Boundary Layer Evolution

1
Boundary Layer Evolution
C. David Whiteman

• Atmos 3200/Geog 3280
• Mountain Weather and Climate

2
Boundary layer definition

• The boundary layer The layer of fluid in the
  immediate vicinity of a fluid-solid boundary. In
  the atmosphere, the layer near the ground
  affected by diurnal heat, moisture and momentum
  transfer to or from the surface of the earth.

3
Tethersonde and radiosonde
Whiteman (2000)
Whiteman photo
4
Typical evolution of boundary layer
Whiteman (2000)
5
Temperature and potential temperature profiles
Stull (2000)
Stull (2000)
Compare T and ? soundings
FA free atmosphere EZ entrainment zone ML
mixed layer, SL superadiabatic layer CI
capping inversion RL residual layer SBL
stable boundary layer
6
Diurnal fair weather evolution of bl over a plain
Whiteman (2000)
7
Pressure and Winds
8
Standard atmosphere
Pressure (hPa mb) Typical height (ft) Typical height (m)
1013.25 0 0
1000 370 110
850 4780 1460
700 9880 3010
500 18280 5570
300 30050 9160
Pressure decreases exponentially with altitude
Pressure changes more rapidly in the vertical
than in the horizontal, but the horizontal
variations are important for visualizing
traveling pressure systems.
Standard atmosphere (tropo) 15C at sea
level Temperature lapse rate 6.5 C/km
9
Pressure and wind units (also temperature)
Wind speed conversion table

• Pressure
• mb (hPa)
• inches of mercury
• mm of mercury
• pounds per square inch
• atmospheres
• Winds
• mph (statute miles per hour)
• m/s
• knots (nautical miles per hour)
• km/h

mph m/s km/h knots
1 0.4 1.6 0.9
2 0.9 3.2 1.7
3 1.3 4.8 2.6
4 1.8 6.4 3.5
5 2.2 8.0 4.3
10 4.5 16.1 8.7
Beaufort Wind Scale See Table 5.2
Demonstrate Kestral
F 32 (9/5)C C (5/9) (F-32)
See Appendix D for unit conversions
10
Highs, lows, ridges, troughs
Isoheight analysis, 500 mb
Sea level pressure analysis
Pressure variation on constant height surface
(left) Height variation on constant pressure
surface (rt)
Isobar
Isohypse, isoheight, contour line
How to reduce surface pressure to sea level?
11
500 mb hemispheric pattern
Mean 500 mb height pattern
25 Jan 1999 500 mb height pattern
12
Balance of forces, highs and lows, NH
Balance of Forces Pressure Gradient Force (PGF)
is directed from high to low and is proportional
to pressure gradient. Coriolis Force (CF) is a
function of speed and latitude and is directed to
right of wind. Friction Force opposes the wind
and is proportional to surface drag.
Above sfc friction, winds become parallel to
isobars with low on left. If friction is felt,
winds turn across isobars toward low pressure.
Thus, winds spiral counterclockwise into sfc low
and clockwise out of sfc high.
13
Leeward, windward, definitions
Convention for naming winds Named for the
direction from which they blow Example A north
wind blows from N to S Example An east wind
blows from E to W A sea breeze blows from sea to
land A land breeze blows from land to sea A
mountain wind blows from mountain to valley A
valley wind blows from valley to mountain
If we need more specificity, use the bearing from
which the wind blows Example a wind from 178 (a
south wind) Example a wind from 87 (an east
wind)
Or use a vector
Will need a legend or scale
14
Buys-Ballot rule (Northern Hemisphere)
If the wind blows into your back, the Low will
be to your Left (and the high will be to your
right).
This rule works well if the wind is above the
earths boundary layer, not channeled by
topography, etc.
15
Todays 500 mb analysis
16
Todays surface analysis (MSLP analysis)
17
Transfer of heat by cyclones and anticyclones
Excess of solar radiation relative to long wave
loss at equator. Deficit at poles. Poles get
cold, equator gets warm. A zonal temperature
gradient develops. Lows and highs and ocean
currents are necessary to transfer heat from the
equator toward the poles.
18
Cornices, snow fence, flagged trees
19
Diurnal variation of wind speed
Winds at the ground increase in the afternoon as
stronger winds are mixed down from aloft. Winds
aloft decrease in the afternoon as the mixing
exposes them to friction at the earths surface.