7-Sep-08 12:00 PM EST

Howard University Researchers Study the Connection Between African Weather Events and US Hurricanes

Each year from June 1^st through November 30^th, Atlantic hurricanes pose an immediate threat to residents of the Caribbean, Central America and the United States. The majority of Atlantic forming hurricanes evolve from westward propagating African Easterly Waves (AEWs), elongated areas of relatively low atmospheric pressure that are convectively transported as an extended wave train.

AEWs have a wavelength of approximately 3000 km and a frequency of 3-5 days. In a given summer season, nearly 100 AEWs will emerge from West Africa, but only 10% will be associated with hurricanes in the US.

While AEWs are associated with some of nature’s most devastating weather to the Western Hemisphere (Hurricanes Georges, Mitch, Katrina), these disturbances bring life-giving rains to West Africa and its people. A wet season is often associated with higher than normal number of Atlantic tropical disturbances.

The processes linking AEWs in West Africa to Atlantic Hurricanes are poorly understood, in part because of a poor observing system in West Africa. There are only 3 stations – located in Dakar, Senegal, Bamako, Mali, Niamey, Niger -- where daily measurements are made of the entire troposphere, and there are no comprehensive field campaigns, i.e., coordinated measurements of atmospheric and meteorological variables at a range of altitudes over many square miles over some period of time.

One of the largest and most extensive international field campaigns for examining AEWs was the GARP Atlantic Tropical Experiment (GATE) field campaign with its command station in Dakar Senegal in 1974. But in 2006, for only the second time in 32 years, a large-scale field campaign, the African Monsoon Multidisciplinary Analysis (AMMA) took place in West Africa and the extreme eastern Atlantic.

Sponsored by NASA, faculty and students from Howard University and other US universities helped coordinate the field campaign and participated in the aircraft and ground measurements during the summer of 2006 in Senegal and Cape Verde. Faculty, staff and students at the Laboratory for Atmospheric and Oceanic Physics- Siemon Fongang (LPAO-SF) at the University of Cheikh Anta Diop also played a critical role in measurements in Senegal.

Some of the low-pressure zones measured during this field campaign eventually developed into tropical cyclones (Debby and Helene). So this new data set is providing new insights on tropical cyclone genesis in the extreme Eastern Atlantic as well as the linkages to Saharan dust and rain processes over the continent.

After a synthesis and analysis workshop in June 2007, students from the US and Senegal presented their results at the January 2008 meeting of the American Meteorological Society in New Orleans.

Rainfall measurements will continue in Senegal, and a solar power array is being commissioned to continue long-term measurements of infrared and solar radiation, aerosols and tropospheric ozone.

Future endeavors include: increasing measurement capacity in other parts of Senegal and in Guinea.

"These improvements are critically important for capacity building and the collaborative work at Howard University and the University of Cheikh Anta Diop," says Dr. Gregory Jenkins, leader of the US-based work and chair of the physics department at Howard.

"We believe that these measurements will help in understanding the processes while providing new information for numerical weather prediction models thereby increasing the predictability of West African AEWs to develop into powerful Atlantic Hurricanes.

At the same time, West African, African-American and Hispanic American students are being prepared to serve and educate their respective communities now and for potential 21st century climate change."

Many uncertainties exist with climate change projections for West African, but inferences may be possible through the AEW/hurricane connection.

Additional Photos

Drs. Gregory Jenkins and Amadou Gaye (Cheikh Anta Diop University) with US Ambassador Janice Jacob (top) and Sengalese Research Ministers Kene Gassama Dia During the 2006 field campaign (image)

Howard and Cheikh Anta Diop students install a ground monitoring station in Senegal (image)

Additional information

[1] Burpee, R., 1972: The Origin and Structure of Easterly Waves in the Lower Troposphere of North Africa. J. Atmos. Sci., 29, 77–90.

[2] GATE, 1974: International and Scientific Management Group of GATE, Bulletin of the American Meteorological Society, 55, 711–744.

[3] Redelsperger, J-L. et al. (2006), African Monsoon Multidisciplinary Analysis: An International Research Project and Field Campaign, BAMS, 87, 1739-1746.

[4] Jenkins, G.S. A, Pratt, A. Heymsfield, 2008: Possible linkages between Saharan dust and Tropical Cyclone Rain Band Invigoration in Eastern Atlantic during NAMMA-06, Geophys. Res. Lett., 35, L08815, doi:10.1029/2008GL034072

[5] Jenkins, G. S. and A. Pratt, 2008: Saharan Dust, Lightning and Tropical Cyclones in the Eastern Tropical Atlantic during NAMMA-06, Geophys. Res. Lett., 35, L12804, doi:10.1029/2008GL033979

[6] Grant, D., et al., 2008: Ozone Transport by Mesoscale Convective Systems in Western Senegal, Atmospheric Environment, in press.

[7] Kamga, A. F., G. S. Jenkins, A. T. Gaye, A. Garba, A. Sarr, A. Adedoyin, 2005: Evaluating the NCAR CSM over West Africa: Present-day and the 21st Century A1 Scenario, JGR, 110, doi:10.1029/2004JD004689.