Diurnal and semidiurnal variations in the time series of 3-hourly assimilated precipitation by NASA GEOS-1

Time series and harmonic analyses were used to examine diurnal and semidiur nal 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 back ground noise. The signal-to-noise ratio was smaller for the time series of tropical precipitation than that of midlatitudes. However, for the area-ave raged 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) ove r the continents (oceans). The semidiurnal variation had amplitudes of abou t 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 ea rly 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 distr ibution of land and sea had definite effects on the modulation of occurrenc e of assimilated amounts of precipitation at diurnal and semidiurnal freque ncies. The assimilated amounts of precipitation showed reasonable diurnal and semi diurnal Variations in strength over land. Over the oceans the diurnal varia tions in the assimilated amounts were Very weak when compared with observat ions. The weak diurnal and semidiurnal variations resulted from the small a mplitude and irregular phase of harmonic dials for each day over the tropic al oceans. Over the midlatitude oceans phase irregularity was more responsi ble for the weakness.