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The purpose of the International Global Precipitation Measurement (GPM) Program is to develop a next-generation space-based measuring system which can fulfill the requirements for frequent, global, and accurate precipitation measurements. The associated GPM Mission is being developed as an international collaboration of space agencies, weather and hydrometeorological forecast services, research institutions, and individual scientists. The design and development of the GPM Mission is an outgrowth of valuable knowledge and published findings enabled by the Tropical Rainfall Measurement Mission (TRMM). From the TRMM experience, it was recognized that the GPM Mission must consist of a mixed nonsunsynchronous and sunsynchronous orbiting satellite constellation in order to have the capability to provide physically based retrievals on a global basis, with ~3-h sampling assured at any given Earth coordinate ~90% of the time. The heart of the GPM constellation is the Core satellite, under joint development by NASA and the Japan Aerospace Exploration Agency (JAXA), which will carry a dual frequency Ku/Kaband precipitation radar (PR) and a high-resolution, multichannel passive microwave (PMW) rain radiometer. The core is required to serve as the calibration reference system and the fundamental microphysics probe to enable an integrated measuring system made up of additional constellationsupport satellites, each carrying at a minimum some type of PMW radiometer. In this article the background, planning, design, and implementation of the GPM is described.

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[1] The morphology and evolution of the stratospheric ozone (O 3 ) distribution at high latitudes in the Northern Hemisphere (NH) are examined for the late summer and fall seasons of 1999. This time period sets the O 3 initial condition for the SOLVE/THESEO field mission performed during winter 1999/2000. In situ and satellite data are used along with a three-dimensional model of chemistry and transport (CTM) to determine the key processes that control the distribution of O 3 in the lower-to-middle stratosphere. O 3 in the vortex at the beginning of the winter season in late November is found to be nearly constant from 500 to above 800 K with a value at 3 ppmv ± $10%. Values outside the vortex above 550 K are up to a factor of 2 higher and increase significantly with potential temperature. The seasonal time series of data from POAM shows that the relatively low O 3 mixing ratios, which characterize the vortex in late November, are already present at high latitudes at the end of summer in mid-September before the vortex circulation sets up. Analysis of the CTM output shows that the minimum O 3 and increase in variance in the middle stratosphere in late summer are the result of (1) stirring of polar concentric O 3 gradients by nascent wave-driven transport and (2) an acceleration of net photochemical loss with decreasing solar illumination. The segregation of low O 3 mixing ratios into the vortex as the circulation strengthens through the fall suggests a possible feedback role between O 3 chemistry and the vortex formation dynamics that may need to be better understood in order to make confident predictions of the recovery of NH O 3 at high latitudes.

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