The Asian Dust Events of April 1998

1
The Asian Dust Events of April 1998

• R. B. Husar, D. M. Tratt, B. A. Schichtel, S. R.
  Falke, F. Li D. Jaffe, S. Gassó, T. Gill, N. S.
  Laulainen, F. Lu, M.C. Reheis, Y. Chun, D.
  Westphal, B. N. Holben, C. Gueymard, I. McKendry,
  N. Kuring, G. C. Feldman, C. McClain, R. J.
  Frouin, J. Merrill, D. DuBois, F. Vignola, T.
  Murayama, S. Nickovic, W. E. Wilson, K. Sassen,
  N. Sugimoto, W.C. Malm

The full paper to appear the JGR Special Issue on
Dust. Asian Dust Website http//capita.wustl.edu/
Asia-FarEast Comments rhusar_at_me.wustl.edu
2
Analysis of the 1998 Dust Storms by a Virtual
Community

• On April 15 and 19 1998, dust storms in the Gobi
  Desert have produced unusually large dust clouds,
  some of which was transported across the Pacific.
  
• When it was evident that the dust cloud was
  reaching North America, an interactive website
  was set up to share observations, and ideas. By
  April 29 the ad-hoc virtual workgroup consisted
  of over 40 scientists and air quality managers
  from North America and Asia.
• This work was produced by the virtual community
  and summarizes the formation, transport,
  dissipation and other features of the the two
  dust events.
• The full paper is being published in the Journal
  of Geophysical Research, Special Issue on Dust.,
  edited by Irina Sokolik.

3
Dust Storms in the Gobi Desert on April 15 and
19, 1998

• Daily measurements of surface visibility, aerosol
  optical depth, TOMS data and SeaWiFS images for
  the Gobi desert, show that major dust storms
  occurred on April 15th and April 19th.
• The April 19th storm had larger impact on the
  East Asia region.
• Model simulations of dust production and the dust
  pattern correspond to the observations.

4
The April 15th Dust Storm Dissipation within
Asia

• Fast surface winds ( gt 20 m/s) over the Gobi
  desert generated individual dust plumes as seen
  from the SeaWiFS reflectance data.
• After about 500 km transport, the plumes merged
  into a dust cloud

After 1000 km transport from Gobi to Shanghai,
the yellow dust cloud has retained considerable
spatial texture. The TOMS absorbing aerosol index
data (green lines - index2) and the SeaWiFS
image show similar pattern over Eastern
China. The April 15th dust was ingested and
removed by a precipitating low pressure system.
Yellow muddy rain was reported from Beijing on
April 16-17.
5
The Cause of Dust StormsLow Pressure Systems
over Gobi Desert

• On both days, April 15 and 19, the high surface
  wind speeds (gt20 m/s) were caused by extreme
  pressure gradients between the low and the
  adjacent high pressure systems.

6
The April 19th Dust Storm

• The surface wind was gt 15 m/s and surface
  visibility reduction was due to dust throughout
  Mongolia..
• The GMS-5 animation and the SeaWiFS image show a
  sharp dust front progressing from the the Gobi
  desert.
• Over the Yellow See and Korea, the TOMS data
  shows another dust cloud while the SeaWiFS does
  not.

Size distribution data and inversions of optical
data show that the dust volume is in the 1-10 mm
size range with a volume peak at 2-3 mm
The dust layer increases by 20-30 the spectral
reflectance of soil, particularly at lgt0.6 mm.
7
April 20-21 Transport Across East Asia

• On April 20 the dust cloud was stretched along
  the seaboard of East Asia
• Dust layers over low level white clouds (inset),
  turned the clouds yellow by reducing the blue
  (412 nm) reflectance up to a factor of two.

By April 21 the dust cloud extended 1000 km into
the Pacific. Over the dark ocean, the excess dust
reflectance (inset) was also yellow.
8
Trans-Pacific Dust Transport
Approximate location of the April 19 dust cloud
over the Pacific Ocean based on daily SeaWiFS,
GMS5/GOES9/GOES10 and TOMS satellite data. Over
the Pacific Ocean, the dust cloud followed the
path of the springtime East-Asian aerosol plume
shown by the optical thickness derived from AVHRR
data.
Model simulations indicate a wavy transport
pattern at multiple altitudes. NRL NAAPS Model
Animation ICOD DREAM Model CAPITA Monte Carlo
Model Animation
Throughout the Trans-Pacific transit, the dust
appeared as a yellow dye marking its own
position. Much of the dust was either in
cloud-free regions or over the clouds.
9
Visual Appearance of the Dust

• The most noticeable impact of the dust was the
  discoloration of the sky.
• From April 25 onward, the normally blue sky
  appeared milky white throughout the non-urban
  West Coast
• This effect is due to the redistribution of the
  direct solar radiation into diffuse skylight.

• Solar radiation data for Eugene, OR on a clear
  and dusty day shows a loss of direct radiation
  and doubling of the midday diffuse radiation due
  to dust particle scattering and absorption.

10
Dust over the West Coast of North America

• a. GOES 10 geostationary satellite image of the
  dust taken on the evening of April 27.
• The dust cloud, marked by the brighter
  reflectance covers the entire northwestern US and
  adjacent portions of Canada.
• A dust stream is also seen crossing the Rocky
  Mountains toward the east.

b. Contour map of the PM10 concentration on April
29, 1998. Note the coincidence of high PM10 and
satellite reflectance over Washington
c. Regional average daily PM10 concentration over
the West Coast. The sharp peak on April 27-30 is
due to the Asian dust.
11
Lidar Dust Profiles of Asian Dust over North
America

• Lidar profile at Salt Lake City, UT on April 24
  indicates a strongly scattering aerosol layer at
  7-9.4 km with depolarization delta-values up to
  18, indicating non-spherical dust particles.

• Lidar backscatter profiles at, Pasadena, CA at
  the peak of the event (April 27) show a dust
  layer between 6 and 10 km.

12
Dust Map over the West Coast
The PM2.5 dust concentration data from the
IMPROVE speciated aerosol network show virtually
no dust on April 25th, high values over the West
Coast on April 29th and dust further inland on
May 2. Evidently, on April 25th the dust layer
seen by the sun photometers was still elevated
since the surface dust concentration was low.
13
Hourly PM 10 Concentration in California

• In California, there was a synchronous rise and
  fall of the hourly PM10 concentration at all
  sites in the in the Sacramento area.
• During the dust event (April 26-May 1) the
  excess dust concentration was values of 30-40
  mg/m3.
• The diurnal cycle is attributed to dust removal
  in the nocturnal BL at night.

14
The April 98 Asian dust - A unique Event over N.
America.

• The average PM2.5 dust concentration at three
  IMPROVE monitoring sites over the 1988-98 period
  was well below 1 mg/m3
• On April 29, 1998 the sites show simultaneous
  sharp rise to 3-11 mg/m3.
• Evidently, the April 1998 Asian dust event caused
  2-3 times higher dust concentrations then any
  other event during 1988-1998.

15
Abstract - Technical Summary

• On April 15 and 19 1998, two intense dust storms
  were generated over the Gobi Desert by springtime
  cold weather systems. The April 15 dust cloud was
  recirculating and it was removed by a
  precipitating weather system over East Asia.
• The dust cloud increased the albedo over the
  cloudless ocean and land by up to 10-20 but it
  reduced the cloud reflectance near UV, causing a
  yellow coloration of all surfaces. The dust was
  detected and its evolution followed by its
  yellow color on SeaWiFS satellite images, routine
  surface-based monitoring and through
  serendipitous observations.
• The April 19 dust cloud was transported across
  the Pacific in 5 days in elevated layers (gt3 km).
  Part of the dust continued eastward across North
  America, a branch turned south along the West
  Coast at 5-10 km altitude and another significant
  fraction subsided to the surface between British
  Columbia and California.
• Over the West Coast, the dust layer has increased
  the spectrally uniform optical depth to about
  0.4, reduced the direct solar radiation by 30-40
  and doubled the diffuse radiation. This effect
  was also noticed by the whitish discoloration of
  the blue sky. On April 29, the average excess
  Asian dust aerosol concentration over the valleys
  of the West Coast was about 20-50 mg/m3 with
  local peaks gt100 µg/m3..
• The chemical fingerprint of the Asian dust
  (particle diameter 2-3 mm) was evident throughout
  the West Coast and extended to Minnesota.
  According to the chemical aerosol records, the
  impact of the April 1998 Asian dust event was 2-3
  times higher then any other event since 1988.

16
Conclusions and Discussion

• Currently available space-borne and surface
  aerosol monitoring allows the detection and
  following the evolution of global-scale aerosol
  events.
• The online data and explanations on the Asian
  dust have provided just-in-time science support
  to managers responsible for protecting public
  health.
• The Asian Dust web-based virtual community has
  shown that ad-hoc collaboration is a practical
  way to share observations and to collectively
  generate the explanatory knowledge on these major
  unpredictable atmospheric events.
• Further activities may include (1) organizing the
  available data into a documented and shared
  resource base (2) coordinated global dynamic
  aerosol model validation and testing (3)
  evaluation of satellite aerosol retrievals using
  the event data.
• It would be useful to set up a web-based
  communication, cooperation and coordination
  system to monitor the global aerosol pattern for
  extreme aerosol events. The system would alert
  interested communities, so that that the
  detection and analysis of such unpredictable
  events is not left to serendipity.
• It is envisioned that such a community-supported
  global aerosol information network a) be open to
  a broad international participation b)
  complement and synergize with other monitoring
  programs and field campaigns and c) support the
  scientific as well as the air quality and
  disaster management communities.