The Surface Energy Budget, Part II
COMET Planetary Boundary Layer Symposium
Matt Haugland
Generic Examples
How will the following situations
affect the components of the Surface
Energy Budget?
Green crop field
Green crop field
Green crop field
HIGHER
NORMAL
LOWER
Rn
S
LE
G
DQ
Result: lower daytime temperature, higher dewpoint
Car in the sun
Surface = interior of your car, parked in the sun
Car in the sun
HIGHER
NORMAL
LOWER
Rn
S
LE
G
DQ
Result: very high “surface” temperature!
Dead grass
Dead grass, minimal soil moisture
Dead grass
HIGHER
NORMAL
LOWER
Rn
S
LE
G
DQ
Result: higher daytime surface temperature, deeper boundary
layer, stronger (& more westerly) wind
South-facing slope
South-facing slope
South-facing slope
HIGHER
NORMAL
LOWER
Rn
S
LE
G
DQ
Result: slightly higher surface temperature, slightly
lower relative humidity
Woodland (nighttime)
Woodland or open-canopy forest (nighttime)
Woodland (nighttime)
Rn
S
LE
G
DQ
LESS
NEGATIVE
NORMAL
MORE
NEGATIVE
Result: lower temperature, higher RH (at night)
Hilltop (nighttime)
Hilltop (nighttime)
Hilltop (nighttime)
Rn
S
LE
G
DQ
LESS
NEGATIVE
NORMAL
MORE
NEGATIVE
Result: higher surface temperature at night
Real Example
Consider a mesoscale strip of land in
a region with substantially different
vegetation
Oklahoma’s Winter Wheat Belt
•
•
•
•
100-150 km wide strip in western Oklahoma/Kansas
Winter wheat is primary vegetation type
Cool-season (C3) crop, surrounded by warm-season (C4) grasses
Ideal outdoor laboratory for study of land-surface impact on atmosphere
Before Harvest
Before harvest (April 2000) – healthy wheat crop
• Winter Wheat Belt is greener than adjacent counties
• Latent Heat Flux much higher across WWB (sensible heat flux lower)
Before Harvest
Average 19Z Temperature (March 2000)
Before Harvest
19Z Temperature (8 April 2000)
After Harvest
Before harvest (June 2000) – dead wheat stubble, bare soil
• Winter Wheat Belt is less green than adjacent counties
• Latent Heat Flux much lower across WWB (sensible heat flux higher)
After Harvest
Average 21Z Temperature (June 2000)
After Harvest
21Z Temperature (8 June 2000)
Impact on Air Pressure – Before Harvest
Diurnal Temperature and Surface Pressure Anomalies over the WWB
Before Harvest (March 1994-2000) - error bars indicate 90% confidence interval
Temperature
Pressure
• During the afternoon, the low temperature anomaly over the WWB
creates a surface high pressure anomaly
• Suggests that temperature anomaly is approximately as deep as the PBL
Impact on Air Pressure – After Harvest
Diurnal Temperature and Surface Pressure Anomalies over the WWB
After Harvest (June 1994-2000)
Temperature
Pressure
• During the afternoon, the high temperature anomaly over the
WWB creates a surface low pressure anomaly
The Diurnal Cycle of Dewpoint
The Diurnal Cycle of Dewpoint
An excellent indicator of land-atmosphere interactions
Diurnal Cycle of Dewpoint
4 Stages of the Diurnal Cycle of Dewpoint
Diurnal Cycle of Dewpoint – Morning Stage
• Dewpoint generally increases
• Dew evaporates
• Transpiration begins
• Shallow boundary layer – moisture builds up
• Relatively little dry air entrainment
SEPTEMBER
New Orleans, LA
Phoenix, AZ
Daytime Dewpoint
Evapotranspiration (ET)
Dry Air Entrainment
(Particularly important in western states)
Dry air aloft
Vertical mixing
Moist air near surface
Diurnal Cycle of Dewpoint – Daytime Stage
• Dewpoint generally decreases
• Battle between evapotranspiration and
dry air entrainment – D.A.E. usually wins
by afternoon, except in highly vegetated
areas or in coastal areas where D.A.E. is
minimal
AUGUST
Mason City, IA
SEPTEMBER
Phoenix, AZ
Diurnal Cycle of Dewpoint – Evening Stage
• Dewpoint almost always increases
• Dry air entrainment shuts off
• Dominated by evapotranspiration
• Shallow/stable boundary layer –
moisture builds up near surface
SEPTEMBER
Grand Island, NE
Los Angeles, CA
Diurnal Cycle of Dewpoint – Night. Stage
• Dewpoint generally decreases
• Dominated by condensation
• Largest decreases are in (moist) valleys
with plenty of radiational cooling
• Can increase at very dry and/or windy
locations
FEBRUARY
Fresno, CA
SEPTEMBER
Phoenix, AZ
Diurnal Cycle of Dewpoint
Oklahoma Mesonet sites representing Winter Wheat Belt (WWB) and
adjacent counties (AC)
Diurnal Cycle of Dewpoint
Before Harvest (March 1994-2000)
Diurnal Cycle of Dewpoint
Average 13Z to 0Z Dewpoint Change (March 1994-2000)
Diurnal Cycle of Dewpoint
After Harvest (June 1994-2000)
Diurnal Cycle of Dewpoint
Average 11Z to 23Z Dewpoint Change (June 1994-2000)
Conclusions
• Weather and climate “start” at the ground (surface)
and are driven by solar radiation.
• Nature of the earth’s surface determines way in which
the atmosphere is heated/cooled.
• Differences over land surfaces result from contrasts in
soil moisture, vegetation type & status, etc.
• Local boundary layer conditions
strongly depend on local surface
characteristics
Questions
Any Questions??