Py. Groisman et Dr. Legates, THE ACCURACY OF UNITED-STATES PRECIPITATION DATA, Bulletin of the American Meteorological Society, 75(2), 1994, pp. 215-227
Precipitation measurements in the United States (as well as all other
countries) are adversely affected by the gauge undercatch bias of poin
t precipitation measurements. When these measurements are used to obta
in areal averages, particularly in mountainous terrain, additional bia
ses may be introduced because most stations ate at lower elevations in
exposed sites. Gauge measurements tend to be underestimates of the tr
ue precipitation, largely because of wind-induced turbulence at the ga
uge orifice and wetting losses on the internal walls of the gauge. The
se are not trivial as monthly estimates of this bias often vary from 5
% to 40%. Biases are larger in winter than in summer and increase to t
he north in the United States due largely to the deleterious effect of
the wind on snowfall. Simple spatial averaging of data from existing
networks does not provide an accurate evaluation of the area-mean prec
ipitation over mountainous terrain (e.g., over much of the western Uni
ted States) since most stations are located at low elevations. This te
nds to underestimate area averages since, in mountainous terrain, prec
ipitation generally increases with elevation. Temporal precipitation t
rends for the United States, as well as seasonal and annual averages,
are presented. Estimates of unbiased (or less biased) precipitation ov
er the northern Great Plains provide a regional analysis.