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

Abstract

Abstract. 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, the focus of this study is 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 a weather research and forecasting (WRF) model with spectral bin microphysics to simulate a deep convective cloud system over the 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 cloud 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. The enhanced mass loading above the ZTL acted to reduce the cloud buoyancy, while the thermal buoyancy (driven by the enhanced latent heat release) increased in the polluted runs. The overall effect showed an increased upward transport (across the ZTL) of liquid water driven by both larger updrafts and 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 the affected cloud system properties, such as cloud fraction and rain rate.

Investigating the impacts of Saharan dust on tropical deep convection using spectral bin microphysics

“…This would indicate that average hydrometeor particle sizes are smaller, allowing for increased vertical transport and slower sedimentation rates near the cloud tops. Similar increases in vertical transport can also occur due to aerosol effects on the liquid phase of DCC increasing particle mobility (Koren et al, 2015;Chen et al, 2017), although in our current study, CCN concentrations have not been changed. This suggests that IN concentration may also play a complimentary role in cloud top height enhancement in addition to changes CCN number concentration noted by previous studies.…”

Quantifying the effect of aerosol on vertical velocity and effective terminal velocity in warm convective clouds

“…Although previous studies have focused on deep convective clouds, these effects are expected to be significant in warm convective clouds as well. Moreover, warm processes serve as the initial and boundary conditions for mixed-phase processes in deep convective clouds, and therefore gaining a better process understanding of the warm phase is essential for understanding the deeper systems (Chen et al, 2017).…”

Interactive comment on “Aerosol-induced changes in the vertical structure of precipitation: a perspective of TRMM precipitation radar” by Jianping Guo et al.

“…The bulk precipitation system parameter called the reflectivity center of gravity (ZCOG) is used to represent the vertically weighted reflectivity distribution (Chen et al, 2016).…”

“…The three-dimensional (3-D) structures of radar echoes, which are determined by a combination of dynamic, thermodynamic, and cloud microphysical processes, are known as a good way to represent details inside precipitating systems (Zipser and Lutz, 1994;Yuter and Houze, 1995;Min et al, 2009;Chen et al, 2016). Any systematic changes in precipitation vertical structure as aerosol varies may provide new insights into the mechanism underlying the aerosolcloud-precipitation interaction (Koren et al, 2009;Chen et al, 2017). Indeed, the deployment of the cloud profiling radar onboard CloudSat has led to new insights into the response of clouds to aerosols (e.g., Nakajima et al, 2010;Suzuki et al, 2010;Chen et al, 2016;Peng et al, 2016).…”

“…Any systematic changes in precipitation vertical structure as aerosol varies may provide new insights into the mechanism underlying the aerosolcloud-precipitation interaction (Koren et al, 2009;Chen et al, 2017). Indeed, the deployment of the cloud profiling radar onboard CloudSat has led to new insights into the response of clouds to aerosols (e.g., Nakajima et al, 2010;Suzuki et al, 2010;Chen et al, 2016;Peng et al, 2016). To the best of our knowledge, however, few studies have ever used the precipitation radar (PR) to analyze the association of the vertical structure of precipitation with aerosol in China.…”