Comparison of the diurnal variations of warm-season precipitation for East Asia vs. North America downstream of the Tibetan Plateau vs. the Rocky Mountains
A wave-number-frequency spectral decomposition technique is used to analyze the high-resolution NOAA/Climate Prediction Center morphing technique (CMORPH) precipitation data set and to explore the differences and similarities of the diurnal variation of warm-season precipitation in the East Asia and North America downstream of big topography. The predominant phase speed of precipitation at different time scales for North America, averaged over all warm-season months (May–August) for 2003–2010, is ~20 ms −1, which is faster than the speed of ~14 ms −1 calculated for East Asia. Consistent with the recent studies of the precipitation diurnal cycles for these two regions, the difference in the diurnal phase propagation is likely due to the difference in the mean steering level wind speed for these two regions. The wave-number-frequency spectral analysis further reveals the complex, multi-scale, multi-modal nature of the warm-season precipitation variation embedded within the diurnal cycle over both continents, with phase speeds varying from 10 to 30 ms −1 and wave periods varying from diurnal to a few hours. At the diurnal frequency regulated by the thermodynamically driven mountains–plains solenoids (MPSs), increased precipitation for both continents first originates in the afternoon from the eastern edge of big topography and subsequently moves downslope in the evening and reaches the broad plains area at night. More complex diurnal evolutions are observed in East Asia due to the more complex, multistep terrains east of the Tibetan Plateau and the associated localized MPS circulations. Nevertheless, increased variation of precipitation at smaller spatial and temporal scales is evident in the active phase of the dominant diurnal cycle for both continents.