This study investigates the effects of aerosol vertical distribution on a deep convective cloud system. We intend to elucidate the mechanisms for aerosols entering the cloud from different heights, and how they affect cloud microphysics and precipitation. A thermal bubble is released at 1.5 km initially to run an idealized case using the Weather Research and Forecast (WRF) model. The aerosol layer with high concentration was initially put at different altitudes in the model to study the mechanisms and the number of aerosols entering the cloud. It was found that there are three mechanisms for aerosols from different heights to enter the cloud, depending on their relative height with the thermal bubble. Aerosols from lower altitudes (below 1 km) enter the cloud through pumping, while aerosols from higher altitudes (2–3 km, 3–5 km) enter the cloud through entrainment. Both mechanisms lead to low cloud condensation nuclei (CCN) concentration in the cloud. Only aerosols from intermediate altitudes (1–2 km), which is the same as the initial height of the thermal bubble, enter the cloud mainly by ascending with the bubble and lead to high CCN concentration in the cloud. The differences in activated CCN concentration affect the microphysical processes and precipitation remarkably. For the simulations with an initial aerosol layer at 1–2 km and 0–5 km, aerosols can enter the cloud more efficiently than the other four simulations. More activated CCNs in these two simulations lead to more graupels with smaller sizes at higher altitudes, which delays the precipitation but makes the precipitation last longer. However, the accumulated precipitation is similar in all six simulations, no matter what aerosol vertical distribution is like. The results in this study indicate that the altitude of aerosol layers determines the mechanisms for aerosols entering clouds, CCN concentration in the cloud, and to what extent the cloud microphysical processes and precipitation are affected.
<p>The characteristics of isolated deep convection initiation (DCI) and its relation to topography in the North China area are studies statistically and numerically. The infrared brightness temperature data from satellite Himawari-8 are utilized to identify DCI events in three summers. A total of 2534 DCI events are obtained and their locations show clustering over mountains and hills, suggesting the significance of local topography. Topography is described with elevation and relief amplitude. DCI events and grid boxes are counted. DCI events per grid box increases with elevation and relief amplitude. Among different types of topography, DCI is favored in mountains and hilly areas. Moreover, the morning cloud cover condition also shows notable impact on the relation of DCI and topography. For the regime characterized with less morning clouds (regime one), DCI strongly depends on elevation and relief amplitude, while for the regime with more morning clouds (regime two), topography shows a moderate impact on DCI. The time of DCI events are also recorded, and regime one shows a stronger diurnal variation and a peak occurring 2 hours earlier than that of regime two. The synoptic patterns show the difference of large-scale environment between the two regimes, which can explain their differences in DCI to some extent. To clarify the mechanism of topographic effect in DCI process, quasi-idealized numerical simulation in North China is conducted with WRF. The averaged 6-hourly ERA-Interim reanalysis data, which can maintain the major patterns of large-scale circulations, are inputted into the model as initial and boundary conditions. The elevation and relief amplitude of the study domain is varied in the model. The preliminary result shows that the speed of upscale convection growth changes with elevation and relief amplitude, which indicates that mechanisms involving topography-induced variation of solar heating may exist and need further numerical study. We suggest that special attention should be paid to elevation and relief amplitude (or topography type), as well as morning cloud cover condition when forecasting DCI in the North China area and mountainous areas around the world.</p>
<p>Cloud feedback in mid-latitude marine stratocumulus is not clearly understood due to few reliable observations. Stratocumulus cloud is the most frequent and extensive cloud type over mid-latitude marine areas and has strong short-wave radiative effect. In this study, large eddy simulation (LES) is used to resolve the vertical structure of mid-latitude marine stratocumulus. We find that, in the wintertime over North Pacific, stratocumulus cloud often forms in regions of high pressure and large-scale sinking motion, and can remain in steady-state for a couple of days. We then choose two typical cases to do LES simulation: One has a lower cloud top height and a stronger temperature inversion (case l), without mesoscale cellular structure; the other has a higher cloud top height and a weaker temperature inversion (case h), with closed-cell cellular structure. The liquid water content profiles are adiabatic, and the boundary layer is well-mixed for both cases. In case l, the main source of turbulent kinetic energy (TKE) is from cloud top long-wave radiative cooling for the entire boundary layer. In case h, TKE production due to cloud-top longwave cooling is only significant in the cloud layer, and the subcloud layer TKE is mainly from surface processes.</p>
Abstract. Insoluble particles affect weather and climate indirectly by heterogeneous freezing process. Current weather and climate models have large uncertainty in freezing process simulation due to little regarding species and number concentration of heterogeneous ice-nucleating particles, mainly insoluble particles. Here, for the first time, size distribution and species of insoluble particles are analyzed in 30 shells of 12 hailstones in China, using scanning electron microscopy and energy dispersive X-ray spectrometry. Total 289,461 insoluble particles are detected and grouped into 3 species: organics, dust, and bioprotein by machine learning methods. The size distribution of insoluble particles of each species vary greatly in different hailstorms but little in shells. Further, classic size distribution modes of organics and dust were performed as logarithmic normal distributions, which may be adapted in future weather and climate models though uncertainty still exists. Our finding suggests that physical properties of aerosols should be considered in model simulation on ice freezing process.
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