Layers in the atmosphere
- Surface layer 50m
- Atmospheric boundary layer 1 km
- Troposphere 10 km
Planetary boundary layer definition
the lowest part of the atmosphere that is in contact with the surface of the earth
PBL is a region where earth’s surface strongly influence:
- temperature
- moisture
- wind
Importance of PBL
PBL plays an important role in many fields:
- air pollution
- agro. met
- hydrolody
- aviation
- ….
Free atmosphere:
the remaining air through the atmosphere above the PBL
energy in FA is ……………………. through ………………..
dissipated through the PBL
friction and turbulence
properties:
- friction
- turbulence
- thickness
- mixing
friction in PBL and FA
PBL:
- significant drag
- energy dissipation (due to friction)
FA:
- Insignificant drag
- little to no energy dissipation
Turbulence in PBL and FA
PBL:
- continuous turbulences throughout the layer
FA:
- turbulence only in convective clouds and in thunderstorms
thickness in PBL and FA
PBL:
- between 100 and 3000 m
- diurinal variation
FA:
- between 3 and 18 km
- little variation
Mixing in PBL and FA
PBL:
- Rapid mixing in vertical and horizontal
FA:
- rapid horizontal mixing
PBL can be subdevided into:
- surface
- mixed layer (convective mixing layer)
- stable layer
- residual layer
PBL depends on:
- temperature
- structure
PBL structure varies with:
- season
- higher in summer
- time of day
- higher in daytime
- weather conditions
- higher in instability (1000’s of meters)
- lower in stability (10’s of meters)
Surface layer:
- closest to the earth’s surface
- turbulent stresses are relatively constant
- height of surface layer is 10% of PBL height
- varies from 20 to 200m
- exchange of
- heat
- momentum
- water
- airpollutants
by turbulent fluxes between surface and atmosphere
Mixing layer (convective mixed layer)
- above the surface layer
- mixed layer (during day)
- stable boundary layer (during night)
both depend on temperature structure of PBL
- after sunrise the convective mixed layer grows rapidly due to intense turbulences
- this layer is capped by a stable environment zone
- during daytime the mixing layer reaches heights greater than 1 km and make upthe entire layer of the PBL above the surface layer
Stable boundary layer
- Near sunset the mixed layer collapses and in its place a stable (nocturnal) BL is formed
- the bottom part of this layer is stabalized by night time radiational cooling of the surface
- the height of PBL decreases during night time
Residual layer
- above the stabe BL is the residual layer
- while
- nocturnal BL has a stable profile
- RL has a neutral profile
- RL doesnt hace contact with the earth’s surface, therefore it is not concidered as part of BL
- residual layer is bounded by a capping inversion (at a height of daytime BL height), which prevents environment aloft
PBL charactaristics:
- Physical state
- energy exchange
- Turbulence
PBL physical state is described with:
(state variability)
- temperature
- humidity
- pressure
- wind
- trace gasses
related to dynamical aspects (of PBL)
- turbulence
- diffusion
- stability
changes in state variability are controled by
processes that transfer (rate of transfer are quantified in the form of fluxes)
- mass
- energy
- momentum
within the BL and between the BL & FA
energy exchange:
radiation exhange regulate the physical and chemical state of the BL
radiation (solar energy):
- absorbed during nighttime (warms PBL) or emiited during nighttime (cools PBL)
- supply energy for evapotranspiration
Turbulence:
in convective BL turb. eddies are generated by:
- wind shear (DAY or NIGHT)
- bouyancy associated with surface heating (DAY)
some of the large eddies penetrate into the FA generating convective clouds
PBL modeling:
(turbulent and fluxes)
PBL model is an attempt to describe the statistical behaviours of the PBL turbulence through simplified equations
