• Asia-Pacific Journal of Atmospheric Sciences
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• v.54 no.4
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• pp.587-598
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• 2018

The impacts of aerosol loading on surface precipitation from mid-latitude deep convective systems are examined using a bin microphysics model. For this, a precipitation case over north central Mongolia, which is a high-altitude inland region, on 21 August 2014 is simulated with aerosol number concentrations of 150, 300, 600, 1200, 2400, and $4800cm^{-3}$. The surface precipitation amount slightly decreases with increasing aerosol number concentration in the range of $150-600cm^{-3}$, while it notably increases in the range of $600-4800cm^{-3}$ (22% increase with eightfold aerosol loading). We attempt to explain why the surface precipitation amount increases with increasing aerosol number concentration in the range of $600-4800cm^{-3}$. A higher aerosol number concentration results in more drops of small sizes. More drops of small sizes grow through condensation while being transported upward and some of them freeze, thus increasing the mass content of ice crystals. The increased ice crystal mass content leads to an increase in the mass content of small-sized snow particles largely through deposition, and the increased mass content of small-sized snow particles leads to an increase in the mass content of large-sized snow particles largely through riming. In addition, more drops of small sizes increase the mass content of supercooled drops, which also leads to an increase in the mass content of large-sized snow particles through riming. The increased mass content of large-sized snow particles resulting from these pathways contributes to a larger surface precipitation amount through melting and collision-coalescence.

• Atmosphere
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• v.23 no.3
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• pp.293-305
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• 2013

Nonhydrostatic effects on convectively forced mesoscale flows in two dimensions are numerically investigated using a nondimensional model. An elevated heating that represents convective heating due to deep cumulus convection is specified in a uniform basic flow with constant stability, and numerical experiments are performed with different values of the nonlinearity factor and nonhydrostaticity factor. The simulation result in a linear system is first compared to the analytic solution. The simulated vertical velocity field is very similar to the analytic one, confirming the high accuracy of nondimensional model's solutions. When the nonhydrostaticity factor is small, alternating regions of upward and downward motion above the heating top appear. On the other hand, when the nonhydrostaticity factor is relatively large, alternating updraft and downdraft cells appear downwind of the main updraft region. These features according to the nonhydrostaticity factor appear in both linear and nonlinear flow systems. The location of the maximum vertical velocity in the main updraft region differs depending on the degrees of nonlinearity and nonhydrostaticity. Using the Taylor-Goldstein equation in a linear, steady-state, invscid system, it is analyzed that evanescent waves exist for a given nonhydrostaticity factor. The critical wavelength of an evanescent wave is given by ${\lambda}_c=2{\pi}{\beta}$, where ${\beta}$ is the nonhydrostaticity factor. Waves whose wavelengths are smaller than the critical wavelength become evanescent. The alternating updraft and downdraft cells are formed by the superposition of evanescent waves and horizontally propagating parts of propagating waves. Simulation results show that the horizontal length of the updraft and downdraft cells is the half of the critical wavelength (${\pi}{\beta}$) in a linear flow system and larger than ${\pi}{\beta}$ in a weakly nonlinear flow system.