Ground and Airborne Validation Plans for GPM in the Central State of São Paulo, Brazil

GPM (Global Precipitation Mission), which will be the successor of TRMM (Tropical Rainfall Measuring Mission), represents an invaluable asset for precipitation estimates in the global scale, but is also of primary importance for the tropics. The latter is specially true for GPM, since it comprises a constellation of satellites, thus reducing the time required for mapping the global precipitation distribution to 3 hours. This is a crucial issue, particularly for tropical areas, and GPM will substantially mitigate the problem of quantification of convective rainfall. Since Central São Paulo State is situated in the tropics, with a high variability in rainfall distribution, due to its predominantly convective nature, it will draw much benefit from the GPM. Therefore, validation efforts form an important role in the research activities of the Meteorological Research Institute (IPMet), which already has more than three decades of experience in rainfall measurements by radar, as well as corresponding processing and analysis methods.

A unique opportunity to become more involved in the validation of satellite observations opened, when two EU-funded international projects, viz., HIBISCUS (a project on “Impact of tropical convection on the upper troposphere and lower stratosphere”) and TROCCINOX (Tropical Convection, Cirrus and Nitrogen Oxides), joined the Brazilian TroCCiBras (Tropical Convection & Cirrus Brasil) Project to conduct a major field experiment in the State of São Paulo early in 2004. TroCCiBras comprises a variety of sub-projects proposed by the major atmospheric sciences specialist research groups in Brazil. The two main components, constituting the data acquisition, were ground-based sensors and airborne platforms, e.g., specifically instrumented aircraft and stratospheric balloons.

The ground segment involved operational observations by two meteorological radars (S-band Doppler; volume scans every 7.5 min), rain gages and Automatic Weather Stations, as well as dedicated instruments operating for a limited period of time, during field campaigns in support of special research programs. This is the case with aerosol lidars (e.g., the elastic backscattering Lidar of IPEN, the “Instituto de Pesquisas Energéticas e Nucleares”), radiosondes and small networks of experimental lightning detectors. Due, mainly, to both logistic and technology transfer difficulties, IPMet developed a policy of international cooperation for joint experiments in Brazil, which allows that specific measurements of high value, essential for sophisticated research projects, but for practical or financial reasons unattainable in Brazil, are obtained with instrumentation temporarily imported by the foreign partner(s).

With respect to the ground-based observations, the outlook for 2008 and beyond, when GPM operations are supposed to become effective, envisages the incorporation of a tropospheric wind profiler with RASS and an acoustic sounder (Sodar), both based in Bauru, into the IPMet Observation system. Also incorporated in this system will be oceanic radars (with an X-band meteorological Radar), a microwave radiometer and a network of GPS stations for obtaining regular humidity profiles throughout the atmosphere, and possibly other instrumentation, such as a Dual X-pol radar and a lower-tropopause wind profiler in Metropolitan São Paulo. The radars will all be integrated in real-time by IPMet. The airborne segment has been based on stratospheric, short-term experimental balloons on one hand, and low-, intermediate- and high- (lower stratosphere) level laboratory aircraft, on the other. Six balloon campaigns have been carried out since 1995 until 2004, primarily in partnership with the French CNRS (Centre National de la Recherche Scientifique) and CNES (Centre National d'Études Spatiales). In 2004 and 2005, two outstanding campaigns were effected within the context of the EC project TROCCINOX, through the Brazilian project TroCCiBras, based on a low-level Embraer Bandeirante, a mid-level Mystère Falcon and a low stratosphere Russian M-55 (Geophysica) aircraft. In general, data obtained is suitable for satellite validation. This is the case with the ENVISAT and ICESAT spacecrafts, for which dedicated flights had been planned and executed. A possible mission using an upper-troposphere aircraft is being considered for 2008, and might provide measurements for GPM validation. The possibility of observations of particle distributions aloft, combined with microwave radiometry from low tropospheric heights, is an outstanding example of measurements of particular value in the context of validation of microwave sensors onboard the constellation satellites.

This paper describes the present and near-future ground instruments, main aspects of selected past experiment, and their potential for GPM ground validations (GV). Aspects of data format, storage and access, will be considered. Instruments typically flown with the stratospheric balloons and their respective accuracies will be discussed. In the case of the aircraft-based campaigns, conducted so far during 2004 and 2005, the specific sensors flown in each plane are listed, focusing on those used for satellite validation.

A brief preliminary, qualitative only, verification of cloud height estimates, as performed by the Geophysica pilot by eye and the on-board lidar (MAL 2, Miniature Aerosol Lidar Mark 2, downward-looking, measuring backscatter and depolarization ratios of subvisible clouds and the top of opaque clouds), as well as a comparison with the S-band Bauru radar, will be presented.