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  • Author or Editor: Eric A. Smith x
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  • Understanding Diurnal Variability of Precipitation through Observations and Models (UDVPOM) x
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Song Yang
and
Eric A. Smith

Abstract

This study investigates the variability of convective and stratiform rainfall from 8 yr (1998–2005) of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and TRMM Microwave Imager (TMI) measurements, focusing on seasonal diurnal variability. The main scientific goals are 1) to understand the climatological variability of these two dominant forms of precipitation across the four cardinal seasons and over continents and oceans separately and 2) to understand how differences in convective and stratiform rainfall variations ultimately determine how the diurnal variability of the total rainfall is modulated into multiple modes.

There are distinct day–night differences for both convective and stratiform rainfall. Oceanic (continental) convective rainfall is up to 25% (50%) greater during nighttime (daytime) than daytime (nighttime). The seasonal variability of convective rainfall’s day–night difference is relatively small, while stratiform rainfall exhibits very apparent day–night variations with seasonal variability. There are consistent late evening diurnal peaks without obvious seasonal variations over ocean for convective, stratiform, and total rainfall. Over continents, convective and total rainfall exhibit consistent dominant afternoon peaks with little seasonal variations—but with late evening secondary peaks exhibiting seasonal variations. Stratiform rainfall over continents exhibits a consistent strong late evening peak and a weak afternoon peak, with the afternoon mode undergoing seasonal variability. Thus, the diurnal characteristics of stratiform rainfall appear to control the afternoon secondary maximum of oceanic rainfall and the late evening secondary peak of continental rainfall. Even at the seasonal–regional scale spatially or the interannual global scale temporally, the secondary mode can become very pronounced, but only on an intermittent basis. Overall, the results presented here demonstrate the importance of partitioning the total rainfall into convective and stratiform components and suggest that diurnal modes largely arise from distinct diurnal stratiform variations modulating convective variations.

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Song Yang
and
Eric A. Smith
Full access
Song Yang
,
Kwo-Sen Kuo
, and
Eric A. Smith

Abstract

This investigation seeks a better understanding of the assorted mechanisms controlling the global distribution of diurnal precipitation variability based on the use of the Tropical Rainfall Measuring Mission (TRMM) microwave radiometer and radar data. The horizontal distributions of precipitation’s diurnal cycle are derived from 8 yr of TRMM Microwave Imager (TMI) and Precipitation Radar (PR) measurements involving three TRMM standard rain retrieval algorithms; the resultant distributions are analyzed at various spatiotemporal scales. The results reveal both the prominent and expected late-evening (LE) to early-morning (EM) precipitation maxima over oceans and the counterpart prominent and expected mid- to late-afternoon (MLA) maxima over continents. Moreover, and not generally recognized, the results reveal a widespread distribution of secondary maxima, which generally mirror their counterpart regime’s behavior, occurring over both oceans and continents. That is, many ocean regions exhibit clear-cut secondary MLA precipitation maxima, while many continental regions exhibit just as evident secondary LE–EM maxima. This investigation is the first comprehensive study of these globally prevalent secondary maxima and their widespread nature, a type of study only made possible when the analysis procedure is applied to a high-quality global-scale precipitation dataset.

The characteristics of the secondary maxima are mapped and described on global grids using an innovative clock-face format, while a current study that is to be published at a later date provides physically based explanations of the seasonal regional distributions of the secondary maxima. In addition to a primary “explicit” maxima identification scheme, a secondary “Fourier decomposition” maxima identification scheme is used as a cross-check to examine the amplitude and phase properties of the multimodal maxima. Accordingly, the advantages and ambiguities resulting from the use of a Fourier harmonic analysis are investigated.

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