Abstract
We examine past and future changes in both winter haze and clear weather conditions over the North China Plain (NCP) using a Perturbed Parameter Ensemble (PPE) and elucidate the influence of model physical parameterizations on these future projections for the first time. We use a meteorology-based Haze Weather Index (HWI), which was developed to examine the haze conducive weather conditions for Beijing. We find that the HWI can be used as an indicator of winter haze across the entire NCP due to the extended spatial coherence of the local meteorological conditions. The PPE generated using the UK Met Office HadGEM-GC3 model shows that under a high-emission (RCP8.5) scenario, the frequency of haze conducive weather is likely to increase whereas the frequency of clear weather is likely to decrease in future. However, a change of opposite sign with lower magnitude in the frequencies, though less likely, is also possible. In future, the total number of hazy days for a given winter can be as much as ~3.5 times higher than the number of clear days over the NCP. We also examined the changes in the interannual variability of the frequency of hazy and clear days and find no marked changes in the variability for future periods. The future frequencies of winter hazy and clear days in the PPE are largely driven by changes in zonal-mean mid-tropospheric winds and the vertical temperature gradient over the NCP. We do not find any discernible influence of model physical parameterizations on the future projections of trends in the frequency of hazy or clear days. We find a clear impact of anthropogenic climate change on future trends for both hazy and clear days, however, it is only discernible for specific periods due to the large underlying internal variability in the frequencies of hazy and clear days.
Highlights
Over the last decade, a number of severe haze episodes were reported over the North China Plain (NCP) during boreal winter (December-January-February, DJF)
We investigate the importance of the different meteorological variables used in the Haze Weather Index (HWI) in determining the future changes in haze conducive conditions in the Perturbed Parameter Ensemble (PPE) (Section 6)
For the PPE historical and RCP8.5 simulations, the daily HWI time series is calculated for each ensemble member for DJF for 1969-2089 using the same methodology as used for ERA-5, with the difference being that the normalisation of the PPE time-series (1969-2089) is performed using the historical standard deviation (1969-2005), following Cai et al (2017)
Summary
A number of severe haze episodes (several days or longer) were reported over the North China Plain (NCP) during boreal winter (December-January-February, DJF). Index (HWI) and projected a 50% increase in the frequency of winter haze conducive weather conditions, similar to the January 2013 event, over Beijing in the future (2050-2099) as compared to the historical (1950-1999) period under RCP8.5 scenario using 15 CMIP5 models. Callahan and Mankin (2020) found 10-15% increase in winter hazy days in CMIP5 multimodel and CESM large ensemble under 3° warming and emphasized a large influence of internal variability in addition to anthropogenic forcing on future haze conducive weather over Beijing. The advantage of using the PPE over the initialised or multimodel ensemble is that it accounts for internal variability and model uncertainty arising due to the different settings of the physical parameterisations in a single model Both multimodel ensemble and initialised ensemble from a single model have been used to assess the future winter haze conducive conditions over Beijing. More details on the data and methods used in this paper are provided
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