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6. Rain and Snow. 6.1 Drop-size distribution and drop breakup. 6.2 Size distribution of snowflakes and breakup. 6.3 Rainfall rate (Precipitation rate). 6.1 Drop-size distribution and drop breakup. Best way to characterize rain reaching the ground is through drop-size distribution.

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slide1

6. Rain and Snow

6.1 Drop-size distribution and drop breakup

6.2 Size distribution of snowflakes and breakup

6.3 Rainfall rate (Precipitation rate)

slide2

6.1 Drop-size distribution and drop breakup

  • Best way to characterize rain reaching the ground is through drop-size distribution.
  • Drop-Size distribution
    • Number of drops per unit size interval per unit volume.
    • Size interval is usually diameter (or radius)
  • How do we mathematically represent this drop-size distribution?
    • Experimental studies
slide3

Observations

Exponential Fit

Observations

slide4

Why do drop-size distribution tend to be

a negative-exponential form?

Drop Breakup

  • When drops reach a diameter of 3mm, the surface tension is barely able to hold drop together for larger sizes, drop breaks up.
  • Drops much larger than 3mm are unstable, exist only briefly before breaking up.
  • Collisions between drops produce small satellite drops
slide5

Young (1975)

  • Modeled drop-size spectrum
  • Assumed activity of condensation nuclei
  • Coalescence, condensation, and collision-induced breakup
  • Result: Marshall-Palmer type distribution.
slide8

6.2 Size distribution of snowflakes and breakup

  • Irregular aggregates of crystals.
  • Difficult to measure.
  • Usually expressed in terms of particle mass or diameter of the water drop formed when the snowflake is melted.
  • Distribution can be described reasonably well by an exponential function.
  • Breakup of snowflakes is likely due to collision.
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6.3 Rainfall rate (Precipitation rate)

  • Volume of water passing through a horizontal surface per unit time.