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48 hour Trajectory Forecast for high MODIS aerosol optical depth (AOD), ta ≥ 0.4

Sample Trajectory Forecast MODIS AOD image

Sample Trajectory Forecast for high MODIS ta plot

The trajectory forecast animation plots latest available daily MODIS (the Moderate Resolution Imaging Spectroradiometer) aerosol optical depth (in color contours), daily MODIS cloud optical thickness (in gray contours), an animated ~48 hour air parcel trajectory forecast (in magenta-white colors), and the 3hr accumulated precipitation (in yellow).

MODIS derived aerosol optical depth, ta

The MODIS ta used for these plot is based on the MOD04_L2 aerosol product (MODIS ATBD Kaufman and Tanré 1998) and is from the MODIS sensors aboard the Terra and the Aqua satellites. Terra has a descending orbit and a 10:30am equatorial overpass time, while Aqua has an ascending orbit and a 1:30pm equatorial overpass time. The predicted and actual overpass time for a given day can be found at http://www.ssec.wisc.edu/datacenter/terra/, and http://www.ssec.wisc.edu/datacenter/aqua/, respectively. The MODIS ta is at a 10 x 10 km2 spatial resolution and is plotted on a scale of 0.0 to 1.0 with ta > 1.0 shown as dark red.

Forward air parcel trajectory forecast

Sample Trajectory Forecast MODIS AOD > 0.6 image

Sample plot with locations of high ta (> 0.4) used to initialize forecast trajectories

This plot shows the locations where high values of MODIS ta (>0.4) are located and used to initialize starting locations for forecast trajectories. The high MODIS ta values are determined by calculating mean ta values on a 50 km x 50 km grid, or 5 pixels square.

The trajectories are initialized at the high ta values at 50mb, 100mb, 150mb, and 200 mb above the surface level, with a maximum number of 100 trajectories per swath. The initialization points for the trajectories are colored to the AOD scale and the most recent day of MODIS data remains on the plot.

Starting Dec 10,2008, the OMI (Ozone Monitoring Instrument) aerosol index (AI, Torres et al., 2007) is incorporated into the initialization of Aqua AOD trajectory forecast. OMI sensor is on board satellite Aura, which follows Aqua with eight minutes time difference of the overpass. Since high OMI AI represents high aerosol load at high level, the initialization levels are defined at 300mb above surface levels (about 3km above surface) for those locations with AI larger than 2.0, instead of the four levels close to the surface. Following are the possible OMI AI and Aqua AOD combinations: (1) high AI, high AOD, (2) high AI, no AOD, (3) high AI, low AOD, (4) low AI, high AOD, (5) no AI, high AOD. For cases (1),(2), and (3), the trajectories are initialized at 300mb above surface. For cases (4) and (5), the trajectories are initialized at four levels close to the surface, i.e. 50mb, 100mb, 150mb, and 200mb above surface level.

The air parcel trajectories are run using the 12Z NOAA (National Oceanic and Atmospheric Administration) /NCEP(National Center for Environmental Prediction) NAM (North American Mesoscale Model) forecast data providing a ~48hr forecast via trajectories. The pressure levels of the trajectories are plotted in mb and colored to a magenta-white scale. The most recent 12 hours of the 48 hour forward trajectories are plotted at each frame of the animation. The 850mb wind field vectors are plotted to show wind direction and speed.

The forecast trajectory animation details the potential vertical movement of high aerosol loads in the troposphere. The darker colors on trajectory pressure scale are limited to the 1000-800 mb pressure levels to help distinguish trajectories moving within the boundary layer. As the forecast trajectories progress in time, darker color trajectories indicate a flow of air towards the surface, and a potential similar movement of the aerosols, captured by the high ta. White color trajectories indicate a upward movement in the air flow.

MODIS cloud optical thickness (COT) and 3hr accumulated precipitation

The MODIS COT used for these plots is based on the MOD06_L2 cloud products and is from the MODIS sensor aboard the Terra and the Aqua satellite. A ta is not derived for cloud filled pixels, therefore the clouds fields derived from MODIS are plotted to help define synoptic weather features important to the movement of aerosols.

The 3 hour accumulated precipitation used for these plots is from the NAM forecast model (NOAA/NCEP). Areas where 3 hour accumulated precipitation is greater than 2kg/m2 are shaded yellow and can be used to determine “rain out” or potential for wet deposition in areas of high aerosol loading.

Acknowledgements:
MODIS aerosol optical depth and cloud optical thickness data is being obtained from NASA (National Aeronautics and Space Administration) through MODIS Direct Broadcast (DB).

NAM meteorological forecast data is being obtain from the National Weather Service component of NOAA

References
Torres, O., et al. (2007), Aerosols and surface UV products from Ozone Monitoring Instrument observations: An overview, J. Geophys. Res., 112, D24S47, doi:10.1029/2007JD008809.

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