Standard Amazonian rainfall climatologies rely on stations preferentially located near river margins. River breeze circulations that tend to suppress afternoon rainfall near the river and enhance it inland are not typically considered when reporting results. Previous studies found surprising nocturnal rainfall maxima near the rivers in some locations. We examine spatial and temporal rainfall variability in the Santarém region of the Tapajós-Amazon confluence, seeking to describe the importance of breeze effects on afternoon precipitation and defining the areal extent of nocturnal rainfall maxima. We used three years of mean S band radar reflectivity from Santarém airport with a Z-R relationship appropriate for tropical convective conditions. These data were complemented by TRMM satellite rainfall estimates. Nocturnal rainfall was enhanced along the Amazon River, consistent with the hypothesis that these are associated with the passage of instability lines, perhaps enhanced by local channeling and by land breeze convergence. In the daytime, two rainfall bands appear in mean results, along the east bank of the Tapajós River and to the south of the Amazon River, respectively. Keywords : Precipitation, River Breeze, Instability Lines. RESUMO: VARIABILIDADE ESPACIAL E TEMPORAL DA PRECIPITAÇÃOOBSERVADA COM O RADAR NA CONFLUENCIA DO TAPAJÓS E AMAZONAS Na Amazônia as series climatológicas estão baseadas na rede de estações meteorológicas de superfície que estão instaladas preferencialmente nas margens de rios. Portanto, o efeito de brisas fluviais que promovem precipitação no interior do continente não está sendo contabilizado nas medidas realizadas nestas estações. Além disto, estudos anteriores encontraram máximo de precipitação no período noturno sobre rios. Assim, este artigo examina a variabilidade espacial e temporal da precipitação na região de confluência dos rios Tapajós e Amazonas em Santarém, visando descrever a importância da brisa fluvial na distribuição da precipitação durante a tarde e definir a extensão da área onde ocorre maxima precipitação no período noturno. A refletividade medida pelo radar meteorológico banda S, instalado em Santarém para um período de três anos foi utilizada, tendo estes dados sido calibrados através dos dados do sensor de precipitação instalado abordo do satélite TRMM. A intensidade de precipitação foi estimada utilizando a relação ZR para o caso convectivo tropical. No período noturno, a máxima precipitação foi encontrada ao longo do rio Amazonas, consistente com a hipótese de que esta precipitação estaria associada a passagem das linhas de instabilidade neste período, talvez esta precipitação também esteja sendo reforçada pela canalização do rio e pela confluência do vento de retorno da brisa fluvial durante a noite. Para o período diurno, observou-se duas bandas de precipitações localizadas no interior do continente, uma ao longo da margem leste e outra ao sul dos rios Tapajós e Amazonas, respectivamente. Palavras-chave: Precipitação, Brisa Fluvial, Linhas de Instabili...

The height (zi) of the Atmospheric Boundary Layer (ABL) is a fundamental parameter for several areas of knowledge, especially for weather and climate forecasting, pollutant dispersion and air quality. In this work, we used data from a remote sensing instrument (ceilometer), located at the experimental site of the Amazon Tall Tower Observatory (ATTO) in the Central Amazonia rainforest, in order to obtain the height of the ABL. Data used were obtained from 2014 to 2020, with the exception of the year 2017. The results showed that the zi average varies from year to year (interannual variability) and the average of the maximum zi values (zi_max) was approximately 1400 ± 277 m, occurring at 15:00 local time. In addition, it was found that these maximum heights are higher in the dry season and during El Niño years (about 1741 ± 242 m) and they are lower during the wet period and in La Niña years (1263 ± 229 m). Taking into account all the years investigated, the month with the highest zi_max value is September (1710 ± 253 m), and the month with the lowest value is May (1108 ± 152 m). Finally, it was observed that the growth rate of the ABL during the early hours after sunrise varies from month to month (intraseasonal variability), reaching its maximum values in September and October (about 210 ± 53 m h−1 and 217 ± 59 m h−1, respectively) and minimum values in April and July (approximately 159 ± 48 m h−1 and 159 ± 50 m h−1, respectively). It is concluded that the values of zi in Central Amazonia are influenced by several seasonal factors (temperature, cloud cover, turbulent heat flux, etc.) which gives it a wide variability in terms of heights and growth rates. Additionally, a linear regression was proposed in order to model the maximum zi value as a function of its growth rate from 08:00 LT (Local Time) up to 10:00 LT. The results showed a good correlation compared with the experimental values.

The atmospheric boundary layer height (zi) is a key parameter in the vertical transport of mass, energy, moisture, and chemical species between the surface and the free atmosphere. There is a lack of long-term and continuous observations of zi, however, particularly for remote regions, such as the Amazon forest. Reanalysis products, such as ERA5, can fill this gap by providing temporally and spatially resolved information on zi. In this work, we evaluate the ERA5 estimates of zi (zi-ERA5) for two locations in the Amazon and corrected them by means of ceilometer, radiosondes, and SODAR measurements (zi-experimental). The experimental data were obtained at the remote Amazon Tall Tower Observatory (ATTO) with its pristine tropical forest cover and the T3 site downwind of the city of Manaus with a mixture of forest (63%), pasture (17%), and rivers (20%). We focus on the rather typical year 2014 and the El Niño year 2015. The comparison of the experimental vs. ERA5 zi data yielded the following results: (i) zi-ERA5 underestimates zi-experimental daytime at the T3 site for both years 2014 (30%, underestimate) and 2015 (15%, underestimate); (ii) zi-ERA5 overestimates zi-experimental daytime at ATTO site (12%, overestimate); (iii) during nighttime, no significant correlation between the zi-experimental and zi-ERA5 was observed. Based on these findings, we propose a correction for the daytime zi-ERA5, for both sites and for both years, which yields a better agreement between experimental and ERA5 data. These results and corrections are relevant for studies at ATTO and the T3 site and can likely also be applied at further locations in the Amazon.

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