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Saharan Air Layer

Principle Investigator : Jason Dunion (HRD)

SeaWiFS/NASA/GSFC

Team Members:

Sim Aberson (HRD)
Mike Black (HRD)
Neal Dorst (HRD)
Jeff Hawkins (NRL)
John Kaplan (HRD)
Chris Landsea (HRD)
Paul Leighton (HRD)
Frank Marks (HRD)
Mark Powell (HRD)
Rob Rogers (HRD)
Joseph Prospero (UM/CIMAS)
Chris Velden (UW/CIMSS)

Objectives:
  • Improve our understanding of the mechanisms by which the Saharan Air Layer's (SAL's) dry air, mid-level easterly jet, and embedded mineral dust affect Atlantic tropical cyclone genesis and intensity change;
  • Include the moisture information from the GPS dropwindsondes in operational parallel runs of the NOAA Global Forecast System (GFS) model. Assess the impact of this data on the GFS (and GFDL) initial/forecast humidity fields and its forecasts of TC track and intensity;
  • Improve the representation of the SAL's dry air and mid-level easterly jet (enhanced vertical wind shear) in forecast models;
  • Implement a new total precipitable water (TPW) predictor in the SHIPS model to help account for the impact of dry layers (e.g. the SAL and mid-latitude dry air intrusions) on tropical cyclone intensity.
  • Investigate how the SAL interacts with the tropical cyclone inner core;
  • Improve our understanding of the SAL's vertical structure;
  • Investigate the mechanisms that lead to strong SAL outbreaks over Africa;
  • Investigate the mechanisms by which the SAL maintains its thermodynamic structure;
  • Investigate the effect of the SAL's suspended mineral dust on cloud microphysics;

Methods:

  • Conduct the SALEX experiment using NOAA's P-3 Orion and G-IV aircraft as described in the HRD hurricane field program (HFP);
  • SAL monitoring: GOES, MSG, SSM/I, AMSR-E, Aqua and Terra satellites
  • SAL sampling: GPS dropsondes;


Dust layer associated with the Saharan Air Layer. Photo taken from the NOAA G-IV northeast of Barbados during SALEX mission 20060916n. Cumulus clouds can be seen poking through the tops of the dust layer, which is seen as a milky white haze. Photo credit: Jason Dunion NOAA/HRD.

Accomplishments:

  • Developed automated geostationary satellite imagery (GOES-10 and Meteosat-9) to track the SAL continuously. These new products include split window IR, mid-level water vapor, and multi-spectral true color satellite imagery and are available in real-time at the UW-CIMSS tropical cyclone website:
    http://cimss.ssec.wisc.edu/tropic/real-time/wavetrak/sal.html
  • Conducted first-ever SALEX missions during the 2005 Atlantic hurricane season that used the NOAA G-IV jet to investigate the environments of TS Irene and pre Tropical Depression 19 and their interactions with the SAL:
    1. Aug. 07, 2005 (G-IV jet)
    2. Aug. 08, 2005 (G-IV jet)
    3. Sept. 27, 2005 (G-IV jet)
    4. Sept. 27, 2005 (G-IV jet)

  • Successfully submitted a proposal to the NOAA/Joint Hurricane Testbed program to assimilate moisture information from GPS dropsondes launched from the NOAA G-IV jet in operational parallel runs of the NOAA GFS model and assess the impact of this data on the GFS initial/forecast humidity fields and its forecasts of TC track and intensity. This JHT project began in 2005 and ended in early 2007 and results from this work led to a decision by NOAA/NCEP to begin routinely assimilating humidity data from GPS dropsondes into the operational version of the GFS model (beginning 22 August 2006).
  • Conducted SALEX missions during the 2006 Atlantic hurricane season that used the NOAA P-3 Orion and G-IV jet to investigate the environments of Tropical Storm Debby and Hurricane Helene and their interactions with the SAL:
    1. Aug. 25, 2006 (G-IV jet)
    2. Aug. 26, 2006 (G-IV jet)
    3. Sep. 15, 2006 (G-IV jet)
    4. Sep. 16, 2006 (G-IV jet)
    5. Sep. 18, 2006 (P-3 Orion; G-IV jet)
    6. Sep. 20, 2006 (P-3 Orion; G-IV jet)


    G-IV flight crew in Barbados after the 26 August 2006 Tropical Storm Debby SALEX mission. From left to right are: Chuck Rasco, Mission Flight Director, Jeff Hawkins, Jeff Smith, Sim Aberson, Michele Finn, Dale Carpenter, John Longenecker, Jason Dunion, and Gordon Kitson. Photo credit: Jack Parrish NOAA/AOC.


References
:

Dunion, J.P., and C.S. Marron, 2007: A Reexamination of the Jordan mean tropical sounding based on awareness of the Saharan Air Layer: Results from 2002. J. Climate. (Accepted).

Jones, T., D. Cecil, and J.P. Dunion, 2007: The environmental and inner core conditions governing the intensity of Hurricane Erin (2001). Wea. Forecast. 24 no. 4, 708-725.

Evan, A.T., J. Dunion, J.A. Foley, A.K. Heidinger, and C.S. Velden, 2006: New evidence for a relationship between Atlantic tropical cyclone activity and African dust outbreaks. J. Geophys. Res. 33 L19813, 1-5.

Dunion, J.P., and S.D. Aberson, 2007: Assimilating moisture information from Global Positioning System (GPS) dropwindsondes into the NOAA Global Forecast System, 61st Annual Interdepartmental Hurricane Conference, New Orleans, LA, Office of Fed. Coord. For Meteor. Services and Supporting Research, NOAA.

Dunion, J.P., J.D. Hawkins, and C.S. Velden, 2006: Hunting for Saharan Air with the NOAA GI-IV Jet. AMS 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, American Meteorological Society.

Dunion, J.P., C.S. Velden, J.D. Hawkins, and J.R. Parrish, 2004: The Saharan Air Layer- Insights from the 2002 and 2003 Atlantic hurricane seasons. AMS 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, American Meteorological Society, 495-496.

Dunion, J.P., and C.S. Velden, 2004: The impact of the Saharan Air Layer on Atlantic tropical cyclone activity. Bull. Amer. Meteor. Soc., 85 no. 3, 353-365.

Dunion, J.P., and C.S. Velden, 2003: The Impact of the Saharan Air Layer on Atlantic tropical cyclones. 57th Annual Interdepartmental Hurricane Conference. Miami, FL, Office of Fed. Coord. For Meteor. Services and Supporting Research, NOAA.


Photo of the Saharan Air Layer taken from the NOAA P-3 Orion after SALEX mission 20060918h during the ferry back to Barbados. Saharan dust gives the sky an orange glow during this late afternoon sunset in the eastern Caribbean. Small cumulus clouds can be seen poking through the tops of the dust layer. Photo credit: Jason Dunion NOAA/HRD.

Future Work:

  • Conduct additional SALEX missions during future NOAA HRD hurricane field programs that utilize the NOAA P-3 Orion and G-IV aircraft. These research missions will be designed to address the main objectives described above.
  • Continue to assess the impact of humidity data from the GPS dropsondes launched from the NOAA G-IV jet on the NOAA GFS ad GFDL models. Specific goals will include continued assessment of the impact of the GPS dropsondes on GFS and GFDL initial/forecast humidity fields and their forecasts of TC track and intensity;
  • Develop a new mean Jordan tropical sounding (dry SAL vs moist non-SAL) that is based on 8 years of Caribbean rawinsonde data;
  • Implement an improved humidity parameter (TPW) for routine use in the SHIPS hurricane intensity forecast model;


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Last modified: 11/04/2007

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