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

Rn S LE G DQ 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

Rn S LE G DQ Car in the sun Result: very high “surface” temperature! HIGHER NORMAL LOWER Rn S LE G DQ Result: very high “surface” temperature!

Dead grass Dead grass, minimal soil moisture

Rn S LE G DQ Dead grass HIGHER NORMAL LOWER Result: higher daytime surface temperature, deeper boundary layer, stronger (& more westerly) wind

South-facing slope South-facing slope

Rn S LE G DQ South-facing slope HIGHER NORMAL LOWER Result: slightly higher surface temperature, slightly lower relative humidity

Woodland (nighttime) Woodland or open-canopy forest (nighttime)

Rn S LE G DQ Woodland (nighttime) LESS NEGATIVE NORMAL MORE Result: lower temperature, higher RH (at night)

Hilltop (nighttime) Hilltop (nighttime)

Rn S LE G DQ Hilltop (nighttime) LESS NEGATIVE NORMAL MORE 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 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

Evapotranspiration (ET) 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 SEPTEMBER Mason City, IA 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 SEPTEMBER Fresno, CA 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??