MEASUREMENTS OF RAINDROP SIZE DISTRIBUTIONS OVER THE PACIFIC WARM POOL AND IMPLICATIONS FOR Z-R RELATIONS

Raindrop images obtained on research flights of the NCAR Electra aircr aft in the Tropical Oceans Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) are analyzed. The drop size distribut ions, based on the images obtained in 6-s samples (about 750 m of flig ht track), are used to calculate both radar reflectivity Z and rain ra te R. Airborne radar data from the NOAA P-3 aircraft flying in coordin ation with the Electra are used to categorize the particle-image data according to whether the drop images were obtained in regions of conve ctive or stratiform precipitation. Within stratiform precipitation, th e same rain rate could be produced by a drop spectrum dominated by num erous small drops (lower reflectivity) or by a few large drops (higher reflectivity). The reflectivity Values varied by as much as 9 dB for a given rain rate. Reflectivity data from the airborne radar and fligh t-level data reveal that the stratiform regions often contain fallstre aks of about 0.1-2 km in horizontal dimension. The fallstreaks are ass ociated with large-drop spectra and local maxima in reflectivity up to approximately 40 dBZ and in rain rates up to 25 mm h(-1). The fallstr eaks extend downward from the melting band and bend with the low-level wind shear, but do not usually reach the surface. Thus, although rela tively more uniform than convective regions, stratiform regions can be variable in reflectivity and rain rate at fine spatial scales in both the horizontal and vertical directions. Stratiform regions are theref ore best characterized by their ensemble properties rather than the va lues of individual high-resolution measurements. The variability of st ratiform drop size spectra arises primarily from the occurrence of fal lstreaks and the discontinuous nature of regions favoring aggregation of snow crystals, and it implies that Z-R distributions associated wit h convective and stratiform precipitation are not statistically distin ct. Thus, separate Z-R relations for convective and stratiform precipi tation are not justified, and techniques to distinguish between convec tive and stratiform precipitation based solely on the characteristics of drop size distributions are not likely to be accurate. The variabil ity of the drop size spectra in tropical precipitation makes an expone ntial fit to the Z-R relation sensitive to the spatial scale over whic h Z and R are determined. This sensitivity can be avoided by using a p robability-matched Z-R relation. The probability-matched Z-R relation for all the raindrop image data from the Electra collected between alt itudes of 2.7 and 3.3 km in TOGA COARE is similar to the Z-R relation obtained at the sea surface in the Global Atmospheric Research Program Atlantic Tropical Experiment.