For the past few decades, researchers have focused on how aerosols influence clouds and precipitation, namely the aerosol-cloud-precipitation interaction (ACPI).
Excessive aerosols, released to the atmosphere by continuing human activities, could modify the characteristics of clouds and precipitation after being activated as cloud condensation nuclei (CCN). Clouds developed under the environment with more CCN could have more cloud droplets but smaller sizes, leading to a narrower drop size distribution (DSD). Small droplet sizes and a narrow DSD could result in less collision-coalescence efficiency and thus suppress the warm rain processes. However, in deep convection, mixed-phase microphysics processes come into play, and more complicated mechanisms that could affect precipitation are included (Tao et al., 2012).
How aerosols influence deep convection, which has a higher ability to produce heavy precipitation and a greater probability of causing hazards, is particularly the main target of research.
As listed in Tao et al. (2012), numerous studies using cloud-resolving model (CRM) simulations were conducted for the sake of testing the convective precipitation sensitivity to increasing aerosols. Cases of squall lines, mesoscale convective systems, cold fronts, sea breeze-induced cumulus, and the other forms of convection were included. Due to the high variety of meteorological conditions and weather systems,
the results showed no consistent agreement among these simulations (Fan et al., 2016).
Since both meteorology and aerosols could influence the development of clouds and precipitation, Stevens and Feingold (2009) stated that the aerosol effects on clouds and precipitation are almost certainly dependent on the regimes. Thus, it is necessary to focus on a specific regime while investigating ACPI, especially for deep convection. It should be remarked that the regime includes not only meteorological factors, but also orography, land use types, and the other aspects of the environment.
Afternoon thunderstorms are locally and diurnally developed deep convection.
Even if significant synoptic-scale weather forcing is absent, the development of afternoon thunderstorms can still be fueled by the surface heat flux (including sensible heat and latent heat), and be affected by the flow pattern of local circulation. Since solar heating and corresponding surface heat flux are stronger on the mountain ridges, topography could influence the development of the afternoon thunderstorms. Several studies have highlighted the importance of topography in the afternoon thunderstorms in Taiwan. Kuo and Wu (2019) used idealized CRM simulations to show that the confluent flow of sea breezes coming in from the Keelung River Valley and the Tamsui River Valley could determine the location of initiation and the development of afternoon thunderstorms inside Taipei Basin. Chen et al. (2010) discovered that the formation and maintenance mechanism of an afternoon thunderstorm system over
Snow Mountain Range (SMR) on June 20th, 2000 was related to the lifting of high equivalent potential temperature airflow over the southwestern slope of SMR. Miao and Yang (2020) simulated an afternoon thunderstorm case on June 14th, 2015. They revealed that the channel effect along the Tamsui River Valley intensified the sea breeze and increased moisture transport, providing favorable dynamic and thermodynamic conditions for stronger convection to develop inside Taipei Basin. Thus, with the tight relationship between afternoon thunderstorms and the local environment, especially topography, we postulate that the influence of microphysics perturbation on convection through increasing CCN can be revealed more clearly in these “topographically-locked”
afternoon thunderstorms given similar large-scale weather conditions.
Rosenfeld et al. (2008) proposed that deep convection can be invigorated under the environment with more aerosols. Since the warm rain processes are suppressed, more freezing of cloud droplets are allowed and latent heat release is enhanced above the freezing level. Thus, the deep convection would become stronger and cause more rainfall under a more polluted environment. Grabowski and Morrison (2016) showed that the precipitation can be strengthened by 10% with high CCN concentration based on the simulation of a diurnally developed deep convection case during the Large-Scale Biosphere-Atmosphere (LBA; Avissar and Nobre, 2002) field campaign over the great plain of Amazon (Grabowski et al., 2006). Different from the aerosol invigoration effect,
they suggested that the increase in precipitation was mainly due to a smaller supersaturation below the freezing level. However, the development of diurnally developed deep convection in Taiwan is highly affected by its complex topography, which is not considered in the mechanisms mentioned above. Thus, the aerosol effect on the diurnally developed deep convection over complex topography might not be simply explained by these mechanisms.
Several studies have introduced the aerosol effects on convective precipitation under different orographic regimes. Seo et al. (2020) showed that the upslope geometry could control the precipitation of shallow convective clouds over a bell-shaped mountain by conducting two-dimensional idealized simulations. Yang et al. (2016) discovered that the weakening of mountain-valley circulation due to increasing absorbing aerosols through aerosol-radiation interaction could lead to a reduction in convection and precipitation in Mount Hua, Shaanxi, China. Observations from Dominica Experiment (DOMEX; Smith et al., 2012) field campaign also revealed that aerosols could have impacts on thermally driven orographic clouds and precipitation (Nugent et al., 2016), but the effect of wind and cloud-layer moisture could be more dominant. However, the aerosol effects on diurnally developed deep convection over terrain remain insufficiently discerned. None of the studies mentioned above has explained how aerosols influence the properties of diurnally developed deep convection
with the role of complex topography.
In this study, we focus on how aerosols affect the properties of diurnally developed deep convection under weak synoptic weather regime over complex topography with
the view of the features of precipitating systems. Due to the complicated interactions between convective clouds and their environment, it is challenging to separate the impacts of aerosols from the effects of meteorology on convection simply using observational data (Grabowski, 2018). Thus, semi-realistic large-eddy simulations (LESs) with fine temporal and spatial resolutions are conducted, highlighting the role of topography on the evolution of diurnally developed deep convection. The analyses
based on cloud object connecting and rain cell tracking provide novel and useful insights to the understanding of the responses resulting from increasing aerosols.
Section 2 presents the model description and the experiment setup. The influence of aerosols, serving as CCN, on diurnally developed deep convection over complex
topography are analyzed in Section 3, mainly with the perspective from precipitating systems. Summary and discussion are shown in Section 4, including possible roles of local circulation.