Natalie M. Schultz

Urban heat islands in China enhanced by haze pollution

The urban heat island (UHI), the phenomenon of higher temperatures in urban land than the surrounding rural land, is commonly attributed to changes in biophysical properties of the land surface associated with urbanization. Here we provide evidence for a long-held hypothesis that the biogeochemical effect of urban aerosol or haze pollution is also a contributor to the UHI. Our results are based on satellite observations and urban climate model calculations. We find that a significant factor controlling the nighttime surface UHI across China is the urban–rural difference in the haze pollution level. The average haze contribution to the nighttime surface UHI is 0.7±0.3 K (mean±1 s.e.) for semi-arid cities, which is stronger than that in the humid climate due to a stronger longwave radiative forcing of coarser aerosols. Mitigation of haze pollution therefore provides a co-benefit of reducing heat stress on urban residents.

Assessing the use of subgrid land model output to study impacts of land cover change

Subgrid information from land models has the potential to be a powerful tool for investigating land-atmosphere interactions, but relatively few studies have attempted to exploit subgrid output. In this study, we modify the configuration of the Community Land Model version CLM4.5 so that each plant functional type (PFT) is assigned its own soil column. We compare subgrid and grid cell-averaged air temperature and surface energy fluxes from this modified case (PFTCOL) to a case with the default configuration—a shared soil column for all PFTs (CTRL)—and examine the difference in simulated surface air temperature between grass and tree PFTs within the same grid cells (ΔTGT). The magnitude and spatial patterns of ΔTGT from PFTCOL agree more closely with observations, ranging from −1.5 K in boreal regions to +0.6 K in the tropics. We find that the column configuration has a large effect on PFT-level energy fluxes. In the CTRL configuration, the PFT-level annual mean ground heat flux (G) differs substantially from zero. For example, at a typical tropical grid cell, the annual G is 31.8 W m−2 for the tree PFTs and −14.7 W m−2 for grass PFTs. In PFTCOL, G is always close to zero. These results suggest that care must be taken when assessing local land cover change impacts with subgrid information. For models with PFTs on separate columns, it may be possible to isolate the differences in land surface fluxes between vegetation types that would be associated with land cover change from other climate forcings and feedbacks in climate model simulations.

Global satellite data highlights the diurnal asymmetry of the surface temperature response to deforestation

Uncertainties remain about the spatial pattern and magnitude of the biophysical effects of deforestation. In particular, a diurnal asymmetry in the magnitude and sign of the surface temperature response to deforestation (ΔTS) has been observed, but the biophysical processes that contribute to day and nighttime ΔTS are not fully understood. In this study, we use a space-for-time approach with satellite and reanalysis data to investigate the biophysical processes that control the day and nighttime ΔTS. Additionally, we incorporate flux-tower data to examine two hypotheses for nighttime forest warming relative to open lands: (1) that forests generate turbulence in the stable nocturnal boundary layer, which brings heat aloft down to the surface, and (2) that forests store more heat during the day and release it at night. Our results confirm a diurnal asymmetry in ΔTS. Over most regions of the world, deforestation results in daytime warming and nighttime cooling. The strongest daytime warming is in the tropics, where the average ΔTS is 4.4 ± 0.07 K. The strongest nighttime cooling is observed in the boreal zone, where open lands are cooler than forests by an average of 1.4 ± 0.04 K. Daytime patterns of ΔTS are explained by differences in the latent heat flux (ΔLE) and absorbed solar radiation (ΔKa). We find that nighttime ΔTS is related to the strength of the nocturnal temperature inversion, with stronger temperature inversions at high latitudes, and weak inversions in the tropics. Forest turbulence at night combined with stored heat release drives nighttime ΔTS patterns.

Global lake evaporation accelerated by changes in surface energy allocation in a warmer climate

Lake evaporation is a sensitive indicator of the hydrological response to climate change. Variability in annual lake evaporation has been assumed to be controlled primarily by the incoming surface solar radiation. Here we report simulations with a numerical model of lake surface fluxes, with input data based on a high-emissions climate change scenario (Representative Concentration Pathway 8.5). In our simulations, the global annual lake evaporation increases by 16% by the end of the century, despite little change in incoming solar radiation at the surface. We attribute about half of this projected increase to two effects: periods of ice cover are shorter in a warmer climate and the ratio of sensible to latent heat flux decreases, thus channelling more energy into evaporation. At low latitudes, annual lake evaporation is further enhanced because the lake surface warms more slowly than the air, leading to more long-wave radiation energy available for evaporation. We suggest that an analogous change in the ratio of sensible to latent heat fluxes in the open ocean can help to explain some of the spread among climate models in terms of their sensitivity of precipitation to warming. We conclude that an accurate prediction of the energy balance at the Earth’s surface is crucial for evaluating the hydrological response to climate change.Lake evaporation could increase substantially despite modest changes in incoming solar radiation at the surface, as a result of changes in energy partitioning and shorter periods of ice cover, according to numerical simulations.