Computing Morphing Vectors for Version 06 IMERG

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George J. Huffman
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In IMERG up through V05, the cloud motion vector computation approach used is that pioneered in CMORPH (Joyce et al. 2011), in which motion vectors are computed from 4-km geosynchronous infrared (GEO-IR) brightness temperatures. Hence, the motion vectors reflect cloud top motions. However, there are two main limitations in using GEO-IR. The first limitation is that cloud top motions may not match precipitation motions due to both wind shear and the growth and decay of precipitation systems. The resulting motion mismatch necessitated the use of climatological scaling factors based on US ground-based radars in CMORPH, although it is unclear how applicable these scale factors are to other regions. Moreover, this scaling only corrects for differences in mean speed and not differences in direction. The second limitation is that the GEO-IR dataset used in CMORPH and IMERG (the NOAA/NWS/CPC 4-km Merged IR) are confined to the latitude band 60°N-S. Poleward of this band, the viewing angle is too oblique to produce useful data outside 60°N-S. In addition, as we approached the start of V06 processing we found that issues with access to the necessary GEO-IR data would inhibit a complete retrospective processing.

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