COMPARATIVE OBSERVATION OF LONG-WAVE RADIATION BALANCE ON GROUND-SURFACE AND ON ROOF-LEVEL IN THE URBAN AREA

Abstract

Considering a long-wave radiation field as a model, it should be noted that radiative energy is basically redistributed on the ground-surface in the rural area, but in the urban area it is redistributed on the roof-level and the ground-surface. The urban roof-level is exposed to long-wave radiation additionally caused by the aerosol layer and the warm vertical temperature structure of urban atmosphere besides background long-wave radiation. The urban ground-surface is, moreover, under influence of screen effects due to buildings. This suggests that the restraint of radiative cooling resulting from the double screen effects on the urban ground-surface is one of the main causes of the urban heat island. The objectives of this paper are to obtain the difference between long-wave radiation balance components on the roof-level and on the ground-surface in the urban area, and to discuss the screen effects (the effects of buildings) upon the long-wave radiation field on the ground-surface in the urban area. Measurements of long-wave radiation balances on the ground-surface and on the roof-level were achieved in the urban area of Tokyo on clear nights. The radiation sensors were polyethylene-shielded net radiometers (C. S. I. R. O. Net Radiometer Model CN-l and CN-2). The measurement accuracy of the sensors was ≤±4%. Upward long-wave radiation (R↑) was computed from measured surface temperatures and Stefan-Bolzmann's Law (assuming a concrete emissivity of 0.92 by Falckenberg (1928)). As observations are restricted to nights, the radiation balance equation is expressed by a simple form: RNet=R↓+R↑(1) where RNet is net long-wave radiation; R↓, downward long-wave radiation. Downward long-wave radiation (R↓) was computed by equation (1). The results were as follows: 1) Net long-wave radiation (RNet) on the urban ground-surface is consistently half as much as that on the urban roof-level. This is largely because downward longwave radiation (R↓) on the urban ground-surface is greater than on the urban roof-level. 2) The increased downward long-wave radiation (R↓) on the urban ground-surface is explained by taking the screen effects due to buildings into consideration. 3) It is clear that long-wave radiation components are extremely influenced by clouds. The increase of cloudiness, in particular, causes the increase of downward long-wave radiation (R↓), and then the decrease of net long-wave radiation (RNet). On a closer examination, cloudiness is also of greater influence upon long-wave radiation components on the roof level than on the ground-surface. From this analysis the existence of the screen effects due to buildings on the urban ground-surface can be confirmed.