ABSTRACT
A Comparison of Rainfall, Outgoing Longwave Radiation, and Divergence
over the Amazon Basin
Brant
Liebmann
Climate Diagnostics Center,
University of Colorado, Boulder, Colorado
José A. Marengo, John D. Glick, Vernon E. Kousky, Ilana
C. Wainer, and Oswaldo Massambani
(Manuscript received 26 June 1997, in final form 5 November 1997)
Abstract
Observed rainfall, outgoing longwave radiation (OLR), and divergence and
precipitation from the Reanalysis project of the National Centers for
Environmental Prediction and Atmospheric Research are compared over the
Amazon basin. The spatial pattern of the mean and the phase of the
annual cycle generally compare well, except that the amplitude of the
annual cycle of model precipitation is much smaller than observed. On
10 - 30 day time scales, it is shown that averaging stations with a 5°
radius is approximately equivalent to total wave number 20 (T20)
spatial scale, although it is more important to have a high density of
stations then an exact match of spatial scales. Ideally, there should
be 1 station per 20,000 square kilometers. On 10 - 30 day scales,
observed rainfall is best correlated with OLR. Correlations between
OLR and 150 mb divergence are larger than between observed rainfall and
divergence or between rainfall and model precipitation. For example,
if 10-30 day filtered OLR and divergence are truncated at T20, and
rainfall is averaged to include stations within a 5° radius, OLR is
correlated with rainfall at about -0.6, OLR is correlated with
divergence at about -0.35, and rainfall is correlated with divergence
at about 0.2. At least part of the lack of correlation is due to
inadequate spatial sampling of rainfall. Correlations improve with
larger spatial scale. The major seasonal transitions from dry to rainy
regimes are captured well by OLR, but not by the model quantities. The
mean diurnal cycle is represented reasonably by 150 mb divergence.