Mark L. Morrissey, School of Meteorology University of Oklahoma
Howard J. Diamond, NOAA NCDC And Michael J. McPhaden and H. Paul Freitag NOAA/Pacific Marine Environmental Laboratory
The oceanic tropical region is of vital importance for the study of the changing climate. Perhaps the most important variable to be measured accurately is rainfall due to its direct association with the amount of latent heat released in the tropical atmosphere. NASA's Tropical Rainfall Measuring Mission (TRMM) primary objective is to study rainfall characteristics in the tropics. The TRMM satellite has a unique onboard radar dedicated for this purpose. It's been long understood that remotely sensed estimates of precipitation require verification using surface observations. This is because of the inherent uncertainties associated with algorithms that convert received radiance to rainfall. Surface verification requires a number of rain-measuring gauges arranged in a network in an effort to match the spatial scale of the remote sensed estimates and thereby reduce sampling error. Of course, raingauge themselves have their own source of random and systematic error. While the magnitude of error associated with a raingauge can be studied easily when deployed within land-based networks, it is extremely difficult to determine the error magnitude of raingauges situated on buoys such as the capacitance gauges mounted on the buoy array operated by the Tropical Atmosphere Ocean project (TAO). Several studies have determined error bounds for the capacitance gauges, most notably Serra et. al (2001). The present study uses the findings of Morrissey et al. (1994) that island-based hourly rainfall estimates made by Fischer-Porter weighing gauges showed an extremely consistent relationship between the number of hours in a month indicating rain and the associated monthly rainfall total. This relationship was consistent among the Pacific equatorial islands having these gauges (i.e., 6) regardless of differences in topography and regional climate regimes. This result suggested that the hourly rain rate, given that there was rain, averaged over a month, is the same from month to month. It was noted in Morrissey et al. (1994) that these results could be used to check the consistency of buoy-mounted gauges. Since 1994 the TAO capacitance gauges have been deployed and are operational allowing a similar study using these data. The results of this updated work indicate a nearly identical relationship between the Pacific island gauges and among the TAO buoy gauges. The results suggest that the TAO capacitance gauges are quite reliable in terms of consistency in the measurement of the expected rainfall rate given rainfall. A comparison of the measurements from the weighing gauge on Majuro atoll and a nearby TAO buoy (~500km due west) indicates that the difference in the magnitude of long-term averaged rainfall measured on Majuro atoll and the capacitance gauge appears to be minimal. Overall, these results suggest that the buoy-mounted gauges are relatively unbiased compared to the island weighing gauges. It should be noted, however, that undercatchment of rain by the gauges due to dynamic wind effects and evaporation is a problem with both types of gauge and this issue remains to be investigated.
Morrissey, M.L., W.F. Krajewski and M.J. McPhaden, 1994: Estimating rainfall in the tropics using the fractional time raining, Journal of Applied Meteorology, 33, No. 3, 387-393.
Serra, Y.L., P.A'Hearn, H.P. Freitag, and M.J. McPhaden, 2001: ATLAS self-siphoning rain gauge error estimates. J. Atmos. Ocean. Technol., 18, 1989-2002.
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