Quantifying the effects of the diurnal cycle in the variability of rainfall

Abstract

AbstractIn order to quantify how much variability is affected by the diurnal cycle (DC), two variance‐partitioning schemes were applied on two rainfall datasets that cover the Intra Americas Seas (IAS) domain at a 3‐hourly resolution during 21 years. The datasets are the Tropical Rainfall Measurement Mission (TRMM) 3B42, and a simulation from the Centre National de Recherches Météorologiques (CNRM) model. From TRMM, the results show that the variance attributable to the DC is large because it shapes the variability of transients. Such diurnal effects shape transients similarly during all seasons, but their effects produce different patterns over continental and oceanic regions. Over continental regions, the total variance attributable to the DC is much larger than that attributable to the mean seasonal cycle and to interannual variability. But over oceanic regions, the latter two types of partitions can be larger than that of the DC. Although the analysed model simulation captures the broad features of the observed variance partitions, it overestimates the importance of the typical diurnal march, it underestimates how the DC tends to suppresses transient variability during nighttime over land, and it underestimates how it tends to increase it otherwise. As shown by the results, the variance‐partitioning approach of this study might complement previous approaches because rather than focusing on diurnal amplitudes and phases, it focuses on diurnal effects. Therefore, this approach could be useful to diagnose DC‐related issues from a relatively new perspective, issues such as the evaluation of systematic errors, or the evaluation of how the DC affects transients under different large‐scale conditions or slow‐varying forcings.