Trans-Pacific Yellow Sand and Trace Gas Transport in April 1998

The impact of Asian anthropogenic pollutants to the west coast of United States is concerned as a new type of environmental problem (Jaffe et al., 1999). Transport of natural origin Asian yellow sand is a good satellite-visible tracer and plays an important role to understand the meteorological mechanism of this very long-range transport between Asian continent and North America. This paper mainly focuses on the numerical results of the yellow sand transport episode to North America occurred in April 1998. To simulate such a long-range transport over the Pacific Ocean, an on-line tracer model coupled with CSU-RAMS (Regional Atmospheric Modeling System, Pielke et al., 1992) was developed. Tracer model is strongly coupled with comprehensive meteorological model, and this is because the regional meteorological conditions such as precipitation, cloud microphysics, diurnal cycle of boundary-layer turbulence play significant roles for the transport and deposition of tracers. The model study was conducted from April 13 to April 30, 1998 during when the significant yellow sand transport was reported at west coast of United States. ECMWF/TOGA global meteorology data are used for the RAMS continuous nudging run. Mellor-Yamada type turbulent closure and build-in vegetation and microphysics options are used for RAMS simulation.

Dust emission area is the area where Matthews vegetation type (Matthews, 1983) is assigned to desert. Most of Gobi and Takla Makan deserts are indexed as a source region. Dust roll-up scheme proposed by Gillette (1978) is used, i.e., Dust mass loading Fdust is defined as Fdust=C (U-Utr) U2 , where C is a constant, U is surface wind speed and Utr is critical wind speed of dust roll-up (Utr=6.5m/s used). Dust lift-up height (mixed layer) is determined from the vertical profile of potential temperature. RAMS build-in horizontal and vertical advection and turbulent diffusion scheme (user option parameters) are used for dust.

A very severe dust storm was observed in April 14-15, 1998 at Xinjian Province, China, and Satellite image clearly retrieved the onset of this dust front and dust cloud. Daily dust transport simulated by on-line tracer model clearly revealed that eastward travelling strong low-pressure system and cold-frontal line is responsible for this strong dust episode. Model results also clearly indicate that the high/low pressure system chains over the Pacific Ocean efficiently transport the dust cloud from west Pacific region to North America. Results show that dust emitted on April 19 arrived to west coast of United States on days around April 26. Simulated main transport path agreed with NASA/SeaWiFS image data. Numerical results are compared with the intensive lidar observation, ground base SPM (suspended particle matter) observation and high time resolution particle counter data. Time-height cross section analysis shows (i) the dense dust layer exists below 2-3 km (within PBL) and (ii) dust concentration correlated with potential temperature variation.

Model results based on the anthropogenic global SO2 emission data from GEIA are simultaneously tracked by on-line tracer model. Numerical results of SO2 and sulfate are also reported and compared with the ground base observation.