A research aircraft provides a mobile platform to measure moving features and to concentrate observations in the desired location. Although the DC-8 has many measurement capabilities this proposal will primarily depend on measurements from the GPS dropwindsonde system and the flight level standard meteorological observations.

General measuring strategy

The NASA DC-8 aircraft will be flown into and around African waves on successive days to describe the amplification process in unprecedented detail. A minimum of three flights would be needed for each wave, though up to 5 flights could be carried out to measure a developing system from the Cape Verde Islands. The flights would be near maximum aircraft duration, since they would be designed to measure both the synoptic-scale environment of the depression and mesoscale details near the circulation center.

The first portion of each flight would be that of deploying DWS's, since the exact circulation center of an over-land African wave would probably be difficult to locate from satellite imagery and the sparse sounding data. DWS observations would be made at approximately 200km separation along the aircraft track. These observations would be analyzed in flight to determine the position of the lower-tropospheric circulation center location, or the region of maximum vorticity if no circulation center was evident. Using this information, a flight pattern at a specific lower-mid tropospheric level (between 3-4 km above sea level) would be designed and carried out. Since there is considerable vertical shear over the region, this flight level should be the same for each flight, so that comparisons can easily be made from day to day and wave to wave.

Measuring intensification

There are a number of ways to describe intensification of a tropical wave into a depression or tropical storm. The change in the absolute vorticity, or the potential vorticity, is probably the best indicator of the intensification process. However, the measurement of vorticity is often scale dependent, with mesoscale measurements (such as those made by a research aircraft) often revealing much higher values than those resolved by synoptic scale observations. Thus it is a challenge from the observing perspective to measure vorticity accurately. Perhaps the most common measurement of intensification is that of minimum surface pressure. However, surface pressure is affected not only by local, but planetary scale pressure changes, and by the semidiurnal and diurnal tides. In the weak wave to storm transition phase that will be studied here, surface pressure may not be an accurate indicator of intensity. The temperature anomaly of the eye with respect to the surrounding tropical environment is another quantity (essentially similar to surface pressure anomaly) that can be used and is easy to measure with an aircraft, but this generally is applicable only to systems with a developed eye. In a baroclinic African wave, with warmest air far to the north, such a local maxima in temperature may not exist in the early stages of amplification.

Given the availability of a research aircraft, we believe that direct measurement of vorticity is the best indicator of intensification. The other measures, related to the temperature and pressure fields, will also be obtained through the aircraft and DWS measurements.

Determining the relative intensities of AEW's: routine observations in support of the DC-8 activity

The DC-8 aircraft observation program is assumed to last approximately one month, during which 5 to 10 well-defined wave passages across west Africa might be expected according to climatology. The DC-8 will be able to describe aspects of perhaps 2-3 of these waves, given anticipated flight resources (though actual available aircraft flight hours are unknown to the PI). In order to compare the intensity of the aircraft-sampled waves with other AEW's that occur during the field program it will be necessary to have an independent estimate of the wave strength as the AEW's pass a certain longitude. The upper air conventional observations from radiosondes and the proposed pilot balloon sites are intended to provide the additional information to assess the relative intensities of the AEW's. This is major advantage of coupling both programs.

Example of the NASA DC-8 possible flight tracks designed to follow the propagation of a relative vorticity maxima.