Welcome to ENSC 312

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Welcome to ENSC 312!

• Boundary Layer Meteorology
• Instructors Peter Jackson/Stephen Déry
• Emails peterj/sdery_at_unbc.ca
• Lecture WF 830-920 a.m. in 5-158
• Lab T 8-11 a.m. in 8-127
• Website http//cirrus.unbc.ca/ensc/312/

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Introduction to BLM

• What is boundary layer meteorology?
• Atmosphere contains phenomena that span a very
  wide range of spatial and temporal scales.
• Atmospheric scientists like to categorize
  phenomena and this is typically done on the basis
  of scale.
• There is a physical basis for some of this - in
  fact energy as a function of scale has been
  computed, and the drawing looks like this

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Oke (1987)
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Peixoto Oort (1992)
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• None of these phenomena are discrete, all are
  part of a continuum.
• A general classification follows
• Micro-scale 10-2 to 103 m.
• Local-scale 102 to 5 104 m.
• Meso-scale 104 to 2 105 m.
• Macro-scale 105 to 108 m.
• This course is concerned with features smaller
  than the meso-scale and will focus on where all
  the action occurs, near the ground.
• This is where solar radiation is absorbed,
  longwave radiation is emitted, turbulent fluxes
  of sensible and latent heat originate. In
  addition, it is where life on earth exists.

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• The vertical zone of surface influence is below
  about 10 km - the troposphere.
• It is in this zone that the weather - including
  mid-latitude disturbances (storms) - is
  contained.
• Over the time scale of one day or less, in the
  absence of any synoptic scale disturbances, the
  vertical zone of surface influence is below about
  1 km planetary boundary layer (PBL) or
  atmospheric boundary layer (ABL).
• The PBL is characterized by mechanically induced
  turbulence due to the frictional drag of airflow
  over a rough surface, and by convectively-induced
  turbulence caused by thermal instability.

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• Turbulence (small scale chaotic movements) is
  responsible for the fluxes of heat and moisture
  to, and from, the surface.
• The PBL height varies by place, surface type,
  time of day and year and atmospheric conditions.
• During the day, solar radiation heats the ground
  causing thermal instability and buoyant plumes of
  rising air that increase the depth of the PBL.
• During the night, surface cooling dominates the
  radiation regime and the PBL depth decreases to
  less than 100 m in depth.
• The above scenario would be for a PBL dominated
  by thermally induced turbulence (i.e. under light
  wind conditions).

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• Under strong wind conditions the turbulence is
  forced by airflow over rough terrain and diurnal
  variations would be less.
• The above picture assumes no large scale
  disturbances.
• Cloud, winds, and precipitation brought by these
  disturbances, are not tied to the surface or the
  diurnal cycle.
• These disturbances have larger time and length
  scales and can be so energetic that they
  wipe-out or mask any local or micro-scale
  features. The effect of these disturbances,
  although significant, is transitory. Local and
  micro-scale features dominate the climate in most
  places, most of the time.
• The region of the PBL below about 50 m (day) is
  called the turbulent surface layer - intense
  small-scale turbulence, highly variable, but
  horizontally homogenous when averaged over
  about 10 min.

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• This layer is also called the constant flux
  layer because the turbulent fluxes do not change
  in the vertical throughout the layer.
• Below the surface layer, is the roughness layer
  where the direct effect of surface roughness
  elements dominates - turbulence has not yet
  broken down the irregularities caused by the
  roughness elements into homogenous turbulence.
• The height of the roughness layer is typically
  2-3 times the height of the roughness elements.
• The lowest layer, within just a few mm of the
  surface, is called the laminar boundary
  layer.In this layer the air is laminar and
  non-turbulent. This layer adheres to all
  surfaces, creating a buffer between the surface
  and the turbulent layer and beyond. All fluxes
  which go from the surface to the atmosphere must
  travel through this layer. Since there is no
  turbulence, this transfer is by conduction
  (same as fluxes through soils, etc.)

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Oke (1987)
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• Conduction is very inefficient compared to
  convection so that large gradients must exist.
• Laminar boundary layer allows the surface to heat
  up (see energy and mass exchanges and balances,
  Oke, Ch. 6).
• The first law of thermodynamics states that
  energy can neither be created nor destroyed so
  that for systems, only 2 possibilities exist
• Energy input Energy output
• Energy input Energy output and storage

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Meteorological Conventions

• Coordinate Systems
• In meteorology, Cartesian or rectangular
  coordinates are often applied for small regions.
  In this system, the x-axis typically denotes
  east, the y-axis represents north, and the z-axis
  points vertically.
• The associated velocity components (i.e. winds)
  are given by u, v, and w, respectively.
• A positive value of u represents a wind blowing
  from the west to the east (this is a west
  wind).

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• Eulerian vs. Lagrangian Framework
• For meteorological applications, two frames of
  reference are often used.
• One is the Eulerian reference frame in which
  one considers the local rate of change over time
  t of a given quantity (let's call it Q) at a
  fixed point in space. This is given by ?Q/?t.
• In the Lagrangian reference frame, one considers
  a small parcel of air which we follow along its
  trajectory. For this case, changes in a quantity
  Q are given by its total derivative, dQ/dt.
• The difference between the two is due to
  advection. Thus the Eulerian and Lagrangian
  derivatives are related by

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• Units
• Some of the common units used in meteorology are
  non-metric. The following table provides a list
  of unit conversions for various aspects of the
  atmosphere

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• Chemical Elements and Compounds
• Air pollution is one of one the main topics of
  this course. This subject matter requires
  knowledge of some basic chemical elements and
  compounds such as the following