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Introduction

Data from weather radar provide a great deal of detailed information regarding precipitation and wind fields. In locations such as the Caribbean where weather data over the ocean are limited, radar and satellite data can be used to analyze the weather situation. Together they can become an integral part of the forecast process. Concepts, techniques and radar products demonstrated in this module build upon the information covered in the Weather Radar Fundamentals module, which is considered a prerequisite for this module.

This module is designed to help integrate radar data into your forecast process by using radar products.  After completing the module you will be able to:

  1. Identify different types of radar products and use them to analyze different convective conditions
  2. Distinguish between real cloud signatures and spurious echoes like sea spray, second trip echoes, and electromagnetic sources.
    1. Identify sea spray, second trip echoes, and electromagnetic sources in radar imagery
    2. Determine the location of the actual feature in second trip echoes

Weather radar work by sending out a pulse of energy and then waiting for the reflected signal to return to the radar. The signal is reflected off of targets in the path of the pulse. Radar reflectivity products show the location of these targets.

Figure 1: How radar works.

The examples used in this module are fairly typical of any weather radar. However, they were taken from the METEOR 600S weather radars in use at a number of Caribbean sites, including Trinidad, Guyana and Barbados.

Map of Trinidad and Tobago with Piarco Airport identified.

Figure 2: Locations of flooding and reported tornado on August 9, 2009. Freeport - red, Warrenville- purple; and the airport.

The first case study investigates the radar signatures from a strong convective event that was associated with heavy rain, flooding, and tornadoes. On the 9th of August, 2009, strong storms developed over northwestern Trinidad. The Trinidad and Tobago Newsday website reported that the Trinidad and Tobago Meteorological Service and the Office of Disaster Preparedness and Management (ODPM) received reports of a tornado in central Trinidad over the Caroni plains near Warrenville. No injuries were reported. However, heavy rains resulted in flooding in Freeport and Chaguanas. View videos of the tornadoes through the following links:

  1. http://www.trinifans.com/tornado-trinidad-t1659.html#p15191
  2. http://bandwagonist.wordpress.com/2009/08/09/trinidad-tornado/
  3. http://www.youtube.com/watch?v=kW_nfHkKQ9w
  4. http://www.youtube.com/watch?v=l_JBW3fqayg
  5. http://www.youtube.com/watch?v=I9UxVeESLJc

Although tornadoes are not common in the Caribbean, they do occur. This case study provides the opportunity to review indications of heavy rain associated with possible tornado development.

Radar Reflectivity

Various types of reflectivity products are created to determine details of storms. The following products were taken from the same time period to illustrate the unique information that can be determined from each one.

Question

How would you describe the reflectivity signatures in the radar animation? (Choose all that apply.)

Plan position Indicator (PPI) scan over Trinidad and Tobago on 9 August 2009.

Figure 3: PPI loop over Trinidad and Tobago radar from 17:15 to 17:45 UTC on 9 Aug 2009.

The correct answers are c and d.

The northern part of Trinidad is experiencing the greatest amount of convection on this day with a large area to the northwest of the radar and more scattered convection to the north. Over the southern portion of the island, low values of reflectivity are reported, but no signs of strong convection are obvious, so choice a is incorrect. Choice b is incorrect because no organized north-south line of convection exists on the image.

Please make a selection.

Radar ReflectivityPPI and RHI

A common image derived from weather radar is the simple Plan Position Indicator (PPI) scan, which is depicted in Figure 4. This product shows color-coded values of reflectivity. From these values, we can infer the extent and strength of convection and precipitation. Higher values of higher reflectivity are associated with heavier rainfall, while the areal coverage of the reflectivity is associated with the horizontal extent of the precipitation.

PPI image over Trinidad and Tobago radar taken at 17:45 UTC on 9 Aug 2009

Figure 4: PPI image over Trinidad and Tobago radar taken at 17:45 UTC on 9 Aug 2009.

PPI products contain data returned from a specific elevation angle. Repeated PPI scans at progressively increasing elevation angles make up a volume scan. Once a volume scan is complete, the data can be processed and displayed in a variety of ways.

Instead of the PPI scan which holds the elevation angle constant and moves in the azimuth (rotating), an RHI scan holds the azimuth constant and moves the beam up and down (elevation angle). Thus, by repeating this at many azimuth angles yields a volume scan just as repeating PPI at various elevation angles yields a volume scan. Though the RHI is the best way to interrogate vertical structure, for weather surveillance the PPI strategy is generally used because the RHI strategy involves too much starting and stopping of the radar drive motors. As will be discussed later, vertical cross-sections (VCut) are available after a full volume scan is made.

Question

Review the PPI image and select the location with the heaviest precipitation? (Choose the best answer.)

PPI image over Trinidad and Tobago radar taken at 17:45 UTC on 9 Aug 2009 with locations marked for the question

Figure 5: PPI over Trinidad and Tobago radar from 17:45 UTC on 9 Aug 2009.

The correct answer is a.

The higher reflectivity values strongly suggest that this location is receiving the heaviest precipitation.

Please make a selection.

When using PPI it is good to remember that the elevation of the beam increases as the distance from the radar increases.

radar elevation scan and the information contained PPI and CAPPi imagery.

Figure 6: Depiction of radar elevation scan and the information contained in PPI imagery.

Radar ReflectivityVCut

Vertical cross-sections of radar data provide a slice through the radar echoes and can be used to analyze the vertical structure of storms and infer the existence of the potential for severe weather. This section introduces the VCut image, which displays a vertical cut along a horizontal line by processing the volume scan data.

PPI image with the line of the Vcut.

Figure 7: PPI image over Trinidad and Tobago radar from 17:45 UTC on 9 Aug 2009. The horizontal line indicates where the vertical cross section depicted in Figure 8 was taken.

Radar cross-section 17:45 09 Aug 2009

Figure 8: VCut image taken at 17:45 UTC on 9 Aug 2009 from the Trinidad and Tobago radar.

Question 1

What can we tell about the storms depicted in Figure 7 from the vertical cross-section in Figure 8? (Choose all that apply.)

The correct answers are a, b, and c.

The top of the cell at point A is being cut off because the cell is too close to the radar. At point B, the higher reflectivity values are elevated. At point C, heavy precipitation is reaching the ground as indicated by the high reflectivity near the surface. New convection may be forming at point D, so we will need to keep an eye on this area and watch how it evolves over time.

Please make a selection.

On July 30, 2011 a Caribbean Airlines airplane crashed in Guyana due to wet conditions that caused the plane to go off the runway. All 163 people aboard survived the crash. The plane landed but could not be stopped before it reached the end of the runway. It continued forward through the airport fence and stopped a short distance later, just missing a 200 ft (61 m) deep ravine.

Learn more about this incident from the following links:

Plane from New York Crashes at Guyana Airport from BBC News Latin America and Caribbean

Passengers Safe After Guyana Plane Crash from CNN World

Slideshow: Guyana Plane Crash from Reuters

Figure 9 contains radar images from Guyana near the time of the crash (0130 local time or 0530 UTC). A VCut image can be taken anywhere in the domain scanned by the radar. In Figure 9, the line along which the VCut is taken is indicated on the PPI scan and goes through several storms. The cross-section displayed provides details of the vertical profile of reflectivity along the line drawn in the PPI image.

Radar scan and Vcut from Guyana

Figure 9: VCut cross-section through convective cells from Guyana radar.

Question 2

What is the maximum value of the effective reflectivity seen in the image (Choose the best answer.)

The correct answer is d.

The cell with the highest reflectivity has an effective reflectivity maximum value of 60 dBZ.

Please make a selection.

Radar Reflectivity MAX Views

Figure 10 shows the MAX radar product, which is another way that the information of a full volume scan of reflectivity data can be processed and displayed. This imagery provides a more complete view of the reflectivity patterns by combining information from both a horizontal and a vertical view.

Radar scan of Trinidad and Tobago 17:45 09 Aug 2009 and cross section

Figure 10: MAX radar product from 17:45 UTC from 9 Aug 2009 from the Trinidad and Tobago radar.

The main center panel shows, on a horizontal 2-D display, the maximum effective reflectivity value found in each column or tube perpendicular to the display plane (i.e. vertical columns in this case). This is called a 'composite' image on a WSR-88D weather radar display, the radar used throughout the United States. Along the top and right are two additional 2-D displays, each looking like a vertical cross section but showing again the maximum value along a column or tube perpendicular to the display plane. In these cases, the columns or tubes are horizontal. That is, across the top each point on the east - west versus height display shows the maximum reflectivity found among all the points located in the horizontal (north - south) tube at that altitude and along that longitude. Likewise, each point on the north - south versus height (right) display shows the maximum reflectivity found for all the points located in the horizontal (east - west) tube at that altitude and along that latitude.

Figure 11: Schematic depiction of how a Max view radar image is formed.

The MAX view is a very good way to see much of the 3-D information contained in a volume scan on a 2-D display. It is biased towards the most intense echoes but those are usually the most important to take note of. Understanding the 3-D information via the 2-D image takes thought and practice.

Radar scan of Trinidad and Tobago 17:45 09 Aug 2009 and cross section

Figure 10 (repeated): MAX radar product from 17:45 UTC from 9 Aug 2009 from the Trinidad and Tobago radar.

Let's start with the storm system in northwestern Trinidad that we've analyzed using other radar products. The central panel gives a general idea of where the cells with the highest effective reflectivity exist. The other panels allow us to see how the reflectivity values are distributed vertically.

Question 1

Based on this image, which cell corresponds to high reflectivity at Z. (Choose the best answer.)

Radar scan of Trinidad and Tobago 17:45 09 Aug 2009 and cross section with locations marked

Figure 12: MAX radar product from 17:45 UTC from 9 Aug 2009 from the Trinidad and Tobago radar.

The correct answer is b.

How can you tell? Let's review this animation and then go back to the original image and take a closer look.

Figure 11: Schematic depiction of how a Max view radar image is formed.

Radar scan of Trinidad and Tobago 17:45 09 Aug 2009 and cross section

Figure 10 (repeated): MAX radar product from 17:45 UTC from 9 Aug 2009 from the Trinidad and Tobago radar.

This image allows for more detailed 3D analysis. The reflectivity plotted across the top of the image displays the maximum reflectivity in the north - south direction, the high values indicated by the black shading corresponds to the system labeled B.

Please make a selection.

Question 2

Which storm is responsible for the high reflectivity values seen at X?

Radar scan of Trinidad and Tobago 17:45 09 Aug 2009 and cross section with locations marked

Figure 12 (repeated): MAX radar product from 17:45 UTC from 9 Aug 2009 from the Trinidad and Tobago radar.

The correct answer is B.

Similarly, along the right axis, which displays the maximum reflectivity in the east - west direction, the highest reflectivity values are seen at location B in the image.

Please make a selection.

Real Cloud versus Spurious Echo

This section deals with evaluating non-meteorological features on radar imagery. It's important not to assume that all echoes on an image are clouds. Also, non-meteorological echoes give additional information about the atmosphere.The Caribbean examples that follow assume that you are familiar with the concepts in the Weather Radar Fundamentals module. If you have not reviewed the Weather Fundamentals module, please do so before continuing.

Examine the images below from the Barbados radar and attempt to identify (guess) what is depicted in the areas indicated in each of the numbered shapes.

Real Cloud versus Spurious EchoGround and Sea Clutter

Short-range 0.0 deg PPI

Short-range 0.0 deg PPI

Reflectivity radar image over Barbados 22:01 19 Dec 2011

Figure 13 (repeated): PPI image at 0 degree elevation angle with a maximum range of 150 km over Barbados on 19 Dec 2011.

Long-range 0.5 deg PPI

Long-range 0.5 deg PPI

Reflectivity radar image over Barbados 22:01 19 Dec 2011

Figure 14 (repeated): PPI image at 0.5 degree elevation angle with a maximum range of 250 km over Barbados on 19 Dec 2011.

Area 1 does not include cloud. The first indication that this is not cloud but likely clutter is the random mottled look. The shape of the region, bounded by radials, is very suspicious. A cloud would not likely appear this way. The lack of any detectable echo at the same location in the 0.5 degree scan (Figure 14) confirms that this is not cloud. However, it's not normal ground clutter either. It is sea clutter which is enhanced when winds are strong.

It is important in radar interpretation to use other sources of information to confirm or modify one's suspicions, speculations, and hypotheses whenever possible. In this case, a surface plot from the region shows winds from the east northeast affecting Barbados.

Surface winds from the eastern Caribbean 2100 UTC 19 Dec 2011

Figure 15: Surface winds over the eastern Caribbean at 2100 UTC on 19 Dec 2011.

A closer look at the metars from Grantley Adams airport on the island of Barbados supports the hypothesis that a choppy sea surface with sea spray was enhancing the reflectivity.

TBPB 192200Z 06008KT 9999 FEW020 SCT300 26/23 Q1013

TBPB 192100Z 07011KT 9999 FEW018 SCT030 27/21 Q1013

TBPB 192000Z 06011KT 9999 FEW018 SCT030 27/23 Q1013

TBPB 191900Z 07013KT 9999 SCT018 27/21 Q1013

TBPB 191800Z 05015KT 9999 SCT016 SCT038 28/20 1013

Another indicator that an echo is not actually a cloud, is how the echo changes with time. In a loop display, you will see clouds moving in a coherent way, such as with the wind, while clutter will remain relatively stationary with individual pixels possibly scintillating.

Real Cloud versus Spurious Echo » Range Folding/Second-trip Echoes

Range-folded second-trip echoes are misplaced on an image closer to the radar and the ground than they really are. Their shapes tend to be distorted and elongated along radials and have lower reflectivity values than the clouds that produced them.

Figure 16: Depiction of a second-trip echo.

Short-range 0.0 deg PPI

Short-range 0.0 deg PPI

Reflectivity radar image over Barbados 22:01 19 Dec 2011

Figure 13 (repeated): PPI image at 0 degree elevation angle with a maximum range of 150 km over Barbados on 19 Dec 2011.

Long-range 0.5 deg PPI

Long-range 0.5 deg PPI

Reflectivity radar image over Barbados 22:01 19 Dec 2011

Figure 14 (repeated): PPI image at 0.5 degree elevation angle with a maximum range of 250 km over Barbados on 19 Dec 2011.

Range folded echoes will move as the maximum range is varied. Thus, comparing the two scans we can surmise:

  • Area 3 contains second trip echo since no echo appears at the same location in the longer range scan. Also, a large cloud is detected on the 250 km image at the location that would lead to a second trip echo on the 150 km image in Area 3.
  • Area 2 is also second trip echoes. However, it does not appear on 250 km image because the clouds causing the echo is farther than 250 km from the radar.
  • Area 4 is actually cloud at the indicated location because it appears in the same place in the longer range (250 km) image.

Question

Area 2 is displayed about 140 km from the radar, just under the maximum unambiguous range of 150 km in Figure 13. Assuming the area is displaying second trip echo, at what range would you expect to find the true cloud? (Choose the best answer.)

The correct answer is b.

The true range is equal to the max range of the image (150 km) plus the apparent range of the cloud for second trip echoes (140 km), so the actual location of the cloud from the radar is 290 km.

Please make a selection.

Real Cloud versus Spurious Echo Electromagnetic Energy Source

Reflectivity radar image over Barbados 22:01 19 Dec 2011

Figure 14 (repeated): PPI image at 0.5 degree elevation angle with a maximum range of 250 km over Barbados on 19 Dec 2011.

Area 5 has an interesting apparent echo that extends continuously from about 50 km range to the maximum range in Figure 14. Note also that the apparent reflectivity increases with range because the source strength is constant, but the calculated reflectivity is proportional to the received power times the range squared. The source does not appear at distances less than 50 km because the calculated reflectivity is below the lowest value displayed. This feature is often seen in the Barbados radar data and is due to a relatively constant electromagnetic source from a fixed position.The most likely source is a cell phone or other microwave communications tower.

Summary

This module provided examples of radar imagery used by Caribbean nations in forecasting operations. A variety of products are available to help you analyze how the weather situation is unfolding.

Operational radars conduct volume scans by taking full sweeps of the atmosphere at a variety of elevations angles to collect a comprehensive data set of the atmosphere surrounding the radar. These data are then processed into a variety of derived products including VCut images and MAX.

Non-meteorological features often appear in radar data. Being able to distinguish these features will prevent misinterpretation of echoes that are not clouds and provide information of other phenomena that might be present in the area.

Forecasting in the Caribbean is challenging. Since data sources are limited over the ocean, remote sensing data can fill critical voids in available data. Weather radar plays an important role in the analysis of weather situations and specifically in the development and evolution of convection.