Geoscience Reference
In-Depth Information
13
Effects of Anomalous Propagation Conditions
on Weather Radar Observations
Joan Bech
1
, Adolfo Magaldi
2
, Bernat Codina
1
and Jeroni Lorente
1
1
Dep. Astronomy and Meteorology, University of Barcelona
2
Institute of Space Sciences, Spanish National Research Council (CSIC), Bellatera
Spain
1. Introduction
The effect of atmospheric propagation on radar observations is an important topic both for
radar application developers and end-users of radar products, particularly of weather radar
systems. An excellent review of this subject is given by Patterson (2008), and most general
topics about weather radars have a chapter on the topic -see for example Battan (1973),
Collier (1996), Doviak and Zrnic (2006), Rinehart (2001) or Sauvageot (1991).
In this chapter our objective is to provide an overview of the effects of anomalous
propagation conditions on weather radar observations, based mostly on studies performed
by the authors during the last decade, summarizing results from recent publications,
presentations, or unpublished material. We believe this chapter may be useful as an
introductory text for graduate students, or researchers and practitioners dealing with this
topic. Throughout the text a spherical symmetric atmosphere is assumed and the focus is on
the occurrence of ground and sea clutter and subsequent problems for weather radar
applications. Other related topics such as long-path, over-the-horizon propagation and
detection of radar targets (either clutter or weather systems) at long ranges is not considered
here; however readers should be aware of the potential problems these phenomena may
have as range aliasing may cause these echoes appear nearer than they are - for more details
see the discussion about second trip echoes by Zrnic, this volume.
Despite the motivation and results shown here are focused on ground-based weather radar
systems (typically X, C or S band radars, i.e. cm-radars), a large part of these results are
applicable to other types of radar, in fact also to micro-wave links or, in general terms, for
propagation of electromagnetic waves in the atmosphere. As discussed in detail below, the
main effect of anomalous propagation on weather radar observation is a lower height of the
observed echoes than expected in normal conditions. This may imply an increase of ground
clutter or, for radars operating near the coast, an increase of sea clutter, which will be hardly
corrected by the standard Doppler filtering, affecting inevitably precipitation estimates.
This chapter is organized as follows. Section 2 introduces the fundamental concepts of
refractivity and modified refractivity and the various propagation conditions associated with
refractivity profiles. Section 3 presents some results on propagation condition variability, and
Section 4 focuses specifically upon the impact of that variability on radar beam blockage
