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. • 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
Observations Exponential Fit Observations
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
Young (1975) • Modeled drop-size spectrum • Assumed activity of condensation nuclei • Coalescence, condensation, and collision-induced breakup • Result: Marshall-Palmer type distribution.
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.
6.3 Rainfall rate (Precipitation rate) • Volume of water passing through a horizontal surface per unit time.