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Relationships between Water Vapor Path and Precipitation over the Tropical Oceans

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
  • | 2 Department of Applied Mathematics, University of Washington, Seattle, Washington
  • | 3 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
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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 compared with in situ radiosonde and radar-derived area-averaged precipitation data from Kwajalein Island, but the microwave estimates of daily P scatter considerably about the radar estimates (which are considered to be more reliable). Using the radar-derived precipitation in combination with microwave-derived W yields a daily rP relationship at Kwajalein similar to that derived solely from microwave measurements, but with somewhat less P associated with the highest values of r. This emphasizes that the absolute calibration of the rP relationship is somewhat dependent on the datasets used to derive r and especially P. Nevertheless, the results provide a useful constraint on conceptual models and parameterizations of tropical deep convection.

Corresponding author address: Christopher S. Bretherton, Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. Email: breth@atmos.washington.edu

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 compared with in situ radiosonde and radar-derived area-averaged precipitation data from Kwajalein Island, but the microwave estimates of daily P scatter considerably about the radar estimates (which are considered to be more reliable). Using the radar-derived precipitation in combination with microwave-derived W yields a daily rP relationship at Kwajalein similar to that derived solely from microwave measurements, but with somewhat less P associated with the highest values of r. This emphasizes that the absolute calibration of the rP relationship is somewhat dependent on the datasets used to derive r and especially P. Nevertheless, the results provide a useful constraint on conceptual models and parameterizations of tropical deep convection.

Corresponding author address: Christopher S. Bretherton, Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. Email: breth@atmos.washington.edu

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