Sensitivity Checks on the Effects of Shallow Clouds on the Initiation of Deep Convection in the North American Mesoscale Parallel (NAMX) 4km CONUSNEST

An overabundance of shallow clouds has been noticed in the 4-km convection-allowing NAM CONUS nest forecast system, planned for operational implementation in August 2014. It is suspected that these low-level clouds directly impact the downward shortwave radiation flux and surface temperatures, consequently inhibiting the onset of deep convection. This research examines the modeling system's sensitivities to physical parameterizations, initial conditions, and other settings/configurations for simulating convective environments. Preliminary results suggest that adding parameterized shallow convection only (no deep, precipitating convection) can improve forecasts of deep convection where initiation has been noted to be problematic generally due to the existence of low-level clouds and fog in the pre-convective environment. In addition, a sensitivity to horizontal grid-spacing has also been noticed. When decreasing the horizontal grid-spacing to 1 km from 3 km, and disabling parameterized shallow convection, it was noted that similar encouraging results were obtained relative to a 3 km model simulation which used parameterized shallow convection. Preliminary results largely stem from a single case study, however results from other cases will be discussed as time allows.