29 August 2013

Wind and Cold Carry Dust to New Heights

Science

Scientists at China's Lanzhou University and Pacific Northwest National Laboratory found that dust lifted from the Taklimakan Desert during a dust storm had a significant effect on the regional climate. The 2006 storm was aggravated by a cold front that pushed the dust to the highest level of the atmosphere over the northern Tibetan Plateau in China, affecting the balance of heat in the region's atmosphere. The ability to accurately model such storms will help in understanding the climatic impact of dust.

Approach

The researchers studied the effects of a large dust storm from the Taklimakan Desert (TD) that occurred July 21-30 in 2006. The dust swept over the northern slope of the Tibetan Plateau (TP), the world's third largest ice mass. The collaborators used the Weather Research and Forecasting model with chemistry (WRF-Chem) to simulate the event.

MODIS satellite view of dust storm over Taklamakan Desert.A MODIS satellite view of dust blowing across the Taklimakan Desert with the area of a cold front marked. Photo courtesy of NASA.. Enlarge Image.They examined the synoptic weather systems and dust transport pathway during the dust storm and estimated the shortwave and longwave radiative forcings of TD dust transported over the TP. During a typical summer, the heat radiating from the TP and weak East Asian westerly winds ripens conditions for a dust event. During the event in 2006, the conditions were further pushed by a strong cold front system over the TD. The dust broke through the planetary boundary layer and extended to the upper troposphere over the northern slope of the TP.

This study shows that the dust radiative forcing modified the surface and TOA energy budget and atmospheric heating profile, which can modulate the stability of the atmosphere as well as surface sensible and latent heating to increase the role of the TP as a heat pump.
What's Next? Researchers want to understand the worldwide climatic influence of the Taklimakan Desert, a major source of dust that ultimately travels long distances. Scientists plan to characterize the seasonal aspect of dust emissions and transport. Further, they will investigate how dust interacts with and changes clouds in its path, and how dust landing on snow changes the regional climate.

Impact

The Tibetan Plateau in China's Himalayan region has one of the world's largest stores of ice. As increasing ice melt over the plateau swells rivers and lakes, it also increases the amount of land in the region. The additional land mass acts as a heat pump drawing heat out of the atmosphere, increasing the intensity and duration of regional monsoons. Worldwide, monsoons provide half the Earth's population with fresh water. They also impact the world's climate via the global circulation.

The addition of dust blowing over the plateau heats the lower atmosphere, fueling the heat pump effect. In addition, when dust falls on the plateau's snow and ice, it absorbs the sun's energy and stimulates melting. In this study, results show that increasing dust over the region points to significant change the atmospheric circulation and the Asian monsoon climate.

Summary

­Modeling the Transport and Radiative Forcing of Taklimakan Dust over the Tibetan Plateau
Understanding the impact of desert dust on the Tibetan Plateau 

A research team, including scientists at Lanzhou University, China, and DOE scientists at Pacific Northwest National Laboratory found that dust from the Taklimakan Desert cooled the atmosphere near the surface and heated the atmosphere above during an intense dust storm event. The team used the Weather Research and Forecasting model with chemistry (WRF-Chem) to investigate the July 26-30, 2006 event that originated over the Taklimakan Desert (TD) and transported dust to the northern slope of Tibetan Plateau  (TP). The dust storm was initiated by the approach of a strong cold frontal system over the TD. In summer, the meridional transport of TD dust to the TP is typically favored by the thermal effect of the TP and the weakening of the East Asian westerly winds. During the studied dust storm, the transport of TD dust over the TP was further enhanced by the passage of the cold front. As a result, TD dust broke through the planetary boundary layer and extended to the upper troposphere over the northern TP. TD dust flux arrived at the TP with a value of 6.6 Gg/day in this 5-day event, but decayed quickly during the southward migration over the TP due to dry deposition. The event saw a maximum heating rate of 0.11 K day-1 at ~7 km over the TP. The event-averaged net radiative forcings of TD dust over the TP were -3.97, 1.61, and -5.58 W m−2 at the top of atmosphere, in the atmosphere, and at the surface, respectively. The promising performance of WRF-Chem in simulating dust and its radiative forcing for this event provides confidence for use in further investigation of climatic impact of TD dust over the TP.

Reference: Chen S, J Huang, C Zhao, Y Qian, LR Leung, and B Yang. 2013. “Modeling the transport and radiative forcing of Taklimakan dust over the Tibetan Plateau: A case study in the summer of 2006.” Journal of Geophysical Research 118(2):797-812. DOI:10.1002/jgrd.50122.

Funding: National Basic Research Program of China, the Special Scientific Research Project for Public Interest of China, the Program for Changjiang Scholars and Innovative Research Team in University, and the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of China; and DOE’s Regional and Global Climate Modeling program.

Contact: Chun Zhao, (509) 371-6372, chun.zhao@pnnl.gov

Contact
Siyu Chen
Publications
Chen, S, J Huang, C Zhao, Y Qian, L Leung, and B Yang.  2013.  "Modeling the Transport and Radiative Forcing of Taklimakan Dust Over the Tibetan Plateau: A Case Study in the Summer of 2006."  Journal of Geophysical Research - Atmospheres 797-812.  https://doi.org/10.1002/jgrd.50122.
Acknowledgments

Sponsors: This work was jointly funded by the National Basic Research Program of China, the Special Scientific Research Project for Public Interest, the Program for Changjiang Scholars and Innovative Research Team in University, the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of China, and the U.S. Department of Energy's (DOE's) Office of Science Biological and Environmental Research Regional and Global Climate Modeling Program.

Research Team: Siyu Chen, Chun ZhaoYun QianL. Ruby Leung and Ben Yang of PNNL; Jianping Huang of Lanzhou University, China.