Review of the Boundary Layer

•Over land, in the vicinity of an area of high pressure, the boundary layer evolves in a well-defined manner

•What is the boundary layer?

•Defined: The Boundary Layer can be defined as: "that part of the troposphere that is directly influenced by the presence of the earth's surface and responds to surface forcings with a time scale of about an hour or less."

•Typical depth: 100 – 3000 m

•Boundary layer/troposphere:

about 10%.

Review of the Boundary Layer

•Boundary Layer forcing mechanisms – what physical processes modify boundary layer air parcels?

1.Heat transfer to/from the ground

2.Frictional drag

3.Evaporation/transpiration

4.Terrain-induced flow modification

5.Pollution emission

Review of the Boundary Layer

•There are sub layers within the boundary layer:–Surface layer–Mixed layer–Residual layer–Entrainment zone

•Common variables used to study and locate the boundary layer:

•T- temperature:

• = potential temperature:

• w = Mixing ratio

•Winds

•Remember that both and w are conserved for a dry adiabatic process

k

P

PT

0

Wind in the Boundary LayerBoundary layer winds can be decomposed into:

•a mean wind

•waves

•turbulence

Mean Wind:

•The mean wind is important for horizontal transport of quantities such as moisture, heat, momentum, and pollutants - i.e., advection.•typical speeds are 2-10 m/s.•friction slows the winds near the surface, the wind velocity is 0 m/s right at ground level.

Wind in the Boundary LayerBoundary layer winds can be decomposed into:

•a mean wind

•waves

•turbulence

Waves:

•Occur mostly at night in the nocturnal boundary layer.•transport little heat, moisture and other scalar variables like pollutants•Waves are effective at transporting energy and momentum, however.

Wind in the Boundary LayerTurbulence:

•the vertical transport of moisture, heat, momentum, and pollutants is dominated by turbulence.. this is a very important process . Q:  What is turbulence?

http://amsglossary.allenpress.com/glossary/browse?s=t&p=56

•can be visualized as consisting of irregular swirls of motion called eddies - see example to the right -> •eddies can vary in size - anywhere from 100-3000 m, however, eddies exist in size as small as a few millimeters!

Wind in the Boundary Layer

Turbulence:

•Q:  what creates turbulence?•solar heating generating thermals - nothing more than larger eddies•wind shear•deflected flow around obstacles such as trees and buildings, creating turbulent wakes downstream of the obstacle

•In summary, turbulence allows the boundary layer to respond to changes in surface forcings (daytime heating, for example). •This does not occur in the free troposphere, the free troposphere acts like the earth's surface does not exist.

Thermodynamic Variables used in the Boundary Layer•Temperature (T)

•Dew-point temperature (Td)

•Mixing ratio (q) (conserved for adiabatic processes)

•Potential temperature ( - given by Poissons equation as: is also conserved for adiabatic processes.

•Virtual Potential Temperature (v) - for unsaturated air, v is given by v = (1 + 0.61q). 

•Q: is v conserved for dry adiabatic processes?

Boundary Layer Depth Variability

Near a region of high pressure:

•Over both land and oceans, the boundary layer tends to be shallower near the center of high pressure regions. 

•This is due to the associated subsidence and divergence.•few clouds form in this region, why? 

•boundary layer depth increases on the periphery of the high where the subsidence is weaker.

•clouds are more apt to form in this region

Boundary Layer Depth Variability

Near a region of low pressure:

•the rising motion associated with the low transports boundary layer air up into the free troposphere. •Hence, it is often difficult to find the top of the boundary layer in this region.•In a situation such as this where the boundary layer is not evident in a sounding, cloud base is often used at the top of the boundary layer.•this may be somewhat arbitrary, but it is objective and makes some physical sense.

Review of the Boundary Layer•Let’s start during the daytime:•At low levels, a surface layer (0-100 m typically) exists

–Have strong gradients of temperature, moisture, and winds–Often superadiabatic

•Above the surface layer, you will find a well-mixed layer where convective thermals are trying to well-mix heat and moisture

–In the mixed layer, , and w are conserved (constant)•The entrainment zone is a transition layer between the well-mixed convective boundary layer and the free atmosphere – is often an inversion layer

Review of the Boundary Layer•Here is a daytime sounding from AMA:•See if you can identify the layers just discussed:

Surface Layer•The surface layer is in direct contact with the earth's surface.  •The lowest few centimeters of air in the surface layer is often called the microlayer, or interfacial layer.•The interfacial layer is at most, a few centimeters deep.•Within this layer, molecular transport of heat, moisture, and momentum is much more effective than turbulent transport. •This makes some sense since this layer is in direct contact with the ground.•Above the interfacial layer, in the heart of the surface layer, turbulent transport dominates.•Gradients of temperature, moisture and winds can be very large in the surface layer, especially in the interfacial layer.•The lapse rate within the surface layer tends to be super-adiabatic.

Mixed LayerThis layer is located above the surface layer and below the entrainment zone.•Heat, moisture and momentum are uniformly mixed within the mixed layer.  This is accomplished by turbulence within the mixed layer.•Turbulence within the mixed layer is largely convectively driven from two main sources:

1. heat transfer from the warm ground to the interfacial layer via conduction and then convective transport of this heat by thermals up into the mixed layer.

2. radiative cooling from the top of the cloud layer creating "upside down" thermals of cool, sinking air.

•wind shear can also generate mechanical turbulence within the mixed layer•The mixed layer begins to grow vertically approximately 1/2 hour after sunrise.  It grows rapidly during the morning hours and reaches a maximum depth in the afternoon

Mixed Layer

Mixed Layer Growth•The turbulence (largely the convectively driven thermals) mixes (entrains) down potentially warmer, usually drier, less turbulent air down into the mixed layer.•These variable are then well-mixed within the mixed layer.•Hence, heat, moisture and momentum are well mixed, or distributed uniformly within the mixed layer. See Figure 1.9.•Rapid growth of mixed layer occurs when thermals mix up into the residual layer. Residual layers are nearly dry adiabatic – are left over mixed layers from the day before.

Entrainment Layer•As shown in the above figures, the entrainment layer (zone) is a stable layer above the mixed layer.•It acts as a lid to rising thermals•It is often an inversion layer, but not always.•Waves can often be seen propagating on top of the mixed layer within and above the entrainment zone.

Nocturnal Boundary Layer•Approximately 1/2 hour before sunset, the thermals in the convectively mixed boundary layer have shut off as the surface is cooling.•Hence, above the stable boundary layer, the residual layer is found, and can be thought of as a left-over convective mixed layer.•The residual layer, therefore, has all the properties of the recently decayed convective mixed layer.•The static stability of this layer of air is ________ ?•The residual layer does not come in direct contact with the ground, and therefore, is strictly speaking, not a boundary layer.

Nocturnal Boundary Layer•As evening progresses and the surface cools via radiational cooling, a shallow stable layer of air forms that is in direct contact with the ground.  This stable layer is often called the radiation inversion.•The nocturnal boundary layer can be anywhere from 0-200m or so deep and is characterized by:

•strong static stability•weak/sporadic turbulence - often occurs in short bursts•weak/calm winds at the surface, but increasing to supergeostrophic speeds aloft – at the top of the stable layer•This wind speed profile is often referred to as a low-level, or nocturnal jet.  The low-level flow is often decoupled from the flow aloft within the low-level jet.  It is possible for the surface winds to be calm, while, a few 10's of meters aloft, the winds are 30-40 m/s.

Nocturnal Boundary Layer

Q:  How is turbulence within the nocturnal boundary layer generated???? Answer: ______________

Waves are often observed within the nocturnal boundary layer, more specifically, gravity (buoyancy) waves.  Gravity waves are generated in statically stable layers of air.  The nocturnal boundary layer, therefore, is a prime location for gravity wave generation.

Review of the Boundary Layer•Here is an early morning sounding near Orlando, FL:•See if you can identify the layers just discussed: