HR: 11:45h
AN: U51C-06    [PDF]
TI: Measuring Longwave Radiative Flux Divergence in an Urban Canyon
AU: * Soux, A
EM: asoux@geog.ubc.ca
AF: Department of Geography, University of British Columbia, 1984 West Mall, Vancouver, BC V6T 1Z2 Canada
AU: Oke, T R
EM: toke@geog.ubc.ca
AF: Department of Geography, University of British Columbia, 1984 West Mall, Vancouver, BC V6T 1Z2 Canada
AU: Nunez, M
EM: nunez@utas.edu.au
AF: School of Geography and Environmental Studies, University of Tasmania, Private Bag 78, Hobart, TAS 7001 Australia
AU: Wilson, M
EM: matthew.wilson@utas.edu.au
AF: School of Geography and Environmental Studies, University of Tasmania, Private Bag 78, Hobart, TAS 7001 Australia
AB: There has been very little measurement of longwave radiation divergence since the urban studies of Fuggle, Oke and Nunez in the mid 1970's or the rural work of Funk in the early 1960's. Although radiative divergence has been widely ignored for sometime there is the belief that it may play an important role in balancing nocturnal energy budgets in a range of environments. For example, in urban environments surface temperature relates well to the energy balance whereas air temperature does not, even in non-turbulent conditions. This is probably due at least in part to the effects of longwave divergence. To help answer issues related to longwave divergence a new dual-channel infrared radiometer (DCIR) has been developed. The DCIR, as the name implies, measures the directional infrared radiation in two wavebands and can, through differencing of the signals and further signal processing, give a direct measurement of longwave radiative flux divergence. The DCIR was deployed for the first time as part of a larger study (BUBBLE) of the urban boundary layer of Basel, Switzerland. The objective is to further study the thermal regime of a city at the canyon scale. To this end, a street canyon was carefully selected, in the city of Basel. The canyon surface and air volume were instrumented, including turbulent and conductive fluxes, and standard meteorological variables in addition to radiation. A unique data set was obtained to allow the complete energy balance of the canyon system to be evaluated without the need to resort to using residuals to quantify the magnitude of the longwave radiative flux divergence. Measured values of longwave flux-divergence are converted to cooling rates to compare with measured air temperature cooling. Preliminary results show that at the onset of canyon air-volume cooling, measured cooling rates are slightly lower than radiative cooling rates. The differences are less than $0.5\deg$ C. This contrasts sharply with previously measured above roof level and rural differences of greater than $5\deg$ C. The difference between the rural and above-canyon case and the in-canyon case is most likely a result of differences in radiative environments and wind and temperature fields. The differences illustrate the strong role of urbanization on the surface energy budget.
DE: 0360 Transmission and scattering of radiation
DE: 3359 Radiative processes
DE: 3360 Remote sensing
DE: 3394 Instruments and techniques
SC: U
MN: 2003 Fall Meeting