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U.S. Department of Energy (DOE) Atmospheric System Research (ASR) Program; U.S. Department of Energy (DOE) Climate Model Development and Validation (CMDV) program; DOE [DE-AC06-76RLO1830]; Office of Science of the U.S. DOE [DE-AC02-05CH1123]; National Basic Research Program of China [2013CB430105]; National Natural Science Foundation of China [41575130, 41775132]; U.S. DOE ASR grant [DE-SC0008678, DE-SC0008648, DE-SC0016476]; DOE CMDV project at University of Arizona [DE-SC0017015]; U.S. DOE [DE-AC02-98CH10886]; Israel Science Foundation [2027/17]; U.S. National Science Foundation; [DE-SC008811]

Observations during the Ice in Clouds Experiment-Tropical (ICE-T) field experiment show that the ice particles concentration in a developing deep convective clouds at the level of T = −15 °C reached about 500 L−1, that is, many orders higher than that of ice-nucleating particle. To simulate microphysics of these clouds, the 2-D Hebrew University Cloud model (HUCM) with spectral bin microphysics was used in which two main types of ice multiplication mechanisms were included in addition to the Hallet-Mossop mechanism. In the first ice multiplication mechanism ice splinters form by drop freezing and drop-ice collisions. Ice multiplication of this type dominates during developing stage of cloud evolution, when liquid water content is significant. At later stage when clouds become nearly glaciated, ice crystals are produced largely by ice splintering during ice-ice collisions (the second ice multiplication mechanism). Simulations show that droplet size distributions, as well as size distributions of ice particles, agree well with the measurements during ICE-T. Simulations with different cloud condensation nuclei concentrations show the existence of the “optimum” cloud condensation nuclei concentration (or droplet concentration), at which concentration of ice splinters reaches maximum. In these simulations ice nucleation caused by the direct formation of ice crystals upon ice-nucleating particles, as well as the Hallett-Mossop process, has a negligible contribution to the ice crystal concentration. (Less)

Stratospheric water vapor has an important effect on Earth's climate. Considering the significance of overshooting deep convection in modulating the water vapor content (WVC) of the lower stratosphere (LS), we use a three-dimensional convective cloud model to simulate the effects of various silver iodide (AgI) seeding scenarios on tropical overshooting deep convection that occurred on 30 November 2005 in Darwin, Australia. The primary motivation for this study is to investigate whether the WVC in the LS can be artificially modified by deliberate cloud seeding. It is found that AgI seeding done at the early stages of clouds produces significant effects on cloud microphysical and dynamical properties, and that further affects the WVC in the LS, while seeding at the mature stages of clouds has only a slight impact. The response of stratospheric water vapor to changes in the amount of seeding agent is nonlinear. The seeding with a small (large) amount of AgI increases (decreases) the WVC in the LS, due to enhanced (reduced) production and vertical transport of cloud ice from the troposphere and subsequent sublimation in the stratosphere. The results show that stratospheric water vapor can be artificially altered by deliberate cloud seeding with proper amount of seeding agent. This study also shows an important role of graupel in regulating cloud microphysics and dynamics and in modifying the WVC in the LS.

Typhoon Saomai (2006) was simulated using the Weather Research and Forecasting Model (WRF) with explicit spectral (bin) microphysics (SBM) to investigate the effects of aerosol from mainland China on the intensity, structure and precipitation of the landfalling storm. MAR (maritime), MIX (semi-continental) and CON (continental) experiments were conducted with different initial aerosol concentrations. Varying aerosol concentrations had little influence on the storm track but resulted in pronounced deviations in intensity and structures. The experiment with a high initial aerosol concentration showed invigorated convection at the periphery of the tropical cyclone (TC), which interfered with the reformation of the eyewall, leading to TC weakening. The minimum pressures in the CON and MIX experiments were increased by more than 30 hPa and 14.6 hPa, and their maximum wind speeds were 20 m s-1 and 13 m s-1 weaker than that in the MAR experiment, respectively. The rain rates in the MIX and CON experiments were 16.6% and 56.2% greater than that in the MAR run, with the differences mainly occurring in the outer rainbands. These results indicate that the aerosol concentration substantially affects the spatial distributions of cloud hydrometeors and rainfall. The increase of rainfall was triggered by an increase in the melting of graupel and cloud droplets collected by raindrops. Similarly, the graupel melting process also enhanced in the outer rainbands with increasing aerosol. Furthermore, a positive microphysics feedback associated with the rainwater in the outer rainbands played an important role in increasing the rain rate in more aerosol scenarios.

As part of the third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-III) field campaign, raindrop size distribution (DSD) measurements were taken with a laser optical disdrometer in Naqu, China, at 4508 m above sea level (ASL) during the summer months of 2013, 2014, and 2015. The characteristics of DSDs for five different rain rates, for two rain types (convective and stratiform), and for daytime and nighttime rains were studied. The shapes of the averaged DSDs were similar for different rain rates, and the width increased with rainfall intensity. Little difference was found in stratiform DSDs between day and night, whereas convective DSDs exhibited a significant day-night difference. Daytime convective DSDs had larger mass-weighted mean diameters (Dm) and smaller generalized intercepts (NW) than the nighttime DSDs. The constrained relations between the intercept N0 and shape μ, slope Λ and μ, and NW and Dm of gamma DSDs were derived. We also derived empirical relations between Dm and the radar reflectivity factor in the Ku- and Ka-bands.

[1] A cloud resolving model coupled with a spectral bin microphysical scheme was used to investigate the effects of deep convection on the concentration and size distribution of aerosol particles within the upper troposphere. A deep convective storm that occurred on 1 December, 2005 in Darwin, Australia was simulated, and was compared with available radar observations. The results showed that the radar echo of the storm in the developing stage was well reproduced by the model. Sensitivity tests for aerosol layers at different altitudes were conducted in order to understand how the concentration and size distribution of aerosol particles within the upper troposphere can be influenced by the vertical transport of aerosols as a result of deep convection. The results indicated that aerosols originating from the boundary layer can be more efficiently transported upward, as compared to those from the mid-troposphere, due to significantly increased vertical velocity through the reinforced homogeneous freezing of droplets. Precipitation increased when aerosol layers were lofted at different altitudes, except for the case where an aerosol layer appeared at 5.4–8.0 km, in which relatively more efficient heterogeneous ice nucleation and subsequent Wegener-Bergeron-Findeisen process resulted in more pronounced production of ice crystals, and prohibited the formation of graupel particles via accretion. Sensitivity tests revealed, at least for the cases considered, that the concentration of aerosol particles within the upper troposphere increased by a factor of 7.71, 5.36, and 5.16, respectively, when enhanced aerosol layers existed at 0–2.2 km, 2.2–5.4 km, and 5.4–8.0 km, with Aitken mode and a portion of accumulation mode (0.1–0.2μm) particles being the most susceptible to upward transport.