Influence of Model Physics on NWP Forecasts - version 2

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* PBL Components

This page contains detailed information about the observed elements of the PBL in the real atmosphere and how NWP models account for them.

The daily evolution and variety of structures in the PBL are most evident in the warm season when the diurnal cycle of incoming solar radiation results in the largest differences in energy available for surface processes between day and night in middle latitudes.

The graphic illustrates the components that can constitute the PBL and shows how their depth and orientation change throughout the diurnal cycle. Note that as the depth of the actual and model PBL changes, so do the number of model levels that encompass it and the model's ability to capture PBL processes. The daytime components are driven by conduction, convection, and turbulence. Nighttime components are driven by conduction and radiational cooling. Click each label to learn more about the layer's characteristics and how models emulate them.

Contact Layer

Actual characteristics

• At the bottom of the PBL; a thin layer (centimeters in depth) in direct contact with the earth's surface.
• Transfers heat and moisture vertically by conduction and molecular processes.
• Located where the exchange between the earth's surface and the atmosphere begins.
• Temperature and moisture differences between air and material surface determine the direction of the exchange (for example, warm air over a cold surface results in heat conduction from air to surface, cooling the air and warming the surface).

NWP emulation

• Contact layer fluxes are usually estimated:
  • using differences between the surface and first layer (or shelter-level temperature and moisture extrapolated from the first model layer if the model vertical resolution near the ground is too coarse) values for temperature and moisture to estimate vertical temperature and moisture gradients;
  • using differences of surface and first layer (or 10-m level extrapolated as above) values for wind to estimate the aerodynamic effects on sensible and latent heat flux.
    • Generally, stronger vertical wind gradients result in larger overall fluxes. Stronger vertical temperature and moisture gradients result in larger vertical fluxes of heat and moisture, respectively.
    • The accuracy of vertical temperature, moisture, and wind gradients (and thus fluxes) depends upon the:
      • forecast "skin" temperature (which depends upon the accurate emulation of surface energy and water balances);
      • predicted first layer temperature, moisture, and wind (and extrapolation to shelter level, if used);
      • assigned surface roughness, which partially determines the vertical wind gradient. Depending upon the model, surface roughness may either be fixed, have an annual cycle (for example, deciduous vegetation), or be allowed to vary based on the wind speed (for example, emulating the effect of ocean waves on surface roughness). Over land, surface roughness may be assigned based on the vegetation type or from an independent dataset.

Free Atmosphere

Actual characteristics

• Above the top of the PBL, where the surface has no direct effect on flow.
• At its bottom, exchanges heat, moisture, and momentum with the PBL.
• Not directly forced by radiative processes.
• Horizontal and vertical transport by sub-grid scale motions (for example, horizontal and vertical turbulent eddies and gravity waves), while smaller than in the PBL, still must be accounted for.
  • Eddies are generally forced by horizontal and vertical wind shears.
  • Fast-moving gravity waves are forced by sharply peaked, high variance surface topography, which behaves differently from momentum flux in the PBL created by the vertical wind shears.
    • The drag on horizontal momentum created by highly variable surface topography is dumped where gravity wave speed equals the horizontal wind speed.

NWP emulation

• Dynamical processes dominate.
• Determines turbulent mixing rates from stability and wind shear parameters (this is important, for example, in regions of strong temperature gradients and near jet streaks).
• Uses vertical gradients of moisture, heat, and momentum to determine the end result of mixing; tends to be small within the free atmosphere, but can be large at the PBL/free atmosphere interface.
• Gravity waves (a special case of turbulence).
  • Effects are usually emulated using topographic variance within each grid box.
    • Gravity wave speed is estimated based on stability criteria and topographic roughness.
    • Need gravity wave speed to determine the amount and vertical distribution of gravity wave generated momentum (where estimated gravity wave speed matches horizontal wind).
      • Vertical temperature and wind gradient errors (stability) and wind speed errors (location of regions to put gravity wave generated momentum) will result in effects on the jet stream location in the vicinity and downwind of mountains, lee side cyclogenesis, and movement of cold air to the lee of mountain ranges.

Stable Boundary Layer (SBL)

Actual characteristics

• Forms on clear, calm nights and when warmer air moves over colder surfaces.
• Grows mainly by conduction with air in the contact layer, with additional contribution from radiative cooling.
• Forms after the superadiabatic surface layer breaks down as the ground cools after mid-afternoon heating maximum.
• Near sunset, the surface layer, contact layer, and SBL can merge. As night progresses and cooling by conduction from cool surface continues, the surface layer becomes a smaller fraction of the SBL, with constant, typically small and/or negative fluxes of moisture, heat, and momentum.
• There is no well-defined SBL top, but a gradual transition at the top of the nocturnal temperature inversion.

NWP emulation

Indirectly by:

• Estimating rates of conduction from the lowest model layers to the ground, using vertical gradients of temperature, moisture, and momentum.
• Usually uses the same scheme as for a mixed layer.

Surface Layer

Actual characteristics

• Heat, moisture, and momentum are transported by convection (thermal instability) during the day and by conduction with the contact layer at night.
• Comprises approximately the next 10% of the PBL above the contact layer in a well-developed daytime PBL or well-developed stable boundary layer (SBL) at night.
• The size of surface-layer transports depends upon fluxes out of the contact layer and are thus sensitive to near-surface temperature, moisture, and wind profiles.
• Acts as the source for buoyant eddies or thermals, which drive the mixed layer during the day.
• Acts as a 'buffer' between the contact layer and 'skin' and the rest of the atmosphere, through turbulence, with quasi-constant heat, moisture, and momentum transports between the contact and the mixed layer during the day and SBL at night.
• At night, evolution is somewhat more complicated, with initial merging of contact, surface, and SBL right after sunset with subsequent evolution of three separate entities.

NWP emulation

• Assigns or determines a depth for the surface layer based on observed behavior of the surface layer and the model's vertical resolution.
• Flux from the contact layer is assumed to be constant through the surface layer.
• Accuracy depends upon the contact layer flux calculation (see contact layer).

Mixed Layer

Actual characteristics

• Main mode of transport is through turbulence during the day when the surface is hotter than the overlaying atmosphere.
• Located between the surface layer and the area where dry free-atmosphere air is taken into the PBL (the entrainment zone).
• Vertical transports vary and depend upon the size and intensity of turbulent eddies.
• Vertical eddy size depends upon the vertical lapse rate, distance from the surface, and vertical wind shear (so the rate of mixed layer growth depends upon the amount of surface heating).
  • Note: Individual large eddies created by differential heating of the surface at small scales (10s to 100s of meters) may transport quantities against or "counter" to the prevailing large-scale gradient before the large-scale vertical gradients become unstable, as noted in the description of PBL evolution above; this results in warm, moist air transport from what at grid scale is a cool layer to warmer layers above. This counter-gradient flux breaks the nocturnal inversion and builds the PBL sooner than might otherwise be expected.
• Has a well-defined top (or cap) characterized by a stable layer/temperature inversion.

NWP emulation

• Depending upon the model, the height of the PBL top is diagnosed using stability criteria in the bottom-most model layers.
• Calculates or prescribes mixing coefficients for each layer between the surface layer and PBL top.
• May allow mixing between non-adjacent layers in one time step or may limit mixing to adjacent layers only.
• May emulate vertical mixing by small-scale eddies before the large-scale vertical temperature and moisture gradients would otherwise allow (allow for counter-gradient flux).

Entrainment Zone (EZ)

Actual characteristics

• Area where parcels from the 'free atmosphere' can be brought into the PBL by turbulent mixing; builds upward as the depth of the superadiabatic layer increases.
• If the lifting condensation level (LCL) has been reached, will contain cumulus or stratocumulus cloud.
• The top of the EZ is frequently characterized by a temperature inversion resulting from large-scale subsidence and cooling/moistening through the evaporation of shallow cumulus. The existence of a temperature inversion tends to cap the vertical development of the PBL and allows buildup of moist static energy in the PBL.
• Cooling and moistening of the EZ through turbulent eddy exchange or through large-scale lifting can reduce or remove this cap, resulting in explosive convection if the PBL is sufficiently unstable.

NWP emulation

• Not directly emulated, although a transition zone between the PBL and free atmosphere is usually present in NWP models, which may include a stable inversion layer.
• Some models use a shallow convective scheme as an extension of the PBL scheme to handle EZ.
• If model vertical resolution is too coarse, this may not be captured at all and the inversion may be missed.

Capping Inversion

Actual characteristics

• A stable layer topping a residual mixed layer after the surface and atmosphere decouple, resulting from the earlier entrainment by the active daytime PBL.
• The inversion remains until the mixed layer reaches its level the next day.
• Prevents interaction with the free atmosphere.
• Advection and subsidence affect the capping inversion strength.

NWP emulation

• Not directly emulated, although a stable inversion layer usually appears in NWP models as a transition zone between the PBL and free atmosphere.
• Often too shallow to be represented well in models, especially when at elevations where model resolution is relatively coarse and when the PBL is unusually deep.

Residual Layer

Actual characteristics

• The inactive (no new mixing) well-mixed layer remaining after sunset, after the atmosphere decouples from the surface. Any remaining turbulent mixing action becomes negligible shortly after sunset.
• Retains the characteristics of the daytime mixed layer, changed only by radiative cooling or horizontal advection.
• Remains until the next day's surface heating "recouples" the surface to the residual mixed layer, resulting in a rapid 'jump' in PBL height.

NWP emulation

• Usually well emulated as the land-surface decouples from the atmosphere, but depends upon the adequacy of the model boundary layer structure.