Figure 2 shows the configuration of the two aircraft stacked one above the other with the lower aircraft at an altitude of 10-100 m and the upper aircraft at an altitude of approximately 500 m. The lower aircraft will be equipped with a laser altimeter, or LIDAR, to measure the distance from the aircraft to the ocean surface. This distance will include the displacement of the ocean surface due to surface waves and the vertical displacement of the aircraft from a horizontal track (c.f. Figure 1e). The vertical displacement of the aircraft will be measured with a closely-coupled DGPS and inertial navigation system. Thus the sea surface displacement (the surface waves) along the aircraft track can be measured. Flying the aircraft along tracks in different directions will give the wave directional spectrum or its moments. As LIDAR miniaturization develops we anticipate that future systems may be able to accommodate a scanning LIDAR, which will give directional wave information from one flight track.
The laser altimeter data will be conditionally averaged and used as input to the flight control system to fly the desired altitude pattern. Such patterns may include a constant altitude above the average sea surface, wave-following altitudes over different ranges of wave frequency, and variable altitudes to permit sampling of vertical profiles of different variables, including U(z) and the Reynolds stress.
The lower aircraft will be modified with a hemispherical nose section containing a five-element gust probe array for measuring the three components of the wind velocity relative to the aircraft. When combined with the aircraft velocity measured by the DGPS/INS system, this gives the wind velocity in an Earth frame. Along with the wind velocities in the lower UAV we will measure the atmospheric temperature, relative humidity and sea surface temperature (SST). The wind stress can be measured from this UAV by measuring the Reynolds stress at a constant altitude, by profiling the mean velocity over a range of altitudes, some combination of the two, or by the use of inertial dissipation techniques.
Figure 2: Schematic of the UAV system (lower figure) with two UAVs flying one above the other. The lower UAV will fly at altitudes of 10-100 m to measure the MABL, while the upper UAV images the surface waves and other surface phenomena. The upper part of the figure shows a manned aircraft operating at safe altitudes with a range of sensors and expendables.
The upper UAV will fly in formation directly above the lower UAV. This coupled control and formation flying for a small array of aircraft has already been implemented by colleagues at Scripps in an atmospheric radiation experiment conducted in the Maldives in 2006 (Ramanathan et al., 2007; http://www-abc-asia.ucsd.edu/MAC/secure/Index.htm). The upper UAV will be equipped with a high resolution video camera for imaging the ocean surface in the neighborhood of the lower UAV so that the lower aircraft will always be in the field of view. It will also be equipped with upward and downward looking radiometers for upwelling and downwelling radiation and for measuring the sea surface temperature (SST). The fact that the lower UAV is flying in the field of view of the upper UAV and the fact that the upper UAV will be able to image whitecaps and other ocean surface features including Langmuir circulations and fronts, will permit the coherent measurement of the influence of these phenomena on the MABL. For example, it is anticipated that flow separation from large breaking waves will lead to bursts of turbulence and Reynolds stress above the breakers.
Together a pair of UAVs can provide detailed measurements of the MABL from the lower UAV, in the context of a larger field of view from the upper aircraft.