Meteorological Value of Ground Target Measurements by Radar

Abstract

Many characteristics of radar echoes from ground targets vary with time as the properties of the atmosphere in which the radar waves propagate evolve. For example, if the phase of a target varies with time and the target is known to be stationary, that phase variation is related to changes in the refractive index of air between the radar and that target. These changes are themselves caused by variations in the pressure, temperature, and especially the humidity of air. The changing phase and intensity of ground targets are hence records of evolving atmospheric conditions and could therefore in theory be used to retrieve parameters of meteorological value. In this paper, the various ways meteorological conditions can affect radar returns from ground targets are explored and quantified. In particular, the extent with which the changes in the phase of ground targets can be used to extract weather-related information is investigated. While wind, precipitation, and the vertical structure of temperature and humidity all affect ground echo appearance, it is demonstrated that the most promising quantity to be obtained is the refractive index of air near the ground. The process and the uncertainty of refractive index measurements by radar are then described. Comparisons with surface stations indicate that near-surface refractive index can be obtained reasonably accurately by radar in very flat terrain. In more complex topography, sensitivity analyses show that radar-based refractive index measurements will be strongly affected by the vertical profile of refractive index.