Comparisons of urban-related warming in Beijing using different methods to calculate the daily mean temperature

Abstract

To evaluate the contribution of urban surface expansion to regional warming using different methods to calculate the daily mean surface air temperature (SAT), satellite-based images displaying urban surface expansion over the past 37 years (1980–2016) across China were collected for use in nested numerical experiments using the weather research and forecasting (WRF) regional climate model. The contribution of urban surface expansion to urban-related warming was determined using the daily mean SAT averages based on four time records each day (00, 06, 12, and 18 h UTC, T4) and averages of the SAT maximum (Tmax) and minimum (Tmin) (Txn). The contribution of urban surface expansion to urban-related warming (relative value) in Beijing was 0.110°C per decade (22.8% of total warming) for T4 and 0.094°C per decade (20.2%) for Txn. The values obtained when using T4 were larger than those obtained when using Txn. Differences in the urban-related warming calculated using T4 and Txn could be attributed to the smaller changing trends in Txn in the urban-surface expansion experiment, which resulted from a large changing trend in Tmin and a much smaller changing trend in Tmax. The changes in the diurnal cycle of the energy budget due to urban surface expansion induced changes in the diurnal cycle of SAT, as evidenced by the four time records each day, as well as Tmax and Tmin. This was especially true for periods of intense urban surface expansion, although the annual mean SAT calculated using Txn was larger than that calculated using T4. The increase in impervious area (walls, streets, etc.) due to urban surface expansion, as well as the widespread use of building materials with a large heat capacity resulted in a marked increase in ground heat flux in the daytime. This restricted the increase in SAT in the daytime, but promoted it at night. The increases in SAT due to urban surface expansion were not symmetrical, being smaller in the daytime and larger at night.