Rain retrieval challenges associated with Hawaiian Islands

Rainfall patterns on and around the Hawaiian Islands are strongly constrained by the interactions of orography, trade wind flow, and island-induced circulations. The horizontal distributions are reasonably well known, whether by raingauge measurements or by vegetation type and cover. This allows a sanity check for rain retrievals from satellite-based remote sensors.

Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) rain estimates very generally show the correct climatological rainfall patterns, although precise locations and magnitudes of the rainfall maxima go beyond the measurement capabilities. The combination of shallow rain layers and sharp surface elevation gradients makes radar-based estimation of surface rainfall difficult.

TRMM Microwave Imager (TMI) rain retrievals do not portray the rainfall maxima associated with some islands. They misplace the maximum associated with the Big Island, placing it on the relatively dry southwestern side instead of the wet northeastward-facing coast and slopes. Very preliminary analysis suggests that much of the problem lies with retrievals for surface types designated as "coast" or "mixed", where strong emission from a radiometrically warm land surface and low emission from a radiometrically cold ocean surface can both contribute to the same sensor field of view. Besides that, shallow heavy rain showers (without large ice particles aloft) are difficult for a passive microwave imager to identify against a land surface background.

While initial analysis has been limited to TRMM, similar challenges should apply to AMSR, SSMI, and other passive microwave sensors, and to merged microwave-IR multi-satellite techniques. These issues could also apply to other small tropical land masses (islands and peninsulas). The well characterized climatological rainfall patterns of the Hawaiian Islands provide a useful basis for diagnosing and hopefully mitigating such retrieval problems.