• Garreaud, R. D., and J. M. Wallace, 1997: The diurnal march of convective cloudiness over the Americas. Mon. Wea. Rev., 125 , 31573171.

    • Search Google Scholar
    • Export Citation
  • Gu, G., and C. Zhang, 2001: A spectrum analysis of synoptic-scale disturbances in the ITCZ. J. Climate, 14 , 27252739.

  • Hastenrath, S., 1990: Diagnostics and prediction of anomalous river discharge in northern South America. J. Climate, 3 , 10801096.

  • Hastenrath, S., 2002: The intertropical convergence zone of the eastern Pacific revisited. Int. J. Climatol., 22 , 347356.

  • Hendon, H., and K. Woodberry, 1993: The diurnal cycle of tropical convection. J. Geophys. Res., 98 , 1662316637.

  • Imaoka, K., and R. W. Spencer, 2000: Diurnal variation of precipitation over the tropical oceans observed by TRMM/TMI combined with SSM/I. J. Climate, 13 , 41494158.

    • Search Google Scholar
    • Export Citation
  • Janowiak, J. E., P. A. Arkin, and M. Morrissey, 1994: An examination of the diurnal cycle in oceanic tropical rainfall using satellite and in situ data. Mon. Wea. Rev., 122 , 22962311.

    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77 , 437471.

  • Kousky, V., 1980: Diurnal rainfall variations in northeast Brazil. Mon. Wea. Rev., 108 , 488498.

  • León, G. E., J. A. Zea, and J. A. Eslava, 2000: General circulation and the intertropical convergence zone in Colombia (in Spanish). Meteor. Colomb., 1 , 3138.

    • Search Google Scholar
    • Export Citation
  • Lin, X., D. A. Randall, and L. D. Fowler, 2000: Diurnal variability of the hydrologic cycle and radiative fluxes: Comparisons between observations and a GCM. J. Climate, 13 , 41594179.

    • Search Google Scholar
    • Export Citation
  • López, M. E., and W. E. Howell, 1967: Katabatic winds in the equatorial Andes. J. Atmos. Sci., 24 , 2935.

  • Machado, L. A. T., H. Laurent, and A. A. Lima, 2002: Diurnal march of the convection observed during TRMM-WETAMC/LBA. J. Geophys. Res., 107 , 8064. doi: 10.1029/2001JD000338.

    • Search Google Scholar
    • Export Citation
  • Madden, R. A., and P. A. Julian, 1994: Observations of the 40–50-day tropical oscillation—A review. Mon. Wea. Rev., 122 , 814837.

  • Maloney, E. D., and D. L. Hartmann, 2000: Modulation of hurricane activity in the Gulf of Mexico by the Madden–Julian oscillation. Science, 287 , 20022003.

    • Search Google Scholar
    • Export Citation
  • Mapes, B. E., T. T. Warner, M. Xu, and A. J. Negri, 2003a: Diurnal patterns of rainfall in northwestern South America. Part I: Observations and context. Mon. Wea. Rev., 131 , 799812.

    • Search Google Scholar
    • Export Citation
  • Mapes, B. E., T. T. Warner, and M. Xu, 2003b: Diurnal patterns of rainfall in northwestern South America. Part III: Diurnal gravity waves and nocturnal convection offshore. Mon. Wea. Rev., 131 , 830844.

    • Search Google Scholar
    • Export Citation
  • Martínez, M. T., 1993: Influence on weather patterns of major synoptic scale systems in Colombia (in Spanish). Atmósfera, 16 , 110.

  • Meisner, B. N., and P. A. Arkin, 1987: Spatial and annual variations in the diurnal cycle of large-scale tropical convective clouds and precipitation. Mon. Wea. Rev., 115 , 20092032.

    • Search Google Scholar
    • Export Citation
  • Mejía, J. F., and Coauthors, 1999: Spatial distribution, annual and semi-annual cycles of precipitation in Colombia (in Spanish). DYNA, 127 , 726.

    • Search Google Scholar
    • Export Citation
  • Myers, N., R. A. Mittermeier, C. G. Mittermeier, G. A. B. da Fonseca, and J. Kent, 2000: Biodiversity hotspots for conservation priorities. Nature, 403 , 853858.

    • Search Google Scholar
    • Export Citation
  • Negri, A. J., R. F. Adler, E. J. Nelkin, and G. J. Huffman, 1994: Regional rainfall climatologies derived from Special Sensor Microwave Imager (SSM/I) data. Bull. Amer. Meteor. Soc., 75 , 11651182.

    • Search Google Scholar
    • Export Citation
  • Negri, A. J., E. N. Anagnostou, and R. F. Adler, 2000: A 10-year climatology of Amazonian rainfall derived from passive microwave satellite observations. J. Appl. Meteor., 39 , 4256.

    • Search Google Scholar
    • Export Citation
  • Negri, A. J., T. L. Bell, and L. Xu, 2002a: Sampling of the diurnal cycle of precipitation using TRMM. J. Atmos. Oceanic Technol., 19 , 13331344.

    • Search Google Scholar
    • Export Citation
  • Negri, A. J., L. Xu, and R. F. Adler, 2002b: A TRMM-calibrated infrared rainfall algorithm, applied over Brazil. J. Geophys. Res., 107 , 8048. doi: 10.1029/2000JD000265.

    • Search Google Scholar
    • Export Citation
  • Poveda, G., 1994: Empirical orthogonal functions in the relationship between river streamflows and sea surface temperatures in the Pacific and Atlantic Oceans. Proc. XVI Latin American Hydraulics and Hydrology Meeting (in Spanish), Santiago, Chile, IAHR, 131–144.

  • Poveda, G., and O. J. Mesa, 1997: Feedbacks between hydrological processes in tropical South America and large-scale oceanic–atmospheric phenomena. J. Climate, 10 , 26902702.

    • Search Google Scholar
    • Export Citation
  • Poveda, G., and A. Jaramillo, 2000: ENSO-related variability of river discharges and soil moisture in Colombia. Biospheric Aspects of the Hydrologic Cycle, No. 8, IGBP, 3–6.

    • Search Google Scholar
    • Export Citation
  • Poveda, G., and O. J. Mesa, 2000: On the existence of Lloró (the rainiest locality on earth): Enhanced ocean–atmosphere–land interaction by a low-level jet. Geophys. Res. Lett., 27 , 16751678.

    • Search Google Scholar
    • Export Citation
  • Poveda, G., A. Jaramillo, M. M. Gil, N. Quiceno, and R. I. Mantilla, 2001: Seasonality in ENSO related precipitation, river discharges, soil moisture, and vegetation index (NDVI) in Colombia. Water Resour. Res., 37 , 21692178.

    • Search Google Scholar
    • Export Citation
  • Press, W. H., B. P. Flannery, P. Brian, S. Teukolsky, and W. T. Vetterling, 1986: Numerical Recipes: The Art of Scientific Computing. Cambridge University Press, 818 pp.

    • Search Google Scholar
    • Export Citation
  • Ricciardulli, L., and P. D. Sardeshmukh, 2002: Local time- and space scales of organized tropical deep convection. J. Climate, 15 , 27752790.

    • Search Google Scholar
    • Export Citation
  • Snow, J. W., 1976: The climate of northern South America. Climates of Central and South America, W. Schwerdtfeger, Ed., Elsevier, 295–403.

    • Search Google Scholar
    • Export Citation
  • Soden, B., 2000: The diurnal cycle of convection, clouds and water vapor in the tropical upper troposphere. Geophys. Res. Lett., 27 , 21732176.

    • Search Google Scholar
    • Export Citation
  • Sorooshian, S., X. Gao, K. Hsu, R. A. Maddox, Y. Hong, H. V. Gupta, and B. Imam, 2002: Diurnal variability of tropical rainfall retrieved from combined GOES and TRMM satellite information. J. Climate, 15 , 9831001.

    • Search Google Scholar
    • Export Citation
  • Torrence, C., and G. P. Compo, 1998: A practical guide to wavelet analysis. Bull. Amer. Meteor. Soc., 79 , 6178.

  • Trojer, H., 1959: Fundamentos para una zonificación meteorológica y climatológica del trópico y especialmente de Colombia. Rev. Cenicafé, 10 , 288373.

    • Search Google Scholar
    • Export Citation
  • Velasco, I., and M. Frisch, 1987: Mesoscale convective complexes in the Americas. J. Geophys. Res., 92 , D8,. 95919613.

  • Warner, T. T., B. E. Mapes, and M. Xu, 2003: Diurnal patterns of rainfall in northwestern South America. Part II: Model simulations. Mon. Wea. Rev., 131 , 813829.

    • Search Google Scholar
    • Export Citation
  • Waylen, P. R., and G. Poveda, 2002: El Niño–Southern Oscillation and aspects of western South America hydro-climatology. Hydrol. Processes, 16 , 12471260.

    • Search Google Scholar
    • Export Citation
  • Yang, G. Y., and J. Slingo, 2001: The diurnal cycle in the Tropics. Mon. Wea. Rev., 129 , 784801.

  • Zulauga, M. D., and G. Poveda, 2004: Diagnostics of mesoscale convective systems in Colombia and the eastern Pacific during 1998–2002 using TRMM data (in Spanish). Av. Recur. Hidrául., in press.

  • View in gallery

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

  • View in gallery

    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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    Seasonal distribution of the relationship between altitude above sea level and the timing of rainfall diurnal maximum, for the entire dataset.

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    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 h−1 (right axis), and cumulative precipitation is in mm (left axis).

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    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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    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.

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The Diurnal Cycle of Precipitation in the Tropical Andes of Colombia

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  • 1 Escuela de Geociencias y Medio Ambiente, Universidad Nacional de Colombia, Medellín, Colombia
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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 (<10 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.

Corresponding author address: Germán Poveda, Escuela de Geociencias y Medio Ambiente, Facultad de Minas, Universidad Nacional de Colombia, Cra 80 x Calle 65, M2-315 Medellín, Colombia. Email: gpoveda@unalmed.edu.co

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 (<10 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.

Corresponding author address: Germán Poveda, Escuela de Geociencias y Medio Ambiente, Facultad de Minas, Universidad Nacional de Colombia, Cra 80 x Calle 65, M2-315 Medellín, Colombia. Email: gpoveda@unalmed.edu.co

1. Introduction

Colombia is located in the northwestern corner of South America, exhibiting complex geographical, environmental, and hydroecological features in which the Andes plays a major role. In terms of precipitation amount and distribution, Snow (1976, p. 362) describes the Andes as “a dry island in a sea of rain,” but the atmospheric dynamics and precipitation variability associated with the Andes are not well understood at a wide range of time–space scales. Improving our understanding of the atmospheric dynamics and precipitation variability is crucial under current environmental threats faced by the tropical Andes, identified as the most critical “hot spot” for biodiversity on earth (Myers et al. 2000).

On seasonal time scales, it is well known that the displacement 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, submitted manuscript to J. Hydrol. Eng.). Central and western Colombia experience a bimodal annual cycle of precipitation with distinct rainy seasons (April–May and October–November), and “dry” (less rainy) seasons (December–February and June–August), which result from the double passage of the ITCZ over the region. A unimodal annual cycle (May–October) is witnessed over the northern Caribbean coast of Colombia and the Pacific flank of the southern isthmus, reflecting the northernmost position of the ITCZ over the continent and eastern equatorial Pacific, respectively (Hastenrath 1990, 2002). A single peak is also evident over the eastern piedmont of the eastern Andes. Moisture transported from the Amazon basin encounters the orographic barrier of the Andes, thus focusing and enhancing deep convection and rainfall in the eastern flank of the Cordillera, with maximum rainfall occurring during June–August. The meridional migration of the ITCZ is highly intertwined with atmospheric circulation features over the Caribbean Sea, the easternmost Pacific Ocean, and the Amazon River basin. The presence of the three branches of the Andes with elevations surpassing 5000 m, containing tropical glaciers, and including diverse intra-Andean valleys in a predominant south–north direction introduce strong local orographic effects that induce atmospheric circulations and deep convection with heavy rainfall. Over the Pacific coast of Colombia, extreme precipitation rates are witnessed, including one of the rainiest regions on earth (Poveda and Mesa 2000). Other large-scale phenomena associated with the seasonal hydroclimatological cycle over the region include the low-level “Chorro del Occidente Colombiano” (CHOCO) jet that flows onshore from the Pacific Ocean (Poveda and Mesa 2000) and the associated development of mesoscale convective complexes, which in turn exhibit characteristic diurnal cycles (Velasco and Frisch 1987; Poveda and Mesa 2000; Mapes et al. 2003a). The interannual variability of the diurnal cycle is dominated by the effects of both phases of El Niño–Southern Oscillation (ENSO). Such effects are due to influences of sea surface temperatures off the Pacific coast, but also to atmospheric teleconnections, which are enhanced by land surface–atmosphere feedbacks (Poveda and Mesa 1997, 2000; Poveda et al. 2001). At intraseasonal time scales, tropical easterly waves are known to affect precipitation regimes over different regions in Colombia in their westward propagation during the Northern Hemisphere summer–autumn (Martínez 1993).

Previous studies regarding the diurnal cycle of rainfall for Colombia and tropical South America are scarce and limited to small datasets. Trojer (1959) identified a distinct effect of altitude on the diurnal cycle of precipitation maxima over the Cauca River valley, with an early-morning peak for stations located near the valley floor, while those at higher elevations showed afternoon maxima. Snow (1976, p. 365) found different diurnal-maxima characteristics at three localities located on each of the branches of the Colombian Andes: Quibdó (on the lowlands of the Pacific coast), showing a late-night and early-morning maximum; Chinchiná (central Andes), displaying an early-morning maximum; and Bogotá (high plateau on the eastern Andes), exhibiting an afternoon maximum. Other studies for northern South America rely on precipitation estimates derived from satellite data (Kousky 1980; Meisner and Arkin 1987; Velasco and Frisch 1987; Hendon and Woodberry 1993; Janowiak et al. 1994; Negri et al. 1994, 2000; Garreaud and Wallace 1997; Imaoka and Spencer 2000; Lin et al. 2000; Soden 2000; Yang and Slingo 2001; Ricciardulli and Sardeshmukh 2002; Sorooshian et al. 2002; Mapes et al. 2003a). For instance, Negri et al. (2000) identified a strong diurnal cycle in Amazonian rainfall and pointed out the important role of land–atmosphere feedbacks in controlling the intensity of the diurnal cycle. They also identified a rainfall maximum offshore in the Gulf of Panama that occurs during the morning hours around 0900 LST and a topographically induced afternoon maximum in the mountains of Venezuela (near 5°N, 63°W) that occurs during May–September. Over the eastern slope of the Andes, a nighttime maximum of precipitation has been identified (Garreaud and Wallace 1997; Negri et al. 1994). The diurnal march of convection in a small region of the Amazon was studied during the 1999 Tropical Rainfall Measuring Mission (TRMM) Wet-Season Atmospheric Mesoscale Campaign of the Large-Scale Biosphere–Atmosphere Experiment (WETAMC/LBA) (Machado et al. 2002). The three-part study of Mapes et al. (2003a,b) and Warner et al. (2003) reports observational and modeling results for the diurnal cycle of precipitation over western Colombia and the neighboring Pacific, using a short-period dataset (July–September 2000) that is based on the rain-rate satellite estimates of Negri et al. (2002a). Most of the aforementioned studies on the diurnal cycle of precipitation over the region are based upon short-term datasets derived from satellite information that, though spatially distributed, may induce estimation errors. For instance, Negri et al. (2002b), showed that 3 yr of rainfall data derived from the TRMM satellite are inadequate to describe the diurnal cycle of precipitation over areas smaller than 12°×12° because of high spatial variability in sampling.

We aim to study and characterize the long-term seasonal diurnal cycle of precipitation (section 3) and its relationship with altitude on the Andes (section 4). We also aim to investigate the interannual variability of the diurnal 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 westerly phases of the Madden–Julian oscillation (30–60 days). In addition, high-frequency variability (less than 10 days) of two contrasting ENSO years is studied, using hourly data from rain gauges located throughout the tropical Andes of Colombia. Conclusions are summarized in section 7.

2. Data and methods

The dataset consists of hourly precipitation rates at 51 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 dataset was provided in paper logs by the Centro Nacional de Investigaciones del Café (CENICAFE) and Empresas Públicas de Medellín (EPM), from which the digital files were created. The gauges lie between 1°15′N and 10°20′N, and 77°29′W and 72°40′W, and are situated between 260 and 2595 m in elevation. Details of the geographical setting and exact location of the gauges are shown in Table 1 and Fig. 1.

The Andes are divided into three major branches in Colombia, namely Cordillera Occidental (western branch), 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 rainfall values are examined is arbitrarily defined as running from 0700 to 0700 LST to better capture the large quantities of precipitation observed during late-night–early-morning hours. Gauging stations are numbered in Fig. 1 according to their location along different Andean slopes: western slope of the Cordillera Occidental (1 to 3); southern “Macizo Colombiano” (4 to 6); eastern slope of the Cordillera Occidental belonging to the Cauca River valley (7 to 12); the western slope of the Cordillera Central (13 to 32); the eastern slope of the Cordillera 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 Nevada de Santa Marta (51).

To examine interannual variability, we quantify the diurnal cycle of precipitation during both phases of ENSO: El Niño and La Niña. ENSO years were obtained from the multivariate ENSO index (MEI), developed by the National Oceanic and Atmospheric Administration (NOAA), as follows: 1957/58, 1965/66, 1972/73, 1982/83, 1986/87, 1991/92, 1994/95, 1997/98, and 2002/03 for El Niño, and 1964/65, 1970/71, 1973/74, 1975/76, 1988/89, and 1998/2000 for La Niña (see http://www.cdc.noaa.gov/∼kew/MEI/).

Intra-annual variability of the diurnal cycle is investigated by discriminating according to the phase of the Madden–Julian oscillation (MJO; Madden and Julian 1994; Maloney and Hartmann 2000). Characterization of easterly and westerly phases of the MJO was made using 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 from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) 40-Year Climate Reanalysis Project (Kalnay et al. 1996). The westerly (easterly) phase of the MJO corresponds to those periods in which the index is greater (lesser) than plus (minus) one standard deviation of the series. The average diurnal cycle of precipitation was estimated for the easterly, westerly, and neutral phases of the MJO.

We also characterize the high-frequency variability (less than 256 h or 10 days) of hourly precipitation during La Niña and El Niño years. The activity of tropical easterly waves (4–6 days) is dominant within such frequency band during the June–October period. Toward that end, we perform wavelet analyses of the series of hourly rainfall rates, using the Morlet mother wavelet (Torrence and Compo 1998).

3. Seasonal variability

Fast Fourier transform analyses of the entire dataset (not shown) revealed significant spectral peaks at 24 and/or 12 h, corresponding to distinct diurnal and semidiurnal cycles. No evidence was found of regional coherence in the predominance of the identified periodicities. At monthly time scales, most rain gauges exhibit a seasonally varying diurnal cycle of precipitation, which is itself extremely variable over space. Figure 2 shows results of the seasonal march of the diurnal cycle from a subsample of 17 stations selected to reflect the spatial disposition and results of the larger sample. Isolines represent the percentage of daily precipitation total, on days with measurable precipitation, falling throughout the course of the day and are estimated through interpolation of the original 24 h × 12 months matrix.

Several features are worth noticing: (i) the presence of differing timings of diurnal maxima at any given station, including shifts from unimodal to multimodal behavior, within the year; (ii) the presence of unimodal, bimodal, or multimodal diurnal behavior at different stations; (iii) the coexistence of different timing of the diurnal maxima at nearby stations during the same season, and in summary; and (iv) an astounding variability of the number of daily maxima of the diurnal cycle, even at very small spatial scales.

However, a generalized pattern can be drawn from these figures: the minimum percentage of daily precipitation is always found during the morning hours of 0900–1100 LST regardless of season or location. Also, two broad regional statements can be made. First, the predominant afternoon maxima found in many stations located over northeastern and western Colombia, such as at Mandé (3, located at the ten o’clock position in Fig. 2), Luis Bustamante (47, at three o’clock in Fig. 2), Maracay (27, at seven o’clock in Fig. 2), Pueblo Bello (51), Manuel Mejía (4), El Sauce (5), and Paraguaicito (30), among others, may be explained as the result of convective precipitation associated with solar thermal forcing, enhanced by the entrance of low-level, moisture-laden winds onshore from the Caribbean and Pacific. These winds ascend due to orographic lifting and reach condensation levels at sunset hours. Therefore, this daytime convection tends to be in the form of small cloud systems (Mapes et al. 2003a). Second, a common maximum of precipitation prevails during the middle of the night amongst stations located on the eastern flank of the Central Cordillera, corresponding to the western-edge margin of the wide Magdalena River valley [Bizcocho (39, at 12 o’clock in Fig. 2), La Trinidad (36, at one o’clock in Fig. 2), Inmarco (38), El Limón (34), Chapetón (35), Llanadas (37), and Peñol (40)]. Such a late-night–early-morning maximum is mainly associated with the life cycle of mesoscale convective systems (Velasco and Frisch 1987), with an upscale development of storm size through the night, supported by diurnally driven breezes and gravity waves (López and Howell 1967; Mapes et al. 2003a).

The identified diurnal patterns of precipitation seem to lack the type of temporal or spatial structure, or large-scale order, that one might expect in a region with such strong seasonality resulting from the passage of the ITCZ and such distinct, well-delineated, topographic features. For instance, even within the grouping of stations near the Venezuelan border and to the west, which have been identified as sharing a similar unimodal behavior, local differences may be detected. The time of maximum precipitation at Aguas Blancas (41, located at two o’clock in Fig. 2) shifts during the year from afternoon (July–August) to evening hours (November–March); however, the eastward-facing Francisco Romero (50, above the previous one in Fig. 2) shows a maximum during early-evening hours (1800 to 2000 LST) throughout the year. Likewise, in the west, Mandé (3, at ten o’clock in Fig. 2) has a maximum of precipitation from 1700 to 2000 LST throughout the year, while the timing of the maximum shifts markedly at Sireno (1, above the previous one in Fig. 2) to 1900–0100 LST during May–October. The following detailed analysis of the spectral characteristics of precipitation at stations in geographic proximity to one another, in the Cauca and Magdalena valleys, illustrates that the spatial coherence in the diurnal cycle of rainfall is limited to very small-scale regions within the intra-Andean valleys.

a. Cauca River valley

  • Eastern flank of the Cordillera Occidental (western branch). The diurnal cycle at stations located 480 and 700 m above the valley floor exhibit bimodal behavior, with a stronger afternoon maximum (1300 to 1600) and a second midnight–predawn maximum, as exemplified by Julio Fernández (7) and Mallarino (8). Above 700 m, precipitation is almost exclusively concentrated in the afternoon hours, as at Santiago Gutierrez (9, at four o’clock in Fig. 2) and Alban (10), or near midnight at the northerly Rafael Escobar (11) and Miguel Valencia (12).

  • At stations across the valley, on the western flank of the central Andes, the diurnal cycle exhibits maxima either at or near midnight, or during the afternoon, or both. Examples of the first kind are Luker (14), Santagueda (16, at four o’clock in Fig. 2), Santa Ana (17), Cenicafé (18), Naranjal (19, at nine o’clock in Fig. 2), and Santa Helena (20). Examples of stations with afternoon maximum of precipitation are Agronomia (15), El Cedral (24), Bremen (26, at five o’clock in 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 (23), Arturo Gomez (25, at six o’clock in Fig. 2), and La Bella (29). The relationship with elevation is highly variable. Stations located between 350 and 400 m above the valley floor exhibit a single midnight peak, as at Santagueda (16, at four o’clock in Fig. 2) and Luker (14). Between 400 and 900 m, patterns become more bimodal, with peaks during evening hours (1700 to 1900), and a secondary midnight peak, as illustrated at Paraguaicito (30), Arturo Gómez (25, at six o’clock in Fig. 2), La Catalina (23), La Bella (29), La Sirena (31), Maracay (27), Planta Tratamiento (22), El Sena (28), and La Selva (32). At stations located from 900 to 1500 m above the valley floor there is a strong afternoon peak, though a much smaller midnight peak appears during August–December, such as at Bremen (26), El Cedral (24), and Agronomía (15). At Rosario (13) and El Jazmín (21) the diurnal cycle exhibits multimodality between afternoon and predawn hours.

b. Magdalena River valley

  • At all stations on the eastern flank of the Cordillera Central, the diurnal cycle exhibits a precipitation maximum occurring near midnight (2300 to 0300 h LST), with very little precipitation before 1700 h LST. This is particularly clear on the eastern flanks, such as at Bizcocho (39, at twelve o’clock in Fig. 2), and Inmarco (38). But the peak appears earlier and less pronounced at Bizcocho during the period November–March. No such seasonal changes are apparent at Inmarco.

  • Across the valley, on the western flank of the Cordillera Oriental, with the exception of Bertha (42, afternoon peak), Luis Bustamante (47, at three o’clock in Fig. 2, with midday peak), and Tibacuy (46, bimodal, afternoon, and midnight), all stations exhibit one of three forms of seasonally varying behavior:

    • (i) shifting unimodal, from a near-midnight precipitation maximum during October–March to an afternoon maximum in May–September, as at Aguas Blancas (41, two o’clock in Fig. 2), La Montaña (below the previous one in Fig. 2), and Yacopí (43, below the two previous ones);

    • (ii) shifting from unimodal with a maximum in the afternoon throughout the year to bimodal (a secondary peak appears in the afternoon and near midnight) from November to March, as at Santa Inés (44);

    • (iii) shifting unimodal, from a nighttime maximum (1900 to 2300 LST) during November–March to an afternoon maximum during May–September, as at Misiones (45).

4. The role of altitude and the position of the ITCZ

In an attempt to explain the observed behavior of the diurnal maxima of rainfall, potential relationships with altitude and with the seasonal meridional migration of the ITCZ are examined. Figure 3 shows no clear relationship between the timing of diurnal maxima and a station’s altitude. An apparent relationship with the altitude is evidenced only in small groups of rain gauges, as in the previous discussion of the Magdalena and Cauca River valleys. Similar analyses (not shown) are performed according to the location of the rain gauges on each branch and aspect of the Andes, but no general spatial pattern emerges. Despite the generality of the bimodal pattern witnessed in average monthly precipitation over the Andes, the relationship between the annual and diurnal cycles is highly variable in space. Our analysis (not shown) indicates that the seasonally dry and wet epochs discussed in the introduction are associated with seasonal shifts in diurnal maxima in 24 of the 51 rain gauges. Geographically, this association is most pronounced over the western flank of the Cordillera Oriental (eastern branch, rain gauges 41 to 48). The possible forcing of the ITCZ position on the diurnal cycle may be related to the predominant southeasterly (boreal summer) or northeasterly (austral summer) surface trade winds, and also to lower surface atmospheric pressures that favor low-level moisture convergence and deep convection.

Topography and aspect appear to play an important role in controlling the observed diurnal cycle; however, relief alone does not explain the observed seasonal variability of diurnal maxima displayed by many rain gauges. In tropical mountainous regions with such rough terrain as the Andes, the marked spatial variability of diurnal precipitation maxima is strongly determined by very local topographic and environmental characteristics that interact with coexisting large-scale forcing mechanisms, such as the aforementioned nightlife cycle of mesoscale convective systems (Velasco and Frisch 1987; Mapes et al 2003a; Zuluaga and Poveda 2004), and the position of the ITCZ, to generate deep convection. We suggest that such combination of factors may contribute to explain the strong space–time variability identified here. The limitations of the available data impede a full explanation of the complex pattern of alternating unimodal and bimodal diurnal cycles of rainfall identified at many stations.

5. Interannual (ENSO) variability

Both El Niño and La Niña strongly affect precipitation amounts at seasonal and annual time scales in Colombia. El Niño is associated with negative anomalies in rainfall, river flows, soil moisture, and vegetation activity, particularly during September–February, while La Niña is generally associated with positive rainfall anomalies (Poveda 1994; Poveda and Mesa 1997; Poveda and Jaramillo 2000; Poveda et al. 2001; Waylen and Poveda 2002). Are these anomalies the result of changes in the amplitude and/or phase of the diurnal cycle as a consequence of changes in the number, duration, and/or intensity of storms? To answer these questions, the average diurnal cycles associated with both ENSO phases are estimated and compared to those of “normal” years. The hydrological year is defined from June (year 0, during El Niño onset) through May (year +1). Figure 4 shows the average diurnal cycle during both phases of ENSO and normal years, for the stations shown in Fig. 2.

Without exception, hourly rainfall increases (diminishes) throughout the diurnal cycle during La Niña (El Niño). During La Niña, average daily total rainfall increases between 5.7% and 40%, as compared to “normal” years, with average increase of 20.8% among all stations. Equivalent percentages during El Niño diminish between 6.9% and 56% for an average decrease of 22.2%. Overall total daily precipitation is 48% higher during La Niña years than during El Niño years. Application of a two-sample Kolmogorov–Smirnov nonparametric test (Press et al. 1986) of differences between two probability distribution functions (PDFs) of hourly records during El Niño and La Niña revealed that, at more than 85% of the stations, the null hypothesis of similar PDFs during both phases of ENSO was rejected. In spite of the effects of both phases of ENSO on the amplitude of diurnal precipitation, the phase of the diurnal cycle remains unaltered.

6. Intra-annual variability

a. Phase of the MJO

During the westerly (easterly) phase of the MJO the diurnal cycle of precipitation shows greater (lesser) amplitude than the long-term average diurnal cycle. Figure 5 displays the results for the selected stations and indicates that the phase of the diurnal cycle remains unaltered. A detailed analysis shows that during the westerly phase there is an increase in daily precipitation at 85% of the stations. This increase averages 37% (14% to 76.9%), compared with the long-term diurnal precipitation. On the other hand, the easterly phase is associated with a decrease in daily precipitation at 88% of the stations, the anomaly being of 72% (0% to 206.7%). No change in phase was detected. Interestingly, during the westerly phase there is greater tropical storm activity over the Caribbean (Maloney and Hartmann 2000), which in turn may disrupt atmospheric patterns, leading to heavy rainfall events over northern South America.

b. High-frequency variability (2 to 256 h)

Figure 6 shows the wavelet spectra at Paraguaicito (No. 30 in Table 1), corresponding to the June–May period of 2 yr of differing ENSO phase: La Niña 1988/89 (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 Fig. 6. Each wavelet spectrum is scaled so that the sum of the squared absolute value of the coefficients is equal to the total variance. Thus, regions shaded with a color corresponding to 90% indicate that those coefficients explain more variance than 90% of all wavelet transform coefficients. In a sense, they represent those regions where the variance is concentrated. The third graph of Fig. 6 shows the global time-integrated wavelet power spectrum (Torrence and Compo 1998).

Results show that the largest proportion of the variance is concentrated in the frequency bands associated with 32 and 256 h (1.3 and 10.7 days), whose origin deserves further investigation. Also, differences appear in the high-frequency bands between the two phases of ENSO. During El Niño, high-frequency variability decreases throughout the entire June–May period, as evidenced by the time-integrated power spectrum, especially during July–October (year 0, of El Niño onset), which can be partly explained by the observed diminished activity of 4–6-day (96–144 h) tropical easterly waves, over the Caribbean and northern South America, during El Niño (Gu and Zhang 2001). It is important to note that the ability of the easterly waves to perturb Colombian weather depends on the specific path followed by their westward propagation. This might explain why the variance in the 4–6-day bands is highly intermittent. A decrease in high-frequency variability is also evidenced in El Niño during mid-February to mid-April (year +1), at 4–256 h (0.17 to 10.6 days). The explanation of this variability deserves further investigation. Figure 6 also shows that intraseasonal variability (30–60 day or 720–1440 h) appears to be stronger during June–August during both ENSO phases, but also during October–December of El Niño. The generality of these latter observations requires further investigation.

7. Conclusions

The diurnal cycle of precipitation in the tropical Andes of Colombia is characterized using long-term observations. Five main general conclusions can be drawn: (i) the observations indicate the existence of seasonally varying diurnal (24 h) and semidiurnal (12 h) cycles. However, these cycles are highly variable in space; (ii) daily precipitation minima are found predominantly during the late-morning hours of (0900–1100 LST) regardless of season and location; (iii) over northeastern and western Colombia, precipitation maxima occur predominantly in the afternoon; (iv) over the western flank of the central Andes, precipitation maxima occur either around midnight, or during the afternoon, or both; and (v) over the eastern flank of the Central Cordillera, precipitation maxima occur around midnight.

There is no evidence in our dataset of a relationship between the timing of diurnal maximum rainfall with elevation, the branch of the Andes, or aspect, except for some small groupings of stations within the intra-Andean Magdalena and Cauca River valleys. These results indicate that the highly variable topography plays a major role in determining the observed diurnal cycles, yet the identified seasonal variations of diurnal precipitation maxima cannot be explained by relief alone. The migration of the ITCZ may be a controlling factor to explain the seasonal changes in diurnal precipitation maxima in about half of the set of rain gauges. We conjecture that the identified strong time–space variability in the diurnal cycle of precipitation in the tropical Andes of Colombia results from a combination of local- and large-scale environmental conditions, which deserve further investigation.

Observations indicate a spatially coherent response of the diurnal cycle to both phases of ENSO, as hourly (and daily) rainfall diminishes during El Niño and increases during La Niña. High-frequency variability is mostly concentrated at 32- and 256-h cycles, during both ENSO phases, but high-frequency variability is diminished (increased) by the occurrence of El Niño (La Niña), in particular during the 4–6-day period of highest activity of tropical easterly waves (June–November of the ENSO year 0) and during February–April of year +1. In addition, the phase of the diurnal cycle remains unchanged either by ENSO or by MJO. Analysis according to the phase of the Madden–Julian oscillation indicates an overall increase (decrease) in the amplitude of diurnal cycle during the westerly (easterly) phase of the MJO, whereas the phase of the diurnal cycle remains unaltered.

These findings need to be merged with diagnostic studies of the diurnal cycle in tropical precipitation derived from satellite estimates, and also from general circulation (GCM) or mesoscale models. For instance, understanding the diurnal march of precipitation over the tropical Andes of Colombia may contribute to test its purported feedback to oceanic precipitation over the eastern tropical Pacific, as suggested by Mapes et al. (2003b) and Warner et al. (2003). They can also be used to improve modeling and forecasting of tropical convection over land, but also serve as ground truth for satellite-estimated rainfall and elucidate the space–time dynamics of tropical precipitation over complex terrain.

Acknowledgments

We would like to thank A. Jaramillo, O. Guzmán, and A. Cadena from CENICAFE, and Empresas Públicas de Medellín for providing access to the data. Thanks to P. Waylen, C. Penland, T. Warner, J. Ramírez, and two anonymous reviewers for their thoughtful comments, which led to an improvement of the manuscript. D. Hartmann kindly provided the MJO series. This work was supported by Dirección Nacional de Investigaciones (DINAIN), of Universidad Nacional de Colombia.

REFERENCES

  • Garreaud, R. D., and J. M. Wallace, 1997: The diurnal march of convective cloudiness over the Americas. Mon. Wea. Rev., 125 , 31573171.

    • Search Google Scholar
    • Export Citation
  • Gu, G., and C. Zhang, 2001: A spectrum analysis of synoptic-scale disturbances in the ITCZ. J. Climate, 14 , 27252739.

  • Hastenrath, S., 1990: Diagnostics and prediction of anomalous river discharge in northern South America. J. Climate, 3 , 10801096.

  • Hastenrath, S., 2002: The intertropical convergence zone of the eastern Pacific revisited. Int. J. Climatol., 22 , 347356.

  • Hendon, H., and K. Woodberry, 1993: The diurnal cycle of tropical convection. J. Geophys. Res., 98 , 1662316637.

  • Imaoka, K., and R. W. Spencer, 2000: Diurnal variation of precipitation over the tropical oceans observed by TRMM/TMI combined with SSM/I. J. Climate, 13 , 41494158.

    • Search Google Scholar
    • Export Citation
  • Janowiak, J. E., P. A. Arkin, and M. Morrissey, 1994: An examination of the diurnal cycle in oceanic tropical rainfall using satellite and in situ data. Mon. Wea. Rev., 122 , 22962311.

    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77 , 437471.

  • Kousky, V., 1980: Diurnal rainfall variations in northeast Brazil. Mon. Wea. Rev., 108 , 488498.

  • León, G. E., J. A. Zea, and J. A. Eslava, 2000: General circulation and the intertropical convergence zone in Colombia (in Spanish). Meteor. Colomb., 1 , 3138.

    • Search Google Scholar
    • Export Citation
  • Lin, X., D. A. Randall, and L. D. Fowler, 2000: Diurnal variability of the hydrologic cycle and radiative fluxes: Comparisons between observations and a GCM. J. Climate, 13 , 41594179.

    • Search Google Scholar
    • Export Citation
  • López, M. E., and W. E. Howell, 1967: Katabatic winds in the equatorial Andes. J. Atmos. Sci., 24 , 2935.

  • Machado, L. A. T., H. Laurent, and A. A. Lima, 2002: Diurnal march of the convection observed during TRMM-WETAMC/LBA. J. Geophys. Res., 107 , 8064. doi: 10.1029/2001JD000338.

    • Search Google Scholar
    • Export Citation
  • Madden, R. A., and P. A. Julian, 1994: Observations of the 40–50-day tropical oscillation—A review. Mon. Wea. Rev., 122 , 814837.

  • Maloney, E. D., and D. L. Hartmann, 2000: Modulation of hurricane activity in the Gulf of Mexico by the Madden–Julian oscillation. Science, 287 , 20022003.

    • Search Google Scholar
    • Export Citation
  • Mapes, B. E., T. T. Warner, M. Xu, and A. J. Negri, 2003a: Diurnal patterns of rainfall in northwestern South America. Part I: Observations and context. Mon. Wea. Rev., 131 , 799812.

    • Search Google Scholar
    • Export Citation
  • Mapes, B. E., T. T. Warner, and M. Xu, 2003b: Diurnal patterns of rainfall in northwestern South America. Part III: Diurnal gravity waves and nocturnal convection offshore. Mon. Wea. Rev., 131 , 830844.

    • Search Google Scholar
    • Export Citation
  • Martínez, M. T., 1993: Influence on weather patterns of major synoptic scale systems in Colombia (in Spanish). Atmósfera, 16 , 110.

  • Meisner, B. N., and P. A. Arkin, 1987: Spatial and annual variations in the diurnal cycle of large-scale tropical convective clouds and precipitation. Mon. Wea. Rev., 115 , 20092032.

    • Search Google Scholar
    • Export Citation
  • Mejía, J. F., and Coauthors, 1999: Spatial distribution, annual and semi-annual cycles of precipitation in Colombia (in Spanish). DYNA, 127 , 726.

    • Search Google Scholar
    • Export Citation
  • Myers, N., R. A. Mittermeier, C. G. Mittermeier, G. A. B. da Fonseca, and J. Kent, 2000: Biodiversity hotspots for conservation priorities. Nature, 403 , 853858.

    • Search Google Scholar
    • Export Citation
  • Negri, A. J., R. F. Adler, E. J. Nelkin, and G. J. Huffman, 1994: Regional rainfall climatologies derived from Special Sensor Microwave Imager (SSM/I) data. Bull. Amer. Meteor. Soc., 75 , 11651182.

    • Search Google Scholar
    • Export Citation
  • Negri, A. J., E. N. Anagnostou, and R. F. Adler, 2000: A 10-year climatology of Amazonian rainfall derived from passive microwave satellite observations. J. Appl. Meteor., 39 , 4256.

    • Search Google Scholar
    • Export Citation
  • Negri, A. J., T. L. Bell, and L. Xu, 2002a: Sampling of the diurnal cycle of precipitation using TRMM. J. Atmos. Oceanic Technol., 19 , 13331344.

    • Search Google Scholar
    • Export Citation
  • Negri, A. J., L. Xu, and R. F. Adler, 2002b: A TRMM-calibrated infrared rainfall algorithm, applied over Brazil. J. Geophys. Res., 107 , 8048. doi: 10.1029/2000JD000265.

    • Search Google Scholar
    • Export Citation
  • Poveda, G., 1994: Empirical orthogonal functions in the relationship between river streamflows and sea surface temperatures in the Pacific and Atlantic Oceans. Proc. XVI Latin American Hydraulics and Hydrology Meeting (in Spanish), Santiago, Chile, IAHR, 131–144.

  • Poveda, G., and O. J. Mesa, 1997: Feedbacks between hydrological processes in tropical South America and large-scale oceanic–atmospheric phenomena. J. Climate, 10 , 26902702.

    • Search Google Scholar
    • Export Citation
  • Poveda, G., and A. Jaramillo, 2000: ENSO-related variability of river discharges and soil moisture in Colombia. Biospheric Aspects of the Hydrologic Cycle, No. 8, IGBP, 3–6.

    • Search Google Scholar
    • Export Citation
  • Poveda, G., and O. J. Mesa, 2000: On the existence of Lloró (the rainiest locality on earth): Enhanced ocean–atmosphere–land interaction by a low-level jet. Geophys. Res. Lett., 27 , 16751678.

    • Search Google Scholar
    • Export Citation
  • Poveda, G., A. Jaramillo, M. M. Gil, N. Quiceno, and R. I. Mantilla, 2001: Seasonality in ENSO related precipitation, river discharges, soil moisture, and vegetation index (NDVI) in Colombia. Water Resour. Res., 37 , 21692178.

    • Search Google Scholar
    • Export Citation
  • Press, W. H., B. P. Flannery, P. Brian, S. Teukolsky, and W. T. Vetterling, 1986: Numerical Recipes: The Art of Scientific Computing. Cambridge University Press, 818 pp.

    • Search Google Scholar
    • Export Citation
  • Ricciardulli, L., and P. D. Sardeshmukh, 2002: Local time- and space scales of organized tropical deep convection. J. Climate, 15 , 27752790.

    • Search Google Scholar
    • Export Citation
  • Snow, J. W., 1976: The climate of northern South America. Climates of Central and South America, W. Schwerdtfeger, Ed., Elsevier, 295–403.

    • Search Google Scholar
    • Export Citation
  • Soden, B., 2000: The diurnal cycle of convection, clouds and water vapor in the tropical upper troposphere. Geophys. Res. Lett., 27 , 21732176.

    • Search Google Scholar
    • Export Citation
  • Sorooshian, S., X. Gao, K. Hsu, R. A. Maddox, Y. Hong, H. V. Gupta, and B. Imam, 2002: Diurnal variability of tropical rainfall retrieved from combined GOES and TRMM satellite information. J. Climate, 15 , 9831001.

    • Search Google Scholar
    • Export Citation
  • Torrence, C., and G. P. Compo, 1998: A practical guide to wavelet analysis. Bull. Amer. Meteor. Soc., 79 , 6178.

  • Trojer, H., 1959: Fundamentos para una zonificación meteorológica y climatológica del trópico y especialmente de Colombia. Rev. Cenicafé, 10 , 288373.

    • Search Google Scholar
    • Export Citation
  • Velasco, I., and M. Frisch, 1987: Mesoscale convective complexes in the Americas. J. Geophys. Res., 92 , D8,. 95919613.

  • Warner, T. T., B. E. Mapes, and M. Xu, 2003: Diurnal patterns of rainfall in northwestern South America. Part II: Model simulations. Mon. Wea. Rev., 131 , 813829.

    • Search Google Scholar
    • Export Citation
  • Waylen, P. R., and G. Poveda, 2002: El Niño–Southern Oscillation and aspects of western South America hydro-climatology. Hydrol. Processes, 16 , 12471260.

    • Search Google Scholar
    • Export Citation
  • Yang, G. Y., and J. Slingo, 2001: The diurnal cycle in the Tropics. Mon. Wea. Rev., 129 , 784801.

  • Zulauga, M. D., and G. Poveda, 2004: Diagnostics of mesoscale convective systems in Colombia and the eastern Pacific during 1998–2002 using TRMM data (in Spanish). Av. Recur. Hidrául., in press.

Fig. 1.
Fig. 1.

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

Citation: Monthly Weather Review 133, 1; 10.1175/MWR-2853.1

Fig. 2.
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.

Citation: Monthly Weather Review 133, 1; 10.1175/MWR-2853.1

Fig. 3.
Fig. 3.

Seasonal distribution of the relationship between altitude above sea level and the timing of rainfall diurnal maximum, for the entire dataset.

Citation: Monthly Weather Review 133, 1; 10.1175/MWR-2853.1

Fig. 4.
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 h−1 (right axis), and cumulative precipitation is in mm (left axis).

Citation: Monthly Weather Review 133, 1; 10.1175/MWR-2853.1

Fig. 5.
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.

Citation: Monthly Weather Review 133, 1; 10.1175/MWR-2853.1

Fig. 6.
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.

Citation: Monthly Weather Review 133, 1; 10.1175/MWR-2853.1

Table 1.

Location and details of the 51 rain gauges.

Table 1.
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