Radar Meteorology

A First Course

Robert M. Rauber, Stephen W. Nesbitt

• Editore: Wiley-Blackwell
• Anno: 2018
• ISBN:
  9781118432631
• Edizione: 1
• Argomento: Physical Sciences
• Categoria: Meteorology & Climatology

A comprehensive introduction to the current technology and application of radar in meteorology and atmospheric sciences

Written by leading experts in the field, Radar Meteorology, A first Course offers an introduction to meteorological radar systems and applications, with emphasis on observation and interpretation of physical processes in clouds and weather systems. This comprehensive introduction to the subject offers an overview of the quantities essential to radar meteorology including the radar reflectivity factor, and Doppler, dual-polarization, and multi-wavelength radar variables. The authors highlight wind retrieval from single and multiple Doppler radars, precipitation estimation and hydrometeorological applications, with chapters dedicated to interpretation of radar data from warm season mid-latitude severe weather, winter storms, tropical cyclones and more.

In addition, Radar Meteorology highlights research applications of this burgeoning technology, exploring dynamic applications such as space-borne and ground-based vertically pointing radar systems, and cloud, airborne and mobile radars. As meteorological radars are increasingly used professionally for weather observation, forecasting and warning, this much-needed text:

• Presents an introduction to the technical aspects and current application of radar as used in the meteorology and atmospheric sciences
• Contains full-colour illustrations that enhance the understanding of the material presented
• Examines the wide-range of meteorological applications of radar
• Includes problems at the end of each chapter as a helpful review of the contents
• Provides full instructor support with all illustrations and answers to problems available via the book's instructor website.

Radar Meteorology offers a much-needed introductory text to the study of radar as applied to meteorology. The text was designed for a one semester course based on the authors' own course in Radar Meteorology at the University of Illinois at Urbana-Champaign.

Chapter 1
Properties of Electromagnetic Waves

Objectives

By the end of this chapter, you should understand the following:

• The basic nature of electric and magnetic fields and how these fields are related through Maxwell's equations;
• Electromagnetic waves consist of oscillating electric and magnetic fields propagating at the speed of light;
• The electromagnetic spectrum, and why radars use the microwave frequency part of the spectrum;
• Why the degree of absorption of electromagnetic waves by a medium such as air, water, or ice depends on the wave frequency;
• That electromagnetic radiation can be characterized as waves or as particles called photons;
• How radiation interacts with matter through processes of refraction, reflection, scattering, and absorption and how each of these relate to radar meteorology;
• What is meant by polarization of an electromagnetic wave.

1.1 Introduction

A radar transmits electromagnetic radiation, normally microwaves, and measures the properties of the radiation scattered back to its antenna by objects in the path of its beam. Radar meteorologists face an enormous challenge because a radar, at best, measures only six pieces of information: the amplitude, phase, and polarization state of the returned electromagnetic energy, the time the radiation took to travel to and from the objects, and the azimuth and elevation of the radar antenna at the time the radiation was transmitted. From this scant data, they must deduce meteorologically relevant information such as the location of precipitation, rainfall rate, precipitation type and wind speed, and from this information quickly report to the public the location of a flash flood, hail, or a tornado. How they accomplish that is the subject of this book. To understand how radars work and, more importantly, how the energy radars transmit and receive can be used to determine atmospheric properties, we must first develop a basic understanding of electromagnetic energy.

1.2 Electric and magnetic fields

As you sit reading this book, you are surrounded by electric and magnetic fields. The light entering your eyes as you stare at this page reaches your retina as propagating electromagnetic waves. Cell phone transmissions, the energy heating food in your microwave, medical X-rays, and sunlight are all forms of electromagnetic energy—electric and magnetic fields oscillating in time and space.

1.2.1 The electric field

To help understand electric fields, let us start by imagining two infinite, parallel horizontal plates separated by 1 m (Figure 1.1). Assume for the moment that a perfect vacuum exists between the plates and an excess positive charge density of 10⁻¹² C m⁻² exists on one of the plates (i.e., there are fewer electrons per square meter on the positively charged plate, where a coulomb is a unit of electric charge). An electric field exists in the presence of a charged body, so an electric field exists between the plates. The electric field intensity (

), a vector quantity, has a magnitude and a direction. The magnitude of

is proportional to the force acting on a unit positive charge at a point in the field. The direction of

is the direction in which that force acts. The electric field intensity is measured in units of volts per meter, or even more fundamentally in J C⁻¹ m⁻¹. By convention, the electric field is directed away from positive and toward negative charges, so the electric field vector at any point between the plates in our experimental apparatus points from the positive to the negative plate.

Figure 1.1 The electric field (black arrows) and lines of constant voltage (green lines) between two infinite, parallel, oppositely-charged plates

We often represent

by drawing lines of force, or “flux lines,” represented by a vector, ...