Physical parametrizations

In global atmospheric models, subgrid-scale parametrisations describe the effect of unresolved processes on the resolved scale (e.g. surface exchange, convection, gravity wave drag, vertical diffusion), but also describe diabatic effects such as radiation and water phase changes.

The IFS has a comprehensive package of sub-grid parametrization schemes representing radiative transfer, convection, clouds, surface exchange, turbulent mixing, sub-grid-scale orographic drag and non-orographic gravity wave drag. The radiation scheme is based on the Rapid Radiation Transfer Model (RRTM) (Mlawer et al., 1997) with cloud radiation interactions using the Monte Carlo Independent Column Approximation (McICA) (Morcrette et al., 2008). Radiation calculations of short- and longwave radiative fluxes are done less frequently than the timestep of the model and on a coarser grid.

The moist convection scheme uses the mass-flux approach and represents deep (including congestus), shallow and mid-level (elevated moist layers) convection (Bechtold et al. (2014). Clouds and large-scale precipitation are parametrized with prognostic equations for cloud liquid, cloud ice, rain and snow water contents and a sub-grid fractional cloud cover.

The land surface parametrization scheme represents the surface  fluxes of energy / water and corresponding sub-surface quantities. It uses a tiled approach (HTESSEL) representing different sub-grid surface types for vegetation, bare soil, snow and open water (Balsamo et al., 2009). Turbulent diffusion and exchange with the surface are represented on the basis of Monin-Obukhov similarity in the surface layer and an Eddy-Diffusivity Mass-Flux (EDMF) framework above the surface layer (Kohler et al., 2011).

Unresolved orographic effects are represented in the Turbulent Orographic Form Drag (TOFD) scheme for the scales smaller than 5 km (Beljaars et al., 2004) and in the low level blocking and gravity wave scheme for the larger scales (Lott and Miller, 1997). Non-orographic gravity waves are parametrized according to Orr et al. (2010).

At the sea surface, the IFS has a two-way coupling to the WAve Model (WAM, Hasselmann et al., 1988). The coupling with WAM is performed every time step whereby WAM is forced by IFS 10-metre wind speeds while the IFS is forced by surface roughness over sea.

Radiation

The radiation scheme performs computations of the short-wave and long-wave radiative fluxes using the predicted values of temperature, humidity, cloud, and monthly-mean climatologies for aerosols and the main trace gases (CO₂, O₃, CH₄, N₂O, CFCl₃ and CF₂Cl₂). The radiation code is based on the Rapid Radiation Transfer Model (RRTM, Mlawer et al. 1997; Iacono et al. 2008). Cloud-radiation interactions are taken into account in detail by using the values of cloud fraction and liquid, ice and snow water contents from the cloud scheme using the McICA (Monte Carlo Independent Column Approximation) method (McRad, Morcrette et al. 2008). The solution of the radiative transfer equations to obtain the fluxes is computationally very expensive, so depending on the model configuration, full radiation calculations are performed on a reduced (coarser) radiation grid and/or on a reduced time frequency. The results are then interpolated back to the original grid. However, the short-wave fluxes are updated at every grid point and timestep using values of the solar zenith angle.

Convection

The moist convection scheme is based on the mass-flux approach and represents deep (including congestus), shallow and mid-level (elevated moist layers) convection. The distinction between deep and shallow convection is made on the basis of the cloud depth (< 200 hPa for shallow). For deep convection the mass-flux is determined by assuming that convection removes Convective Available Potential Energy (CAPE) over a given time scale. The intensity of shallow convection is based on the budget of the moist static energy, i.e. the convective flux at cloud base equals the contribution of all other physical processes when integrated over the subcloud layer. Finally, mid-level convection can occur for elevated moist layers, and its mass flux is set according to the large-scale vertical velocity. The scheme, originally described in Tiedtke (1989), has evolved over time and amongst many changes includes a modified entrainment formulation leading to an improved representation of tropical variability of convection (Bechtold et al. 2008), and a modified CAPE closure leading to a significantly improved diurnal cycle of convection (Bechtold et al. 2014).

Clouds and stratiform precipitation

Clouds and large-scale precipitation are parametrized with a number of prognostic equations for cloud liquid, cloud ice, rain and snow water contents and a sub-grid fractional cloud cover. The cloud scheme represents the sources and sinks of cloud and precipitation due to the major generation and destruction processes, including cloud formation by detrainment from cumulus convection, condensation, deposition, evaporation, collection, melting and freezing. The scheme is based on (Tiedtke 1993) but with an enhanced representation of mixed-phase clouds and prognostic precipitation (Forbes and Tompkins 2011; Forbes et al. 2011). Supersaturation with respect to ice is commonly observed in the upper troposphere and is also represented in the parametrization (Tompkins et al. 2007).

Surface scheme

The surface parametrization scheme represents the surface fluxes of energy and water and corresponding sub-surface quantities. The scheme is based on a tiled approach (TESSEL) representing different  sub-grid surface types for  vegetation, bare soil, snow and open water. Each tile has its own properties defining separate heat and water fluxes used in an energy balance equation which is solved for the tile skin temperature. Four soil layers are represented as well as snow mass and density. The evaporative fluxes consider separately the  fractional contributions from snow cover, wet and dry vegetation and bare soil.  An interception layer collects water from precipitation and dew fall, and infiltration and run-off are represented depending on soil texture and subgrid orography (HTESSEL, Balsamo et al. 2009). A carbon cycle is included and land-atmosphere exchanges of carbon dioxide are parametrized to respond to diurnal and synoptic variations in the water and energy cycles (CHTESSEL, Boussetta et al. 2012).

Turbulent diffusion

The turbulent diffusion scheme represents the vertical exchange of heat, momentum and moisture through sub-grid scale turbulence. The vertical turbulent transport is treated differently in the surface layer and above. In the surface layer, the turbulence fluxes are computed using a first order K-diffusion closure based on the Monin-Obukhov (MO) similarity theory. Above the surface layer a K-diffusion turbulence closure is used everywhere, except for unstable boundary layers where an Eddy-Diffusivity Mass-Flux (EDMF) framework is applied, to represent the non-local boundary layer eddy fluxes (Koehler et al. 2011). The scheme is written in moist conserved variables (liquid static energy and total water) and predicts total water variance.  A total water distribution function is used to convert from the moist conserved variables to the prognostic cloud variables (liquid/ice water content and cloud fraction), but only for the treatment of stratocumulus. Convective clouds are treated separately by the shallow convection scheme.

Orographic drag

The effects of unresolved orography on the atmospheric flow are parametrized as a sink of momentum (drag). The turbulent diffusion scheme includes a parametrization in the lower atmosphere to represent the  turbulent orographic form drag induced by small scale (< 5 km) orography (Beljaars et al. 2004). In addition, in stably stratified flow, the orographic drag parametrization represents the effects of low-level blocking due to unresolved orography (blocked flow drag) and the absorption and/or reflection of vertically propagating gravity waves (gravity wave drag) on the momentum budget (Lott and Miller 1997).

Non-orographic gravity wave drag

The non-orographic gravity wave drag parametrization accounts for the effects of unresolved non-orographic gravity waves. These waves are generated in nature by processes like deep convection, frontal disturbances, and shear zones. Propagating upward from the troposphere the waves break in the middle atmosphere, comprising the stratosphere and the mesosphere, where they exert a strong drag on the flow. The parametrization uses a globally uniform wave spectrum, and propagates it vertically through changing horizontal winds and air density, thereby representing the wave breaking effects due to critical level filtering and non-linear dissipation. A description of the scheme and its effects on the middle atmosphere circulation can be found in Orr et al. (2010).