Diurnal and Semidiurnal Variations in the Time Series of 3-Hourly Assimilated Precipitation by NASA GEOS-1

Abstract

Time series and harmonic analyses were used to examine diurnal and semidiurnal oscillations in the time series of 3-hourly precipitation amounts from NASA GEOS-1. The diurnal and semidiurnal components of assimilated amounts of precipitation manifested themselves clearly against the spectrum of background noise. The signal-to-noise ratio was smaller for the time series of tropical precipitation than that of midlatitudes. However, for the area-averaged precipitation the ratio was much larger for the Tropics than for the midlatitudes. The amplitude of the diurnal oscillation was about 2.0 (0.1) mm day?1 over the continents (oceans). The semidiurnal variation had amplitudes of about 1.0 mm day?1 over land and 0.05 mm day?1 over sea. The diurnal cycle peaked at about 0600 LST over the oceans and about 1430 LST over land. The maximum of the semidiurnal cycle mostly occurred from midnight (noon) to early morning (evening). Besides amplitude and phase differences between over land and over the oceans, there existed a large-scale organization in the distribution of harmonic arrows. The regions with such coherent orientation of the harmonic arrows as well as their similar magnitude are the Gulf of Mexico, the South China Sea, the Bay of Bengal, and water bodies surrounded by the islands in the Tropics. The pattern suggests that large-scale distribution of land and sea had definite effects on the modulation of occurrence of assimilated amounts of precipitation at diurnal and semidiurnal frequencies. The assimilated amounts of precipitation showed reasonable diurnal and semidiurnal variations in strength over land. Over the oceans the diurnal variations in the assimilated amounts were very weak when compared with observations. The weak diurnal and semidiurnal variations resulted from the small amplitude and irregular phase of harmonic dials for each day over the tropical oceans. Over the midlatitude oceans phase irregularity was more responsible for the weakness.