GFDL - Geophysical Fluid Dynamics Laboratory

Publication 8501

Harnack, R., and J. Lanzante, 1985: Specification of United States
seasonal precipitation.
Mon. Wea. Rev., 113, 319-325.


Seasonal precipitation is specified for the United States by matching various
area-averaged precipitation statistics as predictands with three different
predictors in turn: 700 mb heights, North Pacific SST and North Atlantic SST.
Predictors are in the form of empirical orthogonal function (EOF) amplitude
time series. The predictands used in trials include total precipitation and
precipitation frequencies derived using three different critical
values: 2.5, 12.7 and 25.4 mm. Screening multiple linear regression is used
to relate predictands to predictors for samples ranging from 24 to 35 years
in length; initially trials are compared in terms of area-averaged true skill
and percent area of local significance. In order to assess specification
skill on an independent sample, additional tests are made using a jackknife
regression approach.

Results suggest that skillful seasonal precipitation prediction
will continue to be very difficult using predictors and methods presently
in common use based on the use of specification equations on an independent
sample. Generally, area-averaged explained variances are less than 10% and
the area of significant local skill is less than 50%. Based on the low level
of specification skill, predictive skill for precipitation using specification
equations with imprecisely known specifier fields (like 700 mb heights) as
input would be effectively zero.

Other conclusions are:

  1. 700 mb heights specify seasonal precipitation about equally well in
    winter, spring and summer, but worse in fall.
  2. Among the three predictor types employed, 700 mb heights are best for
    all seasons but fall, when Pacific SST does best. Specification using
    Atlantic SST is poor in all instances and inferior to the use of the
    other predictor fields.
  3. Overall among the four precipitation statistics used as predictands, the
    frequency statistics have a slightly better relationship with 700 mb
    heights or Pacific SST than do precipitation totals.