The Taklimakan Desert is one of source of Asian dust. It is located at the center of the Tarim Basin in northwest China. The desert is surrounded by high mountains exceeding 5,000 m in height: the Tianshan Mountains to the north, the Pamir Plateau to the west and the Kunlun Mountains to the south. Due to its topography, the Taklimakan dust can be transported over long distances when it is lifted up to the free troposphere. In this context, it plays an important role in causing climatological effects (Liu et al. 2008; Huang et al. 2009). Recently, the dust layer over the Taklimakan Desert and its climatological importance have been studied by using the space-borne lidar CALIPSO data (Hara et al. 2008; Liu et al.2008; Huang et al. 2009). But, the detailed structure of summer dust storms has not been investigated in the Taklimakan Desert. The purpose of the present study is to investigate the structure of this summer dust storm in the Taklimakan Desert through both in situ lidar observations and a numerical simulation.
2. Method and data
Intensive observations of dust storms were carried out at Aksu, Xinjiang, China in the summer of 2004. Aksu is located in the northern part of the Taklimakan Desert. Surface meteorological elements were automatically measured every 30 min by an Automatic Weather Station (AWS). Vertical wind profiles were measured every three hours by pilot balloons.
The lidar system was a single wavelength Nd:YAG laser-based system designed to measure the vertical profiles of backscattering and depolarization ratios of aerosol particles from the layer near the ground up to the stratospheric aerosol layer. The pulse energy of the laser was 300 mJ at 532 nm, and the pulse frequency was 10 Hz. To expand the receiving range and signal strength, two telescopes were used: one with a diameter of 200 mm to measure the lower atmosphere, and another with a diameter of 355 mm used to measure the upper atmosphere.
The numerical model used in the present study is the modified Regional Atmospheric Modeling System (RAMS), known as TERC-RAMS, presented by Sato and Kimura (2003). The original RAMS was developed at Colorado State University (Pielke et al. 1992). The model has two nested grid systems designed by a polar stereographic coordinated system. The domain of the coarse grid system covers the eastern Asia region centered at 85ºE, 40ºN with a horizontal grid interval of 60 km and 60 × 50 grid points. The nested grid system covers an area of the Taklimakan Desert with a grid interval of 15 km and 138 × 78 grid points. The 6-hourly NCEP/NCAR reanalysis data were used for the initial and boundary conditions of this calculation.
A dust storm with the gust front suddenly passed over Aksu at 13:20 UTC 7 August 2004, accompanying a strong easterly cold wind. There was no rainfall at the ground during the dust storms.
There were abrupt changes of temperature and humidity between 12:00 UTC and 14:00 UTC. The temperature decreased from 32.1 to 26.3 ºC and the relative humidity increased from 37.3% at 12:00 UTC to 63.5% at 3:00 UTC and suddenly decreased to 32.0% at 14:30 UTC. In connection with this change, a strong northeasterly wind began to blow at 13:30 UTC and its peak value occurred at 14:00 UTC. These results suggest the passage of the front of the dust storm with the same structure as the gravity current (Simpson 1987; Aoki et al. 2005).
The lidar observation show the structure of the dust storm; the height of the head was about 1200m and the nose was 300 m.
4. Discussion As mentioned previously, the temperature dropped by 6 ºC and the relative humidity increased by 25% as the front of the dust storm passed over Aksu. The desert was in a dry condition but the narrow frontal zone had high humidity. Whilst the decrease of in temperature by 6 ºC can explain part of the 25% increase of relative humidity, it does not explain all of the increase.
We carried out a numerical simulation In order to investigate this phenomenon. According to the numerical simulation, the arc cloud was formed by the intrusion of the cold air mass, which pushed the warm air up to 2,500 m. No rainfall was observed at the ground surface during the dust storm. This means that most of the rainfall from the arc cloud may be evaporated during falling. The supply of water vapor from the arc cloud causes a peak in relative humidity at the ground surface.
On 7 August 2004 the dust storm occurred with a strong wind in the Taklimakan Desert. It was an inflow of a cold air mass, which was separated from the synoptic-scale cold westerly wind to the north of Tianshan Mountains. During the passage of the dust storm, the surface temperature abruptly decreased and the relative humidity peaked in the following two hours. The lidar observation shows the structure of the head of the dust storm. The numerical simulation and in situ observations show that the jump in surface humidity is caused by the supply of moisture from an arc cloud over the head of the dust storm. This cold local front with a thickness of 1200 m had the same structure as the gravity current. The structure and process of the summer dust storm in the Taklimakan appeared similar to the spring dust storm in passing local cold front.