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This paper reviews the basis of passive microwave algorithms that derive rainfall rates directly from relationships between brightness temperatures and rainfall rates established by statistical relationships and empirical calibration. The performance of these algorithms and their present and future roles are assessed in comparison with the increasing number of modeling techniques used for passive microwave rainfall retrievals.

The second WetNet Precipitation Intercomparison Project (PIP-2) evaluates the performance of 20 satellite precipitation retrieval algorithms, implemented for application with Special Sensor Microwave/Imager (SSM/I) passive microwave (PMW) measurements and run for a set of rainfall case studies at full resolution instantaneous space-timescales. The cases are drawn from over the globe during all seasons, for a period of 7 yr, over a 60 [degrees] N - 17 [degrees] S latitude range. Ground-based data were used for the intercomparisons, principally based on radar measurements but also including rain gauge measurements. The goals of PIP-2 are 1) to improve performance and accuracy of different SSM/I algorithms at full resolution-instantaneous scales by seeking a better understanding of the relationship between microphysical signatures in the PMW measurements and physical laws employed in the algorithms; 2) to evaluate the pros and cons of individual algorithms and their subsystems in order to seek optimal 'front-end' combined algorithms; and 3) to demonstrate that PMW algorithms generate acceptable instantaneous rain estimates. It is found that the bias uncertainty of many current PMW algorithms is on the order of [+ or -]30%. This level is below that of the radar and rain gauge data specially collected for the study, so that it is not possible to objectively select a best algorithm based on the ground data validation approach. By decomposing the intercomparisons into effects due to rain detection (screening) and effects due to brightness temperature-rain rate conversion, differences among the algorithms are partitioned by rain area and rain intensity. For ocean, the screening differences mainly affect the light rain rates, which do not contribute significantly to area-averaged rain rates. The major sources of differences in mean rain rates between individual algorithms stem from differences in how intense rain rates are calculated and the maximum rain rate allowed by a given algorithm. The general method of solution is not necessarily the determining factor in creating systematic rain-rate differences among groups of algorithms, as we find that the severity of the screen is the dominant factor in producing systematic group differences among land algorithms, while the input channel selection is the dominant factor in producing systematic group differences among ocean algorithms. The significance of these issues are examined through what is called 'fan map' analysis. The paper concludes with a discussion on the role of intercomparison projects in seeking improvements to algorithms, and a suggestion on why moving beyond the 'ground truth' validation approach by use of a calibration-quality forward model would be a step forward in seeking objective evaluation of individual algorithm performance and optimal algorithm design.

In this study we measured pressure in the abdominal cavity of Sceloporus olivaceus at rest and during high speed locomotion on a trackway. During trackway locomotion, abdominal pressure fluctuates rapidly around mean pressure and appears to be correlated with footfalls. Preliminary results from two individuals indicate that mean abdominal pressure increases during burst locomotion. Mean abdominal pressure during locomotion increased above mean resting pressure by 3.6 [+ or -] 2.3 [cmH.sub.2]O (0.36 [+ or -] 0.23 kPa) in one individual and by 3.5 [+ or -] 1.9 [cmH.sub.2]O (0.35 [+ or -] 0.19 kPa) in a second individual (mean [+ or -] s.d.). Paired t-tests (paired pre-exercise and running pressures) showed that these increases were statistically significant in both individuals (p [less than] 0.0005). It is likely that these increased abdominal pressures are transmitted to the thoracic region since lizards lack a diaphragm and the lungs and other internal organs are contained within a single cavity. Previous work has shown that lung ventilation decreases dramatically in lizards sprinting on trackways, and the proposed explanation is for this decrease is a mechanical constraint on hypaxial muscles (Carrier, D. R. 1987. Exp. Biol. 47: 33-42). We propose that increased abdominal, and therefore thoracic, pressures may be an additional constraint on ventilation during sprinting in lizards. Supported by a Beckman Foundation Undergraduate Research Scholarship to C.K. and a National Science Foundation grant, IBN-9875245 to E.L.B.