Temporal variability of energy and mass fluxes at a residential site in a tropical city

Urbanization is accompanied by significant changes in thermal, radiative and aerodynamic surface properties which alter the energy partitioning at the surface causing distinct urban climates. Anthropogenic activities associated with cities further emit pollutants and greenhouse gases. Although small in terms of area, the urban environment is estimated to be the source from which up to 85% of the global anthropogenic emission of carbon dioxide (CO2) originates. Recent urban studies on energy and mass fluxes have focused on cities in developed countries located in mid-latitudes such as for example Basel, Copenhagen, Marseille, Melbourne or Tokyo. Only a rudimentary understanding of the physical processes operating in the atmosphere of cities in other climate regions exists. Exceptions are short-term studies conducted in Miami (subtropical climate) and Mexico City (tropical highland climate). This is unfortunate because much of the anticipated future growth of the world's population will occur in cities in the less developed world which are almost exclusively located in the (sub)tropics. These cities will be at the center of the climate change debate because they will lead future urban growth, are major emitters of greenhouse gases and pollutants and in turn are likely to be significantly impacted by future global climate change.

This paper presents results from ongoing research in Singapore, a city located in the humid tropics. The main objectives are to investigated the long-term (>1 year) variability of the energy balance and CO2 fluxes, the turbulent transfer characteristics in the tropical atmosphere and to evaluate the utility of radiation fluxes to derive other important variables such as the turbulent sensible heat flux or stability. The observation site is located in a residential area characterized by dense row and individual houses with a mean height of about 9 m and tree-lined secondary roads. Measurements began in January 2006 using eddy correlation sensors installed at the top of a 21.8 m high pneumatic tower to obtain raw turbulence and 30 min flux data. Besides a couple of periods when individual sensors malfunctioned an almost continuous time series encompassing monsoon and inter-monsoon seasons is available for analysis.

Ensemble averages show that across all seasons the energy partitioning at the surface at this site favours sensible over latent heat and therefore the Bowen ratio is clearly larger than unity. This is surprising for a city in a wet climate with high availability of water for evaporation. The result is, however, similar to that from Miami. Forthcoming analysis will demonstrate if reduced vapor pressure deficit is responsible for suppressing the latent heat flux as was hypothesized to be the case for Miami. An important focus of this research is on the contribution of a tropical city to the carbon budget. The CO2 fluxes show exchanges on averages are always positive at this residential site despite the relatively large amount of greenspace. This result is consistent with data from other urban studies conducted in mid-latitude cities. Further analysis is needed to be able to relate the peaks in mid-morning and the late evening to, for example, particular exchange characteristics of the atmosphere or contributions from traffic emissions at those times.

The data from the present study are the first long-term and therefore robust energy balance CO2 flux record for any (sub)tropical city. Forthcoming analysis will also include an evaluation of the turbulent characteristics (integral and spectral) and the net energy and CO2 budget for clear and cloudy days, individual seasons and the full year.