UCP4: Observations of Surface Energy and Water Balances
The daytime energy budget of small parks in Mexico City and Lisbon, Portugal, as derived by tree sap-flow measurements and transpiration modeling
1Instituto de Ecología, UNAM; 2Centro de Ciencias de la Atmósfera, UNAM; 3Centro de Estudos Geográficos, Universidade de Lisboa
A series of sap flow (transpiration, TRPSF) and energy balance components measurements were carried out in small urban parks in Mexico City. Measurements of sap flow in the trunks of the trees was selected as an alternative tool instead of eddy covariance or Bowen ratio-energy balance methods in order to determine the energy balance components of the tree canopy. These measurements were made in the rainy season (July 2009), from 8:00 to 17:00 LST. With these data, a simple model to estimate transpiration (TRPE) was generated and applied to urban parks in Lisbon, Portugal. Both TRPSF and TRPE showed a unimodal pattern during the day. Mean daily transpiration was higher at urban parks in Lisbon (536 g m-2 d-1) than in Mexico City (411.5 g m-2 d-1), with maxima rates of 0.027 and 0.030 g m-2 s-1, respectively. Net radiation (QN) was mainly dissipated by latent (QE) and sensible (QH) heat around 76 and 17%, respectively in Mexico City and 78 and 13%, respectively, in Lisbon. These differences in QE and QH in the two cities were probably due to differences of water availability since urban parks are irrigated more frequently in Lisbon than in Mexico City. Heat storage of the urban parks was found to be a negligible component of the energy balance in both cities. It is demonstrated that small urban vegetated areas can play an important role in the urban microclimate and therefore, in the mitigation of the urban heat island. Presumably, the constructed model presented a good performance.
Multi-year energy balance and carbon dioxide fluxes over a residential neighborhood in a tropical city
1National University of Singapore, Singapore; 2Swedish Meteorological and Hydrological Institute; 3Singapore-MIT Alliance for Research and Technology
To be able to mitigate potential unwanted consequences from the local to global scale as an outcome of urbanization, it is necessary to understand the heat energetics and characteristics of trace gas emissions of the urban system. This has consequences for e.g. the magnitude and dynamics of the urban heat island, human thermal comfort or the net exchange of carbon dioxide (and other trace gases) which reflects the balance between carbon emissions from human activities and the dynamics of a number of terrestrial processes that remove or emit carbon dioxide. While such process-based work has been carried out in an increasing number of cities in the developed world in the primarily mid-latitudes, similar work is notably absent in the (sub)tropics where many of the cities with the largest projected future growth rates are located. This presentation will focus on the results from a long-term (7-year) study of energy balance and carbon dioxide fluxes for a residential neighborhood in a modern city located in a tropical-wet climate, viz Singapore. Despite the uniformity of the general tropical background climate, a clear seasonality in energy balance fluxes emerges in response to monsoonal shifts in wind direction. Ensemble averages show that across all seasons the energy partitioning at the surface at this site favors sensible over latent heat and therefore the Bowen ratio is clearly larger than unity. This is somewhat surprising for a city in a wet climate with high availability of water for evaporation. 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. The data presented enhance the geographic range of similar work to a grossly understudied region.
Micrometeorological impacts of an ephemeral desert city: The Burning Man experiment
San Francisco State University, United States of America
This paper presents the results from a micrometeorological study of Black Rock City, NV (BRC) during the annual Burning Man festival. Observations were made from 22 August to 2 September 2013 to include the initial construction of the city, the fully populated city during the main event week (69,000 total, ~12,000 km^-2) and the rapid exodus from the city at the end of the event. Built on a flat, arid, playa surface devoid of vegetation or surface water, the city provides a striking contrast in surface roughness as well as the size of sources and sinks of CO2, water and heat. Due to the temporary nature of the city and the high degree of spatial organization, the effects of changing population density and surface cover could be captured in a very short observational period. Eddy covariance measurements of CO2, water vapor, heat and momentum fluxes were made at 12 m and a profile of temperature, humidity, wind speed and direction was measured from 1- 30 m. A second smaller meteorological station was placed on the outskirts of the city and mobile bicycle observations were used to obtain cross-sectional temperature profiles of the urban area.
In this paper, we present the impacts of the urban development on surface roughness, surface energy, water and carbon exchanges, and resulting temperature and humidity signatures. Net CO2 fluxes were negligible prior to urbanization and grew steadily with population density within the flux footprint until they reached rates of 26 gC m^-2 d^-1, similar to large, dense cities observed elsewhere. Diurnal CO2 flux patterns were controlled most strongly by dynamic surface layer drivers, including large swings in surface layer stability and convective transport. The signal from human activities is also clear, such as vehicle migration patterns and even the effect of the culminating event gathering. Also discussed is the role of changing surface roughness on turbulence and vertical exchange processes as well as much smaller changes found in the surface energy balance, Bowen ratio and urban heat island.