Campbell Scientific PWS100 Present Weather Sensor - Manual (Page 68)

Section 7. Operation
The snow water content scaling factor (SWCF) can be adjusted by choosing
option 4 on the WEATHER PARAMETERS menu. This will bring up a line
asking for a value.
Depending on atmospheric conditions likely to be encountered, a wide range of
snow water content figures can be observed dependant on the bulk density of
the particle. Particle bulk density depends on the structure of the particle (e.g.,
whether it is dendritic, columnar or plate-like as the surrounding air or
encapsulated air occupies some of the particle space). The conversion from a
measurement of size (which in itself is not precise for irregular snowflakes)
and water content can at best be considered approximate.
The PWS100 assumes a relationship between snow flake size and bulk density
published in the literature (Brandes, E. A., K. Ikeda, G. Zhang, M. Schönhuber,
and R. M. Rasmussen, 2007: A statistical and physical description of
hydrometeor distributions in Colorado snow storms using a video disdrometer.
J. Appl. Meteor., 46, 634−650). That equation and the values it assumes for
other identified particle types are given in Table 7-3. The equation for
snowflakes has the scaling factor for snow water content that can be adjusted to
suit local conditions. Entering a value of 1 assume this relationship is correct.
For example, if the snow at the site of installation is typically more dense and
wet (as is common in temperate climates) a higher scaling factor may be
needed.
TABLE 7-3. Assumed bulk density of various
particle types.
Particle Type Bulk Density assumed
Liquid 1.0 g cm -3
Graupel 0.6 g cm -3
Snow grain 0.4 g cm -3
Ice pellet 0.7 g cm -3
Hail 0.8 g cm -3
Snow Flake SWCF* 0.178*D -0.922 g cm -3
(D is the flake diameter in mm)
This figure should not be confused with equivalent water to snow depth
calculations which require further knowledge of packing structures, wind
effects, ground temperature, ground type and a myriad of other parameters
related to snow depth. The ratio of water accumulation to snow depth will be
lower than the snow water content figure and is typically in the order of 0.1
(i.e., the snow pack is 10 times deeper than the water accumulation of the
melted snow pack).
Changing the snow water content parameter will also affect the
determination of precipitation intensity, which is defined by
water equivalent precipitation rates. See 8.6.2 and Appendix A
for more details. NOTE
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