Advection and the surface energy balance across an irrigated urban park

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