University of Washington GPM 2Ku 'raintype' Modification

The basic algorithm works like this: Copy rain_type to rain_type_uw. For each pixel in rain_type_uw, change rain_type_uw to stratiform if:

• rain_type = convective and
• shallow_rain_type = 0 and
• [(dbz @ reference point) - (dbz @ point 1.5km higher)]/ [(ht @ reference point) - (ht @ point 1.5km higher)] >= abs(SLOPE) as long as ht @ reference point <= MAX_BB_HT

Some of these terms need to be defined

• REFL_MAX_OFFSET: input parameter set to 2dBZ
• reference point: First find height where max_refl in column occurs. From there, move up level by level until (max_refl - current_refl) exceeds REFL_MAX_OFFSET. The reference point is the point one level below.
• SLOPE: input parameter set to 7.5
• MAX_BB_HT: input parameter set to 5.0 km

The following plot illustrates the algorithm. In it, the horizontal magenta line represents the height of max_refl. The dashed horizontal blue line represents the height of the last point that is within REFL_MAX_OFFSET of max_refl. This is the reference height. The dashed horizontal green line represents the height 1.5 km above the reference height. If you calculate the delta_refl between the dashed vertical blue and green lines and divide it by the delta_ht between the dashed horizontal blue and green lines you get a slope of 5.39. This is not greater than or equal to SLOPE so rain_type_uw remains convective. If that slope was greater than or equal to SLOPE, rain_type_uw would have been changed to stratiform.

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