Atmospheric research conducted in the upper troposphere and lower stratosphere currently lacks critical information in that there is no means of accurately tracking moving air parcels as they circumnavigate the Earth. For example, important details of polar stratospheric cloud formation are unknown because temperature histories of the air parcels prior to observation are not known to sufficient accuracy. In the tropics, uncertainties in air parcel motion mask the mechanism by which water vapor enters the stratosphere, confounding attempts to predict how stratospheric ozone will respond to climate change.
The LAMP aims to address these types of questions through the development of a new research platform. This platform, a small Lagrangian balloon, employs active buoyancy control and highly-accurate meteorological measurements to lock onto and follow an air parcel trajectory (Fig. 1). Under computer control, the balloon is made to rise and fall with the surrounding air and thus tracks the air parcel through three-dimensional space.
The structure of the LAMP balloon is similar to the super-pressure ultra-long-duration balloons (ULDB's) being developed jointly by NASA and Raven Industries; these ULDB's consist of a light-weight polyethylene envelope supported by strong tendons that run from the base to the apex of the balloon. To achieve altitude control, the LAMP balloon incorporates a motor and tensioning line that pulls the apex and base together, changing the balloon's internal pressure and therefore its buoyancy (Fig. 2). This design is particularly effective because the radius of curvature of the load-bearing tendons decreases as the balloon shrinks, helping to offset the stresses induced by the rise in the balloon's internal pressure. Model results show that the effective density of the balloon can be varied by 40% at 21 kilometers without exceeding the yield strengths of the balloon materials.
Model of a Lagrangian Balloon in Flight
A model based on the equations of motion for a Lagrangian balloon supports the viability of the proposed design. In this test, a virtual balloon is run along a model lee wave trajectory (a course approximation of the MM5 model trajectory used by Voigt et al., [2000]) in order to test the proposed design under particularly demanding conditions. A control algorythm adjusts the balloon's volume to achieve critical damping, causing the balloon to rapidly lock onto the air parcel (Fig. 3 top panel) with errors of only a few meters in altitude (second panel). To achieve this degree of control, the balloon's volume is modulated over a range of approximately 7% (third panel), well within the design parameters of the pumpkin balloon. On average, the tensioning motor draws less than 10 Watts of power (bottom panel).
References
Voigt, C., T. Athanasios, A. Dornbrack, S. Meilinger, B. Luo, J. Schreiner, N. Larsen, K. Mauersburger, and T. Peter, Non-equilibrium composition of liquid polar stratospheric clouds in gravity waves, Geophys. Res. Lett., 27, 3873-3876, 2000.
Figure 1. LAMP Balloon Schematic
Figure 2. Balloon Compression
Figure 3. Flight Model