Heat storage and anthropogenic heat flux in relation to the energy balance of a central European city centre

Abstract

The role of net heat storage Q S and anthropogenic heat Q F are considered in the surface energy balance for a downtown area in Łódź, Poland, for a 2 year period. Eddy covariance measurements provide estimates of the turbulent heat fluxes and radiometric measurements of the net all-wave radiation. A method to determine Q S based on representative surface temperature sampling is employed and compared with results from two other models. Results show that Q S is an important flux on the scale of hours to days and that it can be more than 10 W m −2 , on average, for periods of a week or more. By incorporating Q S estimates over hourly intervals, Q F was then determined as the residual of the energy balance. Using the approach, Q F averaged 32 W m −2 from October to March (60% of available energy), and −3 W m −2 from June to August. The physically unrealistic negative values for the summer period may suggest underestimation of turbulent fluxes, but no causal factor was identified. Although energy balance closure was close to 100% throughout the year, there was weaker agreement in the winter. This is attributed to errors in estimates of Q S and variation in Q F . Results highlight the need for future investigations of the urban surface energy balance to incorporate more complete measurements and estimates of Q S .

Anthropogenic heat release in an old European agglomeration (Toulouse, France)

“…The daily heat storage, therefore, should be weaker. Moreover, Q F values estimated from the inventory (Section 'Results and Discussion') at the local scale around our measurement site are larger than the Q S estimates presented by Offerle et al (2005), even in summer conditions. Hence, we assume that the average of the residual over a 1-day (or multiples of 1-day) period can be used to estimate Q F with the following relation:…”

“…The basic assumption is that Q S should vanish over longer timescales (one day or multiple of entire days), otherwise it would lead to unrealistic temperature changes of the urban fabric (Christen and Vogt, 2004). Results supporting this hypothesis are presented by Offerle et al (2005). They present an original method to get hourly values of Q S , at the local scale, by combining numerical simulation and measurements.…”

“…From the measurements performed during CAPITOUL around the study site, Pigeon et al (2004) have shown that the temperature and moisture horizontal gradients are weak at the local scale, and Q A is therefore neglected in the present study. Q S has received much attention in the literature (Oke and Cleugh, 1987;Grimmond et al, 1991;Arnfield and Grimmond, 1998;Grimmond and Oke, 1999;Offerle et al, 2005;Weber, 2005). On time scales of a few hours, Q S is a strong component of the SEB (58% of daytime Q * for Mexico City in Oke et al, 1999).…”

“…However, other sources of information, such as the patterns of population density, can be used to estimate the spatio-temporal variability at smaller scales (Sailor and Lu, 2004). The second method is based on surface energy balance (SEB) measurements (Offerle et al, 2005). The advantage of this method is that it makes it possible to estimate Q F with an hourly time resolution, but its weakness is that it is limited to the local scale (typically a neighbourhood or a few hundreds meters) around the measurement site.…”

“…From February 2004to March 2005, the CAPITOUL [Canopy and Particles Interactions in Toulouse Urban Layer ] field campaign was conducted over the agglomeration to monitor, during a full year, the SEB, the urban heat island and the urban boundary layer structure. A method, simpler than the one developed by Offerle et al (2005), is presented to estimate Q F from SEB measurements made in the downtown area of Toulouse. Also, an inventory of energy consumption is conducted at the agglomeration scale with a 1-day time resolution and a 100-m horizontal resolution.…”