The Saharan Air Layer
Studying Impacts of Transatlantic Saharan Dust Storms on Atlantic Oceanic and Atmospheric Phenomenon
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The Saharan Air Layer and Extreme Weather
What Is the Saharan Air Layer?
SAL activity usually ramps up in mid-June, peaks from late June to mid-August, and begins to rapidly subside after mid-August. During peak period, individual SAL outbreaks reach farther to the west (as far west as Florida, Central America and even Texas) and cover vast areas of the Atlantic (sometimes as large as the lower 48 United States).
The Saharan Air Layer Affects our Weather and Climate
The Saharan Air Layer has unique properties of warmth, dry air, and strong winds that can have significant moderating impacts on tropical cyclone formation and intensification. There are three characteristics of these Saharan dust outbreaks that can affect tropical cyclones, tropical disturbances, and the general climatology of the Atlantic tropical atmosphere:
- Extremely Dry Air
- African Easterly Jet
- Warm Temperatures
Extremely Dry Air
First, The Saharan Air Layer’s dry, dusty air has about 50% less moisture than the typical tropical atmosphere. This extremely dry air can weaken a tropical cyclone or tropical disturbance by promoting downdrafts around the storm.
African Easterly Jet
Second, Strong winds in the Saharan Air Layer (25-55 mph or 10-25 meters per second) can substantially increase the vertical wind shear in and around the storm environment. This “mid-level jet” of enhanced winds, typically found at a height of 6,500-14,500 feet (2000-4500 meters), can cause tilting of the tropical cyclone vortex with height and can disrupt the storm’s internal heat engine.
Warm Temperatures
Third, The Saharan Air Layer’s warmth acts to stabilize the atmosphere, which can suppress the formation of clouds. This stabilizing effect is produced when the Saharan Air Layer’s warm, buoyant air rides above relatively cooler, denser air. The Saharan Air Layer’s suspended mineral dust also absorbs sunlight, which helps maintain its warmth as it crosses the Atlantic Ocean.
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Who We Are
Our main goal is to study all aspects of the Saharan Air Layer, including its thermodynamic and kinematic structure, vertical extent, mineral dust impacts, and affect on the climatology of the North Atlantic atmosphere. This allows us to integrate new Saharan Air Layer observations collected from aircraft-deployed GPS dropsondes into models in order to improve forecasts and advance our knowledge of the Saharan Air Layer’s impact on extreme weather and climate in the Atlantic Basin.
A special thank you to our external partners, Chris Velden (UW-CIMSS), Jeff Hawkins (NRL-Monterey, retired), and Joseph Prospero (University of Miami/CIMAS, retired).
What We Do
The team members working on the Saharan Air Layer project conduct experiments using NOAA’s two P-3 Orions aircraft and G-IV high altitude jet as described in HRD’s Hurricane Field Program. Our Saharan Air Layer monitoring is conducted using a combination of NOAA’s Geostationary Operational Environmental Satellites (GOES), the Meteosat Second Generation (MSG) satellites, as well as the NOAA-20, Suomi-NPSS, Special Sensor Microwave Imager/Sounder (SSMIS), Global Change Observation Mission (GCOM), Global Precipitation Measurement (GPM), and Coriolis polar orbiting satellites. Read more about our current operations on the Hurricane Research Division’s Blog.
Original Saharan Air Layer July 18th
Advanced Saharan Air Layer July 19thProject Goals
Key Accomplishments
Developed several new satellite products to track the SAL continuously using NOAA’s next generation GOES-16 satellite. These products include split window infrared (IR), mid-level water vapor, and multi-spectral pseudo natural color satellite imagery and are available in real-time on the UW-CIMSS tropical cyclone website:
GOES-R Proving Ground: Developed New Satellite Products For SAL Monitoring With NOAA’s Next Generation Satellites
2018Conducted SALEX missions during the 2006 Atlantic hurricane season that used the NOAA P-3 Orions and G-IV high altitude jet to investigate interactions between the SAL and Tropical Storm Debby and Hurricane Helene. These NOAA missions were coordinated with the NASA African Monsoon Multidisciplinary Analyses (NAMMA) field campaign based in the Cape Verde Islands. P-3 and G-IV SALEX missions were also conducted in 2015 to investigate interactions between the SAL and Tropical Storm Erika.
- 25 Aug 2006 (G-IV jet): TS Debby
- 26 Aug 2006 (G-IV jet): TS Debby
- 15 Sep 2006 (G-IV jet): TS Helene
- 16 Sep 2006 (G-IV jet): Hurricane Helene
- 18 Sep 2006 (P-3 Orion & G-IV jet): Hurricane Helene
- 20 Sep 2006 (P-3 Orion & G-IV jet): Hurricane Helene
- 25 Aug 2015 (P-3 Orion & G-IV jet): TS Erika
- 26 Aug 2015 (P-3 Orion): TS Erika
SALEX Aircraft Missions
2006, 2015Developed a 1995-2002 database of GOES satellite imagery, aerosol analyses, and Caribbean rawinsonde observations to create a revised set of atmospheric soundings for the tropical North Atlantic. These new soundings, partly based on knowledge of the SAL, represent a new benchmark for describing the climatological state of the tropical atmosphere during the hurricane season:
A Revision of the Climatology of the Tropical Atlantic Atmosphere
2011Conducted first-ever SALEX aircraft missions during the 2005 Atlantic hurricane season that used the NOAA G-IV high altitude jet to investigate interactions between the SAL and Tropical Storm Irene (07-08 Aug) and an African easterly Wave (27-28 Sep):
Pioneered the Saharan Air Layer Experiment (SALEX)
2005Successfully submitted a proposal to the NOAA Joint Hurricane Testbed (JHT) program to examine the impacts of assimilating moisture information from GPS dropsondes launched from the NOAA G-IV jet in operational parallel runs of the NOAA Global Forecast System (GFS) model and assess the impact of of this data on GFS initial/forecast humidity fields and its forecasts of TC track and intensity. Results from this 2005-2007 JHT project led to a decision by the NOAA National Centers for Environmental Prediction (NCEP) to begin routinely assimilating humidity data from GPS dropsondes into the operational version of the GFS model beginning on 22 Aug 2006.
Dropsonde Assimilation
2005Developed automated geostationary satellite imagery (GOES-EAST and Meteosat) to track the SAL continuously. This product used split window infrared (IR) imagery and was first made available in real-time on the UW-CIMSS tropical cyclone website in 2001:
Developed Satellite Imagery for Monitoring the SAL
2001
Featured Publication
Featured Publication
Rewriting the Climatology of the Tropical North Atlantic and Caribbean Sea Atmosphere
The Jordan mean tropical sounding has provided a benchmark reference for representing the climatology of the tropical North Atlantic and Caribbean Sea atmosphere for over 50 years. However, recent observations and studies have suggested that during the months of the North Atlantic hurricane season, this region of the world is affected by multiple air masses with very distinct thermodynamic and kinematic characteristics. This study examined ;6000 rawinsonde observations from the Caribbean Sea region taken during the core months (July–October) of the 1995–2002 hurricane seasons. It was found that single mean soundings created from this new dataset were very similar to C. L. Jordan’s 1958 sounding work. However, recently developed multispectral satellite imagery that can track low- to midlevel dry air masses indicated that the 1995–2002 hurricane season dataset (and likely Jordan’s dataset as well) was dominated by three distinct air masses: moist tropical (MT), Saharan air layer (SAL), and midlatitude dry air intrusions (MLDAIs). Findings suggest that each sounding is associated with unique thermodynamic, kinematic, stability, and mean sea level pressure characteristics and that none of these soundings is particularly well represented by a single mean sounding such as Jordan’s. This work presents three new mean tropical soundings (MT, SAL, and MLDAI) for the tropical North Atlantic Ocean and Caribbean Sea region and includes information on their temporal variability, thermodynamics, winds, wind shear, stability, total precipitable water, and mean sea level pressure attributes. It is concluded that the new MT, SAL, and MLDAI soundings presented here provide a more robust depiction of the tropical North Atlantic and Caribbean Sea atmosphere during the Atlantic hurricane season and should replace the Jordan mean tropical sounding as the new benchmark soundings for this part of the world.
September 16, 2019
Publications & References
Dunion, J.P., 2011: Re-Writing the Climatology of the Tropical North Atlantic and Caribbean Sea Atmosphere. J. Climate. 24, 893-908.Dunion, J.P., and C.S. Marron, 2008: A Reexamination of the Jordan mean tropical sounding based on awareness of the Saharan Air Layer: Results from 2002. J. Climate. 21, 5242-5253.
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 G-IV Jet. AMS 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, American Meteorological Society.
Dunion, J.P., and S.D. Aberson, 2006: Assimilating moisture information from Global Positioning System (GPS) dropwindsondes into the NOAA Global Forecast System, 60th Annual Interdepartmental Hurricane Conference, Mobile, AL, Office of Fed. Coord. For Meteor. Services and Supporting Research, NOAA.
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.
Ismail, S., R.A. Ferrare, E.V. Browell, S.A. Kooi, J.P. Dunion, G. Heymsfield, A. Notari, C.F. Butler, S. Burton, M. Fenn, T.N. Krishnamurti, M. Biswas, G. Chen, and B. Anderson, 2010: LASE measurements of water vapor, aerosol, and cloud distributions in Saharan Air Layers and tropical disturbances. J. Atmos. Sci, 67 no. 4, 1026-1047.
Jones, T., D. Cecil, and J.P. Dunion, 2007: The environmental and inner core conditions governing the intensity of Hurricane Erin (2001). Wea. Forecast. 22, 708-725.
Evan, A.T., J.P. 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. Geophys. Res. Lett., L19813, doi:10.1029/2006GL026408.
Zipser, E.J., C.H. Twohy, S. Tsay, K. L. Thornhill, S. Tanelli, R. Ross, T.N. Krishnamurti, Q. Ji, G. Jenkins, S. Ismail, N. C. Hsu, R. Hood, G. M. Heymsfield, A. Heymsfield, J. Halverson, H. M. Goodman, R. Ferrare, J. P. Dunion, M. Douglas, R. Cifelli 1, G. Chen, E. V. Browell, and B. Anderson, 2009: The Saharan Air Layer and the fate of African easterly waves-NASA’s AMMA 2006 field study of tropical cyclogenesis. Bull. Amer. Meteor. Soc., 90, 1137-1156.
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