The basic equations - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 39

Figure 2.2. Hailstone that fell in Vivian, SD on July 23, 2010. This hailstone as of 2011 is the
largest hailstone ever documented in terms of both weight (31 oz.) and circumference (19
inches). It was studied in Charlie Knight's Cold Laboratory at NCAR, where the author took
this photograph. The radar echo associated with a convective storm having large hail may have
a spike or flare attached to it as a result of three-body scattering (hail to ground, ground to hail,
and hail back to radar).

Keeping N 0 fixed, however, results in some errors because the consequences of
evaporation are not represented accurately and because processes that affect one
part of the size spectrum differently from other parts are also not represented
accurately: size sorting is not simulated well.
When k ¼ 0

M 0 , the number of hydrometeors in the volume (cf. (2.47)) is
added as a second prognostic variable and the bulk parameterization scheme is
known as a ''two-moment'' scheme. Both N 0 and

;

can vary. Two-moment

schemes, however, over-represent size sorting.
When k ¼ 6 M 6 (cf. (2.47)), which is proportional to the radar reflectivity

factor Z (i.e., proportional to the sixth power of the diameters of the hydro-
meteors when they are liquid and when their diameters are much shorter than the

;