Abstract. In most previous direct numerical simulation (DNS) studies on droplet growth in turbulence, condensational growth and collisional growth were treated separately. Studies in recent decades have postulated that small-scale turbulence may accelerate droplet collisions when droplets are still small when condensational growth is effective. This implies that both processes should be considered simultaneously to unveil the full history of droplet growth and rain formation. This paper introduces the first direct numerical simulation approach to explicitly study the continuous droplet growth by condensation and collisions inside an adiabatic ascending cloud parcel. Results from the condensation-only, collision-only, and condensation-collision experiments are compared to examine the contribution to the broadening of droplet size distribution (DSD) by the individual process and by the combined processes. Simulations of different turbulent intensities are conducted to investigate the impact of turbulence on each process and on the condensation-induced collisions. The results show that the condensational process promotes the collisions in a turbulent environment and reduces the collisions when in still air, indicating a positive impact of condensation on turbulent collisions. This work suggests the necessity of including both processes simultaneously when studying droplet-turbulence interaction to quantify the turbulence effect on the evolution of cloud droplet spectrum and rain formation.
Abstract. This paper investigates the relative importance of turbulence and aerosol effects on the broadening of the droplet size distribution (DSD) during the early stage of cloud and raindrop formation. A parcel–DNS (direct numerical simulation) hybrid approach is developed to seamlessly simulate the evolution of cloud droplets in an ascending cloud parcel. The results show that turbulence and cloud condensation nuclei (CCN) hygroscopicity are key to the efficient formation of large droplets. The ultragiant aerosols can quickly form embryonic drizzle drops and thus determine the onset time of autoconversion. However, due to their scarcity in natural clouds, their contribution to the total mass of drizzle drops is insignificant. In the meantime, turbulence sustains the formation of large droplets by effectively accelerating the collisions of small droplets. The DSD broadening through turbulent collisions is significant and therefore yields a higher autoconversion rate compared to that in a nonturbulent case. It is argued that the level of autoconversion is heavily determined by turbulence intensity. This paper also presents an in-cloud seeding scenario designed to scrutinize the effect of aerosols in terms of number concentration and size. It is found that seeding more aerosols leads to higher competition for water vapor, reduces the mean droplet radius, and therefore slows down the autoconversion rate. On the other hand, increasing the seeding particle size can buffer such a negative feedback. Despite the fact that the autoconversion rate is prominently altered by turbulence and seeding, bulk variables such as liquid water content (LWC) stays nearly identical among all cases. Additionally, the lowest autoconversion rate is not co-located with the smallest mean droplet radius. The finding indicates that the traditional Kessler-type or Sundqvist-type autoconversion parameterizations, which depend on the LWC or mean radius, cannot capture the drizzle formation process very well. Properties related to the width or the shape of the DSD are also needed, suggesting that the scheme of Berry and Reinhardt (1974) is conceptually better. It is also suggested that a turbulence-dependent relative-dispersion parameter should be considered.
Several single-year-long (2017) simulations with different configurations of the Weather Research and Forecasting (WRF) model over the United Arab Emirates (UAE) and the Middle-East at the convective gray-zone resolution (9 km) are evaluated in their ability to capture the temporal and spatial distributions of precipitation. Annual rainfall over the Middle-East is dominated by wintertime precipitation and is mostly initiated by the frontal systems intrusion. WRF at 9 km resolution shows good skill in capturing the synoptic and meso-scale precipitation distributions. The aerosol-aware Thompson microphysics scheme outperforms the WRF double moment 6 class microphysics by about 20% in annual mean precipitation over the Middle-East. Different Planetary Boundary Layer (PBL) physics leads to large differences in the annual rainfall over both the Middle-East (about 30%) and the UAE (about 45%). The Quasi-Normal Scale Elimination (QNSE) PBL scheme produces stronger precipitation than the Asymmetrical Convective Model and is in better agreement with observations. This difference is attributed to the former scheme producing a warmer and moister lower-level atmosphere that builds more substantial instability. Regions of stronger instability also depict decreases in lifting condensation level heights and increases in boundary layer height, suggesting that the boundary layer reaching the cloud base helps to trigger the convection and increase precipitation. Refinement in grid spacing (5 km) marginally improves the summertime precipitation over the Middle-East but significantly increases the computational cost. Current study also indicates that gray-zone simulations can perform as good as convection-permitting simulations by carefully choosing the right model physics packages for the synoptic and meso-scale precipitation.Plain Language Summary Capturing rainfall variability over the Middle-East can be quite challenging for atmospheric models due to the complex interactions between synoptic systems, topography, dust, and convective-scale motions. The Weather Research and Forecasting (WRF) model is configured and tested over the United Arab Emirates (UAE) and the Middle-East for the realistic representation of precipitation. Various combinations of cloud microphysics and boundary layer physics schemes were tested to unearth the most suitable configuration at the convective gray-zone resolution (9 km). The model simulates precipitation comparable to observations in terms of its synoptic and meso-scale variability. It is found that altering the cloud microphysical schemes inflicts ∼20% change in annual mean precipitation over the Middle-East region. However, changing the boundary layer physics dramatically impacts the annual mean rainfall over the Middle-East (∼30%) and specifically over the UAE (∼45%). Reduction in horizontal grid spacing (5 km) does not improve the winter precipitation but marginally increases the summertime precipitation over the Middle-East region. Note that the reduction in horizontal grid spacing significa...
Urbanization extensively modifies surface roughness and properties, impacting regional climate and hydrological cycles. Urban effects on temperature and precipitation have drawn considerable attention. These associated physical processes are also closely linked to clouds’ formation and dynamics. Cloud is one of the critical components in regulating urban hydrometeorological cycles but remains less understood in urban-atmospheric systems. We analyzed satellite-derived cloud patterns spanning two decades over 447 US cities and quantified the urban-influenced cloud patterns diurnally and seasonally. The systematic assessment suggests that most cities experience enhanced daytime cloud cover in both summer and winter; nocturnal cloud enhancement prevails in summer by 5.8%, while there is modest cloud suppression in winter nights. Statistically linking the cloud patterns with city properties, geographic locations, and climate backgrounds, we found that larger city size and stronger surface heating are primarily responsible for summer local cloud enhancement diurnally. Moisture and energy background control the urban cloud cover anomalies seasonally. Under strong mesoscale circulations induced by terrains and land–water contrasts, urban clouds exhibit considerable nighttime enhancement during warm seasons, which is relevant to strong urban surface heating interacting with these circulations, but other local and climate impacts remain complicated and inconclusive. Our research unveils extensive urban influences on local cloud patterns, but the effects are diverse depending on time, location, and city properties. The comprehensive observational study on urban–cloud interactions calls for more in-depth research on urban cloud life cycles and their radiative and hydrologic implications under the urban warming context.
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