ALBERT

Description: The capability of Weather Research and Forecasting (WRF) model in the simulation of cirrus clouds has been examined, with a focus on the effects of radiative processes and vertical model resolution. We incorporate in WRF a new radiation module, referred to as the Fu-Liou-Gu scheme, which is an improvement and refinement based on the Fu-Liou scheme, particularly in reference to parameterization of the single-scattering properties of ice crystal size and shape. We conducted a number of real-time WRF simulations for cirrus cases that were observed in the coastal and western United States on 29–30 March 2007, and we compared these with available observations from Moderate Resolution Imaging Spectroradiometer (MODIS) and GOES visible and IR images over the same areas. Simulation results show that WRF is capable of generating reasonable cirrus cloud fields and their movement and dissipation processes, especially those associated with the large-scale frontal system. Radiative processes are important in cirrus cloud simulations by affecting the vertical thermal structure and hence convection. The newly implemented radiation module, the Fu-Liou-Gu scheme, has been demonstrated to work well in WRF and can be effectively used for studies related to cirrus cloud formation and evolution and aerosol-cloud-radiation interactions. With the newly implemented radiation scheme, the simulations of cloud cover and cloud and ice water path (CWP and IWP) have been improved for cirrus clouds, with a more consistent comparison with the corresponding MODIS observations in terms of CWP and IWP means and CWP frequency distribution, especially for optically thin cirrus with an improvement of about 20% in simulated mean IWP. The model-simulated ice crystal sizes have also been shown to be comparable to those determined from MODIS cloud products. Finally, we have demonstrated that model vertical resolution plays a significant role in cirrus cloud simulation in terms of altering vertical velocity field and the associated regional circulation.

Description: A stochastic approach has been developed to model the positions of BC/dust internally mixed with two snow-grain types: hexagonal plate/column (convex) and Koch snowflake (concave). Subsequently, light absorption and scattering analysis can be followed by means of an improved geometric-optics approach coupled with Monte Carlo photon tracing to determine BC/dust single-scattering properties. For a given shape (plate, Koch snowflake, spheroid, or sphere), the action of internal mixing absorbs substantially more light than external mixing. The snow-grain shape effect on absorption is relatively small, but its effect on asymmetry factor is substantial. Due to a greater probability of intercepting photons, multiple inclusions of BC/dust exhibit a larger absorption than an equal-volume single inclusion. The spectral absorption (0.2 – 5  μ m) for snow grains internally mixed with BC/dust is confined to wavelengths shorter than about 1.4  μ m, beyond which ice absorption predominates. Based on the single-scattering properties determined from stochastic and light absorption parameterizations and using the adding/doubling method for spectral radiative transfer, we find that internal mixing reduces snow albedo substantially more than external mixing and that the snow-grain shape plays a critical role in snow albedo calculations through its forward scattering strength. Also, multiple inclusion of BC/dust significantly reduces snow albedo as compared to an equal-volume single sphere. For application to land/snow models, we propose a two-layer spectral snow parameterization involving contaminated fresh snow on top of old snow for investigating and understanding the climatic impact of multiple BC/dust internal mixing associated with snow grain metamorphism, particularly over mountain/snow topography.