It is widely accepted that the radar returns in clear air are from a combination of scattering by dust, biota, cloud droplets and fluctuations in refractive index due to density gradients within the atmosphere. However, there is still much discussion about the relative importance of each of these mechanisms and the circumstances under which each is most significant. This ongoing debate is leading to advances in the understanding of clear-air echoes which are allowing scientists to extract more and more information about the dynamics of the clear atmosphere from radar remote sensing. This paper presents a review of current literature within this field and attempts to bring together some of the most important issues. Upper-air echoes are discussed but emphasis is placed on the origins of echoes observed in the lower troposphere. In addition to discussing clear-air scatterers, some attention is given to other radar characteristics and limitations which need to be considered when analyzing clear-air mode data. The findings are then applied in a detailed analysis of literature relating to the radar signature of sea breeze phenomena. It is concluded that echoes from the sea breeze, as well as most other echoes seen in the boundary layer, are primarily the result of Rayleigh and/or Mie scattering from insects, although other scatterers and scattering mechanisms may be present to a lesser degree.

Introduction

In the last two decades there has been increasing interest in the use of weather radar for clear-air studies. It has proven to be an excellent tool for studying a myriad of different meteorological phenomena including sea breeze circulations. It is widely accepted that the radar returns in clear air are from a combination of scattering by dust, biota (largely insects and birds feeding on these insects), cloud droplets, and fluctuations in refractive index due to density gradients within the atmosphere. However, there is still much discussion about the relative importance of each of these scattering mechanisms and the circumstances under which each is most significant. This paper presents a review of current literature regarding clear-air echoes. Most types of clear-air echoes are discussed, but the focus is toward those occurring in the lower parts of the atmosphere, especially those occurring in association with sea breezes.

A general background of sea breeze and radar theory is first presented to aid the reader in a better understanding of the following sections. A discussion of the general types and nature of clear-air echoes is then presented, along with some details of the methods that have been developed to discern the particular scattering mechanisms.

Also reviewed are the characteristics and limitations of weather radar, with particular emphasis on clear-air mode operation. This review references such topics as the effects of refraction on the range-height relationship, side lobe effects, and a number of other general characteristics, including some specific to Doppler radar technology.

This science is then brought together to discuss in more detail the clear-air radar signatures observed in the lower troposphere. Particular reference is made to clear-air echoes commonly found in association with sea breezes and several case studies are examined.

The numerous studies and hypotheses are compared and contrasted in order to arrive at considered conclusions about the origin, nature and scattering mechanism of clear-air echoes associated with sea breezes and other boundary layer phenomena. The conclusions drawn apply to most research and operational meteorological radars operating at wavelengths of between 0.5 cm and about 20 cm. This includes the National Weather Service’s new WSR-88D Doppler radars.

<-- Previous Section / Next Section -->