Relationships between Water Vapor Path and Precipitation over the Tropical Oceans

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Abstract
The relationship between water vapor path W and surface precipitation rate P over tropical oceanic regions is analyzed using 4 yr of gridded daily SSM/I satellite microwave radiometer data. A tight monthly mean relationship P (mm day−1) = exp[11.4(r − 0.522)] for all tropical ocean regions and seasons is found between P and a column-relative humidity r obtained by dividing W by the corresponding saturation water vapor path. A similar relation, albeit with more scatter, also holds at daily time scales, and can be interpreted as a moisture adjustment time scale of 12 h for convective rainfall to affect humidity anomalies on 300-km space scales. Cross-spectral analysis shows statistically significant covariability of actual and r-predicted precipitation at all frequencies, with negligible phase lag. The correlation of actual and r-predicted precipitation exceeds 0.5 on intraseasonal and longer time scales. The SSM/I retrievals of W and P are found to be skillful even at daily time scales when compar... Abstract The relationship between water vapor path W and surface precipitation rate P over tropical oceanic regions is analyzed using 4 yr of gridded daily SSM/I satellite microwave radiometer data. A tight monthly mean relationship P (mm day−1) = exp[11.4(r − 0.522)] for all tropical ocean regions and seasons is found between P and a column-relative humidity r obtained by dividing W by the corresponding saturation water vapor path. A similar relation, albeit with more scatter, also holds at daily time scales, and can be interpreted as a moisture adjustment time scale of 12 h for convective rainfall to affect humidity anomalies on 300-km space scales. Cross-spectral analysis shows statistically significant covariability of actual and r-predicted precipitation at all frequencies, with negligible phase lag. The correlation of actual and r-predicted precipitation exceeds 0.5 on intraseasonal and longer time scales. The SSM/I retrievals of W and P are found to be skillful even at daily time scales when compar...