Mechanism for Asian dust transport during blocking episode days in East Asia and North America in Spring 2001

Abstract

The mechanism for transport of Asian dust during blocking episode days in spring 2001 has been analyzed using the TOMS aerosols index and meteorological parameters from the Meteorological Mesoscale Model 5 (MM5) with NCEP/CDAS input data. The blocking episode days are associated with the development of the upper cut-off low and surface cyclones through the E vector, an extension of the Eliassen-Palm flux. Concurrently, the occurrence of dust storms is determined by strong cold advection at the rear of the jet streak that shows a maximum wind speed within the upper jet stream. The mechanism for transport of Asian dust from China is advection of isentropic potential vorticity (IPV) on isentropic surfaces associated with tropopause folding. The transport of Asian dust during blocking episode days occurs at altitudes associated with the distribution of isentropes below the IPV boundary (for which IPV ≈ 1PVU). At the same time, lee waves propagated by topography affect the downward motion and blocking of Asian dust in China. Asian dust transported from dust source regions is deposited by fallout and rain-out with a reinforcing frontogenesis within surface cyclones as seen in TOMS and GMS5 satellite images. Long-range transport of Asian dust towards North America is related to the high-latitude distribution of the jet streak determined by the upper level trough/the deepening cut-off low. In April 2001, the jet streak passed through source regions of dust storms in north and west China during a blocking episode, causing Asian dust to move horizontally eastward with the long-wave trough at 500 hPa to North America. The development and motion of the deepening cut-off low connected with a surface cyclone therefore played a role in the transport and deposition of Asian dust not only in east Asia but also in the north Pacific area. Finally, Asian dust was deposited by the blocking effect of a developing surface cyclone in North America, in a similar manner to the deposition in east Asia.