This web site contains material related to the NASA-funded research
program NASA-AMMA, and our part of it that involved both aircraft
measurements with the NASA DC-8 during August-September 2006 and some
special soundings that our project helped to organize in west Africa
during the same time frame. The overall objective of the research is to
explain why some African easterly waves emerging from West Africa into
the eastern Atlantic Ocean tend to develop more rapidly into so-called
Cape Verde storms than do others. This site contains materials that we
are preparing that are related to this research activity, and also
material that is related to weather and climate in Africa - material
that may be useful to ongoing planning activities for THORPEX-Africa and
Specific Objectives of the Activity
The planned research activity will address a major uncertainty in the development of tropical storms over the Atlantic Ocean - how they evolve from preexisting tropical waves that develop over Africa during the summer season. If the data gathering phase of the proposed activity is successful, our research should lead to a better understanding of why some tropical waves become hurricanes over the eastern and central Atlantic and why others do not. This in turn will add predictive skill to current medium-range weather forecasts over the tropical Atlantic, and longer lead times for preparedness in the downstream regions, including the US and Caribbean Sea region.
It has been more than 3 decades since the GARP Atlantic Tropical Experiment (GATE), the largest tropical field program, which was focused on convection over tropical oceans and the meteorology of the eastern tropical Atlantic. Despite this, much of our knowledge of African easterly waves (hereafter termed AEW's) is based on observations from this one field program. Most of the GATE-based studies have involved a compositing approach, where observations about different waves were averaged together to obtain the characteristics of a "mean wave". The first such summaries of GATE waves (e.g. Reed et al (1977), Norquist et al., 1977) have been widely used as a standard for future studies.
Compositing studies, despite their very valuable results, have, by their very design, not been able to provide many details about the intensification process that occurs during the transition of AEW's into tropical storms. Case studies (e.g. Carlson 1969, Pedgley and Krishnamurti 1976, Berry and Thorncroft 2005, Sall and Sauvageot, 2005), although being able to provide clues to such processes in principle, but have usually been handicapped by poor in-situ observations over the region. As a result, most observational studies of tropical cyclogenesis have involved storms developing in the Caribbean or Gulf of Mexico regions, where they can be more intensively examined with research aircraft. Unfortunately, most such cyclogenesis events are not typical of the tropical cyclogenesis that is responsible for most of the intense hurricanes in the Atlantic basin. The strongest hurricanes in the Atlantic (categories 3-5) are usually associated with AEW's (Landsea, 1993) that show clear signs of development in the eastern Atlantic; these are know as "Cape Verde" storms, after the islands just west of the African coast, close to where these storms first become apparent in satellite imagery.
The objective of our component of the NAMMA activity is to describe the cyclogenesis process from DC-8 flight level, dropwindsonde, and other in-situ measurements. The relative strength of the AEW's, including those sampled by the DC-8 and those not sampled, will be determined from wind soundings made by a special pilot balloon network in west Africa that we will establish prior to the aircraft program. Our web site will eventually describe aspects of both the pilot balloon network and the DC-8 aircraft program.
Feel free to navigate our website for more information.