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A winning proposal for the Innovative Research Program, 2009: Development of the First Autonomous Mini-Glider for Sampling Small-Scale Atmospheric Structure From the Surface to 10 KmInvestigators: Ben Balsley, Don David, and Ken Smith ABSTRACT: We propose a very simple, effective, and inexpensive method for obtaining very high-vertical-resolution atmospheric measurements from the ground up to 10 km. This is an ideal project for an IRP since (1) if successful, the device can be used by the entire atmospheric science community, and (2) the proposed platform (a small GPS-controlled glider) exists and CIRES has already developed much of the additional electronics for other projects. Thus we need only combine existing sub-modules and rigorously flight-test the device. THE PROBLEM: It is becoming increasingly clear that improved understanding of atmospheric dynamics requires a clear knowledge of local, small-scale, non-linear processes such as the breakdown of atmospheric gravity waves (AGWs) and other turbulence-generating mechanisms, as well as recently-discovered atmospheric ‘overturning’ events. Typical vertical scales of these studies range from a few tens of meters on the largest scales down to 1-2 meters, and probably even smaller. Current methods of profiling atmospheric quantities (e. g., winds, temperatures, humidity, and pressure) involve balloon-launched radiosondes with data normally archived every few hundred meters1. In view of the emerging needs for high-resolution, vertically-measured profiles as outlined below, however, data from conventional radiosondes fall far short of the mark in at least two major ways: First, the vertical resolution of the archived data is nowhere near sufficient. Second, since the data are currently obtained using non-tethered (wind-driven) balloons, the resulting profiles are nowhere near vertically-oriented, and can be inclined from the vertical by as much as 70 degrees. It follows that high-resolution vertical profiles of atmospheric quantities throughout the troposphere and lower stratosphere do not exit. A few specific examples of important emerging studies that require vertically-measured, high resolution profiles are listed below: Breaking Atmospheric Gravity Waves: Breaking AGWs appear to supply the lion’s share of energy for the generation of atmospheric turbulence. This turbulence underlies critically-important processes such as turbulent transport and diffusion. The vertical scales of breaking AGWs are typically a few tens of meters and therefore much smaller than the vertical resolution of conventional radiosonde data. Optimum studies require high resolution vertical (i.e., fixed-frame Eulerian, not moving-frame Lagrangian) measurements. Turbulence, Vertical Wind Shears, and Temperature Gradients: Small-scale atmospheric turbulence is truly ubiquitous, and its generation depends primarily on the vertical shear of the horizontal wind (∂U/∂z) coupled with the vertical gradient of the potential temperature (∂T/∂z). The pertinent parameter is the gradient Richardson number, Ri, where Ri α(∂T/∂z)/ (∂U/∂z)2. Recent CIRES studies (Balsley, et al., 2008) show that atmospheric Ri values are strongly scale-dependent. Moreover, the smaller the scale, the more of the region under study is unstable. Specifically, critical values of Ri (< 0.25) responsible for turbulence generation occur only on vertical scales less than about 100 m or so, with critical values occurring more often as the examined scale size decreases. For example, our studies show that, when examining a relatively large atmospheric volume at vertical scales of 1.5 m, more than 60-70% of the volume exhibits Ri values < 0.25. This result is shown in Figure 1, which illustrates the results of examining many nights of Ri values as a function of scale size throughout the entire nighttime residual layer (~100-1000 m). Atmospheric Overturnings: Recent work at CIRES using slow-rise-rate radiosondes (Balsley et al., 2009) shows that, surprisingly, both the troposphere and lower stratosphere exhibit ‘overturnings’ on vertical scales of ~10-100m. Overturning analysis, which grew out of early deep-ocean measurements, involves re-ordering observed potential temperature profiles to force themto be monotonically increasing with height. The resulting ‘Thorpe displacement’ scale results from determining the vertical distances required for this re-ordering. An example of overturnings between 0-20 km appears in Figure 2, where overturnings are visible throughout the entire height range. Fine-scale overturning in the atmosphere is a relatively unstudied phenomenon that clearly needs to be documented and fully understood. Possibly more important, however, is the need to understand the processes responsible for the overturnings themselves. Breaking AGWs, as described above, provide a feasible source for these overturnings. Turbulence Studies Using Clear Air Radars: If they have sufficient vertical resolution, wind-profiling radars (where the radar echoes derive from small-scale atmospheric turbulence), are useful tools for studying AGWs and vertical turbulence structure. Unfortunately, although wind profilers measure both turbulence intensities and vertical wind gradients, they cannot measure the concomitant temperature gradients needed to compute local Ri values (see above). This information requires separate relative high-resolution temperature profiles measured in close proximity to the vertically-directed radar antenna beam. Ri profiles would enable a first-time in situ comparison between measured and computed turbulent values, and their relation to overturnings. THE PROPOSED SOLUTION: The above requirements can be met by developing a small, reusable, automated, GPS-controlled, glider system equipped with radiosonde sensors and a data logging module. This system will archive accurate temperature and reasonably accurate wind profiles between about 0-10 km with a vertical resolution of 2-4 m. In a typical operation, the glider (Databird) is launched beneath a conventional radiosonde balloon and carried to a pre-determined altitude (~10 km). The Databird is then released and glides to a preset waypoint (e.g., over a radar antenna beam, where it begins a slow spiraling descent to a second waypoint near the surface. At the second waypoint the glider then automatically glides to a prescribed landing site and lands. The data archived in the data logger is then downloaded onto a conventional laptop computer, with the entire process being repeated as often as needed. Figure 3 is a photograph of the GPS-controlled Databird (@ $1,400). The Databird is jet-engine ingestible and can be flown wherever radiosondes are launched (i.e., virtually everywhere). The glider is capable of carrying a radiosonde. In our case, the radiosonde would be stripped of its transmitter, and the sensor outputs would be interfaced to a small flash memory card. It is important to point out that these sub-units as well as the expertise to merge them into (and to bench-test) the total package already exist in the CIRES Electronic Shop (D. David and K. Smith). A sketch of one possible programmable flight plan appears in Figure 4. In this plan the Databird is lifted to 10 km, released, and then glides to a location directly above a radar beam (note that the radar could be also a lidar or a sodar). At this point it descends in a relatively tight spiral (~100-200m radius) before landing nearby. Other flight plans are equally possible. THE POTENTIAL FOR FUTURE DATABIRD STUDIES: The incorporation of the Databird technology into atmospheric research could potentially revolutionize studies of high-resolution dynamic processes in the free atmosphere. One can envision a series of Databirds making a sequence of such profiles every 20 minutes or so to document the evolution of the observed processes. It is equally quite feasible to make simultaneous profiles separated horizontally by a few hundred meters (or more) to determine the horizontal extent of the processes (e.g., overturnings and AGWs) under study. Finally, CIRES Databird studies could also be expanded to fly lightweight chemistry sensors that would provide high-resolution profiles of chemical species (e.g., ozone, CO2, H20) under varying conditions. REFERENCES: |
216 UCB