Local-scale heat storage estimation: results from suburban Oklahoma City

Accurate assessment of the storage heat flux (ΔQ_S), the rate of sensible heat flow (per unit area) into or out of the elements comprising the urban canopy layer, is critical to the proper resolution of the surface energy balance. More specifically, storage heat flux is a necessary requisite for applications including modeling boundary layer growth, turbulent sensible heat flux, evapotranspiration, urban heat island formation, and building energy use. However, the complex morphology and materials which characterize urban environments complicate storage heat flux estimation. As a result, a variety of methods, appropriate for different scales and surfaces, have been proposed as solutions to resolve heat storage. The purpose of this paper is to estimate diurnal local-scale (horizontal 10²-10⁴ m) storage heat flux for two suburban sites with different building materials (brick and wooden structures) and to present the inter-comparison of four different methods: Residual Energy Balance; The Objective Hysteresis Model (OHM); Element Surface Temperature Model (ESTM); and Ground heat flux plus air storage. All observations were collected during the summertime as part of the 2003 Joint Urban field campaign (Oklahoma City, OK). Results indicate that heat storage at both sites exhibit signature urban hysteresis behavior and represent a significant component of the surface energy balance with diurnal ranges between -150 W m^-2 and 250 W m^-2. Overall, all four methods agree best at the brick houses site (high thermal mass site), though the wooden houses site (low thermal mass site) has better agreement into the late morning (1000 h LST). After this period, the methods begin to diverge and the OHM and Residual methods exhibit larger storage (~100 W m^-2) with a later afternoon peak than the ESTM or Soil and Air storage methods (1100 h and 1300 h LST, respectively). The impact of the building envelope is less clear, as the heat storage for the low mass structure is more or less than the high mass structure depending on the method used. However, interpretation of these results may be further complicated by the different geometries of these sites in relation to other nearby structures.