GFDL - Geophysical Fluid Dynamics Laboratory

Publication 8203

Lanzante J., and R. Harnack, 1982: Specification of United States summer
season precipitation.
Mon. Wea. Rev., 110, 1843-1850.


The specification of summer season precipitation in the contiguous United
States from summer season fields of 700 mb height, sea level pressure (SLP)
and Pacific sea surface temperature (SST) was carried out using stepwise
multiple liner regression. The specifier fields were characterized by their
first five Empirical Orthogonal Functions (EOF’s). The objectives were to
assess the overall skill in specifying summer season precipitation,
examine the differences among predictands with regard to both spatial
averaging and type of statistic, compare the usefulness of the specifie
fields, and to look at spatial variations in specification skill.

Overall, the strongest relationships between actual summer season
precipitation and the predictors were found for 700 mb heights (R**2 ~ 0.24)
followed by Pacific SST’s (R**2 ~ 0.21) and SLP (R**2 ~ 0.12). The use of
large area averages ( ~ 10**5 km**2) for the predictand produced slightly
greater R**2 values than for individual climatic division averages
(~ 10**4 km**2).

The user of transformed summer season precipitation statistics to account
for precipitation skewness, did not improve upon the use of actual summer
season precipitation as the predictand. However, frequency of precipitation
greater than 0.1 inch resulted in an almost doubling of explained variances
over actual precipitation (0.47 versus 0.24) when 700 mb heights were used
as the specifier field.

The areas of weakest relationship (west of the Rockies and southern states)
between predictor and summer precipitation statistic generally had R**2
values less than 0.3, even for the best models. Elsewhere, the R**2 values
generally ranged from 0.5 to 0.7 for the best model (700 mb heights and
precipitation frequency). After accounting for artificial predictability
which results from imperfect estimates of the statistics, skill values
(explained variances) east of the Rockies ranged from 0.01 to 0.44.