Specific features of rainfall and snowfall radar in the millimeter-wave band

Abstract

One way of measuring rainfall and snowfall zones is to use radars, which allow the sizes and locations of the pre� cipitation zones to be estimated. At present, meteorolog� ical radars usually use the centimeterwave band (1). The objective of the present study is to estimate the feasibility of rainfall and snowfall sensing with a millimeterwave radar and to determine what features of these precipita� tion zones can be discovered in the process of sensing. It is intuitively expected that the use of shorter wavelengths makes it possible (1) to shape radio beams with smaller divergence, a circumstance that implies a better spatial resolution and, thereby, a possibility of resolving the fine structure of zones with different rainfall and snowfall rates; (2) to record the signal reflected from zones with lower rainfall rate and snowfall rate than in the case of the cen� timeterwave band, the circumstance that allows the zones of these precipitations to be determined with greater accuracy; and (3) to use the techniques for retrieving profiles of the coefficient of the radiowave attenuation along the sens� ing path, which were developed for the lidar sensing in the optical band. It should be noted that the most familiar meteorologi� cal radars designed to analyze the rainfall zones operate on frequencies of about 3 GHz, and their antennas are 2.5 m (2) and 3.6 m (3) in diameter. Accordingly, the antennas with their rotators are rather cumbersome installations; in this respect, transition to the millimeter� wave band is also promising, since it results in a reduction in the radar mass and dimensions. The range of the abovementioned radars is 100- 150 km. An obvious drawback of the millimeterwave radar will be a certain decrease in its range. Since we have to deal with a volumedistributed target, the radar equation differs from the common one. The reflecting elements (raindrops or snowflakes), which are involved in the formation of the sum signal arriving at the radar receiver, are distributed over certain volume V. This volume can be approximated by a cylinder with base area Ω L 2 and height сτ/2, where L is the distance from the radar to the center of the desired volume, Ω is the antenna's solid angle (beam divergence), τ is the pulse duration, and с is the speed of light. In this case, it is assumed that the attenuation of radio waves in the illumi� nated volume is insignificant, and the precipitation parti� cles in the entire desired volume are identically illumi� nated. The antenna's solid angle is expressed in terms of its effective area А or directivity D: