Diurnal cycle of rainfall over the Brazilian Amazon

Abstract

The rainfall characteristics over the Brazilian Amazon were analysed using 3 yr of 3 h data from 24 rain gauges. During this period, areas in the eastern sector experienced up to 22% of rainy hours, while those in the southern sector recorded up to 7%. In the northern Amazon, rain tends to occur during the night, while in southern, central and central-western Amazon it occurs mainly in the afternoon. In contrast, over the eastern Amazon, rain occurs late at night and early in the morning. Harmonic analysis revealed an increase in the phase of the first harmonic phase from eastern towards central Amazon, suggesting that the rain propagates in that direction. The amplitude of the first and second harmonics revealed the existence of a relatively dry area inland about 500 km from the Atlantic coast. This area seems to be the limit for the inland penetration of systems originating at the coast. The phase of the first harmonic revealed small differences between the diurnal cycles of rainfall during the wet and dry seasons, suggesting that there is no difference between low-level circulations in either season. KEY WORDS: Amazon · Diurnal cycle · Rainfall · Precipitation · Rain gauge Resale or republication not permitted without written consent of the publisherClim Res 26: [139][140][141][142][143][144][145][146][147][148][149] 2004 noon, from about 15:00 to 19:00 h LST (Dai 2001). The phase of convective activity over land in the South and Central Americas, South and Central Africa, India and Borneo is generally from late afternoon to evening (Houze et al. 1981, Hendon & Woodberry 1993, Yang & Slingo 2001. Specifically over the eastern coast of the Amazon, it has been found that peaks of rainfall tend to occur at 05:00 h LST in Marajó Island and at 17:00 h LST in Belém (Kousky 1980). In the central Amazon, near the convergence between the Amazonas and Negro rivers and over the Reserva Florestal Ducke, located at 25 km from Manaus, peaks of precipitation occur at 12:00 and 15:00 h LST, respectively (Lloyd 1990, Cutrim et al. 2000.Despite the low rain-gauge density over the Amazon Basin, the pluviometers in that area are sufficient to allow evaluation of some rainfall characteristics and also to be very useful as ground truth for satellite applications. The main purpose of the present study is to analyse 3 yr of previously unavailable rain-gauge data over the Brazilian Amazon so as to increase knowledge of the diurnal variations of rainfall over this area. DATA AND ANALYSIS METHODThe nature of the diurnal rainfall cycle was established using harmonic analysis, which yields information about the amplitude and phase (timing) of the peak in a time series. The methodology followed is that outlined by Wilks (1995). Accordingly, rainfall is modelled at time t in a series y by the harmonic function:where y is the mean of the series, n is the period, C is the amplitude, φ is the phase and k is the harmonic number. The amplitude C is estimated by:(2) where A 1 = 2/n × Σy t cos(2πt/n) (3)The phase can then be derived...

Characteristics and Diurnal Cycle of GPM Rainfall Estimates over the Central Amazon Region

“…The S-band SIPAM radar mean values and peak hours during the wet (dry) period at 10:00-13:00 UTC (15:00-21:00 UTC) over the Manaus region corroborate the results by Tota et al, Angelis et al, Machado et al and Tanaka et al [29,33,34,61]. Tota et al [61] attributed the maximum frequency around 19:00 UTC (15:00 Local Time) during the dry season to isolated convective activity and the secondary (around 08:00 UTC) to organized mesoscale convective systems.…”

“…Tota et al [61] attributed the maximum frequency around 19:00 UTC (15:00 Local Time) during the dry season to isolated convective activity and the secondary (around 08:00 UTC) to organized mesoscale convective systems. Angelis et al, Machado et al and Tanaka et al [29,33,34] showed that rainfall frequencies and intensities vary over the study region and that primary peaks may change according to surface type (forest, city, among others). [29,33,34] showed that rainfall frequencies and intensities vary over the study region and that primary peaks may change according to surface type (forest, city, among others).…”

“…Angelis et al, Machado et al and Tanaka et al [29,33,34] showed that rainfall frequencies and intensities vary over the study region and that primary peaks may change according to surface type (forest, city, among others). [29,33,34] showed that rainfall frequencies and intensities vary over the study region and that primary peaks may change according to surface type (forest, city, among others). Figure 5b shows box plots of rain rate differences (in mm¨h´1) between IMERG and S-band SIPAM radar observations.…”

“…Thirdly, the synoptic scale Intertropical Convergence Zone (ITCZ) and the influence of biomass aerosol [32] are the major weather systems/conditions responsible for affecting the rainfall frequencies and intensities and cloud cover (lowest in July and August and highest in February and March). Thus, several precipitation systems and local factors are responsible for modulating the diurnal cycle of precipitation (often leading to late night or early morning precipitation in Manaus) and for controlling the wet-dry seasons in the region around Manaus [33,34].…”

The “Weekend Effect” on Ozone in the Warsaw Conurbation, Poland

“…Harmonic analysis is commonly used in the atmospheric sciences to identify the significance of cycles through a defined, fundamental period. Studies of the diurnal cycle of atmospheric phenomena, often precipitation, define the fundamental period as 24 h [18]. Studies such as ours define the fundamental period as seven days in the week [19].…”