A shallow-convection parameterization for mesoscale models. Part I: Submodel description and preliminary applications

A shallow-convection parameterization suitable for both marine and continental regimes is developed for use in mesoscale models. The scheme is closely associated with boundary layer turbulence processes and can transition to either a deep-convection scheme in conditionally unstable environments or to an explicit (resolved scale) moisture scheme in moist stable environments. The shallow-convection mass-closure assumption uses a hybrid formulation based on boundary layer turbulent kinetic energy (TKE) and convective available potential energy (CAPE), while the convective trigger is primarily a function of boundary layer TKE. Secondary subgrid clouds having nearly neutral buoyancy can form as shallow-convective updrafts detrain mass to their environment. Called neutrally buoyant clouds (NBCs), these can be dissipated through lateral and vertical mixing, light precipitation, ice-crystal settling, and cloud-top entrainment instability (CTEI). The shallow-convection scheme is developed and demonstrated in a 1D version of the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) mesoscale model (MM5) which includes a 1.5-order turbulence parameterization that predicts the TKE, an atmospheric radiation submodel, and an explicit moisture submodel. The radiation calculation includes the feedback effects of the subgrid NBCs predicted by the shallow-convection parameterization. Results from initial applications in both marine and continental environments are consistent with the observed characteristics of the mesoscale thermodynamic structures and local cloud-field parameters. A subsequent paper (Part II) presents more complete verifications in different environments and results of sensitivity experiments.