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Sensors used for boundary layer measurements fall into two broad categories: in situ sensors that can be mounted on the ground, on masts, towers, tethered balloons, free balloons, or aircraft; and remote sensors, ground-based or aircraft-mounted, that infer atmospheric properties through their effects on acoustic, microwave, and optical signals propagating through the air. In situ sensors are the traditional instruments of choice for surface and lower boundary layer studies, being the only ones capable of the accuracy and resolution needed for quantitative work. A major portion of this chapter will therefore be devoted to discussions of their characteristics. Remote sensors have the advantage of increased range and spatial scanning capability, but the constraints on minimum range and spatial resolution limit their usefulness for surface layer measurements. Used in combination, however, the two types of sensors provide a more complete description of the flow field being studied than either of the two can provide separately. New remote sensors with shorter minimum ranges and finer range resolutions are now becoming available for boundary layer applications. A brief discussion of such devices is also included in this chapter. The variables of greatest interest to boundary layer meteorologists are wind speed, temperature, humidity, and the fluxes of momentum, heat, mass, and radiant energy. Given suitable fast-response measurements of wind velocity and scalar fluctuations, we can calculate the eddy fluxes directly from the products of their fluctuating components as explained in Chapter 1. If only the gradients of their means are available, however, then over a flat homogeneous surface the fluxes may be inferred from the Monin-Obukhov relationships of Chapters 1 and 3. Practical methods for doing that are described in many texts; see, for example, Monteith (1975, 1976). (Those simple relationships do not hold, as we know, under advective conditions, in plant canopies, and over hills.) There are also sensors in use that measure surface and near-surface fluxes directly, such as the drag plate (surface stress), the lysimeter (latent heat flux), flux plates (soil heat flux), and radiometers (radiant heat flux). We will discuss these and a few other types as well because of their application to studies of plant canopies. |
