The Role of Rossby Waves within Westerly Jet Waveguides
A severe haze event occurred in the North China Plain (NCP) during November and December 2015, characterized by a wide spatial range, high intensity, and long duration. Analyzing this episode can provide valuable insights into the complex interplay between large-scale atmospheric circulation patterns and local meteorological conditions that drive haze formation.
The Meteorological Factors Behind the Haze Event
Haze events are typically modulated by a combination of pollutant emissions and meteorological conditions. In the case of the 2015 NCP haze, pollutant emissions played a vital role, with monthly mean PM2.5 concentrations reaching 30-50 μg/m³ during the period. However, weather patterns and climate change are also known to significantly influence haze formation, distribution, and persistence.
Key Meteorological Factors:
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Weakened East Asian Winter Monsoon (EAWM): The EAWM index was negative in November and December 2015, indicating a weakened winter monsoon. This allowed for the accumulation of pollutants, as the outward transport of aerosols was impeded.
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Stable Lower Troposphere: The vertical temperature difference between 1000 and 850 hPa was small, creating a relatively stable lower troposphere. This, coupled with weak near-surface winds, prevented the dispersion of pollutants.
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Anticyclonic Circulation: An anticyclonic circulation pattern developed over the NCP, with southerly winds on the northwestern side that prevented the southward dispersion of pollutants. The descending air motion associated with the anticyclone also contributed to the stable atmospheric conditions.
These meteorological factors, in combination with high pollutant emissions, led to the severe and prolonged haze event in the NCP during the late fall and early winter of 2015.
The Role of Rossby Waves within Westerly Jet Waveguides
The key to understanding the 2015 NCP haze event lies in the propagation of Rossby waves within the upper-level westerly jet waveguides, which significantly influenced the regional circulation patterns.
Rossby Waves in the Subtropical and Polar Front Jets
The analysis revealed that anomalous Rossby waves propagated eastward along two pathways: the subtropical westerly jet and the polar front jet.
- Subtropical Westerly Jet Waveguide:
- The Rossby waves within the subtropical jet originated from the Mediterranean region and propagated eastward.
- This resulted in an anomalous anticyclone over the Sea of Japan, which strengthened the descending air motion over the NCP.
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The Rossby waves also weakened the East Asian trough and Ural ridge, leading to a southerly wind anomaly at 850 hPa over eastern China. This further weakened the EAWM, contributing to haze accumulation.
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Polar Front Jet Waveguide:
- The Rossby waves within the polar front jet caused a negative geopotential height anomaly over Siberia, weakening the Ural ridge.
- This prevented the intrusion of cold air from the Arctic, also weakening the EAWM and favoring haze formation.
The combined effect of the Rossby waves propagating along these two waveguides led to the northâsouth circulation pattern that was crucial for the haze event. The upper-level divergence over southern China, coupled with the lower-level convergence over the NCP, created a stable and stagnant atmospheric environment conducive to haze accumulation.
The Maintenance of the Haze Event
The haze event was not only initiated but also maintained by the complex interplay between the Rossby waves and regional circulation patterns.
- Latent Heat Release: The Rossby waves along the subtropical westerly jet triggered large-scale precipitation in southern China. The latent heat released by this precipitation acted as a heat source, further intensifying the ascending motion over southern China.
- Strengthened Descending Motion: The strengthened ascending motion over southern China, in turn, enhanced the descending air motion over the NCP, promoting the persistence of the haze event.
This self-reinforcing local circulation system, driven by the Rossby waves within the westerly jet waveguides, was a key factor in the maintenance of the severe and prolonged haze episode in the NCP during the late fall and early winter of 2015.
Implications and Potential Applications
The findings from the analysis of the 2015 NCP haze event highlight the importance of considering large-scale atmospheric circulation patterns, particularly the propagation of Rossby waves within the upper-level westerly jet waveguides, when studying and predicting haze formation and persistence.
Tracking Rossby Wave Propagation
By monitoring the evolution of Rossby waves within the subtropical and polar front jet waveguides, it may be possible to anticipate the development of circulation patterns that are conducive to haze accumulation in the NCP and other regions. This could assist in the early warning and mitigation of severe haze events.
Improving Haze Forecasting Models
Incorporating the influence of Rossby waves and their interactions with regional circulation features into numerical weather prediction and air quality models could enhance the accuracy of haze forecasting. This would support the implementation of timely and targeted pollution control measures.
Informing Policy and Emission Reduction Strategies
Understanding the complex interplay between large-scale atmospheric processes and local meteorological conditions is crucial for developing effective air pollution control policies. By addressing both emission sources and meteorological drivers of haze, policymakers can craft more holistic and impactful strategies to improve air quality in North China and other affected regions.
Conclusion
The severe haze event in the NCP during November and December 2015 was a result of the combined effects of high pollutant emissions and unfavorable meteorological conditions. The key driver behind these meteorological conditions was the propagation of anomalous Rossby waves within the upper-level westerly jet waveguides, which led to a self-reinforcing local circulation system that promoted haze accumulation and persistence.
By understanding the role of large-scale atmospheric circulation patterns, particularly the Rossby waves within the westerly jet waveguides, researchers and policymakers can gain valuable insights to enhance haze forecasting, develop more targeted emission reduction strategies, and ultimately improve air quality in North China and beyond. The Air Cooled Heat Exchangers blog aims to provide such in-depth analyses and practical solutions to address complex air quality challenges.
