A Unified Representation of Subgrid Convection in NOAA’s Unified Forecast System

It remains challenging to represent subgrid convection in weather and climate models at horizontal grid resolution across the gray zone, in which convective clouds are only partially resolved by the model dynamics and it is required for the representation of subgrid convection to have a generalized transitional behavior as the model’s horizontal resolution varies.  A practical approach for such a representation is to scale the eddy transport of physical properties from a conventional convection parameterization scheme by a quadradic function of the fractional area covered by convective updrafts in the grid cell (Arakawa and Wu, 2013).  Despite this approach’s popularity, its generalization is limited theoretically by the fact that the coarse-graining statistical analysis that gave rise to the approach involved only an idealized scenario of deep convection in quasi-equilibrium.  Additionally, when applying this approach, there is a theoretical ambiguity associated with the validity of conventional convection parameterizations for a fractional area covered by convective updrafts in the grid cell that is not close to zero.

An alternative approach for subgrid convection representation across the gray zone is to apply a unified plume scheme that treats subgrid convection as nonlocal asymmetric eddies due to unresolved convection relative to the grid-mean vertical flow (Park, 2014).  This unified plume scheme represents unresolved convection relative to the grid-mean vertical motion without relying on quasi-equilibrium assumptions in conventional convection parameterizations.  Its generalized transitional behavior across the gray zone is naturally controlled by the size of the plumes representing unresolved convection that varies with the model’s horizontal resolution.  It simulates all unresolved convective transport of atmospheric properties within a single steady framework, allowing multiple convective plumes.  It also includes the prognosis of unresolved cold pool and convection organization within the planetary boundary layer.  The unified plume scheme circumvents the theoretical limitation and ambiguity of the above approach based on conventional convection parameterization.  It also rectifies the lack of plume memory across the time step in conventional convection parameterizations.

This presentation will focus on an ongoing effort to experiment with the alternative unified approach for representing subgrid convection across the gray zone in NOAA’s Unified Forecast System.  Results from 1-D and 3-D case studies will be shown to highlight the advantage of the unified plume scheme.

References:

Arakawa, A., and C.-M. Wu, 2013: A unified representation of deep moist convection in numerical modeling of the atmosphere. Part I. J. Atmos. Sci., 70, 1977–1992.

Park, S., 2014: A unified convection scheme (UNICON). Part I: Formulation. J. Atmos. Sci., 71, 3902–3930.