A Comparison of Rainfall, Outgoing Longwave Radiation, and Divergence over the Amazon Basin

Abstract

Observed rainfall, outgoing longwave radiation (OLR), divergence, and precipitation from the reanalysis project of the National Centers for Environmental Prediction and the National Center for 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 timescales, it is shown that averaging stations within a 5° radius is approximately equivalent to total wavenumber 20 (T20) spatial scale, although it is more important to have a high density of stations than an exact match of spatial scales. Ideally, there should be one station per 20?000 km2. 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.