How do changes in warm-phase microphysics affect deep convective clouds?

Abstract

<p><strong>Abstract.</strong> Understanding aerosol effects on deep convective clouds and the derived effects on the radiation budget and rain patterns can largely contribute to estimations of climate uncertainties. The challenge is difficult in part because key microphysical processes in the mixed and cold phases are still not well understood. For deep convective clouds with a warm base, understanding aerosol effects on the warm processes is extremely important as they set the initial and boundary conditions for the cold processes. Therefore, in this study the focus is on the warm phase, which can be better resolved. The main question is: ''How do aerosol-derived changes in the warm phase affect the properties of deep convective cloud systems?'' To explore this question, we used the weather research and forecasting (WRF) model with spectral bin microphysics to simulate a deep convective cloud system over Marshall Islands during the Kwajalein Experiment (KWAJEX). The model results were validated against observations, showing similarities in the vertical profile of radar reflectivity and the surface rain rate. Simulations with larger aerosol loading resulted in a larger total cloud mass, a larger cloud fraction in the upper levels, and a larger frequency of strong updrafts and rain rates. Enlarged mass both below and above the zero temperature level (ZTL) contributed to the increase in clouds’ total mass (water and ice) in the polluted runs. Increased condensation efficiency of cloud droplets governed the gain in mass below the ZTL, while both enhanced condensational and depositional growth led to increased mass above it. Moreover, the polluted runs showed increased upward transport (across the ZTL) of liquid water, driven by two main processes: 1) larger updrafts and 2) larger droplet mobility. These aerosol effects were reflected in the larger ratio between the masses located above and below the ZTL in the polluted runs. When comparing the net mass flux crossing the ZTL in the clean and polluted runs, the difference was small. However, when comparing the upward and downward fluxes separately, the increase in aerosol concentration was seen to dramatically increase the fluxes in both directions, indicating the aerosol-amplification effect of the convection and affecting cloud-system properties such as cloud fraction and rain rate.</p>

Aerosol Impacts on Mesoscale Convective Systems Forming Under Different Vertical Wind Shear Conditions

“…The aerosol invigoration effect is mainly present at altitudes lower than 8 km where condensation plays a significant role, indicating the important role of condensation changes by CCN, as revealed by Chen et al () and Fan et al (). We find that activation of UAP plays a dominant role in droplet formation in the polluted condition, as in Fan et al ().…”

“…The first, second, and third terms at the right hand of equation (1) stand for the thermal buoyancy, water vapor buoyancy, and condensate loading effects, respectively. The total buoyancy over updraft cores is contributed largely by the thermal buoyancy (Chen et al, ; Fan et al, ). The condensate loading effect is opposite to the thermal buoyancy, showing negative buoyancy.…”

“…There is also a significant decrease of the occurrence frequency of light rain (rain rate of 0.25–1 mm hr −1 ). This result is consistent with many previous studies where enhanced convection by aerosols is seen (e.g.,Chen et al, ; Clavner et al, ; Fan et al, ; Guo et al, ; Qian et al, ). Note that the onset and peak time of mean rain rate are similar between the clean and polluted runs for all of the wind shear cases (Figure , third row), indicating that aerosols do not influence convective development.…”

The Roles of Mineral Dust as Cloud Condensation Nuclei and Ice Nuclei During the Evolution of a Hail Storm

“…With enhanced CCN loading, hail storms tend to be more intensive with larger size of hailstones (Ilotoviz et al, ; Khain et al, ; Lerach & Cotton, ; Loftus & Cotton, ; Yang et al, ). The lower collision efficiency of smaller cloud droplets and the stronger updraft with higher CCN concentrations result in larger liquid water content (LWC) at higher levels, leading to enhanced riming process to produce larger ice particles (Andreae et al, ; Chen et al, ; Han et al, ; Khain et al, ; Lerach & Cotton, ; Reutter et al, ; Rosenfeld & Bell, ). Therefore, hail particles can grow largely by the accretion of cloud droplets with increased amount of supercooled cloud water content (Ilotoviz et al, ; Loftus & Cotton, ).…”