The foregoing remote sensing techniques all require sea truth to generate credible precipitation products. This is a formidable challenge because the measurement of precipitation at sea is anything but routine. Rain gauges on small, low atolls (with no orographic or ``warm lagoon'' effects) are extremely valuable for calibration, but are not available in most areas. Rain gauge measurements from ships are generally regarded as futile. The most promising approaches are:
Instrumented Surface Buoys: Two rain gauge types are in use:
Capacitive filling bucket gauges --- These are being used on the
improved meteorological measurement
(IMET) buoys being deployed in
upper ocean experiments by R. Weller.
Optical rain gauges --- Assumptions about drop size distribution are required to convert the output to a rain rate. These are being used on TOGA TAO moorings in the equatorial Pacific.
Subsurface acoustic --- The ambient noise created by rainfall may be distinguished from that caused by wind by the peak near 15 kHz [ Scrimger, et al., 1987]. This technique has the advantage of being an integral measure rather than a point sample. However, much work remains to be done (in particular, calibration against other approaches) before it can be considered a routine measurement. It may be possible to deploy acoustic rain (and wind) gauges on drifting surface buoys.
Surface radar --- Rainfall is estimated from meteorological radars operated from ships during special experiments. While only good to 50 to 100% in individual comparisons, data on rain drop size distributions and averaging on larger space and time scales can improve accuracy to 10--25%. This is likely to remain a research tool rather than a routine monitoring technique.