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The height of the anemometer at each station was assumed to be at 6m, and the height of the
gauge orifice was assumed to be at 2m. The wind speed was converted from a 6m height to a 2 m
height, as per Goodison et al. (1998) using the snowpack aerodynamic roughness of 0.005 m
(Fassnacht et al., 1999).
For solid precipitation, wetting losses were assumed to be small as compared to wind induced
losses, and for monthly computations were assumed to be minimal. Similarly, evaporation losses
were assumed to be negligible (Goodison et al., 1998). Undercatch equations were derived from
daily data for snowfall when the daily air temperature (Ta) was colder than freezing and for mixed
precipitation when Ta was between 0 and 3 degrees C (Goodison et al., 1998). The specific
equations derived by Yang et al. (1998b) for the unshielded 8" NWS Standard precipitation gauge
with respect to the Double Fence Intercomparison Reference gauge (DFIR) were used for all sites
except Syracuse (Table 1). This station used a Belfort Universal gauge. As per Groisman et al.
(1999), the Yang et al. (1998b) equation was used for snowfall undercatch of the Belfort Universal
gauge. Mixed precipitation undercatch was increased by 7% for the Belfort Universal gauge used
at Syracuse (Groisman et al., 1999). As per Fassnacht (2004) and Bogart et al. (2006), a maximum
wind speed of 6.5 m/s was used in the undercatch equations due to the increased uncertainty at the
higher wind speeds.
Sublimation and the occurrence of blowing snow are episodic and were thus computed using
hourly data. Sublimation was estimated using the bulk transfer approach for the latent heat flux as
a function of humidity (RH), air temperature, wind speed, and station pressure (PR):
( RH,
T , U , PR)
FE = f a z , (3)
as initially formulated by Sverdrup (1936). The occurrence of blowing snow (BSY/N) was
initially estimated as a function of wind speed and different temperature considerations, as per Li
and Pomeroy (1997):
Y / N = . (4)
( U T )
BS f ,
z
a
Once blowing snow was determined to have initiated, the quantity of blowing snow was
computed as a function of wind speed:
BS
( U )
q = f , (5)
z
using the equation derived by Pomeroy et al. (1991). Sublimation and blowing snow quantities
were summed to yield net snowpack loss estimates.
The NWS denotes trace events (PT) as precipitation amounts less than 0.01 inches or 0.254 mm
per hour or day (NWS, 2005), while Legates et al. (2005) called all measurements less than half
the measurable precipitation depth (0.005 inches or 0.127 mm) as a trace event. In this paper, daily
trace events will be assigned a value of 0.127 mm.
RESULTS
Hourly, daily and monthly meteorological data were used to compute PU (Figure 1) and
snowpack losses (FE plus qBS) (Figure 2). The estimated difference between losses and undercatch
using the time step specific to the derived equations versus using a monthly time step is presented
in Figure 3 for monthly totals, and Figure 4 for seasonal totals. For the difference comparison, the
same net result would appear along the 1:1 line, while data at the origin would indicate that
measured precipitation could be used as an estimate of snow on the ground. Below the x-axis or to
the left of the y-axis, more snow is actually accumulating than estimated from the measured
precipitation alone. The difference between the two time step estimates and gauge precipitation for
individual months is illustrated in Figure 5, and for each season is illustrated in Figure 6.
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