Abstract
Comparisons between bin and bulk cloud microphysics schemes are conducted by simulating a heavy precipitation case using a bin microphysics scheme and four double-moment bulk microphysics schemes in the Weather Research and Forecasting (WRF) model. For this, we implemented an updated bin microphysics scheme in the WRF model. All of the microphysics schemes underestimate observed strong precipitation, but the bin microphysics scheme yields the result that is closest to observations. The differences among the schemes are more pronounced in terms of hydrometeor number concentration than in terms of hydrometeor mixing ratio. In this case, the bin scheme exhibits remarkably more latent heat release by deposition and riming than the bulk schemes. This causes stronger updrafts and more upward transport of water vapor, which leads to more deposition, and again, increases the latent heat release. An additional simulation using the bin scheme but excluding the riming of cloud droplets on ice crystals, which is not or poorly treated in the examined bulk schemes, shows that surface precipitation is slightly weakened and moved farther downwind compared to that of the control simulation. This implies that the more appropriate representation of microphysical processes in the bin microphysics scheme contributes to the more accurate prediction of precipitation in this case.
Highlights
There are two methods that are widely used to represent clouds and precipitation in numerical weather and climate models: bulk and bin microphysics schemes
While bin microphysics schemes are believed to be useful to investigate the evolution of size distributions of particles in clouds, bulk microphysics schemes are widely used in operational weather forecasting, owing to their much lower computational cost than bin microphysics schemes
While previous studies have tended to focus on the expression of particle size distributions, this study investigates whether differences in the results of bin and bulk microphysics schemes can be caused by more appropriate representations on microphysical processes in the bin microphysics scheme
Summary
There are two methods that are widely used to represent clouds and precipitation in numerical weather and climate models: bulk and bin microphysics schemes. Fan et al [6] compared the results of a bin microphysics scheme [19] to those of a double-moment bulk microphysics scheme [20] in simulating two precipitation cases that occurred in southern. Note that the bin microphysics scheme used in this study is somewhat different from the publicly available version [12], in which the number of bins for each hydrometeor and aerosol is 33 and some microphysical processes (e.g., gradual melting), as well as liquid water fractions of large ice particles and rimed fractions of snow particles, are not predicted. Precipitation occurred at a stationary front induced by the convergence of these two different air masses over the Korean Peninsula while thermodynamic instability (such as convective available potential energy) was low throughout the precipitation period. The first 6 hr is excluded from analysis considering the spin-up time of the model, and the remaining 18-hr data are used for analysis
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