资源环境科技发展态势分析平台

In a dry convective boundary layer, convective patterns of typical scales spontaneously develop, qualitatively similar to those in a fluid which is placed between two horizontal plates and sufficiently heated from below. As soon as precipitating cumulus clouds form, this pattern is disturbed and a transition to a different state occurs. Here we use idealized large-eddy simulations to explore how the horizontal scale of convection is modified during this transition in the course of a diurnal cycle. Before onset of precipitation, cells with relatively constant diameter self-organize, with diameters roughly on the scale of the atmospheric boundary layer height. We find that the onset of precipitation then signals an approximately linear increase in horizontal scale with time. For our transient simulations, this scale increase progresses at a speed which is relatively insensitive to modifications in mean surface temperature, modifications in the rate at which surface temperature changes, or the initial lapse rate. When exploring the strength of the spatial correlations, we find that precipitation onset causes a sudden disruption of order and a subsequent decline of organizationuntil precipitation eventually ceases. We discuss possible implications for the development of extreme precipitation events.

Plain Language Summary When a fluid is placed between two horizontal plates and the lower plate is sufficiently heated, the fluid undergoes a dynamical motion where convective cells of typical diameter are formed. Within the cells, the fluid rises and sinks in a systematic way. The air of the atmosphere is similar to such a fluid system; it, however, contains moisture which can produce clouds and precipitation. When clouds form through condensation, this can be seen as a heating process, leading to a disruption of the patterns initially formed. We show that after onset of precipitation the typical diameter of structures within the atmosphere systematically increases throughout the course of the day. The increase is found to be rather independent of how much the surface is heated or which configuration of temperature is used to initialize the atmosphere in the morning. Our results show that the onset of precipitation is a characteristic time which can be used to compare very different numerical experiments. For the real-world atmosphere our results have important implications regarding extreme rainfall. Such extreme rainfall has been found to react strongly to changes in surface temperature. Our results make a step toward explaining how this can be the case.