EAPS

Diurnal variations of warm-season precipitation over the continents and coastal lines

I will first present an observational analysis on the differences and similarities of the diurnal variation and propagation of warm-season precipitation over the continents downstream of big topography and over the coastal lines. It is found that the strong diurnal precipitation cycles are strongly associated with several thermally driven regional mountain-plains and sea-breeze circulations due to the differential heating between the big mountain ranges, the highlands, the plains, and the ocean. Increased precipitation 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. The diurnal evolution and propagation of these rainfall cycles are strongly modulated by the mean steering level wind speed which differs from continent to continent. For example, the predominant phase speed of precipitation at different time scales for North America, averaged over all warm-season months for several years, is ∼ 20 m/s, which is faster than the average speed of ∼ 14 m/s for East Asia.

High-resolution convection-permitting numerical experiments are further conducted to understand the dynamics and predictability of warm-season precipitation diurnal variations over the continents and coastal lines. These numerical simulations not only elucidate the role of thermally driven regional solenoids being instrumental in the diurnal variations of the warm-season precipitation over the continents and coastal lines but also reveal the importance of non-hydrostatic cold pool dynamics, which may be one of the key reasons why the current-generation global and climate models often have difficulties in properly modeling these diurnal rainfall variations.