Resumen.

p. 9-[13]

We are happy to acknowledge the support of Commandante Hector Soldi (Expedition Leader) and Teniente Italo Huertas (Base Commander) in insuring the installation and ongoing support of Peru's Machu Picchu Base. Additional support from both Peru's Comision Nacional de Asuntos Antartidos (CONAAN) and Consejo Nacional de Ciencias y Technologia (CONCYTEC), the Argentinean Navy, and the Uraguayan Air Force are greatly appreciated. The untiring assistance of S. Villegas and J. Fernandez, who have been associated with this project since its inception, is also gratefully acknowledged. This research is based upon work supported by NSF's Office of Polar Programs under Grant #DPP 922446.

We report the results of a recent study to examine possible causal factors that could explain the observed differences in PMSE intensity in the northern and southern hemispheres. A variety of satellite data, models and ground‐based radar observations in the two hemispheres during local summer have been examined in this attempt. We will show results of inter‐hemispheric comparisons of temperatures, winds, and water vapor. The primary conclusions that can be drawn from these studies are that, while water vapor differences in the two hemispheres are difficult to interpret at near‐mesopause altitudes, the inter‐hemispheric temperature comparisons show clearly that the southern hemispheric mesopause is indeed a few degrees warmer. Moreover, southern summer mesospheric mean winds are considerably weaker than they are in the north. These results provide support for earlier speculations ...

8 p.

The Jicamarca 50-MHz radar was used to obtain backscattered echoes through the entire atmosphere (0-100 km). Previous radar observations at VHF have shown a gap near the 35-55 km region, where the lack of echoes is attributed to a lack of radar sensitivity. By using the full Jicamarca antenna array and maximun transmitter power and by optimizing transmitter duty cycle through the implementation of complementary codes, doppler measurements display discernable peaks in the computed spectra from atmospheric signals in the gap region as well as above and below it. The results indicate that a minimum power-aperture threshold in excess of 1010 W-m2 is required in order to obtain backscatter echoes near 50 MHz through the entire neutral atmosphere.

Vertical velocity measurements of atmospheric winds are becoming increasingly important. To obtain the long-term mean vertical motion of the atmosphere is difficult to do, both because of the small velocities involved and because of the finite width of the antenna beam. Since the vertical wind measurement is very sensitive to the tilt of the vertically-directed antenna beam, we decided to get the Jicamarca beam direction by measuring the phase and amplitude of a ref­erence signal at each of the antenna feed points. Our results tell us that, while the antenna pointing direction is already good, there is room for improvement.

Since the last MST Radar Workshop in Kyoto, in November 1988, there has been substantial progress made toward implementation of a Trans-Pacific network of wind profilers (Gage et al, 1989, 1990). As of June 1991, the network is virtually complete with the exception of the 50 MHz wind profiler that is under construction at Biak, Indonesia. The original network of 50 MHz wind-profiling Doppler radars is being funded by NSF and TOGA to support fundamental research on tropical convection, equatorial waves, and large-scale tropical circulation systems. The network as originally conceived was to be comprised of four VHF radars: Piura, Peru; Christmas Island, Kirabati; Pohnpei, Federated States of Micronesia; and Biak, Indonesia. The first three wind profilers are virtually completed and the Biak wind profiler will be installed as soon as site preparations are completed.

Since the Sixth MST Radar Workshop held at National Central University, Chung-Li, Taiwan, in August 1993, the NOAA/CU Network of VHF wind profilers has been operated in a routine mode. The network is comprised of VHF wind profilers located at Piura, Peru; Christrnas lsland, Kiribati; Pohnpei, Federated States of Micronesia; and Biak, Indonesia. The Biak wind profiler was completed in early 1992 and has been in continuous operation since March 1992. In addition to the VHF wind profilers named above a 50 MHz wind profiler is operated in Darwin, Australia through a cooperative research agreement between NOAA's Aeronomy Laboratory and the Australian Bureau of Meteorology's Research Centre. The VHF profiler network was described in more detail by Gage et al, (1990) and the VHF profilers are described in Gage et al. (1994a). In addition to the 50 MHz wind profilers, several sites have been ins...