Squall lines and turbulent exchange at the Amazon forest-atmosphere interface

Abstract

Squall lines (SLs) are convective phenomena frequently occurring in the tropical atmosphere and have been widely investigated by the scientific community. In this work, satellite images of the central Amazon region were used to identify the occurrence of SLs between 2014, considered as a typical year, and 2015, characterize by a strong El Nino. Subsequently, fast response data from the Amazon Tall Tower Observatory (ATTO) site were used to establish the effects of SLs on surface parameters and the differences between the typical and strong El Niño years. The objective of this study was to investigate whether there is an influence in El Niño years on the number of occurrences of SLs and consequently on the impact that these phenomena exert on the variables, such as: precipitation, temperature, relative humidity, radiation and turbulent fluxes calculated by the eddy covariance method. Average daily cycles of these variables were used for different seasons (dry and rainy) for both years. When SLs were detected, increasing in (i) precipitation rates; (ii) wind speed; (iii) relative humidity; and (2) decreasing in (i) air temperature; (ii) shortwave radiation; (iii) sensible and latent heat flux were observed. The CO₂ flux, on the other hand, reversed its sign during the presence of SLs, in both observed years. The influence of the El Niño phenomenon in the SLs formation and their impact on the meteorological quantities (turbulent fluxes and thermodynamics variables) measured just above the canopy top is discussed.