BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 23

CHAPTER TWENTY THREE Meteorological Aspects of Radar INTRODUCTION Under certain standard meteorological conditions the range of merchant ship radars slightly exceed the distance of the

CHAPTER TWENTY THREE

Meteorological Aspects of Radar

INTRODUCTION

Under certain standard meteorological conditions the range of merchant ship radars slightly exceed the distance of the observer's visible horizon: using the same height of eye However, just as the eye's range of vision is reduced by conditions of low visibility and may be increased by abnormal refraction, so also certain meteorological conditions affect radar range

METEOROLOGICAL PHENOMENA ON RADAR

Since clouds consist of water droplets, rain falling from them will appear on a radar screen if it is sufficiently heavy The larger the droplets the better the radar target it presents and the greater the interference it is likely to cause Fortunately the droplets composing fog, mist and drizzle are very small and cause little interference Showers will appear as light irregular shapes which are constantly changing, solid targets such as other vessels will be more persistent within these Cold fronts in particular may show as a quite distinct formation Heavy rain may temporarily cause a reduction in the potential range

The composition of snowflakes is such that they do not cause as much trouble as heavy rain but this again is governed by flake size intensity and extent of the snowfall Solid particles such as sand, and dust in the atmosphere may also diminish radar range All these attenuation effects can, to some extent, be ameliorated by judicious use of gain control though this action is liable to weaken the echo strength of any "solid" target such as a ship

WAVE CLUTTER

The presence of sea waves likely to cause clutter on the radar is obvious to those on the bridge This indirect meteorological effect on radar performance

is largely overcome by prudent manipulation of “swept gain" Nothing can prevent it, however, being a potential menace when the detection of small targets such as growlers, boats or buoys is concerned

When heavy rain or some similar cause of interference is some distance from

have their radar range diminished The diffuse appearance of these attenuating echoes on the radar screen is easily recognisable and when remote from the ship their area has a hard edge Very heavy rain often extends to considerable height; this enables the central dangerous area and "eye" of a tropical storm

to be observed at extreme radar range (i.e., up to about 80 miles or so) and thus helps the mariner to take evasive action (See Chapter 17.)

Note: Heavy rain can be penetrated by 10 cm radar but not by 3 cm radar

STANDARD ATMOSPHERE

Radar rays are affected by refraction when passing through atmospheric layers of varying density to a greater extent than are light rays, which have a very much smaller wave length The errors that can occur in astronomical navigation due to this bending of light rays at various altitudes are allowed for in navigation tables on the basis of "mean refraction" This assumes a uniform atmosphere with density decreasing regularly with height Variations from this idealised state are ignored, except when mirage shows abnormal refraction to be present resulting in accurate "sights" being more or less impracticable

The effect of variations in refraction on a radar is to decrease or to increase its range Normal radar performance, giving ranges slightly beyond the clear weather horizon at the radar's height of eye can be expected in Standard Atmospheric Conditions (Standard Refractive Index) This implies normal convection and:

1 A pressure at sea level of 1013 hPa, decreasing with height at a rate of 12 hPa per 100 metres

2 A temperature at sea level of 15°C, decreasing with height at a rate of 0.7oC per 100 metres

3 A constant relative humidity of 60%.

These figures merely det1ne Standard Refractive index which implies a Standard Atmosphere and it is obvious that they cannot be checked by the mariner They represent a mean value for the whole world and should not be considered as normal

The refractive index, which indicates the degree of refraction in the atmosphere at any given time depends on the density of the air through which the radar waves pass Density, in turn is governed by temperature, pressure and relative humidity The greater the density the greater the refraction and vice-versa

STANDARD PROPAGATION

In standard atmosphere the distance from the transmitter to the radar horizon is

a little in excess of the optical distance This distance is 2.21 √h miles where 'h' is the height of the radar scanner above the waterline in metres Standard conditions exist mostly in temperate latitudes at sea well away from the land and when good mixing of the air is indicated by strong winds and cumulus clouds

NON-STANDARD PROPAGATION

The effect of signit1cant variations from standard conditions will either decrease or increase the effective radar range, according to whether the atmospheric density near the surface is less or greater than standard respectively It is important for the mariner to know some of the meteorological conditions which are likely to affect radar propagation and what sort of unusual performance he might expect from his radar Non-standard conditions are more commonly encountered in coastal areas near large land masses A change in air temperature and/or relative humidity at any given height will cause a change in air density (and refractive index at that height If however we consider atmospheric changes which occur near the sea surface only, we can draw the following conclusions:

(a) An increase in temperature (which is the dominating factor) will cause a decrease in density and refractive index which will result in a reduction in radar range

(b) A fall in temperature will cause the density to increase which will result in

an increase in radar range

SUB-REFRACTION

A cold air-mass moving over a relatively warm sea rises in temperature at or near the surface The result is a decrease in air density and a reduction in radar range These circumstances imply a greater than normal decrease in temperature with height (a steep lapse rare) giving vigorous convection and

an increase in relative humidity with height evidenced by a convective type of cloud formation In extreme cases the radar lobe may be refracted into an upward curve, away from the sea surface and can cause a reduction of 30 per cent or more in radar range These conditions are most likely to be experienced in very high latitudes or near large very cold land masses in winter Fortunately for the mariner, visibility tends to be good when sub-refraction is present, except when it is accompanied by heavy precipitation

SUPER-REFRACTION

Warm dry air moving over a relatively cold sea surface is cooled at or near the surface, thereby increasing the atmospheric density, which, in turn, causes an increase in refraction and radar range When the sea is very cold relative to the air mass an inversion (see Appendix 1) may be formed The resulting super-refraction may be further intensified if the warm air is very dry and picks up moisture from the sea surface These conditions occur near large land masses in tropical latitudes and during spring and summer months in the temperate zone

In circumstances favourable for super-refraction there is risk that if surface conditions are favourable, low visibility is liable to occur

DUCT PROPAGATION

In extreme conditions of super-refraction the path of the radar ray may be refracted to such an extent that it assumes a downward curve which is greater than the curvature of the Earth's surface On striking the sea surface the ray is reflected upwards and onwards but is then refracted downwards to strike the water surface again and again in a succession of “hops” The energy is thus trapped in a "radar duct" close to the surface and can follow the curvature of the Earth for very considerable distances often causing echoes to occur on the second and third traces Ranges of up to 100 miles can be attained sometimes

on conventional targets This phenomenon is somewhat similar to that which causes mirage

None of these refraction anomalies have a very serious practical effect on radar performance from the navigator's viewpoint; they merely increase or decrease the extreme radar range but it is useful to know when they are likely

to exist and to be on one's guard against being taken by surprise when they occur

Much practical advice is given in the Royal Institute of Navigation's book

The Use of Radar at Sea Published by Hollis & Caster London, Sydney and

Toronto

QUESTIONS

1 Describe the meteorological conditions which can cause: (a) Sub-refraction (b) Super-refraction What effect does each have on radar range?

2 What do you understand by "standard propagation" in relation to radar range? Describe generally the regions and meteorological conditions in which

it is most likely to exist