Heat, water and carbon exchanges in the tall megacity of Shanghai: challenges and results

Abstract

The observed surface energy balance fluxes are essential to improve model forecasting ability but such data are scarce for subtropical cities and urban areas with tall buildings. One year of eddy covariance flux data for an area within Shanghai with a wide range of building heights (8-150 m, mean by direction) are analysed by wind direction. Consideration is given to how to distinguish between local-and micro-scale fluxes. At the local-scale, median daily peaks of sensible heat flux occur in the early afternoon (winter: 87 W m −2 , spring: 205 W m −2 , summer: 292 W m −2 , autumn: 135 W m −2 ). The latent heat flux is small in winter (median daily maxima 21 W m −2 ) and slightly larger in spring, summer and autumn (49, 65, 49 W m −2 , respectively). The monthly mean daytime Bowen ratio under all-sky conditions ranges from 2 to 4.7. At this site, with predominately impervious surface (85% buildings/pavement, 14% vegetation), the enhancement of evaporation following rainfall usually lasts for about 12 h. Consistently larger Bowen ratios at the micro-scale than the local-scale are attributed to roughness effects and the impact of extensive areas of dry walls. The daily median CO 2 flux is dominated by traffic emissions, with two peaks associated with morning and evening rush hours. The data provide insights into urban surface controls on momentum, energy and carbon dynamics, with implications for urban planning strategies in the context of rapid global urbanization and climate change.

Summertime surface energy balance fluxes at two Beijing sites

“…The higher IAP Q res / Q * is indicative of both larger heat storage associated with the tall buildings (Miao et al, ) but with relatively open spacing. These values are comparable with the COSMO (Comprehensive Outdoor Scale Model experiments, Tokyo) site (Kawai and Kanda, ) and Shanghai (Ao et al, ), with similarly large building and impervious fractions. In such settings, there is a large area of exposed walls that can heat up (cool down) and a large canopy layer volume.…”

“…Previous urban studies have reported both positive and negative nocturnal Q H fluxes (Christen and Vogt, ; Grimmond et al, ; Offerle et al, ; Goldbach and Kuttler, ; Kotthaus and Grimmond, ; Ao et al, ). At less built up suburban sites, Q H values are, generally, negative at night as a result greater surface cooling (Grimmond and Oke, ; Christen and Vogt, ; Offerle et al, ; Newton et al, ; Balogun et al, ; Goldbach and Kuttler, ; Ward et al, ; Oke et al, ).…”

“…Similarly, Ao et al . () found 𝛽 values in a commercial and residential area of Shanghai were influenced by rainfall events for about 12 h. However, with the frequent addition of water from other sources at IAP it is harder to separate rainfall events. As Figure shows, frequently daytime 𝛽 < 1 occurred during “dry” (no rainfall) spells in July and August.…”

Multi‐year energy balance and carbon dioxide fluxes over a residential neighbourhood in a tropical city

“…Some short‐term observations (usually biased towards dry conditions) are available from Mexico City (Oke et al , , ; Tejeda‐Martinez and Jauregui, ; Velasco et al , ), Ouagadougou (Offerle et al , ), Miami (Newton et al , ) and Cairo (Frey et al , ). However, there is a significant lack of long‐term campaigns, which have only been conducted over residential areas in Phoenix (Chow et al , ), Singapore and Mexico City with initial results recently published as part of carbon sequestration research (Velasco et al , ) and a densely built‐up commercial site in Shanghai (Ao et al , ). A few generalizations nevertheless follow from the mostly suburban (sub)tropical work as summarized in Roth ().…”

The Analytical Objective Hysteresis Model (AnOHM v1.0): methodology to determine bulk storage heat flux coefficients

“…Residual: practical difficulties of direct measurement of Q S in urban areas, result in the SEB residual (i.e. Q * +Q F −(Q H + Q E )) frequently being the "preferred" observations (Ao et al, 2016;Ching et al, 1983;Doll et al, 1985;Li et al, 2015;Oke and Cleugh, 1987) (where Q F is the anthropogenic heat flux).…”