El actor y el rol en las potencias de lo falso 31.

As introduced by Hock and Franklin (1999), measurements taken by the dropwind- sonde when it falls through the atmosphere are neither in a Eulerian framework nor in a perfect Lagrangian framework, therefore a proper post-processing of such measurement is needed, and such post-processing should be based on a theoretical analysis of the dif- ference between the dropwindsonde motion, which is reported as the wind velocity, and the driving wind, which is the ”real” wind velocity. Thus, the dropwindsonde motion should be analyzed first to make a theoretical foundation for properly post-processing dropwindsonde measurements. Hock and Franklin (1999) introduced wind finding equa- tions which are used to derive horizontal winds from measurable motion quantities of the dropwindsonde. The derivation is based on an assumption that the dropwindsonde motion can be modelled as a point translating in a Cartesian coordinate system. This assumption is widely used in analyzing the motion of vertically moving sensors. For example, the study of Scoggins (1965) discussed the response characteristics of spheres freely rising in the atmosphere based on such assumption. In this study, data measured

by the balloon up to 120m at Huntsville, Alabama and up to 12km at Cape, Kennedy

were reported. Although the accuracy and resolution, ∼ 600m, of the measurement

ment to measure winds. What is useful from this study in understanding dropwindsonde measurements is its discussion of the wind measurement capacity of vertically moving sensors. Through investigating spectral characteristics of a vertical moving sensor under horizontal wind perturbations, the study concluded that a response function needs to be determined for translating ”apparent” measurements to actual wind speeds. For the measurement taken by the freely rising sensor, high frequency information is not picked up and some smooth filter needs to be applied according to their analysis.

Adopting the same assumption, a systematical investigation of responsive behaviours of a rising, or falling, sphere based on its motion governing equations has been conducted by Fichtl (1971). In this study, linearized equations governing a sensor’s motion subjecting to aerodynamic drags were developed. After processing the wind environment and sensor velocity through the Fourier-Stieltjes integral, the response function and phase lag were derived. The results showed that both the response function and phase lag depend on the external wind perturbation frequency and the sensor’s aerodynamic properties. As expected, the sensor is capable of measuring low frequency wind perturbations while is not that responsive to high frequency wind perturbations. Moreover, the ”apparent” mass, which is the mass of the ambient air affected by the sensor’s motion, made the sensor responsive. At last, the analysis of the response function revealed that the sensor is more sensitive to vertical wind perturbations than to horizontal wind perturbation. Since this linearization and analysis framework is capable of thoroughly investigating responsive behaviours of a rising, or falling, object in the atmosphere, they are used to analyze the dropwindsonde behaviour as described in chapter 3.

Besides assuming the vertically moving sensors as point objects with a constant mass, an alternative way to model the motion of vertically moving parachute systems is to sep- arate the payload from the parachute. As indicated by Cockrell (1987), which thoroughly discussed the parachute aerodynamics, two types of models can generally be formulated in analyzing a parachute system’s motion. One, the parachute and payload are modelled as one rigid body; two, the parachute and payload are modelled as two separate rigid

bodies with interactions. In the case of the dropwindsonde, the mass of the parachute is far less than the mass of the dropwindsonde body, and therefore can be modelled as an external force. In other words, a revised two rigid bodies model can be used to de- scribe the motion of the dropwindsonde where the parachute is modelled as an external passive load acting on the dropwindsonde body whose direction is opposite to the rela- tive dropwindsonde motion. It is worth mentioning that although the drag coefficient of the dropwindsonde derived from its terminal velocity given by Hock and Franklin (1999) is widely used in post-processing its wind measurement, no experiment has been con- ducted, as far to my knowledge, to obtain a reliable estimate of its aerodynamics under a controlled condition.

From the studies reviewed above, it can be seen that the parachute’s aerodynamics are an important and challenging component of a reliable dropwindsonde motion model. Thus, aerodynamics measured in a wind tunnel test are necessary for a model to real- istically depict the dropwindsonde motion. Consulting a summary of wind tunnel test methods on the parachute given by Croll et al. (1981), a wind tunnel test is conducted to measure reliable aerodynamics of the dropwindsonde. Details of the motion derivation and the wind tunnel test are given in chapter 3

Having motion characteristics similar to those of the dropwindsonde, investigations on heavy particle motions in the atmosphere sheds some lights on the motion analysis of the dropwindsonde. A series of studies by Pinsky and Khain (1996) and Khain and Pinsky (1995) described the motion simulation of heavy particles in a field with some natural turbulence features. Using motion governing equations similar to those in Pinsky and Khain (1996), chapter 3 presents a numerical simulation of the dropwindsonde motion, and their simulation methodology is followed in this simulation of the dropwindsonde motion. More specifically, they investigated the heavy particle motion through analyt- ically analyzing and numerically integrating the motion governing equations, and this investigation approach is repeated in chapter 3 to understand the dropwindsonde motion in a wind field. Same as that in dropwindsonde motion studies, such as that of Hock and

Franklin (1999) and Fichtl (1971), their result showed that the turbulence has a great impact on the motion of heavy particles.