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calculates rain number mixing ratio from mass
Original note from Greg:
Not thrilled with it, but set y-intercept parameter to Marshall-Palmer value
that basically assumes melting snow becomes typical rain. However, for
-2C < T < 0C, make linear increase in exponent to attempt to keep
supercooled collision-coalescence (warm-rain) similar to drizzle rather
than bigger rain drops. While this could also exist at T > 0C, it is
more difficult to assume it directly from having mass and not number.