Boundary Layer Evolution

Atmos 3200/Geog 3280Mountain Weather and Climate

C. David Whiteman

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.

Tethersonde and radiosonde

Whiteman photo

Whiteman (2000)

Typical evolution of boundary layer

Whiteman (2000)

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

Diurnal fair weather evolution of bl over a plain

Whiteman (2000)

Pressure and Winds

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)

15°C at sea level

Temperature lapse rate 6.5 °C/km

Pressure and wind units (also temperature)

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

See Appendix D for unit conversions

Demonstrate KestralF = 32 + (9/5)C

C = (5/9) (F-32)

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

Wind speed conversion table

Beaufort Wind Scale

See Table 5.2

Highs, lows, ridges, troughs

Sea level pressure analysis Isoheight analysis, 500 mb

How to reduce surface pressure to sea level?

Pressure variation on constant height surface (left)Height variation on constant pressure surface (rt)

Isobar

Isohypse, isoheight, contour line

500 mb hemispheric pattern

Mean 500 mb height pattern25 Jan 1999 500 mb height pattern

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.

Leeward, windward, definitions

Convention for naming winds:

Named for the direction from which they blowExample: A north wind blows from N to SExample: An east wind blows from E to W

A sea breeze blows from sea to landA land breeze blows from land to seaA mountain wind blows from mountain to valleyA 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 legendor scale

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 earth’s boundary layer, not channeled by topography, etc.

Today’s 500 mb analysis

Today’s surface analysis (MSLP analysis)

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.

Cornices, snow fence, flagged trees

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 earth’s surface.