Implementing Polarization-Corrected Temperature into Typhoon Rainfall Estimates over Taiwan from TRMM/TMI Data


The polarization-corrected temperature (PCT) method is used to quantitatively estimate rainfall associated with typhoons over Taiwan. The satellite rainfall retrieval algorithms for TRMM/TMI (Tropical Rainfall Measuring Mission/TRMM Microwave Imager) have been developed according to rain types to improve the accuracy of the rainfall estimation. Measurements from precipitation radar (PR) are used to classify the rain types into three kinds including convective rain, stratiform rain with bright band and stratiform rain without bright band. Finally, rainfall retrievals are verified using PR near-surface rain rate (PR_RR), which is regarded as the "correct value."

Data from typhoons during 2001 to 2006 are used to set up the rain rate retrieval equations. First, cloud-free areas are identified using infrared satellite images as cases of no-scattering condition and the PCT equations are set up for Taiwan. Second, from the statistical analyses of the PCT values with no-rain status, the rainfall threshold of 270 K is derived. Finally, the rain rate retrieval algorithms are set up using coincident PCT values and PR_RR for different rain types.

Data from typhoons occurring from 2007 to 2008 are used for verification. The results show that the estimated values for convective and stratiform rain with bright band rain areas have better accuracy when the PCT method was used. For the stratiform rain without bright band, we find that the PCT method is not suitable. Applying this approach to estimate the rain rates associated with Typhoon Haitang (2005), the PCT method shows to have more reasonable rain rate estimation than the SIL (scattering index on land) method, with or without rain type classification. In addition, using 85 GHz data only, the PCT method shows higher rain rate retrieval resolution than the SIL method. Overall, we recommend that the PCT method can be used for quantitative precipitation estimation.

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