Lab 6 Radar Imagery Interpretation

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Lab 6 Radar Imagery Interpretation
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Background

• Weather radar (radio detection and ranging) is
  another very useful remote sensing tool used in
  meteorological forecasting.
• Microwave radar was developed in early World War
  II to aid in spotting distant ships and
  airplanes. It was noticed early on that during
  adverse weather conditions widespread
  interference often appeared on the radar screen
  and obscured the military objects of interest.
• A large body of theoretical and experimental work
  in the 1940s showed that this weather clutter
  arose from the scattering of radar waves by
  precipitation.

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• These early findings have been refined and
  elaborated to the point that most of the
  measurable properties of radar signals -
  amplitude, phase, polarization, and frequency -
  can be interpreted in terms of the sizes, shapes,
  motions or thermodynamic phase of the
  precipitation particles.
• Because of their ability to observe and measure
  precipitation quickly, accurately, and from great
  distances, radars have become essential in
  weather observation and forecasting.

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• Heavier precipitation reflects more microwave
  energy back to a radar than lighter rain.
  However, more distant rain also gives a weaker
  return signal. A range-corrected and
  equipment-calibrated measure of reflectivity from
  rain is given by
• log(Z) log(received power) 2 log(range)
  constant
• where Z is the radar reflectivity factor. Because
  Z has such a wide range of values, the
  reflectivity is usually expressed as decibels dB
  of Z.

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• dBZ 10 log(Z)
• Larger and more numerous drops reflect more radar
  energy
• Z SD6/ V
• where D is drop diameter, V is volume of air
  holding the drops, and the sum is over all
  raindrops within that volume.

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• But the number and diameter of drops also
  determines the rainfall rate.
• When the above three equations are combined and
  empirically tuned to the observations, the result
  is a formula for converting radar echo intensity
  in dBZ to rainfall rate R
• R cR100.0625dBZ
  Eqn 1
• where cR 0.036 mm h-1 .
• Six discrete levels of radar echo intensity are
  often used, corresponding to descriptive rainfall
  categories.

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Figure 8-12 Rainfall intensity chart (Stull 2000)
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• Owing to their particular shapes, snowflakes
  produce echoes of different intensity than
  raindrops of the same size.
• The snowfall rate S is therefore often inferred
  from the radar reflectivity factor from
• S cS100.05dBZ
• where cS 0.018 cm h-1 .
• This relationship assumes that 1 cm of melted
  snow equates 1 mm of water, i.e. that the falling
  snow has a density of 100 kg m-3 .

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Example

• Question What is the rainfall rate R for an echo
  of 43 dBZ?
• Answer Use Eqn 1
• R cR100.0625dBZ 0.036 mm/h(100.062543)
  17.5 mm/h

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Example 2

• Question What is the reflectivity Z for an echo
  of 43 dBZ?
• Answer Use dBZ 10 log(Z)
• Then log(Z) dBZ/10 43/10 4.3
• Thus Z 104.3 19952.6

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Radar Displays
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• There are several major types of radar displays
• Plan Position Indicator (PPI) This is a map
  displaying radar reflectivity or echoes on polar
  coordinates in plan view.
• With elevation angle fixed, the antenna scans
  360o in azimuth with the beam sweeping across a
  conical surface in space.
• The data are then used to give the horizontal
  echo structure.

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• Range Height Indicator (RHI) This is similar to
  a PPI, where the antenna scans in elevation with
  azimuth fixed, and emphasizes the vertical
  structure of a precipitating system.

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• Constant-Altitude Plan Position Indicator
  (CAPPI) It is a composite radar display
  constructed by assembling radar data from many
  PPIs at successive elevation angles to obtain the
  pattern of the data at a specified constant
  altitude.
• CAPPIs are the common radar images displayed by
  Environment Canada (at 1.5 km above the surface
  from the radar site).

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• A Doppler radar is a weather radar that detects
  and interprets the Doppler effect in terms of
  radial velocity of a target.
• The signal received by a radar from a moving
  target differs in frequency from the transmitted
  frequency by an amount that is proportional to
  the radial component of the velocity relative to
  the radar.

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• A useful introduction to Doppler radar can be
  found on NOAA's website.
• Canada is in the process of converting all of its
  weather radars to Doppler radars.
• The full Canadian network of weather radars,
  including the one at Baldy Hughes, B.C. (40 km
  southwest of Prince George) can be found on
  Environment Canada's website.

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Materials

• You are provided with two sequences of Doppler
  radar imagery covering parts of North America
• A sequence of Doppler radar images of a severe
  thunderstorm outbreak that occurred in the
  midwestern U.S. (near St. Louis, Missouri) in May
  of 2003.
• Another sequence of Doppler radar images of the
  winter storm that affected the Prince George area
  on 26-27 February 2006.

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Questions

• 1) Examine carefully the sequence of CAPPI images
  (base reflectivity) for the midwestern U.S.
  assuming ambient air temperatures are well above
  0oC and answer the following questions
• A) What is the precipitation rate for the maximum
  echoes observed on these radar images (65 dBZ )?
• B) What type of precipitation does this intensity
  typically represent?
• C) What is the radar reflectivity factor (Z ) for
  this echo?

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• D) What is the precipitation rate for an average
  echo observed on these radar images (30 dBZ)?
• E) Observe the evolution of the severe
  thunderstorm just to the south of St. Louis.
  Track the location of its maximum echoes over
  time on a map of the area. From this trajectory,
  linearly extrapolate the direction in which the
  storm is expected to move if it does not
  dissipate. Highlight the areas where a severe
  thunderstorm warning should be issued.

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• 2) Now examine the other sequence of CAPPI images
  based on the Baldy Hughes Doppler weather radar
  (centre point on the images) recorded between
  1850 UTC and 2220 UTC on 27 February 2006
  assuming ambient air temperatures are below
  freezing and answer the following questions
• A) Tabulate and plot the approximate
  precipitation rate for Prince George over time
  during the storm. When does the snow finally end
  falling in Prince George?
• B) Integrate in time the snowfall rate over
  Prince George to obtain the total snow
  accumulation during this period.
• C) Are the observations from Prince George
  Airport consistent with the radar observations?
  Recall that there is an 8-hour difference between
  Pacific Standard Time (PST) and Universal Time
  Coordinates (UTC).

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• D) Focus on a persistent and coherent radar echo
  over several images. Based on the time elapsed
  between images and the distance between
  concentric circles on the maps, determine the
  approximate speed of the system. Please indicate
  the approximate area on which your calculations
  are based.
• E) What do the radar echoes between Burns Lake
  and Fort St. James (140 km northwest of Baldy
  Hughes) represent? Do you observe similar echoes
  elsewhere?
• F) What other errors of interpretation may arise
  from the remote sensing of precipitation by
  land-based weather radar? You may consult
  Environment Canada's website for hints.

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• G) What is the horizontal resolution of the CAPPI
  images from the Baldy Hughes weather radar?
• H) A typical value of the radar reflectivity
  factor for this snowstorm is Z 30 dBZ . Convert
  this quantity into a snowfall rate and express it
  in units of mm h-1 snow water equivalent (swe)
  using the assumption that the snow has a density
  of 100 kg m-3.
• I) Compare the precipitation rate for the average
  echo of Z 30 dBZ deduced for rain (U.S.
  midwestern case) and for snow (Prince George
  case).

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• J) Discuss some of the possible reasons that
  would explain differences in the average
  precipitation rates observed in the two case
  studies.
• K) Bonus Question Imagine a situation in which
  solid precipitation is falling from clouds
  situated several kilometres above the surface.
  The air temperature in the clouds is about -15oC
  whereas the near-surface air temperature is 5oC.
  As the snow falls into a layer of above freezing
  air near the surface, it will melt. What will be
  the impact of this phase change on the radar
  reflectivity if it set to detect snowfall only?

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Sources

• Environment Canada
• United States National Weather Service
• McGill Weather Radar