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Abstract: The surface
energy balance in an irrigated urban park in suburban Sacramento, CA is observed. Three sites extend from the
edge of the park to its centre, along a transect
which is aligned with the prevailing wind. Direct measurements of the
fluxes of net radiation, soil heat flux and evaporation are made at
each site and the convective sensible heat is found by residual. Strong
advective effects on evaporation are observed, especially in the
afternoon and evening. The driving forces for this are the differences
in surface and air temperature, and humidity, between the cool, wet
park and its warmer, drier built-up surroundings. The control of the
surroundings on park evaporation is demonstrated by comparing values
with those from synchronous observations in the surrounding suburbs and
at an irrigated sod farm just outside the city. Greatest evaporative
enhancement is observed at the upwind edge. Throughout the afternoon
evaporation considerably exceeds the net radiation. This is interpreted
to be due to the microscale leading-edge
effect which appears to be restricted to a fetch of about 20 m. Further
into the park evaporation also exceeds the net radiation in the
afternoon due to the oasis effect. At all sites the sensible heat flux
density in the afternoon is negative. Daily and daytime total
evaporation from the park is more than 300% that from the integrated
suburban area, and more than 130% that from the irrigated rural grass
site. The unlimited water supply and the high temperatures of the park
allow it to behave like a wet leaf in that its surface temperature
seems to be  thermostatically controlled - it never
rises more than a few degrees above
that of the park air and for much of the day is cooler than the park
air. Copyright © 2000 Royal Meteorological Society
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