Directly observing boundary layer trajectories in the Eastern Amazon

Pilot study observations. To test our ability to operate altitude-controllable meteorological balloons in the Amazon Basin, two light-weight controlled meteorological balloons (CMET; developed by Voss; 146 g packages) equipped with altitude control, real time satellite communication, and various sensors (aspirated thermometers, humidity and pressure) were launched twelve hours apart on June 25, 2011 from Tomé-Açu Pará, Brazil (1.96°S; 48.20°W), part of the CHUVA-Belém project. Armed with very good numerical trajectory guidance (Freitas et al., 2000, method), we believed that the balloons would move to the NW and then follow the main channel of the Amazon River into the Basin. Instead, each balloon moved about 40° SW of the predicted line. Each balloon remained aloft about 8 hours and traveled approximately 160 km until each encountered its own strong storm, whose severe downdrafts forced them into the Tocantins River about 8 km apart. Surprisingly, given such a vast sparsely inhabited region, both balloons were recovered thanks to help from a local fisherwoman.

Supporting information/analysis. In this presentation we discuss the mesoscale environment into which the balloon were launched, taking advantage of special observations being made simultaneously at other sites also part of CHUVA-Belém. These included radars, radiosonde launches, and a variety of surface-based microphysical observations. Post project analysis included selected mesoscale model simulations. In particular, we examined the importance of a growing convective cluster to the south of Belém that distorted the regional pressure gradient field and, by inference, its vertical profile. We report on how clouds evident in the GOES image sequence appeared to alter the balloons' trajectories.

Plans. We intend this work to be a precursor to a more comprehensive observation/model campaigns aimed to understand better the role of transient pressure gradients in altering low-level trajectories in the equatorial continental tropics. Further, the sounding capability of the CMET balloons will be an asset in evaluating the downwind evolution of the contaminants, work we will propose for the upcoming GO-Amazon project.

Reflection. Our results should counsel prudence when using readily available trajectories estimated from reanalysis databases to understand airmass source regions in the often sounding-poor tropics, especially for case studies involving a set of selected days.

Freitas, S. R., S. Dias, P. L. . Dias, K. M. Longo, P. Artaxo, M. O. Andreae, and H. Fischer, 2000: A convective kinematic trajectory technique for low-resolution atmospheric models. Journal of Geophysical Research, 105, 24375–24386.