Review of the Boundary LayerThere are sub layers within the boundary layer:Surface layerMixed layerResidual layerEntrainment zoneCommon variables used to study and locate the boundary layer:T- temperature: q = potential temperature: w = Mixing ratioWindsRemember that both q and w are conserved for a dry adiabatic process

Review of the Boundary LayerLets start during the daytime:At low levels, a surface layer (0-100 m typically) existsHave strong gradients of temperature, moisture, and windsOften superadiabaticAbove the surface layer, you will find a well-mixed layer where convective thermals are trying to well-mix heat and moistureIn the mixed layer, q, 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

Surface LayerThe surface layer is in direct contact with the earth's surface. The lowest few centimeters of air in the surface layer is often called themicrolayer, 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 besuper-adiabatic.

Mixed LayerThis layer is located above the surface layer and below theentrainment 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: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.radiative cooling from the top of the cloud layer creating "upside down" thermals of cool, sinking air.wind shearcan also generate mechanical turbulence within the mixed layerThe 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 afternoonMixed Layer

Mixed Layer GrowthThe 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 LayerAs shown in the above figures, the entrainment layer (zone) is a stable layer above the mixed layer.It acts as a lid to rising thermalsIt 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 LayerApproximately 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, theresidual layeris 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 LayerAs 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 theradiation inversion.The nocturnal boundary layer can be anywhere from 0-200m or so deep and is characterized by:strong static stabilityweak/sporadic turbulence- often occurs in short burstsweak/calm winds at the surface, but increasing to supergeostrophic speeds aloft at the top of the stable layerThis wind speed profile is often referred to as alow-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.

Boundary layer meteorologists roughly divide the PBL into three regions1. The viscous sub-layerThe layer extending from the surface to a few millimeters above the surfaceCharacterized by: -molecular diffusion -extreme shear -wind at surface (molecular scale) = 02. The surface layerThe layer extending from the top of the viscous sub-layer to about 10% of the depth of the PBLCharacterized by: -vertical momentum transfer by turbulent eddies -not directly dependent on Coriolis and PG forces3. The Ekman layerThe layer extending from the top of the surface layer to the top of the PBLCharacterized by: -turning wind with height as the effect of friction diminishes and the wind approaches its geostrophic value

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