinct rainy seasons (April–May and Oc- atmospheric dynamics and precipitation variability as- tober–November), and “dry” (less rainy) seasons (De- sociated with the Andes are not well understood at a cember–February and June–August), which result from wide range of time–space scales. Improving our under- the double passage of the ITCZ over the region. A standing of the atmospheric dynamics and precipitation unimodal annual cycle (May–October) is witnessed variability is crucial under current environmental over the northern Caribbean coast of Colombia and the threats faced by the tropical Andes, identified as the Pacific flank of the southern isthmus, reflecting the most critical “hot spot” for biodiversity on earth (Myers northernmost position of the ITCZ over the continent et al. 2000). and eastern equatorial Pacific, respectively (Hastenrath 1990, 2002). A single peak is also evident over the eastern piedmont of the eastern Andes. Moisture trans- Corresponding author address: Germán Poveda, Escuela de ported from the Amazon basin encounters the oro- Geociencias y Medio Ambiente, Facultad de Minas, Universidad Nacional de Colombia, Cra 80 x Calle 65, M2-315 Medellín, Co- graphic barrier of the Andes, thus focusing and enhanc- lombia. ing deep convection and rainfall in the eastern flank of E-mail: [email protected] the Cordillera, with maximum rainfall occurring during

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June–August. The meridional migration of the ITCZ is sity of the diurnal cycle. They also identified a rainfall highly intertwined with atmospheric circulation fea- maximum offshore in the Gulf of Panama that occurs tures over the Caribbean Sea, the easternmost Pacific during the morning hours around 0900 LST and a to- Ocean, and the Amazon River basin. The presence of pographically induced afternoon maximum in the the three branches of the Andes with elevations sur- mountains of Venezuela (near 5°N, 63°W) that occurs passing 5000 m, containing tropical glaciers, and includ- during May–September. Over the eastern slope of the ing diverse intra-Andean valleys in a predominant Andes, a nighttime maximum of precipitation has been south–north direction introduce strong local orographic identified (Garreaud and Wallace 1997; Negri et al. effects that induce atmospheric circulations and deep 1994). The diurnal march of convection in a small re- convection with heavy rainfall. Over the Pacific coast of gion of the Amazon was studied during the 1999 Tropi- Colombia, extreme precipitation rates are witnessed, cal Rainfall Measuring Mission (TRMM) Wet-Season including one of the rainiest regions on earth (Poveda Atmospheric Mesoscale Campaign of the Large-Scale and Mesa 2000). Other large-scale phenomena associ- Biosphere–Atmosphere Experiment (WETAMC/ ated with the seasonal hydroclimatological cycle over LBA) (Machado et al. 2002). The three-part study of the region include the low-level “Chorro del Occidente Mapes et al. (2003a,b) and Warner et al. (2003) reports Colombiano” (CHOCO) jet that flows onshore from observational and modeling results for the diurnal cycle the Pacific Ocean (Poveda and Mesa 2000) and the of precipitation over western Colombia and the neigh- associated development of mesoscale convective com- boring Pacific, using a short-period dataset (July– plexes, which in turn exhibit characteristic diurnal September 2000) that is based on the rain-rate satellite cycles (Velasco and Frisch 1987; Poveda and Mesa estimates of Negri et al. (2002a). Most of the aforemen- 2000; Mapes et al. 2003a). The interannual variability of tioned studies on the diurnal cycle of precipitation over the diurnal cycle is dominated by the effects of both the region are based upon short-term datasets derived phases of El Niño–Southern Oscillation (ENSO). Such from satellite information that, though spatially distrib- effects are due to influences of sea surface tempera- uted, may induce estimation errors. For instance, Negri tures off the Pacific coast, but also to atmospheric tele- et al. (2002b), showed that 3 yr of rainfall data derived connections, which are enhanced by land surface– from the TRMM satellite are inadequate to describe atmosphere feedbacks (Poveda and Mesa 1997, 2000; the diurnal cycle of precipitation over areas smaller Poveda et al. 2001). At intraseasonal time scales, tropi- than 12°ϫ12° because of high spatial variability in sam- cal easterly waves are known to affect precipitation re- pling. gimes over different regions in Colombia in their west- We aim to study and characterize the long-term sea- ward propagation during the Northern Hemisphere sonal diurnal cycle of precipitation (section 3) and its summer–autumn (Martínez 1993). relationship with altitude on the Andes (section 4). We Previous studies regarding the diurnal cycle of rain- also aim to investigate the interannual variability of the fall for Colombia and tropical South America are diurnal cycle associated with both phases of ENSO scarce and limited to small datasets. Trojer (1959) iden- (section 5). In section 6, the intra-annual variability of tified a distinct effect of altitude on the diurnal cycle of the diurnal cycle is studied for the easterly and westerly precipitation maxima over the Cauca River valley, with phases of the Madden–Julian oscillation (30–60 days). an early-morning peak for stations located near the val- In addition, high-frequency variability (less than 10 ley floor, while those at higher elevations showed af- days) of two contrasting ENSO years is studied, using ternoon maxima. Snow (1976, p. 365) found different hourly data from rain gauges located throughout the diurnal-maxima characteristics at three localities lo- tropical Andes of Colombia. Conclusions are summa- cated on each of the branches of the Colombian Andes: rized in section 7. Quibdó (on the lowlands of the Pacific coast), showing a late-night and early-morning maximum; Chinchiná 2. Data and methods (central Andes), displaying an early-morning maximum; and Bogotá (high plateau on the eastern Andes), The dataset consists of hourly precipitation rates at exhibiting an afternoon maximum. Other studies for 51 gauges, with records spanning between 22 and 28 yr. northern South America rely on precipitation estimates Percentages of missing data are generally less than 5%, derived from satellite data (Kousky 1980; Meisner and which was ignored for estimation purposes. The dataset Arkin 1987; Velasco and Frisch 1987; Hendon and was provided in paper logs by the Centro Nacional de Woodberry 1993; Janowiak et al. 1994; Negri et al. Investigaciones del Café (CENICAFE) and Empresas 1994, 2000; Garreaud and Wallace 1997; Imaoka and Públicas de Medellín (EPM), from which the digital Spencer 2000; Lin et al. 2000; Soden 2000; Yang and files were created. The gauges lie between 1°15ЈN and Slingo 2001; Ricciardulli and Sardeshmukh 2002; So- 10°20ЈN, and 77°29ЈW and 72°40ЈW, and are situated rooshian et al. 2002; Mapes et al. 2003a). For instance, between 260 and 2595 m in elevation. Details of the Negri et al. (2000) identified a strong diurnal cycle in geographical setting and exact location of the gauges Amazonian rainfall and pointed out the important role are shown in Table 1 and Fig. 1. of land–atmosphere feedbacks in controlling the inten- The Andes are divided into three major branches in

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TABLE 1. Location and details of the 51 rain gauges. Missing Elev from No. Rain gauge Lat (N) Lon (W) Elev (m) Period of record data (%) valley floor (m) 1 Sireno 06°23Ј 75°40Ј 1210 Mar 1978–Apr 1999 0.0 2 Santa Bárbara 06°24Ј 75°43Ј 2595 Mar 1978–Dec 1999 0.0 3 Mandé 06°27Ј 75°08Ј 495 May 1978–Apr 1999 0.0 4 Manuel Mejía02°25Ј 76°45Ј 1700 Jan 1972–Dec 1999 2.1 5 El Sauce 01°37Ј 77°06Ј 1610 Jan 1971–Dec 1999 12.4 6 Ospina Pérez 01°15Ј 77°29Ј 1700 Jan 1972–Dec 1999 0.7 7 Julio Fernández 03°48Ј 76°32Ј 1360 Jan 1972–Dec 1999 1.0 480 8 Manuel M. Mallarino 04°13Ј 76°19Ј 1380 Jan 1973–Dec 1999 2.5 570 9 Santiago Gutierrez 04°43Ј 76°10Ј 1550 Aug 1971–Dec 1999 5.7 820 10 Alban 04°46Ј 76°Ј11 1500 Jun 1973–Dec 1999 1.7 780 11 Rafael Escobar 05°28Ј 75°39Ј 1320 Oct 1970–Dec 1999 0.1 12 Miguel Valencia 05°36Ј 75°12Ј 1570 Jan 1971–Dec 1999 1.9 13 Rosario 05°28Ј 75°40Ј 1600 Jan 1971–Dec 1999 0.9 1086 14 Luker 05°12Ј 76°28Ј 1420 Mar 1970–Dec 1999 2.7 360 15 Agronomía05°03Ј 75°29Ј 2150 Jan 1972–Dec 1999 1.1 1480 16 Santagueda 05°05Ј 76°15Ј 1010 Apr 1972–Dec 1999 1.2 350 17 Santa Ana 05°01Ј 75°40Ј 1250 Jan 1972–Dec 1999 2.2 580 18 Cenicafé 05°00Ј 75°36Ј 1310 Jan 1972–Dec 1999 0.4 630 19 Naranjal 04°59Ј 75°39Ј 1400 Jan 1971–Dec 1999 0.6 720 20 Santa Helena 04°57Ј 75°37Ј 1525 Nov 1980–Dec 1999 2.1 840 21 El Jazmı´n 04°55Ј 75°38Ј 1600 Jan 1972–Dec 1999 2.1 910 22 Planta Tratamiento 04°48Ј 75°40Ј 1450 Mar 1969–Dec 1999 4.0 740 23 La Catalina 04°45Ј 75°45Ј 1350 Nov 1976–Dec 1999 1.0 630 24 El Cedral 04°42Ј 75°32Ј 2120 Jan 1973–Dec 1999 5.8 1420 25 Arturo Gómez 04°40Ј 75°47Ј 1320 Jan 1972–Dec 1999 1.5 590 26 Bremen 04°40Ј 75°37Ј 2040 Mar 1972–Dec 1999 1.7 1300 27 Maracay 04°36Ј 75°46Ј 1450 Feb 1982–Dec 1999 0.6 710 28 El Sena 04°34Ј 75°39Ј 1550 Dec 1971–Dec 1999 8.8 800 29 La Bella 04°30Ј 75°40Ј 1450 Jan 1972–Dec 1999 0.4 690 30 Paraguaicito 04°23Ј 75°44Ј 1250 Jan 1972–Dec 1999 0.5 470 31 La Sirena 04°17Ј 75°55Ј 1500 Jan 1971–Dec 1999 1.0 700 32 La Selva 03°40Ј 76°18Ј 1800 Jan 1979–Dec 1999 3.3 900 33 Jorge Villamil 02°22Ј 75°33Ј 1500 Jan 1972–Dec 1999 1.1 34 El Limón 03°40Ј 75°35Ј 990 Jan 1971–Dec 1999 7.2 610 35 Chapetón 04°27Ј 75°16Ј 1300 Jan 1971–Apr 1988 2.2 1010 36 La Trinidad 04°54Ј 75°03Ј 1430 Jun 1972–Dec 1999 0.6 1190 37 Llanadas 05°05Ј 75°41Ј 1020 Jan 1972–Dec 1999 10.0 1220 38 Inmarco 06°17Ј 72°48Ј 260 Aug 1968–Aug 1992 0.0 140 39 Bizcocho 06°18Ј 75°05Ј 1070 Apr 1971–Dec 1999 0.0 1120 40 Peñol 06°24Ј 75°51Ј 1880 Apr 1960–Feb 1999 0.0 1750 41 Aguas Blancas 06°50Ј 73°29Ј 920 Jan 1972–Dec 1999 8.4 42 Bertha 05°53Ј 73°34Ј 1700 Jan 1972–Dec 1999 5.1 1550 43 Yacopí 05°28Ј 74°22Ј 1340 Jan 1972–Dec 1999 1.5 44 Santa Inés 04°43Ј 74°28Ј 1250 Jan 1972–Dec 1993 1.9 990 45 Misiones 04°33Ј 74°25Ј 1540 Jan 1977–Dec 1999 3.0 1263 46 Granja Tibacuy 04°22Ј 74°26Ј 1550 Jan 1972–Dec 1999 1.4 1250 47 Luis Bustamante 03°54Ј 74°34Ј 1643 Jan 1972–Dec 1999 18.9 48 La Montaña 03°33Ј 74°54Ј 1260 Jan 1973–Dec 1999 15.9 49 Blonay 07°34Ј 72°37Ј 1235 Jan 1972–Dec 1999 0.7 50 Franciso Romero 07°46Ј 72°40Ј 1000 Jan 1972–Dec 1999 1.1 51 Pueblo Bello 10°25Ј 73°34Ј 1000 Jan 1972–Dec 1999 3.4

Colombia, namely Cordillera Occidental (western dean slopes: western slope of the Cordillera Occidental branch), Cordillera Central, and Cordillera Oriental (1 to 3); southern “Macizo Colombiano” (4 to 6); east- (eastern branch), which are separated by the two intra- ern slope of the Cordillera Occidental belonging to the Andean valleys of the Magdalena and Cauca Rivers. Cauca River valley (7 to 12); the western slope of the The “day,” or 24-h period, over which the hourly rain- Cordillera Central (13 to 32); the eastern slope of the fall values are examined is arbitrarily defined as run- Cordillera Central, Magdalena River valley (33 to 40); ning from 0700 to 0700 LST to better capture the large the western flank of the Cordillera Oriental (41 to 48); quantities of precipitation observed during late-night– the eastern flank of the Cordillera Oriental (49 and 50); early-morning hours. Gauging stations are numbered in and a single station at the isolated northern Sierra Ne- Fig. 1 according to their location along different An- vada de Santa Marta (51).

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FIG. 1. Geographical setting and detailed location of the set of rain gauges. Numbers correspond to rain gauges described in Table 1.

To examine interannual variability, we quantify the Madden–Julian oscillation (MJO; Madden and Julian diurnal cycle of precipitation during both phases of 1994; Maloney and Hartmann 2000). Characterization ENSO: El Niño and La Niña. ENSO years were ob- of easterly and westerly phases of the MJO was made tained from the multivariate ENSO index (MEI), de- using the index suggested by Maloney and Hartmann veloped by the National Oceanic and Atmospheric Ad- (2000), defined as the first principal component of ministration (NOAA), as follows: 1957/58, 1965/66, zonal wind pentads over the eastern Pacific at 850 hPa, 1972/73, 1982/83, 1986/87, 1991/92, 1994/95, 1997/98, during the period 1949–1997, estimated with data from and 2002/03 for El Niño, and 1964/65, 1970/71, 1973/74, the National Centers for Environmental Prediction– 1975/76, 1988/89, and 1998/2000 for La Niña (see http:// National Center for Atmospheric Research (NCEP– www.cdc.noaa.gov/ϳkew/MEI/). NCAR) 40-Year Climate Reanalysis Project (Kalnay et Intra-annual variability of the diurnal cycle is inves- al. 1996). The westerly (easterly) phase of the MJO tigated by discriminating according to the phase of the corresponds to those periods in which the index is

Unauthenticated | Downloaded 09/25/21 08:11 AM UTC 232 MONTHLY WEATHER REVIEW VOLUME 133 greater (lesser) than plus (minus) one standard devia- reach condensation levels at sunset hours. Therefore, tion of the series. The average diurnal cycle of precipi- this daytime convection tends to be in the form of small tation was estimated for the easterly, westerly, and neu- cloud systems (Mapes et al. 2003a). Second, a common tral phases of the MJO. maximum of precipitation prevails during the middle of We also characterize the high-frequency variability the night amongst stations located on the eastern flank (less than 256 h or 10 days) of hourly precipitation dur- of the Central Cordillera, corresponding to the west- ing La Niña and El Niño years. The activity of tropical ern-edge margin of the wide Magdalena River valley easterly waves (4–6 days) is dominant within such fre- [Bizcocho (39, at 12 o’clock in Fig. 2), La Trinidad (36, quency band during the June–October period. Toward at one o’clock in Fig. 2), Inmarco (38), El Limón (34), that end, we perform wavelet analyses of the series of Chapetón (35), Llanadas (37), and Peñol (40)]. Such a hourly rainfall rates, using the Morlet mother wavelet late-night–early-morning maximum is mainly associ- (Torrence and Compo 1998). ated with the life cycle of mesoscale convective systems (Velasco and Frisch 1987), with an upscale development of storm size through the night, supported by di- 3. Seasonal variability urnally driven breezes and gravity waves (López and Howell 1967; Mapes et al. 2003a). Fast Fourier transform analyses of the entire dataset The identified diurnal patterns of precipitation seem (not shown) revealed significant spectral peaks at 24 to lack the type of temporal or spatial structure, or and/or 12 h, corresponding to distinct diurnal and se- large-scale order, that one might expect in a region with midiurnal cycles. No evidence was found of regional such strong seasonality resulting from the passage of coherence in the predominance of the identified peri- the ITCZ and such distinct, well-delineated, topo- odicities. At monthly time scales, most rain gauges ex- graphic features. For instance, even within the grouping hibit a seasonally varying diurnal cycle of precipitation, of stations near the Venezuelan border and to the west, which is itself extremely variable over space. Figure 2 which have been identified as sharing a similar unimo- shows results of the seasonal march of the diurnal cycle dal behavior, local differences may be detected. The from a subsample of 17 stations selected to reflect the time of maximum precipitation at Aguas Blancas (41, spatial disposition and results of the larger sample. Iso- located at two o’clock in Fig. 2) shifts during the year lines represent the percentage of daily precipitation to- from afternoon (July–August) to evening hours (No- tal, on days with measurable precipitation, falling vember–March); however, the eastward-facing Francis- throughout the course of the day and are estimated co Romero (50, above the previous one in Fig. 2) shows through interpolation of the original 24 h ϫ 12 months a maximum during early-evening hours (1800 to 2000 matrix. LST) throughout the year. Likewise, in the west, Several features are worth noticing: (i) the presence Mandé (3, at ten o’clock in Fig. 2) has a maximum of of differing timings of diurnal maxima at any given sta- precipitation from 1700 to 2000 LST throughout the tion, including shifts from unimodal to multimodal be- year, while the timing of the maximum shifts markedly havior, within the year; (ii) the presence of unimodal, at Sireno (1, above the previous one in Fig. 2) to 1900– bimodal, or multimodal diurnal behavior at different 0100 LST during May–October. The following detailed stations; (iii) the coexistence of different timing of the analysis of the spectral characteristics of precipitation diurnal maxima at nearby stations during the same sea- at stations in geographic proximity to one another, in son, and in summary; and (iv) an astounding variability the Cauca and Magdalena valleys, illustrates that the of the number of daily maxima of the diurnal cycle, spatial coherence in the diurnal cycle of rainfall is lim- even at very small spatial scales. ited to very small-scale regions within the intra-Andean However, a generalized pattern can be drawn from valleys. these figures: the minimum percentage of daily precipitation is always found during the morning hours of a. Cauca River valley 0900–1100 LST regardless of season or location. Also, two broad regional statements can be made. First, the • Eastern flank of the Cordillera Occidental (western predominant afternoon maxima found in many stations branch). The diurnal cycle at stations located 480 and located over northeastern and western Colombia, such 700 m above the valley floor exhibit bimodal behav- as at Mandé (3, located at the ten o’clock position in ior, with a stronger afternoon maximum (1300 to Fig. 2), Luis Bustamante (47, at three o’clock in Fig. 2), 1600) and a second midnight–predawn maximum, as Maracay (27, at seven o’clock in Fig. 2), Pueblo Bello exemplified by Julio Fernández (7) and Mallarino (51), Manuel Mejía (4), El Sauce (5), and Paraguaicito (8). Above 700 m, precipitation is almost exclusively (30), among others, may be explained as the result of concentrated in the afternoon hours, as at Santiago convective precipitation associated with solar thermal Gutierrez (9, at four o’clock in Fig. 2) and Alban (10), forcing, enhanced by the entrance of low-level, mois- or near midnight at the northerly Rafael Escobar (11) ture-laden winds onshore from the Caribbean and Pa- and Miguel Valencia (12). cific. These winds ascend due to orographic lifting and • At stations across the valley, on the western flank of

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FIG. 2. Seasonal march of the diurnal cycle of rainfall at 17 selected stations in the tropical Andes of Colombia. The diurnal cycle is defined from 0700 to 0700 LST, and interpolated isolines indicate percent of total daily rainfall, with the color scale shown at the bottom. Boundaries of the neighboring (left) Cauca and (right) Magdalena River valleys are shown in white. The inset at the bottom left shows details of rain gauges located in the western flank of the Central Cordillera.

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the central Andes, the diurnal cycle exhibits maxima afternoon throughout the year to bimodal (a sec- either at or near midnight, or during the afternoon, or ondary peak appears in the afternoon and near both. Examples of the first kind are Luker (14), San- midnight) from November to March, as at Santa tagueda (16, at four o’clock in Fig. 2), Santa Ana (17), Inés (44); Cenicafé (18), Naranjal (19, at nine o’clock in Fig. 2), (iii) shifting unimodal, from a nighttime maximum and Santa Helena (20). Examples of stations with (1900 to 2300 LST) during November–March to afternoon maximum of precipitation are Agronomia an afternoon maximum during May–September, (15), El Cedral (24), Bremen (26, at five o’clock in as at Misiones (45). Fig. 2), Maracay (27), El Sena (28), and combined afternoon and near-midnight peaks appear at El Jazmin (21), Planta de Tratamiento (22), La Catalina 4. The role of altitude and the position of (23), Arturo Gomez (25, at six o’clock in Fig. 2), and the ITCZ La Bella (29). The relationship with elevation is highly variable. Stations located between 350 and In an attempt to explain the observed behavior of the 400 m above the valley floor exhibit a single midnight diurnal maxima of rainfall, potential relationships with peak, as at Santagueda (16, at four o’clock in Fig. 2) altitude and with the seasonal meridional migration of and Luker (14). Between 400 and 900 m, patterns the ITCZ are examined. Figure 3 shows no clear rela- become more bimodal, with peaks during evening tionship between the timing of diurnal maxima and a hours (1700 to 1900), and a secondary midnight peak, station’s altitude. An apparent relationship with the al- as illustrated at Paraguaicito (30), Arturo Gómez (25, titude is evidenced only in small groups of rain gauges, at six o’clock in Fig. 2), La Catalina (23), La Bella as in the previous discussion of the Magdalena and (29), La Sirena (31), Maracay (27), Planta Trata- Cauca River valleys. Similar analyses (not shown) are miento (22), El Sena (28), and La Selva (32). At sta- performed according to the location of the rain gauges tions located from 900 to 1500 m above the valley on each branch and aspect of the Andes, but no general floor there is a strong afternoon peak, though a much spatial pattern emerges. Despite the generality of the smaller midnight peak appears during August– bimodal pattern witnessed in average monthly precipi- December, such as at Bremen (26), El Cedral (24), tation over the Andes, the relationship between the and Agronomía (15). At Rosario (13) and El Jazmín annual and diurnal cycles is highly variable in space. (21) the diurnal cycle exhibits multimodality between Our analysis (not shown) indicates that the seasonally afternoon and predawn hours. dry and wet epochs discussed in the introduction are associated with seasonal shifts in diurnal maxima in 24 b. Magdalena River valley of the 51 rain gauges. Geographically, this association is most pronounced over the western flank of the Cordil- • At all stations on the eastern flank of the Cordillera lera Oriental (eastern branch, rain gauges 41 to 48). Central, the diurnal cycle exhibits a precipitation The possible forcing of the ITCZ position on the diur- maximum occurring near midnight (2300 to 0300 h nal cycle may be related to the predominant southeast- LST), with very little precipitation before 1700 h erly (boreal summer) or northeasterly (austral summer) LST. This is particularly clear on the eastern flanks, surface trade winds, and also to lower surface atmo- such as at Bizcocho (39, at twelve o’clock in Fig. 2), spheric pressures that favor low-level moisture conver- and Inmarco (38). But the peak appears earlier and gence and deep convection. less pronounced at Bizcocho during the period No- Topography and aspect appear to play an important vember–March. No such seasonal changes are appar- role in controlling the observed diurnal cycle; however, ent at Inmarco. relief alone does not explain the observed seasonal • Across the valley, on the western flank of the Cor- variability of diurnal maxima displayed by many rain dillera Oriental, with the exception of Bertha (42, gauges. In tropical mountainous regions with such afternoon peak), Luis Bustamante (47, at three rough terrain as the Andes, the marked spatial variabil- o’clock in Fig. 2, with midday peak), and Tibacuy (46, ity of diurnal precipitation maxima is strongly deter- bimodal, afternoon, and midnight), all stations ex- mined by very local topographic and environmental hibit one of three forms of seasonally varying behav- characteristics that interact with coexisting large-scale ior: forcing mechanisms, such as the aforementioned night- (i) shifting unimodal, from a near-midnight precipi- life cycle of mesoscale convective systems (Velasco and tation maximum during October–March to an Frisch 1987; Mapes et al 2003a; Zuluaga and Poveda afternoon maximum in May–September, as at 2004), and the position of the ITCZ, to generate deep Aguas Blancas (41, two o’clock in Fig. 2), La convection. We suggest that such combination of fac- Montaña (below the previous one in Fig. 2), and tors may contribute to explain the strong space–time Yacopí (43, below the two previous ones); variability identified here. The limitations of the avail- (ii) shifting from unimodal with a maximum in the able data impede a full explanation of the complex pat-

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FIG. 3. Seasonal distribution of the relationship between altitude above sea level and the timing of rainfall diurnal maximum, for the entire dataset. tern of alternating unimodal and bimodal diurnal cycles during La Niña years than during El Niño years. Ap- of rainfall identified at many stations. plication of a two-sample Kolmogorov–Smirnov non- parametric test (Press et al. 1986) of differences between two probability distribution functions (PDFs) of 5. Interannual (ENSO) variability hourly records during El Niño and La Niña revealed Both El Niño and La Niña strongly affect precipita- that, at more than 85% of the stations, the null hypoth- tion amounts at seasonal and annual time scales in Co- esis of similar PDFs during both phases of ENSO was lombia. El Niño is associated with negative anomalies rejected. In spite of the effects of both phases of ENSO in rainfall, river flows, soil moisture, and vegetation on the amplitude of diurnal precipitation, the phase of activity, particularly during September–February, while the diurnal cycle remains unaltered. La Niña is generally associated with positive rainfall anomalies (Poveda 1994; Poveda and Mesa 1997; Po- 6. Intra-annual variability veda and Jaramillo 2000; Poveda et al. 2001; Waylen and Poveda 2002). Are these anomalies the result of a. Phase of the MJO changes in the amplitude and/or phase of the diurnal During the westerly (easterly) phase of the MJO the cycle as a consequence of changes in the number, du- diurnal cycle of precipitation shows greater (lesser) am- ration, and/or intensity of storms? To answer these plitude than the long-term average diurnal cycle. Figure questions, the average diurnal cycles associated with 5 displays the results for the selected stations and indi- both ENSO phases are estimated and compared to cates that the phase of the diurnal cycle remains unal- those of “normal” years. The hydrological year is de- tered. A detailed analysis shows that during the west- fined from June (year 0, during El Niño onset) through erly phase there is an increase in daily precipitation at May (year ϩ1). Figure 4 shows the average diurnal 85% of the stations. This increase averages 37% (14% cycle during both phases of ENSO and normal years, to 76.9%), compared with the long-term diurnal pre- for the stations shown in Fig. 2. cipitation. On the other hand, the easterly phase is as- Without exception, hourly rainfall increases (dimin- sociated with a decrease in daily precipitation at 88% ishes) throughout the diurnal cycle during La Niña (El of the stations, the anomaly being of 72% (0% to Niño). During La Niña, average daily total rainfall in- 206.7%). No change in phase was detected. Interest- creases between 5.7% and 40%, as compared to “nor- ingly, during the westerly phase there is greater tropical mal” years, with average increase of 20.8% among all storm activity over the Caribbean (Maloney and Hart- stations. Equivalent percentages during El Niño dimin- mann 2000), which in turn may disrupt atmospheric ish between 6.9% and 56% for an average decrease of patterns, leading to heavy rainfall events over northern 22.2%. Overall total daily precipitation is 48% higher South America.

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FIG. 4. Average diurnal cycle of precipitation during El Niño (red), La Niña (blue), and normal (black) years, for the set of rain gauges depicted in Fig. 2. Hourly precipitation rates are shown in mm hϪ1 (right axis), and cumulative precipitation is in mm (left axis).

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Fig 4 live 4/C JANUARY 2005 P OVEDA ET AL. 237

Ϫ FIG. 5. Average diurnal cycle of precipitation rates (mm h 1) during the westerly (blue), normal (black), and easterly (red) phases of the MJO, for the set of rain gauges depicted in Fig. 2.

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Fig 5 live 4/C 238 MONTHLY WEATHER REVIEW VOLUME 133 b. High-frequency variability (2 to 256 h) intraseasonal variability (30–60 day or 720–1440 h) appears to be stronger during June–August during both Figure 6 shows the wavelet spectra at Paraguaicito ENSO phases, but also during October–December of (No. 30 in Table 1), corresponding to the June–May El Niño. The generality of these latter observations re- period of 2 yr of differing ENSO phase: La Niña 1988/ quires further investigation. 89 (left) and El Niño 1982/83 (center). The distribution of the squared absolute value of the corresponding wavelet coefficients is shown in the first two graphs of 7. Conclusions Fig. 6. Each wavelet spectrum is scaled so that the sum of the squared absolute value of the coefficients is equal The diurnal cycle of precipitation in the tropical to the total variance. Thus, regions shaded with a color Andes of Colombia is characterized using long-term corresponding to 90% indicate that those coefficients observations. Five main general conclusions can be explain more variance than 90% of all wavelet trans- drawn: (i) the observations indicate the existence of form coefficients. In a sense, they represent those re- seasonally varying diurnal (24 h) and semidiurnal (12 h) gions where the variance is concentrated. The third cycles. However, these cycles are highly variable in graph of Fig. 6 shows the global time-integrated wave- space; (ii) daily precipitation minima are found pre- let power spectrum (Torrence and Compo 1998). dominantly during the late-morning hours of (0900– Results show that the largest proportion of the vari- 1100 LST) regardless of season and location; (iii) over ance is concentrated in the frequency bands associated northeastern and western Colombia, precipitation with 32 and 256 h (1.3 and 10.7 days), whose origin maxima occur predominantly in the afternoon; (iv) deserves further investigation. Also, differences appear over the western flank of the central Andes, precipita- in the high-frequency bands between the two phases of tion maxima occur either around midnight, or during ENSO. During El Niño, high-frequency variability de- the afternoon, or both; and (v) over the eastern flank of creases throughout the entire June–May period, as the Central Cordillera, precipitation maxima occur evidenced by the time-integrated power spectrum, around midnight. especially during July–October (year 0, of El Niño There is no evidence in our dataset of a relationship onset), which can be partly explained by the observed between the timing of diurnal maximum rainfall with diminished activity of 4–6-day (96–144 h) tropical east- elevation, the branch of the Andes, or aspect, except erly waves, over the Caribbean and northern South for some small groupings of stations within the intra- America, during El Niño (Gu and Zhang 2001). It Andean Magdalena and Cauca River valleys. These re- is important to note that the ability of the easterly sults indicate that the highly variable topography plays waves to perturb Colombian weather depends on the a major role in determining the observed diurnal cycles, specific path followed by their westward propagation. yet the identified seasonal variations of diurnal precipi- This might explain why the variance in the 4–6-day tation maxima cannot be explained by relief alone. The bands is highly intermittent. A decrease in high- migration of the ITCZ may be a controlling factor to frequency variability is also evidenced in El Niño dur- explain the seasonal changes in diurnal precipitation ing mid-February to mid-April (year ϩ1), at 4–256 h maxima in about half of the set of rain gauges. We (0.17 to 10.6 days). The explanation of this variability conjecture that the identified strong time–space vari- deserves further investigation. Figure 6 also shows that ability in the diurnal cycle of precipitation in the tropi-

FIG. 6. Wavelet transform spectra of hourly precipitation series at Paraguaicito (No. 30 in Table 1), during the Jun–May period of two contrasting ENSO years: (left) La Niña 1988/89, (center) El Niño 1982/83, and (right) the time-integrated global wavelet power spectrum. Cross-hatched regions on either end of each plot indicate the “cone of influence,” where edge effects become important.

Unauthenticated | Downloaded 09/25/21 08:11 AM UTC JANUARY 2005 P OVEDA ET AL. 239 cal Andes of Colombia results from a combination of cipitation over the tropical oceans observed by TRMM/TMI local- and large-scale environmental conditions, which combined with SSM/I. J. Climate, 13, 4149–4158. Janowiak, J. E., P. A. Arkin, and M. Morrissey, 1994: An exami- deserve further investigation. nation of the diurnal cycle in oceanic tropical rainfall using Observations indicate a spatially coherent response satellite and in situ data. Mon. Wea. Rev., 122, 2296–2311. of the diurnal cycle to both phases of ENSO, as hourly Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Re- (and daily) rainfall diminishes during El Niño and in- analysis Project. Bull. Amer. Meteor. Soc., 77, 437–471. creases during La Niña. High-frequency variability is Kousky, V., 1980: Diurnal rainfall variations in northeast Brazil. Mon. Wea. Rev., 108, 488–498. mostly concentrated at 32- and 256-h cycles, during León, G. E., J. A. Zea, and J. A. Eslava, 2000: General circulation both ENSO phases, but high-frequency variability is and the intertropical convergence zone in Colombia (in Span- diminished (increased) by the occurrence of El Niño ish). Meteor. Colomb., 1, 31–38. (La Niña), in particular during the 4–6-day period of Lin, X., D. A. Randall, and L. D. Fowler, 2000: Diurnal variability highest activity of tropical easterly waves (June– of the hydrologic cycle and radiative fluxes: Comparisons between observations and a GCM. J. Climate, 13, 4159–4179. November of the ENSO year 0) and during February– López, M. E., and W. E. Howell, 1967: Katabatic winds in the April of year ϩ1. In addition, the phase of the diurnal equatorial Andes. J. Atmos. Sci., 24, 29–35. cycle remains unchanged either by ENSO or by MJO. Machado, L. A. T., H. Laurent, and A. A. Lima, 2002: Diurnal Analysis according to the phase of the Madden–Julian march of the convection observed during TRMM- oscillation indicates an overall increase (decrease) in WETAMC/LBA. J. Geophys. Res., 107, 8064, doi:10.1029/ 2001JD000338. the amplitude of diurnal cycle during the westerly (east- Madden, R. A., and P. A. Julian, 1994: Observations of the 40– erly) phase of the MJO, whereas the phase of the di- 50-day tropical oscillation—A review. Mon. Wea. Rev., 122, urnal cycle remains unaltered. 814–837. These findings need to be merged with diagnostic Maloney, E. D., and D. L. Hartmann, 2000: Modulation of hur- ricane activity in the Gulf of Mexico by the Madden–Julian studies of the diurnal cycle in tropical precipitation de- oscillation. Science, 287, 2002–2003. rived from satellite estimates, and also from general Mapes, B. E., T. T. Warner, M. Xu, and A. J. Negri, 2003a: Di- circulation (GCM) or mesoscale models. For instance, urnal patterns of rainfall in northwestern South America. understanding the diurnal march of precipitation over Part I: Observations and context. Mon. Wea. Rev., 131, 799– the tropical Andes of Colombia may contribute to test 812. ——, ——, and ——, 2003b: Diurnal patterns of rainfall in north- its purported feedback to oceanic precipitation over the western South America. Part III: Diurnal gravity waves and eastern tropical Pacific, as suggested by Mapes et al. nocturnal convection offshore. Mon. Wea. Rev., 131, 830–844. (2003b) and Warner et al. (2003). They can also be used Martínez, M. T., 1993: Influence on weather patterns of major to improve modeling and forecasting of tropical con- synoptic scale systems in Colombia (in Spanish). Atmósfera, vection over land, but also serve as ground truth for 16, 1–10. Meisner, B. N., and P. A. Arkin, 1987: Spatial and annual varia- satellite-estimated rainfall and elucidate the space–time tions in the diurnal cycle of large-scale tropical convective dynamics of tropical precipitation over complex terrain. clouds and precipitation. Mon. Wea. Rev., 115, 2009–2032. Mejía, J. F., and Coauthors, 1999: Spatial distribution, annual and Acknowledgments. We would like to thank A. Jara- semi-annual cycles of precipitation in Colombia (in Spanish). millo, O. Guzmán, and A. Cadena from CENICAFE, DYNA, 127, 7–26. Myers, N., R. A. Mittermeier, C. G. Mittermeier, G. A. B. da and Empresas Públicas de Medellín for providing ac- Fonseca, and J. Kent, 2000: Biodiversity hotspots for conser- cess to the data. Thanks to P. Waylen, C. Penland, T. vation priorities. Nature, 403, 853–858. Warner, J. Ramírez, and two anonymous reviewers for Negri, A. J., R. F. Adler, E. J. Nelkin, and G. J. Huffman, 1994: their thoughtful comments, which led to an improve- Regional rainfall climatologies derived from Special Sensor Microwave Imager (SSM/I) data. Bull. Amer. Meteor. Soc., ment of the manuscript. D. Hartmann kindly provided 75, 1165–1182. the MJO series. This work was supported by Dirección ——, E. N. Anagnostou, and R. F. Adler, 2000: A 10-year clima- Nacional de Investigaciones (DINAIN), of Universidad tology of Amazonian rainfall derived from passive micro- Nacional de Colombia. wave satellite observations. J. Appl. Meteor., 39, 42–56. ——, T. L. Bell, and L. Xu, 2002a: Sampling of the diurnal cycle of precipitation using TRMM. J. Atmos. Oceanic Technol., REFERENCES 19, 1333–1344. ——, L. Xu, and R. F. Adler, 2002b: A TRMM-calibrated infrared Garreaud, R. D., and J. M. Wallace, 1997: The diurnal march of rainfall algorithm, applied over Brazil. J. Geophys. Res., 107, convective cloudiness over the Americas. Mon. Wea. Rev., 8048, doi:10.1029/2000JD000265. 125, 3157–3171. Poveda, G., 1994: Empirical orthogonal functions in the relation- Gu, G., and C. Zhang, 2001: A spectrum analysis of synoptic-scale ship between river streamflows and sea surface temperatures disturbances in the ITCZ. J. Climate, 14, 2725–2739. in the Pacific and Atlantic Oceans. Proc. XVI Latin American Hastenrath, S., 1990: Diagnostics and prediction of anomalous Hydraulics and Hydrology Meeting (in Spanish), Santiago, river discharge in northern South America. J. Climate, 3, Chile, IAHR, 131–144. 1080–1096. ——, and O. J. Mesa, 1997: Feedbacks between hydrological pro- ——, 2002: The intertropical convergence zone of the eastern cesses in tropical South America and large-scale oceanic– Pacific revisited. Int. J. Climatol., 22, 347–356. atmospheric phenomena. J. Climate, 10, 2690–2702. Hendon, H., and K. Woodberry, 1993: The diurnal cycle of tropi- ——, and A. Jaramillo, 2000: ENSO-related variability of river cal convection. J. Geophys. Res., 98, 16 623–16 637. discharges and soil moisture in Colombia. Biospheric Aspects Imaoka, K., and R. W. Spencer, 2000: Diurnal variation of pre- of the Hydrologic Cycle, No. 8, IGBP, 3–6.

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——, and O. J. Mesa, 2000: On the existence of Lloró (the rainiest rainfall retrieved from combined GOES and TRMM satellite locality on earth): Enhanced ocean–atmosphere–land inter- information. J. Climate, 15, 983–1001. action by a low-level jet. Geophys. Res. Lett., 27, 1675–1678. Torrence, C., and G. P. Compo, 1998: A practical guide to wavelet ——, A. Jaramillo, M. M. Gil, N. Quiceno, and R. I. Mantilla, analysis. Bull. Amer. Meteor. Soc., 79, 61–78. 2001: Seasonality in ENSO related precipitation, river dis- Trojer, H., 1959: Fundamentos para una zonificación meteo- charges, soil moisture, and vegetation index (NDVI) in Co- rológica y climatológica del trópico y especialmente de Co- lombia. Water Resour. Res., 37, 2169–2178. lombia. Rev. Cenicafé, 10, 288–373. Press, W. H., B. P. Flannery, P. Brian, S. Teukolsky, and W. T. Velasco, I., and M. Frisch, 1987: Mesoscale convective complexes Vetterling, 1986: Numerical Recipes: The Art of Scientific in the Americas. J. Geophys. Res., 92 (D8), 9591–9613. Computing. Cambridge University Press, 818 pp. Warner, T. T., B. E. Mapes, and M. Xu, 2003: Diurnal patterns of Ricciardulli, L., and P. D. Sardeshmukh, 2002: Local time- and rainfall in northwestern South America. Part II: Model simu- space scales of organized tropical deep convection. J. Cli- lations. Mon. Wea. Rev., 131, 813–829. mate, 15, 2775–2790. Waylen, P. R., and G. Poveda, 2002: El Niño–Southern Oscilla- Snow, J. W., 1976: The climate of northern South America. Cli- tion and aspects of western South America hydro-climatol- mates of Central and South America, W. Schwerdtfeger, Ed., ogy. Hydrol. Processes, 16, 1247–1260. Elsevier, 295–403. Yang, G. Y., and J. Slingo, 2001: The diurnal cycle in the Tropics. Soden, B., 2000: The diurnal cycle of convection, clouds and water Mon. Wea. Rev., 129, 784–801. vapor in the tropical upper troposphere. Geophys. Res. Lett., Zulauga, M. D., and G. Poveda, 2004: Diagnostics of mesoscale 27, 2173–2176. convective systems in Colombia and the eastern Pacific dur- Sorooshian, S., X. Gao, K. Hsu, R. A. Maddox, Y. Hong, H. V. ing 1998–2002 using TRMM data (in Spanish). Av. Recur. Gupta, and B. Imam, 2002: Diurnal variability of tropical Hidra´ul., in press.

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