Diurnal Precipitation Regimes in the Global Tropics*

Abstract

Abstract Diurnal variations of the global tropical precipitation are documented by using two complementary Tropical Rainfall Measuring Mission (TRMM) datasets (3B42 and 3G68) for 1998–2006 in an attempt to provide a unified view of the diurnal cycle and a metric for evaluating numerical model performance. The 3B42 data have better spatial coverage; the 3G68 data offer more accurate diurnal phase information. The first and second empirical orthogonal function (EOF) modes represent the diurnal cycle and account for 89% of the total variance in 3B42. The third and fourth EOF modes, which account for 10% of the total variance, represent the semidiurnal cycle. Both datasets yield consistent spatial structures and temporal evolution, but they have different advantages: the patterns derived from 3B42 exhibit less noise, while 3G68 yields an arguably more accurate diurnal phase. The diurnal phase derived from 3G68 systematically leads 3B42 by about 3 h. Three tropical diurnal cycle regimes (oceanic, continental, and coastal) are identified according to the amplitude, peak time, and phase propagation characteristics of the diurnal precipitation. The oceanic regime is characterized by moderate amplitude and an early morning peak [0600–0900 Local Solar Time (LST)], found primarily in the oceanic convergence zones in the Pacific, Atlantic, and Indian Oceans. In contrast, the continental regime features a large amplitude and an afternoon peak (1500–1800 LST), which is particularly pronounced in South America and equatorial Africa near Lake Victoria. Both the oceanic and continental regimes show little spatial phase propagation. The coastal regime, however, shows not only large amplitude but also prominent phase propagation. Two subregimes can also be recognized, often concurring along the same land–sea boundary. The seaside coastal regime is characterized by offshore phase propagation, with peaks occurring from late evening to noon of the next day (2100–1200 LST), whereas the landside coastal regime has landward phase propagation with peaks occurring from noon to evening (1200–2100 LST). The coastal regime is prominent along the land–sea boundaries of the Maritime Continent, the Indian subcontinent, northern Australia, the west coast of America extending from Mexico to Ecuador, the west coast of equatorial Africa, and Northeast Brazil. Note that the amplitude of the diurnal cycle is dependent on season, but the diurnal phase characteristics are not. The underlying mechanism suggested by this analysis, especially over the coastal areas, is also discussed.