Contemporary measurements of a ground-based weather radar and balloon-borne lidar at Bauru during the HIBISCUS campaigns: a powerful synergy in cloud physics studies

During February 2003 and February 2004 two balloon campaigns have been performed in the central State of São Paulo in Bauru (Brazil) in the framework of the European project HIBISCUS, which is a project studying the impact of tropical convection on the upper troposphere and lower stratosphere. Among several stratospheric flights, two of them had as payload a miniaturized optical radar (LIDAR), taking continuous measurements of clouds below the stratospheric floating platform. The balloons were launched during late afternoon, around 17:00 to 18:00 LT (LT = UT-3h) and generally reached their cruising level at between 20 and 23 km height about one after the launch, drifting with the easterly stratospheric winds towards the western part of the State for 3 – 5 hours, when they started a slow descent through the lower stratosphere and upper troposphere.

The lidar observations of such flights are compared with contemporary measurements obtained by a ground-based meteorological radar operated by IPMet. The S-band Doppler radar at Bauru is being operated on a continuous basis, with volume scans between 0.3º and 34.9º elevation (11 PPI scans) at 7.5 min intervals, up to a range of 240 km. The beam width is 2º. For this comparison, the radar data were analyzed in 3 dB intervals, starting at –18 dBZ in order to attempt the detection of thick cirrus clouds. Although the sensitivity of the radar is nominally –32 dBZ at the radar, it decreases rapidly with distance, as expected, viz. –18 dBZ can be seen up to about 15 km radius, while –15 to –12 dBZ can be detected up to 20 – 25 km. Another limitation is caused by the highest elevation being 34.9º, thus cutting nearby echoes at about 10-12.5 km height. This is more than ample for operational purposes, but in this specific application a vertically scanning radar would have been preferable. However, with careful extrapolation, based on the vertical reflectivity gradient, the base of the cirrus clouds could usually be estimated.

The first case study is the flight VOL0503 on Feb 19th 2003 . The flight pattern was first towards east-north-east while ascending, then it changed to west-south-west reaching the stratospheric easterly winds at 23 km. While floating, it passed almost over the Bauru radar at a distance of few km to the north, finally traveling towards west and descending far away from Bauru (>100 km). Thick cirrus at 11 km was detected by the lidar in the very first part of the descent. Then the cirrus disappeared for several kilometers and appeared again at the same level during the last part of the flight, but too far for allowing a detection by the Bauru radar.

The second case study is the flight SF4 on Feb 24th 2004. The balloon was launched from Bauru and reached an altitude of 20.2 km, shortly before sunset, followed by a 3½ hour float and slow descent (initially westwards, then veering to southeast, down to 10.7 km, when it was cut down. The total flight time was 4 hours 52 minutes. The lidar detected two layers of cirrus clouds, at 13 and 11 km, respectively. The trajectory analysis performed at different levels indicated that the two layers had different origins, the higher one being generated by a convective outflow and the lower one by a more recent air mass up-lift. This will be investigated further, using the radar observations of reflectivity and radial velocities, as well as streamline analysis at various levels.

Although the direct comparisons between the lidar and radar in these two case studies did not show clear results, the capability of the radar to observe the 3-dimensional pattern of the tropospheric clouds below the floating platform appeared as a powerful tool in modeling the formation of cirrus clouds as detected by lidar.