INTERPRETATION OF DOPPLER WEATHER RADAR ECHOES

INTERPRETATION OF DOPPLER WEATHER RADAR ECHOES
Wing Commander VS Srinivas Directing Staff, AFAC Indian Air Force

1. The Doppler Weather Radar emits beams (pulses) of Microwave energy from a transmitter into the atmosphere. 2. When these beams collide with objects in the atmosphere, such as raindrops, hail stones, snowflakes, cloud droplets, birds, insects, dust particles, trees, and even the ground, some of the energy bounces back towards the radar. 3. A receiver on the radar collects the reflected energy and displays (EM Spectrum as Fig. 1)

Fig. 1 : EM Spectrum

4. Doppler radar came into use when the Weather Surveillance Radar – 1988 Doppler radar (WSR-88D), was installed in place of the Weather Surveillance Radar – 1974 (WSR-74). 5. Currently, there are 158 such WSR-88D radars that operate around the USA. These are part of a network of Doppler Radars, called as Next Generation Radar (NEXRAD). 6. Individual radar sits inside a dome that rests on a tower about 100 feet tall. The transmitter on the radar emits beams of microwave energy in all horizontal directions to send energy to every part of the lower atmosphere. Fig. 2 shows the display.

Fig. 2 : Radar Display

7. Since radars have a certain spatial resolution, the radar patterns look gridded when zoomed. 8. Many radar programs use smoothing algorithms which smooth data and make it look more natural. 9. However, the interpretation of the radar image is always using their true form which has not been smoothed. Each individual block, box, or square of data you see on radar is called a pixel, bin, or gate. As you move outward along a straight line, the bins that form on a connected line is radial.

10. The range or distance from the radar site and azimuth or angle made between the radial that points to true north and the radial that points to the bin of interest are used to determine location on radar. 11. The radar data is represented by a bin on a colour scale. This coloured bin is called an echo or return (As shown in Fig. 3). 12. All radars have a transmitter, a receiver, and an antenna. The transmitter emits pulses of microwave energy. The receiver receives reflected energy.

Fig. 3: Pixels / Bins / Gates and Radials in Radar Display

CHARACTERISTICS OF DOPPLER RADAR 13. All Doppler radars rotate horizontally as they transmit energy. They can also tilt vertically. 14. A radar scans horizontally 360° at anywhere from four to fourteen different vertical angles. 15. The standard elevation angle is 0.5° above the horizontal, which is the base angle (base X is one of many products, such as reflectivity or velocity). 16. When radar finishes scanning in 360° at one elevation angle, it tilts up to the next elevation angle and scans 360° at that angle, too.

17. While radar transmits energy, it cannot detect reflected energy. It is possible to determine how long the radar is transmitting a pulse by measuring the radius of that circle. 18. The larger the circle, the longer the radar is transmitting. Since radar alternates between transmitting and receiving energy, a term called the Pulse Repetition Frequency (PRF) is defined as the rate at which the radar sends pulses. 19. Microwave energy emitted by radar is a wave and has all the characteristics of waves like wavelength (distance between successive peaks or valleys).

20. In the Microwave portion of the electromagnetic spectrum, wavelengths vary between 1 mm to 1 m. 21. In a Doppler radar different wavelengths are used. S-band uses 10 cm, C-band uses 5 cm and X-band uses 2 cm wavelengths. The Wavelength used is related to:- (a) The size of particles the radar can detect. (b) The beam attenuation after it bounces off reflectors. (c) Value of velocity that can be measured.

OPERATING FREQUENCIES OF WEATHER RADAR
FREQUENCY (M.C.S.) WAVELENGTH (CM) BAND 37500 0.8 (1) K (SHORT RANGE CLOUD DETECTION) 3 X (FOR POLAR AREA, RAIN ESTIMATE) 5.6 (5) C (NON-TROPICAL AREA, MOD. RAINFALL) (10) S (TROPICAL AREA, HEAVY PPTN / TCs) 1500 20 L

22. The shorter the wavelength, the smaller the particles the radar can detect. Attenuation means weakening of the beam due to energy being deflected away or absorbed by particles as the beam travels away from the radar. 23. When a radar beam travels through several intense thunderstorms the beam encounters very large number of raindrops. 24. The more raindrops the beam bounces off of, the less energy is left to travel farther to more distant storms.

25. The product base reflectivity displays the amount of energy returned to the radar. 26. This makes it appear thunderstorms that are farther have less intensity where as they may be as intense as or more intense than the storms closer to the radar. 27. Radars have a certain resolution of data (by radial and azimuth).

BEAM SPREADING 28. The radial resolution of radar is the number of bins for a given distance that beam travels. In latest radars one bin for every 250 meters gets covered. 29. The azimuthal resolution (also called beam width or beam separation) of radar is the number of radials that the radar can depict in terms of degrees of a circle. 30. For example, if there are 360 radials that can be shown by radar, then that radar has one radial for every degree in azimuth. Therefore, the azimuthal resolution for that radar is 1°.

Fig. 4 : Concept of Beam Spreading – outer-most circle corresponds to effective radar range

31. Beam spreading is a term that describes the change in the size of a bin as distance from the radar site increases due to the spreading of radar beams at adjacent radials as they move away from the radar site. In the figure the outer bin is larger than the inner bin. 32. Thus, it is clear that since a single bin only represents one point of data, then the radar data is at a seemingly lower resolution farther from the radar than it is closer to the radar. 33. This causes differences in appearance of radar returns far from and close to the radar site. The same can be clearly seen in the following Radar echo (As shown in Fig. 5).

Fig. 5 : Storms very close to the radar (near the centre of the image) appear much finer and less blocky than the storms far from the radar

34. Base reflectivity is a radar product that displays the amount of power reflected off particles to the radar. 35. The actual amount of energy returned is displayed in terms of reflectivity and is measured in decibels (dBZ). 36. A decibel is a measure of the energy transmitted by a wave, or a measure of the amplitude of a wave (As shown in Fig. 6). BASE REFLECTIVITY

Fig. 6 : Reflectivity Image - strength of returns to the radar, measured in decibels (dBZ)

37. The usual range of reflectivity that is displayed by this product is between 0 dBZ and 80 dBZ, with 0 dBZ indicating very little return of energy and 80 dBZ indicating extremely intense returned energy, a value never reached in meteorological scenarios. 38. The value of reflectivity in a bin of data is actually the average of all the reflectivity values detected within that bin (there will be multiple targets that reflect the energy within the areal space of one bin. All do not reflect the same amount of energy).

39. Also it is useful to understand reflectivity gradient or change of reflectivity over a distance of a few bins. 40. This is important at large distances from the radar site. Base reflectivity shows echoes whenever energy from the radar beam bounces back to it. 41. Anything in the atmosphere that can reflect microwave energy will cause echoes to appear on a radar display. Many times, these echoes are not precipitation, but are meteorological in nature. If base reflectivity shows echoes that are not meteorological in nature, the echoes are called anomalous propagation.

42. A boundary between warm moist air and cool dry air causes reflection of a wave. This boundary will be seen as a very narrow line of light reflectivity. This boundary can be a warm or cold front, a dry line, or outflow boundary from thunderstorms.

(a) When the radar is in precipitation mode, the range of dBZ values displayed can be as low as 5 to a maximum of 75, whereas clear air mode offers a range from -28 to +28. The reason negative dBZ values can occur in clear air mode is because the dBZ is a logarithmic. (b) So an increase of 3 dBZ actually represents a doubling of power returned. Anytime Z is less than 1 mm6/m3, dBZ becomes negative. Negative dBZs are only found when the radar is in clear air mode. (c) Given that light snow is falling, the radar operator needs the radar in its most sensitive mode, namely, clear air mode. Notice the negative dBZ values in the dark taupe color. If the radar were in precipitation mode, the amount of coverage would be limited to the blue areas.

(d) A negative dBZ means that the radar is detecting very small hydrometeors. As mentioned above, this is great way for forecasters to detect very dry light snow or drizzle which have lower reflectivities. It may also be useful to detect outflow boundaries and drylines.

THREE-BODY SCATTER SPIKE (TBSS) 43. A TBSS is an anomalous echo caused by very large hail in a storm. When the radar beam moves through a storm containing large hail, the beam may deflect off the large hail and move towards the ground. 44. After hitting the ground, the energy reflects back up to the hail, and back to the radar. In this phenomenon a noticeable time delay between the energy reflected from the contact of the beam and hail and the energy reflected from the ground after collision.

Fig. 7 : Three-Body Scatter Spike (Red Oval – atmospheric region containing Rain and Hail; Blue arrows - path of radar beam)[Three Body Scatter Spike is named due to the three reflections of the radar beam that occur to give this reflectivity pattern]

45. The radar will interpret this delay as echoes farther from the radar site. Therefore a TBSS will appear on radar as a spike or long, narrow extension of light reflectivity on the backside of the storm (As shown in Fig. 7). 46. This spike occurs along a radial. A TBSS is not seen across radials. An example of radar echo with TBSS can be seen in Fig. 8.

Fig. 8 : TBSS as streak of low reflectivity, extending past the storm along five radials (White bins in the storm - Reflectivity values of over 70 dBZ, a strong indication of large hail in the storm). TBSS is an even stronger indicator of presence of large hail

BRIGHT BANDING 47. Bright Banding is a region of relatively intense reflectivity. This is found along a boundary between frozen and liquid precipitation. 48. The high reflectivity is not an indication of heavy precipitation but is an indication of liquid coating on frozen precipitation. 49. Liquid water reflects microwave energy better than frozen water. At the boundary between frozen and liquid precipitation, there will be a mix of both phases of water.

50. The liquid coating on the frozen particles will make the particles appear much larger to the radar due to reflecting much more energy. An observer can interpret frozen and liquid precipitation. 51. Frozen precipitation will appear smooth and is associated with small reflectivity gradients. However the overall reflectivity is lower than liquid precipitation (As shown in Fig. 9).

Fig. 9 : Bright Banding is occurring just to NE of radar site, indicated by orange and red colour; Frozen precipitation to NW of bright banding appears smoother compared to liquid precipitation to SE; High Reflectivity is due to Bright Banding (Reflectivity is depicted after smoothing)

INTERPRETATION OF DOPPLER
WEATHER RADAR ECHOES 52. The same principle is used to explain why Hail is associated with high values of reflectivity on radar. Reflectivity values above 60 dBZ are indications that hail, or a mixture of heavy rain and hail is being present (As shown in Fig. 10).

Fig. 10 : High Reflectivity with values > 60 dBZ in pink are indicators of hail, or a mixture of very heavy rain and hail; liquid coating on hailstones causes them to reflect more of energy back to radar

53. Base Velocity is a radar product that displays the average wind speed (and the direction) of particles that are detected by the radar. 54. The radar measures the speed based on the Doppler Effect, in which the frequency of a wave changes as it bounces off a particle that is moving (e.g., the particle in the atmosphere) with respect to an object that is not moving (i.e., the radar). 55. Since radar sends pulses of energy in one direction per pulse, the wind speed detected is necessarily a speed in the direction of the beam. BASE VELOCITY

56. An individual Doppler radar cannot measure or calculate wind in more than one dimension. However, two Doppler radars that are not in the same location can. It is important to note that when you are looking at a display of base velocity, you are looking at the wind speed either directly towards or directly away from the radar. 57. Since there are more than 360 different angles along which you can approach radar, the wind velocity detected at one location may not be the same wind velocity detected at a different location, even if the product indicates the same magnitude of wind at both locations.

58. Positive values of velocity mean that there is a positive correlation between distance and direction (i.e., if you’re moving away from the radar site, the wind is in the same direction as the one in which you are travelling). 59. This means the wind direction is away from the radar when values are positive and towards the radar when values are negative. 60. This is commonly marked with a green colour for velocities towards the radar and red for velocities away from the radar (Fig. 11).

Fig. 11 : Green colour to SW of radar, along with Red colour to NE, indicate that winds are flowing towards NE

61. Usually, the most interesting areas are those where red and green colours meet, especially if the values are large where they meet. 62. Base Velocity is a very useful radar product. Associated with base velocity is the estimation of wind shear. Wind shear is a change of velocity with height. A change in wind speed or direction as you ascend is wind shear [As shown in Fig. 12(a) & (b)].

Fig. 12 : Base Velocity image shows :- (a) clockwise turning of winds with height indicated by curving of zero isodop, as one moves away from radar site. (b) as a 4.5° elevation scan shows clearly wind Shear appearing

63. Approach of a front towards the radar can be determined based on the change of wind across the front from the echoes. Consider the example of a Frontal Passage based on Base Velocity, as shown in Fig. 13.

Fig. 13 : Base Velocity image with different colour scale. Zero isodop running nearly due W to E indicates winds from S to N to east of radar site and from N to S to west of the radar site, with a boundary (SSW-NNE oriented and passes almost directly through the radar) across which wind directions drastically change. It is a Cold Front!

64. Based on the Base Velocity in thunderstorms, the amount of rotation present can be determined. Look for areas where red and green meet along adjacent radials (As shown in Fig. 14). 65. If the green is to the left of the red (as you move away from the radar along a radial), the rotation is counter clockwise, or cyclonic. This also applies to clockwise rotation, if the colours are switched.

Fig. 14 : Rotation on Base Velocity. Large values of velocity are observed to SW of the reference city, where bright red and green colours meet. Winds in the area are rotating counter clockwise, and rapidly, due to the brightness of colours, indicating large velocity values (there are two adjacent bins with velocities between 70 & 100 kt).This rotation is depicting a Meso-cyclone, which has a large rotating region of air in the updraft area of a thunderstorm (a tornado is likely or imminent)

66. If red and green meet along the same radial (as you go outward), this represents convergence or divergence flowing together or flowing apart respectively.

PURPLE HAZE AND DOPPLER DILEMMA 67. Range Folding. Radar can only detect and synthesize data when the reflected energy from one pulse returns to the radar before the next pulse leaves. 68. If the reflected energy from one pulse does not make it back before the next pulse is emitted, the radar becomes confused and usually throws the data out or marks it as bad data. 69. The reason reflected energy may not make it back in time is because it travelled too far from the radar (Fig. 15). This is known as Doppler Dilemma.

Fig. 15 : Base Velocity image shows Purple Haze towards outer edges of radar range. PRF of radar was such that pulses travelling into purple region did not make it back before the next pulse was emitted. All of the coloured purple echoes are range folded

Fig. 16 : Radar scans in 360° horizontally at multiple elevations [Lines represent planes through which the radar scans according to different elevation angles; Cone of silence is denoted as region above the highest elevation]

ANOMALOUS PROPAGATION (AP) 70. Anomalous Propagation is a term used to describe any radar returns that does not represent precipitation or other meteorological objects 71. These anomalous echoes result from certain atmospheric properties and man-made objects. 72. The two types of anomalous propagation are ground clutter and super refraction. 73. Ground clutter objects will not move with time and sometimes appear very intense on base reflectivity ( As shown in Fig. 17).

Fig. 17 : Ground Clutter is shown as random scattered Low Reflectivity, very close to radar site. No precipitation is falling, where this reflectivity is occurring.

SUPER REFRACTION 74. Super Refraction is a term that describes bending of the radar beam up or down as it moves away from the radar. 75. The atmospheric refractive index depends on many atmospheric variables like air density, water vapour pressure, and temperature. 76. A change in any of these variables with distance from the radar will bend the radar beam. 77. Both density and water vapour pressure greatly decrease with height in the atmosphere, so the index of refraction always decreases as height of the beam increases.

78. Since the index of refraction decreases, the speed of light increases and the beam bends downward. In this case the top part of the wave moves into the area where its speed is faster before the bottom part of the wave. 79. Therefore, the top of the wave will move faster than the bottom of the wave and the wave will appear to bend away from the faster region. 80. Also as the earth is curved the waves move out and above the horizontal angle curve up naturally as Earth is curving away from the beam. 81. However, the curvature of the earth is so small relative to the distance the beam travels that this affects the wave much less than does the change in the index of refraction. Hence it mainly depends on decrease of refractive index.

82. The biggest affect a variable can have on bending radar beams is temperature. 83. Increasing temperature decreases the index of refraction and thus, increases the speed of the wave. 84. Temperature usually decreases with height, but sometimes remains constant with height or even increases with height in an inversion layer. 85. If temperature is increasing with height, beams that travel from radar will bend downward even more than usual. This is called super refraction. 86. In extreme cases, the bending radar beam bends down to surface. This is called Ducting.

87. When a radar beam hits the surface, the energy from the beam reflects back towards the radar and causes reflectivity to appear. Since the conditions that cause this are usually uniform across the radar’s coverage area, the reflectivity will appear as a uniform circle centred on the radar site with about the same reflectivity value throughout (usually a low value). 88. This can interfere with any actual precipitation echoes near the site by masking the reflectivity of that precipitation from the ducting ( As shown in Fig. 18). 89. Early in the morning super refraction or ducting is most likely to be seen on radar. Also, the presence of a large change of water vapour pressure with height (called a Vertical Moisture Gradient) enhances this effect.

Fig. 18 : Super Refraction & Ducting

90. Ducting of the radar beams is causing the ground to reflect energy back towards the radar, giving the nearly uniform circle of reflectivity centred over the site. 91. No precipitation is seen in this reflectivity but distant storms are seen farther from the radar. 92. Refraction is likely to occur with the beams travelling further to the storms, so the heights of those storms are probably being over or under estimated.

BOW ECHO 93. A Bow Echo is a term describing the characteristic radar return from a CB that is shaped like an archer’s bow.’s 94. These systems can produce severe weather and occasionally cause major damage. 95. A bow echo is associated with or lines of convection. These echoes can range in size from 20 to 200 km, and have a life span of 3 to 6 hours. 96. Bow echoes tend to develop when moderate to strong shear exists in the lower 2 to 3 km of the atmosphere. Bow echoes are smaller in scale and are moved by the wind inside them (As shown in Fig. 19). They tend to push outward and after some time die out.

Fig. 19 : Bow Echo

97. A bow echo also lowers the chance of a tornado being formed in the storm. 98. The "bow shaped" echo is a result of focusing of the strong flow at the rear of the system. 99. Especially strong bow echoes that cause devastating damage all along the width of the storm are often called Der-echos.

REAR INFLOW JET The formation of a bow echo requires a strong elevated rear inflow jet at mid-levels. 101. Upon reaching the edge of the convection the jet descends and spreads along the surface, generating straight-line winds. After the rear inflow jet has bowed the storm system, book end or line end vortices develop on either side of the jet. These vortices are similar in strength.

103. Due to the small size of the bow echo, the vortices help enhance the mid-level flow between them. This strengthens the rear inflow jet. 104. The surface winds increase from the descending jet. As the life of the storm increases, the Coriolis Force acts to intensify the cyclonic vortex and weaken the anticyclonic vortex. . 105. The system then develops an asymmetric comma-shaped echo. Some embedded tornadoes or gust-nadoes develop within these vortices.

Fig. 20 : Evolution of Bow Echo and Comma Echo 106. Typical evolution of thunderstorm radar echo (a) into a bow echo (b, c) and into a comma echo (d). 107. Dashed line indicates axis of greatest potential for downbursts. Arrows indicate wind flow relative to the storm.

108. Note regions of cyclonic rotation (C) and anticyclonic rotation (A). Both regions, especially C, are capable of supporting tornado development in some cases.

STRONGEST WINDS 109. Damaging straight-line winds often occur near the centre of a bow echo. In a type of long-lived powerful bow echo known as a Der-echos, wind speeds can reach up to or exceeding 80 Kt and can produce a damage path extending for hundreds of miles. Bow echoes are capable of producing straight-line winds that are just as strong as many tornadoes. . A strong bow echo will produce more widespread and intense damage than the majority of tornadoes.

COMPOSITE REFLECTIVITY 112. Occasionally we encounter a Composite Reflectivity Product. The Composite Reflectivity product is used to issue alert for developing storms. Upon first glance, the Base and Composite Reflectivity imagery appear very similar. . However, the Composite Reflectivity product is differentiated from the Base Reflectivity in that the composite Reflectivity displays a synopsis of the maximum reflectivity collected from all elevation scans, in the entire radar coverage area, providing the highest dBZ value (As shown in Fig. 21).

Fig. 21 : Composite Reflectivity

PRECIPITATION INTENSITY AND MOVEMENT 115. Time lapse or loop of base reflectivity is an excellent tool for determining the movement of precipitation and to determine storm structure. The reflectivity can be used to determine the onset and end of precipitation. Other useful information includes intensity increase/decrease and indication for storm growth or dissipation.

LINE ECHO WAVE PATTERN (LEWP) 118. A Line Echo Wave Pattern (LEWP) is a squall line that has developed into a wave-like pattern due to acceleration at one end of the line and deceleration along the portion immediately adjacent. A LEWP indicates possible tornadoes, large hail and high winds. The faster the movement of the entire LEWP structure, the higher the potential for severe weather.

VERTICAL AZIMUTH DISPLAY (VAD) WIND PROFILE PRODUCT 121. Weather Radar is capable of displaying vertical wind profiles within 20 nm of the station. The VAD Product is an effective tool to show real-time winds at multiple flight altitudes provides effective real-time winds in knots (As shown in Fig. 22). 122. Wind shafts depict direction, barbs depict the speed. There are several wind display and data analysis options available to detect wind shear.

Fig. 22 : Vertical Azimuth Display (VAD) Wind Profile Product

THANK YOU

INTERPRETATION OF DOPPLER WEATHER RADAR ECHOES

Similar presentations

Presentation on theme: "INTERPRETATION OF DOPPLER WEATHER RADAR ECHOES"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

INTERPRETATION OF DOPPLER WEATHER RADAR ECHOES

Similar presentations

Presentation on theme: "INTERPRETATION OF DOPPLER WEATHER RADAR ECHOES"— Presentation transcript:

Similar presentations

About project

Feedback