Abstract
Abstract. In this study, high-frequency, multilevel measurements, performed from late October to mid-November of 2015 at a 80 m tall tower of the Amazon Tall Tower Observatory (ATTO) project in the central state of Amazonas, Brazil, were used to diagnose the evolution of thermodynamic and kinematic variables as well as scalar fluxes during the passage of outflows generated by deep moist convection (DMC). Outflow associated with DMC activity over or near the tall tower was identified through the analysis of storm echoes in base reflectivity data from an S-band weather radar at Manaus, combined with the detection of gust fronts and cold pools utilizing tower data. Four outflow events were selected, three of which took place during the early evening transition or nighttime hours and one during the early afternoon. Results show that the magnitude of the drop in virtual potential temperature and changes in wind velocity during outflow passages vary according to the type, organization, and life cycle of the convective storm. The nocturnal events had well-defined gust fronts with moderate decreases in virtual potential temperature and increases in wind speed. The early afternoon event lacked a sharp gust front and only a gradual drop in virtual potential temperature was observed, probably because of weak or undeveloped outflow. Sensible heat flux (H) increased at the time of the gust front arrival, which was possibly due to the sinking of colder air. This was followed by a prolonged period of negative H, associated with enhanced nocturnal negative H in the wake of the storms. In turn, increased latent heat flux (LE) was observed following the gust front, owing to drier air coming from the outflow; however, malfunctioning of the moisture sensors during rain precluded a better assessment of this variable. Substantial enhancements of turbulent kinetic energy (TKE) were observed during and after the gust front passage, with values comparable to those measured in grass fire experiments, evidencing the highly turbulent character of convective outflows. The early afternoon event displayed slight decreases in the aforementioned quantities in the passage of the outflow. Finally, a conceptual model of the time evolution of H in nocturnal convective outflows observed at the tower site is presented.
Highlights
Deep moist convection (DMC) is a ubiquitous feature of the atmospheric environment of the Amazon rainforest
In this study we have chosen not to apply any threshold to θv or Vh variations to detect a storm outflow but to subjectively select those events that displayed noticeable perturbations in the temperature and wind fields at the time of the storm occurrence. This choice was motivated by (a) our interest in evaluating DMC events that influenced the evolution of planetary boundary layer (PBL) properties through their outflows, regardless of the magnitude of the temperature and wind variations produced by them; (b) the recognition that perturbations associated with convective storms were easy to identify, as they represented drastic interference in the mean evolution of PBL quantities; and (c) the short period of study, which did not demand defining a set of objective criteria, as would be the case for large datasets as in Addis et al (1984)
The time evolution of atmospheric variables and scalar fluxes during the occurrence of surface outflows produced by deep convective storms in a tropical rainforest was analyzed utilizing high-frequency, multilevel measurements performed at the 80 m walk-up tower of the Amazon Tall Tower Observatory (ATTO) located in northern Brazil
Summary
Deep moist convection (DMC) is a ubiquitous feature of the atmospheric environment of the Amazon rainforest. Scalar flux enhancements in DMC situations were further investigated by Saxen and Rutledge (1998), SR98, who computed surface fluxes from meteorological data measured by an instrumented buoy as part of the Coupled Ocean-Atmosphere Response Experiment (COARE) of the Tropical Ocean Global Atmosphere (TOGA) project that took place in the western Pacific warm pool from November 1992 through March 1993 They found sensible and latent heat flux enhancements reaching peak values of 60 and 250 W m−2 for large, organized mesoscale convective systems (MCSs). Outflows can promote sudden increases of ozone concentration in the PBL through downward transport of mid-tropospheric ozone-rich air by storm downdrafts (Betts et al, 2002; Gerken et al, 2016) In this context, high-frequency tower measurements performed at the Amazon Tall Tower Observatory (ATTO) (Andreae et al, 2015) experiment site provide an excellent way to assess the impacts of tropical DMC on the mean evolution of the PBL in the Amazon rainforest.
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