Ongoing experiments using a new solar radiation parameterization in the Eta model will be described. The solar radiation parameterization, which was originally developed by Chou (1990, 1992, 1999, and later updates), was implemented in the operational GFS model in 2002 (Hou et. al., 2002). Cloud optical properties are calculated to be internally consistent with the microphysical characteristics of cloud droplets, rain, cloud ice, and snow assumed in the Eta's grid-scale microphysical scheme (Ferrier et al., 2002), while simplified optical properties of convective clouds are used. Preliminary results show considerably less incident shortwave radiation than what's currently running in the operational 12-km Eta, and consistent with the vicinity of several SURFAD sites on 11 May 2002. These changes are expected to substantially reduce the high bias in incoming surface solar radiation passing through clouds in the Eta model (Hinkelman and Ackerman, 1999).

Serial experiments tested several versions of the radiation and parameterized cloud optical properties. Large variations in incoming surface solar fluxes below thin ice clouds on the order of several hundred W/m**2 were found when different effective radii for ice crystals were assumed, with less radiation reaching the surface when smaller ice particles were assumed. Different effective radii for cloud droplets also showed large sensitivities in surface solar fluxes, which were particularly noticeable over widespread areas with low-level clouds and shallow convection, in which there was less surface radiation when smaller droplets were assumed.

Verification statistics from a current 32-km Eta real-time parallel run, as well as 32-km retrospective runs from last winter will be presented, together with results from a possible 12-km pre-implementation parallel expected to be run this fall.