Heat fluxes through roofs and their relevance to estimates of urban heat storage.

Heat storage uptake and release constitutes a large term in the heat balance of cities. This flux is difficult to measure but can be parameterized using relations between the net radiation and the heat flux conducted into and out of the materials that form the surface of cities. When weighted by the abundance of such surfaces in a given urban area, these relations have been found to give storage values in broad agreement with those found as a residual in the heat balance, if all other terms are measured directly (Grimmond and Oke, 1999, JAM, 38, 922-940). There is reason to believe that heat storage estimates obtained via parameterization could be improved if there were more and better estimates of the net radiation vs. storage relation for roofs.

In order to accomplish this the heat storage characteristics of 6 different roof assemblies (typical of many North American residential and industrial/commercial buildings) in Vancouver, B.C. were studied. Field observations of the radiative and conductive fluxes and the concurrent thermal and wind conditions were gathered and analyzed. The daily net radiation vs. heat conduction relations showed hysteresis loop behaviour similar to that shown and statistically-described by Camuffo and Bernardi (BLM, 1982). The statistical coefficients necessary to the parameterization scheme were extracted. The observations were also used to verify the Simple Transient Analysis of Roofs (STAR) model developed at the Oak Ridge National Laboratory (ORNL). The ORNL model was then used to estimate Camuffo and Bernardi-type coefficients for other roof types, thereby potentially extending the usefulness of the scheme to a wider range of cities.